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ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V004T00A001. doi:10.1115/PVP2017-NS4.
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This online compilation of papers from the ASME 2017 Pressure Vessels and Piping Conference (PVP2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Fluid-Structure Interaction: General FSI Applications and Transient Thermal Hydraulics

2017;():V004T04A001. doi:10.1115/PVP2017-65143.

Transient fluid velocity and pressure fields in a pressurized water reactor (PWR) steam generator (SG) secondary side during the blowdown period of a feedwater line break (FWLB) accident were numerically simulated employing the saturated liquid flashing model. This model is based on the assumption that compressed water in the SG is saturated at the beginning and decompresses into the two-phase region where saturated vapor forms, creating a mixture of steam bubbles in liquid by bulk boiling.

The numerical calculations were performed for two cases where the outflow boundary condition is different from each other; one is specified as the direct blowdown discharge to atmospheric pressure and the other is specified as the blowdown discharge to an extended calculation domain with atmospheric pressure on its boundary.

To effectively simulate the saturated water flashing from the SG following the FWLB accident, the physical SG model was simplified as a vertical once-through SG to which a feedwater pipe is attached. However, the physical geometry of the analysis model was modeled as realistically as possible in terms of the SG tube bundle height, the SG inner diameter and porosity, the inner diameter and length of broken feedwater pipe part, etc. It was considered that the SG shell-side and the attached feedwater pipe were initially filled with high pressure saturated water. The pressure in the steam space was 7.5 MPa. For the calculation of the two-phase flow during high pressure saturated water flashing from the SG through the broken feedwater pipe, the inhomogeneous two-fluid model was used.

The present simulation results were discussed through a comparison with the predictions using a simple non-flashing model neglecting the effects of phase change.

Based on the comparative discussions, the applicability of each of the non-flashing liquid discharge and saturated liquid flashing discharge models to the confirmatory safety evaluations of new SG designs was examined.

Commentary by Dr. Valentin Fuster
2017;():V004T04A002. doi:10.1115/PVP2017-65154.

Many pipes branch off from the main pipes in power plants. Main flow with high velocity initiates a cavity flow in a downward branch pipe with a closed end. Hot water penetrates into the branch pipe and a thermally stratified layer forms in the branch pipe if the main flow is hot. Fluctuations of the thermally stratified layer may initiate wall temperature fluctuations and thermal fatigue cracks in the branch pipe. Penetration depth of the main flow and the fluctuation characteristics into the branch pipe with a closed end were investigated by experiments. Experiments were conducted for various inner diameters of a branch pipe and main flow velocities under room temperature conditions. Flow structure was observed by test section made of acrylic resin. A tracer method was used to measure the penetration depth of the main flow. The penetration depth of the main flow changed periodically. The maximum penetration depth of the main flow was correlated by the Reynolds number. The fluctuation range and period of the penetration depth were also investigated. Next, the flow patterns on the cross-sectional plane in the branch pipe were observed to investigate the fluctuation mechanism of penetration depth. Three flow patterns were observed on the cross-sectional plane in the branch pipe. They were flow parallel to the cross-sectional direction, flow consisting of small vortexes and large swirl flow. The generation period of the large swirl flow was nearly equal to the fluctuation period of the penetration depth. The fluctuation range of the penetration depth and the duration showed similar trends for different inner diameters of the branch pipe. These results showed that the fluctuation of the penetration depth was caused by the periodic generation of the large swirl flow.

Commentary by Dr. Valentin Fuster
2017;():V004T04A003. doi:10.1115/PVP2017-65360.

Within the break preclusion concept, leak-before-break (LBB) behavior must be demonstrated for safety relevant pressure retaining piping systems in nuclear power plants (NPP) [1]. This requires leak detection systems in NPP with the capability to detect leak rates below the maximum allowable leak rate calculated by the LBB assessment according to nuclear standards, like the German KTA rule 3206 or the U.S. Standard Review Plan (SRP). An important part of the LBB assessment is the availability of accurate calculation models to predict the leak rate under normal operating conditions for postulated through wall cracks. Current leak detection systems in NPP are capable of reliably detecting liquid leak rates 0.05 kg/s. However, most of the available experimental leak rate data published in literature focus on the range between 0.2 kg/s and 2 kg/s, which is significantly above the detection limit. Therefore, additional experimental investigations are necessary to develop and verify leak rate calculation models for smaller leaks.

In order to investigate such types of leaks, a modular test facility (fluid-structure-interaction test loop) has been developed and installed at MPA University of Stuttgart within the framework of a research project sponsored by the German Ministry of Education and Research (BMBF). The test rig includes a leakage piping module which includes artificially machined slits and fatigue through-wall cracks. It allows the variation of the significant influencing parameters such as crack size, surface roughness and the system parameters pressure and temperature up to 75 bar and 280 °C and also the measurement of the pressure gradient across the crack surface at two locations. This is important to develop a better understanding of the two-phase flow and pressure drop across the leak channel.

A first test series has been performed and the results were used to evaluate existing leak-rate models. Within this paper an overview of the test facility, the testing procedure, and the results of the investigations will be presented and discussed.

Topics: Pressure , Pipes , Leakage
Commentary by Dr. Valentin Fuster
2017;():V004T04A004. doi:10.1115/PVP2017-65404.

This paper presents an overview of a numerical method developed to allow one-way structure-fluid interaction of a scanned representative surface of a Pressure Relief Valve (PRV) measuring 100 μm by 100 μm to be incorporated into a coupled finite element and computational fluid dynamics model to investigate gas leak rates through micro-gaps in full size metal-to-metal contacting components. The virtual representative surface is created via a real scan using a 3D micro coordinate and surface roughness measurement system. The scan of the physical surface is converted to a CAD format and a finite element model generated which is deformed for a given loading condition. The micro-gaps of the deformed FEA model are extracted and imported into the CFD solver to find the resulting volumetric/mass flow rate for the same set of pressure conditions. This coupled approach allows the leakage rate to be found based on only the surface roughness of metal-to-metal sealing surfaces. This methodology can now be expanded to understand the behaviour and response of metal-to-metal deformable contacting surface components under pressure. Thereafter, the design objective is to minimise or eliminate component leakage.

Commentary by Dr. Valentin Fuster
2017;():V004T04A005. doi:10.1115/PVP2017-65471.

The syringe in a subcutaneous autoinjector may be subjected to internal pressure transients due to the normal operation of the injection mechanism. These transients are similar to transients in fluid-filled pipelines observed during water hammer events. In this paper, the effect of an air gap in the syringe and a converging section are studied experimentally and numerically in a model system which consists of a fluid-filled metal tube that is impulsively loaded with a projectile to simulate the action of the autoinjector mechanism operation.

The air between the buffer and the water results in a complex interaction between the projectile and the buffer. Also, there are tension waves inside the tube due to the presence of a free surface, and this causes distributed cavitation which, in turn, gives rise to steepening of the pressure waves. The converging section can amplify the pressure waves if the wave front is sharp. Pressures as high as 50 MPa have been measured at the apex of the cone with impact velocities of 5.5 m/s.

Commentary by Dr. Valentin Fuster
2017;():V004T04A006. doi:10.1115/PVP2017-65601.

Japan Atomic Energy Agency is now conducting design study and R&D of an advanced loop-type sodium cooled fast reactor. The cooling system is planned to be simplified by employing a two-loop configuration and shortened piping with less elbows than a prototype fast reactor in Japan, Monju, in order to reduce construction costs and enhance economic performance. The design, however, increases flow velocity in the hot-leg piping and induces large flow turbulence around elbows. Therefore, flow-induced vibration (FIV) of a hot-leg piping is one of main concerns in the design. Numerical simulation is a useful method to deal with such a complex phenomenon. We have been developing numerical analysis models of the hot-leg piping using Unsteady Reynolds Averaged Navier-Stokes simulation with Reynolds stress model. In this study, numerical simulation of a 1/3 scaled-model of the hot-leg piping was conducted. The results such as velocity profiles and power spectral densities (PSD) of pressure fluctuations were compared with experiment ones. The simulated PSD of pressure fluctuation at the recirculation region agreed well with the experiment, but it was found some underestimation at other parts, especially in relatively high frequency range. Eigenvalue vibration analysis was also conducted using a finite element method. Then, stress induced by FIV was evaluated using pressure fluctuation data calculated by URANS simulation. The calculated stress generally agrees well the measurement values, which indicates the importance of precise evaluation of the PSD of pressure fluctuation at the recirculation region for evaluation of FIV of the hot-leg piping with a short elbow.

Commentary by Dr. Valentin Fuster
2017;():V004T04A007. doi:10.1115/PVP2017-65606.

The effect of high-temperature water environment on the fatigue life of steels used for pressure retaining components has been discussed controversially for the last 20 to 30 years. Fatigue testing of laboratory specimens for typical steels showed significant drops in fatigue life when tested in high temperature water environment compared to air environment. Based on these findings the applicability of fatigue design curves such as those enclosed to ASME code Section III NB are questionable concerning their degree of conservatism. Nevertheless, experience from components experiencing power plant operation does not match up with laboratory fatigue testing of small uniaxial specimens. Fatigue life estimations based on models representing laboratory tests do highly overestimate the fatigue life reduction resulting from high temperature water environments compared to the analyses of components having reached their postulated fatigue life. To overcome this disagreement component testing under defined laboratory conditions is highly desired to achieve “gap closure”.

At MPA University of Stuttgart a test facility was set up where environmental fatigue testing on component level can be realized within a hot water test loop. Within the framework of a research project sponsored by the German Federal Ministry of Education and Research (BMBF) piping modules containing a dissimilar metal weld are exposed to water environments with alternating temperature conditions. At specific locations water at about room temperature is injected to a hot pipe segment which results in thermal induced loading situations. Consequently thermal stratification and shocks cause localized stresses and strains in the tested modules. Within this paper an overview of the testing procedure, the tested materials and results from both experimental measurements and fractographic analyses are presented and discussed. In addition to experimental investigations the results drawn from a coupled computational fluid dynamics (CFD) and structural mechanics finite-element-analysis (FEA) including a fatigue life assessment are shown. Finally, this work states on the applicability of common fatigue assessment procedures including the fatigue life reducing factors based on the results from realistic fatigue testing on component level.

Within low cycle fatigue tests a nickel-base weld material was characterized regarding its fatigue life in air and high temperature water environment in comparison. It was found that the effect of environmentally assisted fatigue is in good agreement with what is known from literature for smooth specimens made from austenitic steels. Results from tests using notched specimens showed a significant change in the environmental effect compared to tests using smooth specimens. During component testing within a hot water loop modules which contain a dissimilar metal weld were exposed to alternating water temperature conditions between 20 °C and 65 °C. At the end of the component test cracks were found in the regions where the highest temperature changes were measured and calculated. The numerical analysis of the fluid-structure-interaction pointed out that the transition region between the austenitic steel and the nickel base weld material is the highest loaded section within the module. Finally the fatigue assessment of the pipe sections containing cracks showed that based on common fatigue hypothesis the loading state is regarded to be subcritical.

Commentary by Dr. Valentin Fuster
2017;():V004T04A008. doi:10.1115/PVP2017-65755.

Liquid slugs have a relatively low mass and can therefore — when they occupy a full cross-section of a pipeline — be accelerated to very high velocities by means of pressurized gas. When entrapped gas pockets are present, pressures and temperatures may become dangerously high. Simple models and analytical solutions are derived and used to predict transient velocities, pressures and temperatures. The models have a generic character as they also describe the basics of breaking surface waves impacting on a wall, and pigs and bullets propelled by compressed gas.

Topics: Pipelines , Slug flows
Commentary by Dr. Valentin Fuster
2017;():V004T04A009. doi:10.1115/PVP2017-66020.

The presence of air in piping systems is a major concern in the industry. Problems like flow disruption, reduction of hydraulic machinery efficiencies or a significant drop in pipe capacity are many times related to this fact. The present paper aims to find a simple and non-intrusive experimental method to detect air in piping systems. The method, based on the dynamic properties of fluid-structure systems and underpinned by a novel low computational cost numerical simulation, accurately predicts the volume of water present in a pipe. Good agreement between numerical and experimental solutions has been obtained using much less computational effort than traditional fully coupled Fluid Structure Interaction with CFD analysis. From the numerical and experimental data, two different mathematical expressions relating the system natural frequencies, both vertically and horizontally, and the area occupied by the water have been obtained. These expressions account for the pipe geometry which theoretically would make them suitable for other diameter and wall thickness values. The paper is combined with a preliminary study of the system’s mode shapes for the different volumes of water.

Commentary by Dr. Valentin Fuster
2017;():V004T04A010. doi:10.1115/PVP2017-66153.

The safety of high pressure and high temperature components is paramount, and therefore, developing effective and reliable methodologies to improve the prediction of crack propagation is an important task. The present paper describes and demonstrates a multi-physics numerical analysis approach for assessing crack propagation using a sensor device. This method employs a coupled structural-thermal-electric analysis in conjunction with a thermal-fluid-structure interaction analysis to study the structural health of a high pressure and high temperature component.

Commentary by Dr. Valentin Fuster
2017;():V004T04A011. doi:10.1115/PVP2017-66278.

Major accidents that were affected by hydrogen fires and explosions included Chernobyl, Three Mile Island, and Fukushima Daiichi. Smaller piping explosions have occurred at Hamaoka and Brunsbüttel Nuclear Power Plants. An overview of pertinent topics is presented here to compare similarities and differences between these accidents.

In particular, a hydrogen ignition mechanism is presented here, where fluid transients, or water hammer, may cause pressures to compress flammable hydrogen gas in reactor systems. As the gas compresses, it heats to temperatures sufficient to cause autoignition, or dieseling. Autoignition then leads to fires or explosions in nuclear power plant systems.

To explain this evolving theory on hydrogen ignition during fires and explosions, various nuclear power plant hydrogen accidents require discussion. For example, Chernobyl explosions were unaffected by water hammer, while a Three Mile Island hydrogen fire was a direct result of water hammer following a reactor meltdown, and explosions that followed a meltdown at Fukushima Daiichi occurred during a water hammer event. Other piping damages also occurred during water hammer events. The primary purpose of this paper is to serve as a literature review of past accidents and to provide new insights into those accidents.

In short, what is known versus what is unknown is discussed here with respect to the ignition sources of nuclear power plant fires and explosions. How can nuclear power plant safety be assured unless previous fire and explosion causes are understood? Prior to this work, they were not understood.

Commentary by Dr. Valentin Fuster
2017;():V004T04A012. doi:10.1115/PVP2017-66279.

Water hammers, or fluid transients, compress flammable gasses to their autognition temperatures in piping systems to cause fires or explosions. While this statement may be true for many industrial systems, the focus of this research are reactor coolant water systems (RCW) in nuclear power plants, which generate flammable gasses during normal operations and during accident conditions, such as loss of coolant accidents (LOCA’s) or reactor meltdowns. When combustion occurs, the gas will either burn (deflagrate) or explode, depending on the system geometry and the quantity of the flammable gas and oxygen. If there is sufficient oxygen inside the pipe during the compression process, an explosion can ignite immediately. If there is insufficient oxygen to initiate combustion inside the pipe, the flammable gas can only ignite if released to air, an oxygen rich environment. This presentation considers the fundamentals of gas compression and causes of ignition in nuclear reactor systems. In addition to these ignition mechanisms, specific applications are briefly considered. Those applications include a hydrogen fire following the Three Mile Island meltdown, hydrogen explosions following Fukushima Daiichi explosions, and on-going fires and explosions in U.S nuclear power plants. Novel conclusions are presented here as follows.

1. A hydrogen fire was ignited by water hammer at Three Mile Island.

2. Hydrogen explosions were ignited by water hammer at Fukushima Daiichi.

3. Piping damages in U.S. commercial nuclear reactor systems have occurred since reactors were first built. These damages were not caused by water hammer alone, but were caused by water hammer compression of flammable hydrogen and resultant deflagration or detonation inside of the piping.

Commentary by Dr. Valentin Fuster
2017;():V004T04A013. doi:10.1115/PVP2017-66284.

An explosion that burst a steel pipe like a paper fire cracker at the Hamaoka Nuclear Power Station, Unit-1 is investigated in this paper, which is one of a series of papers investigating fires and explosions in nuclear power plants. The accumulation of flammable hydrogen and oxygen due to radiolysis has long been recognized as a potential problem in nuclear reactors, where radiolysis is the process that decomposes water into hydrogen and oxygen by radiation exposure in the reactor core. Hydrogen ignition and explosion has long been considered the cause of this Hamaoka piping explosion, but the cause of ignition was considered to be a minor fluid transient, or water hammer, that ignited flammable gasses in the piping, which was made possible by the presence of catalytic noble metals inside the piping. The theory presented here is that a much larger pressure surge occurred due to water hammer during operations. In fact, calculations presented here serve as proof of principle that this explosion mechanism may be present in many operating nuclear power plants.

Chubu Electric, the operator of the Hamaoka plant, took appropriate actions to prevent this type of explosion in their plants in the future. In fact, this accident indicates one potential preventive action from explosions for other operating plants. Ensure that a system high point is available, where mixed hydrogen and oxygen may be removed during routine operations and during off-normal accident conditions, such as nuclear reactor meltdowns and loss of coolant accidents.

Commentary by Dr. Valentin Fuster
2017;():V004T04A014. doi:10.1115/PVP2017-66285.

Requiring further investigation, hydrogen explosions and fires have occurred in several operating nuclear reactor power plants. Major accidents that were affected by hydrogen fires and explosions included Chernobyl, Three Mile Island, and Fukushima Daiichi. Smaller piping explosions have occurred at Hamaoka and Brunsbüttel Nuclear Power Plants. This paper is the first paper in a series of publications to discuss this issue. In particular, the different types of reactors that have a history of fires and explosions are discussed here, along with a discussion of hydrogen generation in commercial reactors, which provides the fuel for fires and explosions in nuclear power plants.

Overall, this paper is a review of pertinent information on reactor designs that is of particular importance to this multi-part discussion of hydrogen fires and explosions. Without a review of reactor designs and hydrogen generation, the ensuing technical discussions are inadequately backgrounded. Consequently, the basic designs of pressurized water reactors (PWR’s), boiling water reactors (BWR’s), and pressure-tube graphite reactors (RBMK) are discussed in adequate detail. Of particular interest, the Three Mile Island design for a PWR is presented in some detail.

Commentary by Dr. Valentin Fuster

Fluid-Structure Interaction: Structures Under Extreme Loading Conditions

2017;():V004T04A015. doi:10.1115/PVP2017-65146.

The Hanford Tank Waste Treatment and Immobilization Plant (WTP) will process waste slurries capable of generating a dispersed non-condensable gas phase. Under postulated conditions, the generated bubbles may coalesce to form flammable gas pockets. Ignition of such a gas pocket may result in a deflagration or detonation that can transmit a pressure pulse into the surrounding gas-liquid slurry. The transmitted pressure pulses will impose structural loads on the piping and associated supports. These loads, and their evolution as the pressure pulse propagates throughout the system, must be considered in the design of WTP piping. Recent work has demonstrated good agreement between experimental data and predictions of the pressure diminishment associated with the propagation of a pressure pulse through a bubbly medium using a simplified onedimensional pursuit model (i.e., model of rarefaction wave overtaking shock front).

This paper describes the application of the pursuit model to flammable gas pocket scenarios postulated in the design of WTP piping. The model is adapted to consider the effect of the time-evolution of the ignited gas pocket and parametric simulations are used to demonstrate the effect of key variables on the propagation and diminishment of the pressure pulse in piping. The practical applicability of the pursuit model for WTP piping design / analysis is illustrated through the development of a correlation that appropriately represents the results of the pursuit model for a wide range of conditions and that could be readily incorporated into existing structural analysis tools. Arguments are presented supporting the applicability of the simplified model even for scenarios in which the ignited gas pocket does not satisfy the model’s onedimensional assumption.

Topics: Pressure , Design , Pipes , Hydrogen
Commentary by Dr. Valentin Fuster
2017;():V004T04A016. doi:10.1115/PVP2017-65186.

High-pressure pipeline ruptures are a credible explosion hazard at many industrial facilities. The blast field generated by a pipe rupture is highly directional. However, there have been few evaluations of the directional blast loads produced by pipe ruptures. This paper addresses the blast loads generated by a typical “fish mouth” type pipe rupture. The effects of five key parameters on the resulting directional blast field were examined: rupture opening speed, final rupture opening area, pipe diameter, initial gas pressure, and initial gas temperature. The resultant blast loads were compared to those based on existing blast curves for Pressure Vessel Bursts (PVB), the most common of which is based on an assumption of a spherical vessel geometry and instantaneous failure of the entire pressure vessel boundary. The effective gas volume (i.e., number of pipe diameters) required to achieve reasonable agreement between the blast load based on existing PVB blast curves and that resulting from a high-pressure pipeline fish mouth rupture for a specified direction was determined.

Topics: Blast effect , Pipes , Rupture
Commentary by Dr. Valentin Fuster
2017;():V004T04A017. doi:10.1115/PVP2017-65187.

Blast walls are frequently considered as a potential mitigation option to reduce the applied blast loading on a building or structure in cases where unacceptably high levels of blast damage are predicted. There are three general explosion types of interest with respect to blast loading: High Explosive (HE), Pressure Vessel Burst (PVB), and Vapor Cloud Explosion (VCE). The blast waves resulting from these explosion types can differ significantly in terms of blast wave shape and duration. The effectiveness of a blast wall depends on these blast wave parameters (shape and duration), as well as the blast wall parameters (e.g., height, width and standoff distance from the protected structure). The effectiveness of a blast wall in terms of mitigating the blast loading on a protected structure depends on the combination of the blast wave and blast wall parameters. However, little guidance is available on the effectiveness of blast walls as a mitigation option for non-HE explosion sources. The purpose of this paper is to characterize the effect of blast wave parameters on the effectiveness of a blast wall and to provide guidance on how to determine whether a blast wall is an effective and practical blast damage mitigation option for a given blast loading.

Commentary by Dr. Valentin Fuster
2017;():V004T04A018. doi:10.1115/PVP2017-65205.

The growing number of terrorist attacks in the past decade has focused the public’s attention on the severity of such a man–made hazard. The rising threat of improvised explosive devices — one of the most successful attack strategies — has significantly increased the number of threats on the ground, in the form of suicide–bombs, vehicle–bombs, etc., thereby requiring the development of more effective blast risk mitigation measures. However, the modern proliferation of such measures poses the problem of evaluating their cost–effectiveness, which prompts the need for a comprehensive optimization methodology — capable of maximizing the resilience of the built environment. The aim of this paper is to lay out the foundations of a resilience–based framework for quantifying the performance of different infrastructure elements incurring blast threats, by means of functionality and resilience indicators. The proposed framework can quantify the consequences of multiple outdoor explosions typified by the emblematic car–bomb scenario. The level of localized damage is evaluated via pressure–impulse diagrams; local failures are then aggregated into the definition of resilience and functionality indicators, designed to provide the analyst with a comprehensive picture of global damage, residual functionality, and downtime of the structural system.

Topics: Resilience
Commentary by Dr. Valentin Fuster
2017;():V004T04A019. doi:10.1115/PVP2017-65234.

Blast-proof unit is designed to alleviate the shock wave transmitting to the other unit, in which the damage of accidental explosion is mainly caused by the transmission wave through the wall and diffraction wave over the upper edge. Investigation on the blast flow field in blast-proof units subjected to internal blast loading is studied by numerical simulation in the current paper. The influence of the height of the proof unit is analyzed and on the cap situation is studied. The result shows that it may not have a positive effect on reducing the diffraction wave by increasing the height of units, because it may enhance the reflection wave, which might have a greater effect than the diffraction wave. We can reduce the shock wave in the next unit by adding a cap, which is very effective. The study may contribute to further understanding on the experiment results and the design of blast-proof units.

Commentary by Dr. Valentin Fuster
2017;():V004T04A020. doi:10.1115/PVP2017-65301.

The resilience of the built environment to high explosives poses a significant challenge to the professionals tasked with the design of blast resistant facilities. Current standards — including the ASCE 59-11 and CSA S850-12 — fail to address this challenge in design provisions targeting a single parameter of structural performance, while neglecting other key indicators of performance recovery that define the very concept of resilience. In order to investigate their significance in the design process, two resilience parameters known as robustness and rapidity are evaluated for an archetype blast scenario — a nuclear power plant (NPP) featuring reinforced concrete block masonry walls exposed to a blast hazard, namely, the detonation of an explosive charge within an open (outdoor) area of the industrial complex. The adopted methodology integrates resilience–based analysis and probabilistic risk assessment, in order to account for the uncertainties associated with threat (attack likelihood); hazard (attacker’s success likelihood); load input variables — including location, mass, and type of explosive; resistance variables — including material properties and wall geometry; and loss variables — including the costs of repair and replacement. Based on the current analysis, recommendations are made to incorporate resilience metrics in standards for blast protection, so as to foster more resilient industrial facilities.

Commentary by Dr. Valentin Fuster
2017;():V004T04A021. doi:10.1115/PVP2017-65517.

A series of experiments were performed to study plastic deformation of metallic plates under hypervelocity impact at the University of Nevada, Las Vegas (UNLV) Center for Materials and Structures using a two-stage light gas gun. In these experiments, cylindrical Lexan projectiles were fired at A36 steel target plates with velocities range of 4.5–6.0 km/s. Experiments were designed to produce a front side impact crater and a permanent bulging deformation on the back surface of the target without inducing complete perforation of the plates. Free surface velocities from the back surface of target plate were measured using the newly developed Multiplexed Photonic Doppler Velocimetry (MPDV) system.

To simulate the experiments, a Lagrangian-based smooth particle hydrodynamics (SPH) is typically used to avoid the problems associated with mesh instability. Despite their intrinsic capability for simulation of violent impacts, particle methods have a few drawbacks that may considerably affect their accuracy and performance including, lack of interpolation completeness, tensile instability, and existence of spurious pressure. Moreover, computational time is also a strong limitation that often necessitates the use of reduced 2D axisymmetric models. To address these shortcomings, IMPETUS Afea Solver® implemented a newly developed SPH formulation that can solve the problems regarding spurious pressures and tensile instability. The algorithm takes full advantage of GPU Technology for parallelization of the computation and opens the door for running large 3D models (20,000,000 particles). The combination of accurate algorithms and drastically reduced computation time now makes it possible to run a high fidelity hypervelocity impact model.

Commentary by Dr. Valentin Fuster
2017;():V004T04A022. doi:10.1115/PVP2017-66110.

This paper describes the methods and criteria for the numerical analysis of damage caused by tornado missiles to steel stacks. The paper will describe the range of classic design-basis tornado missiles, the target steel stacks, and the key parameters that affect the analysis. The models of missile and targets will be addressed, as well as the benchmark of calculated damage compared to experimental data. Strain-based material models and strain-based criteria will be described, to predict the various failure modes and the structural and functional damage criteria.

Commentary by Dr. Valentin Fuster

Fluid-Structure Interaction: Symposium on Flow-Induced Vibration

2017;():V004T04A023. doi:10.1115/PVP2017-65019.

When the elastic deformation of the tube bundle is considered, the interaction between the flow field and the structure becomes more complicated. In order to investigate the flow induced vibration (FIV) problems in flexible tube bundle, a numerical model for fluid-structure interaction system was presented firstly. The unsteady three-dimensional Navier-Stokes equation and LES turbulence model were solved with the finite volume approach on structured grids combined with the technique of dynamic mesh. The dynamic equilibrium equation was discretized according to the finite element theory. The configurations considered are tubes in a cross flow. Firstly, the flow-induced vibration of a single flexible tube under uniform turbulent flow are calculated when Reynolds number is 1.35× 104. The variety trends of lift, drag, displacement, vertex shedding frequency, phase difference of tube are analyzed under different frequency ratios. The nonlinear phenomena of locked-in, phase-switch are captured successfully. Meanwhile, the limit cycle and bifurcation of lift coefficient and displacement are analyzed using trajectory, phase portrait and Poincare sections. Secondly, the mutual interaction of two in-line flexible tubes is investigated. Different behaviors, bounded by critical distances between the tubes, critical velocity, and wake vortex pattern are highlighted. Finally, four tube bundle models were established to study the effect of the number of flexible tube on the FIV characteristics. Thanks to several calculations, the critical velocity of instability vibration and the effect of tube bundle configurations on fluid forces and dynamics were obtained successfully. It is therefore expected that further calculations, with model refinements and other validation studies, will bring valuable informations about bundle stability. Further comparisons with experiment are necessary to validate the behavior of the method in this configuration.

Topics: Vibration , Cross-flow
Commentary by Dr. Valentin Fuster
2017;():V004T04A024. doi:10.1115/PVP2017-65045.

Safety measures are required to insure the drop of control rods and that the core is cooled when the fuel assemblies of a Pressurized Water Reactor (PWR) are subjected to a seismic excitation. A way to insure these two criteria is to prevent the spacer grids from buckling. The reactor core made of fuel assemblies is subjected to an axial water flow to cool the reactor. The flow modifies the dynamical behaviour of the fuel assemblies. Tests made on a real fuel assembly highlighted an added stiffness effect under axial flow. In previous studies simulations were compared to experiment involving by-passes significantly larger than the distance between two fuel assemblies in a PWR core. Thus, one could wonder if the observations made on a fuel assembly with large by-passes are representative of core geometry. Simulations using a fluid-structure model of the core to a seismic excitation have been proposed. A parametric study has been conducted to observe the effect of confinement on the added stiffness effect for several confinements and bulk velocities. Simulations showed that the added stiffness reaches a maximum for a confinement around 20 mm, and that the added stiffness should be negligible in a real core configuration.

Commentary by Dr. Valentin Fuster
2017;():V004T04A025. doi:10.1115/PVP2017-65162.

Stream-wise fluidelastic instability has recently been in the spotlight due to a practical event in steam generators at a nuclear power plant [1]. The instability has been reported to occur easily in rotated triangular arrays, but at all hardly in normal triangular arrays [2][3]. In square arrays, no instability has been reported in the authors’ test facility [4].

This paper presents the test results on rotated square arrays of the pitch-diameter ratios from 1.2 to 1.5. The tests have been conducted with a wind tunnel, where cylinders have been constrained to move in only one specific direction, either stream-wise or transverse to the flow. The results indicate that fluidelastic vibrations occur only in the stream-wise direction. The critical factor corresponds to the previous data reported in some guidelines [5], but here the factor is related mainly to stream-wise instability.

Commentary by Dr. Valentin Fuster
2017;():V004T04A026. doi:10.1115/PVP2017-65172.

Spent nuclear fuel is settled in racks and stored in spent fuel pool. A free standing rack (FS rack) is a type of a spent fuel rack, which is not fixed to walls unlike conventional ones. For this characteristic, movement of an FS rack during an earthquake can be reduced by fluid force and friction force. However, collision between a rack and another rack or a wall must be avoided. Therefore, it is necessary for designing an FS rack to figure out how it moves under seismic excitation. In this research, a dynamic model of FS racks is constructed considering seismic inertial force, friction force and fluid force. This model consists of two sub-models: translation model, which simulates planar translational and rotational motion; and rocking model, which simulates rocking motion. Moreover, we developed two kinds of rocking model: slide-rocking considered model, which considers the equations of both slide-rocking motion and non-slide-rocking motion; and non-slide-rocking model, which considers only the equation of non-slide-rocking motion. Then, simulations with sinusoidal inertial force input were conducted, changing values of friction coefficient. To validate this dynamic model, a miniature experiment was conducted. It is found that the non-slide-rocking model simulates movement of an FS rack well and better than the slide-rocking considered model in the aspect of translational and rocking movement. However, planar rotational movement is not simulated well with either model. Through this research, the knowledge is acquired that friction force plays a significant role in motion of an FS rack so that estimating and controlling friction coefficient is important in designing an FS rack.

Commentary by Dr. Valentin Fuster
2017;():V004T04A027. doi:10.1115/PVP2017-65179.

Two-phase flows are preponderant in industrial components. The present work deals with external two-phase flows across tube banks commonly found in heat exchangers, boilers and steam generators. The flows are generally highly complex and remain theoretically intractable in most cases. The two-phase flow patterns provide a convenient albeit qualitative means for describing and classifying two phase flows. The flow patterns are also closely correlated to fluid-structure interaction dynamics and thus provide a practically useful basis for the study of two-phase flow-induced vibrations.

For internal two-phase flows, maps by Taitel et al. (1980) and others have led to detailed and well defined maps. For transverse flows in tube bundles, there is significantly less agreement on the flow patterns and governing parameters. The complexity of flow in tube arrays is an obvious challenge. A second difficulty is the definition of distinct flow patterns and the identification of parameters uniquely identifying the flow patterns.

The present work addresses the problem of two-phase flow pattern identification in tube arrays. Flow measurements using optical as well as flow visualization via high-speed videos and photography have been conducted. To identify the flow patterns, an artificial intelligence machine learning approach was taken. Pattern classification was achieved by designing a support vector machine (SVM) classifier. The SVM achieves quantitative and non-subjective classification by mapping the flow patterns in a high dimensional mathematical space in which the different flow patterns have unique characteristics.

Details of the flow measurement, parameter definition and SVM design are presented in the paper. Flow patterns identified using the SVM are presented and compared with previously identified flow patterns.

Topics: Two-phase flow
Commentary by Dr. Valentin Fuster
2017;():V004T04A028. doi:10.1115/PVP2017-65189.

Flow-induced acoustic resonances in piping with closed side branches or T-junctions are one of the phenomena causing severe structural vibration and fatigue damage of the piping and components in many engineering applications such as power plants. Practical piping systems of power plants often have a steam flow, and moreover, the steam state can be not only dry steam but also wet steam. From our previous experiments under low-pressure dry and wet steam flows using a single side branch, higher acoustical damping was confirmed under wet steam than that under dry steam, which is considered to be caused by the existing liquid phase. Although the static pressure in practical steam piping is often higher than that in our previous experiments, the effects of the static pressure on acoustical damping under a wet steam flow have not been clarified. Thus, we constructed a new test facility that can be used to perform continuous flow test under dry and wet steam flows with higher pressures than our previous test facility. In this paper, we give an overview of the new steam test facility and some experimental results for the acoustic resonance in a single side branch under higher-pressure dry and wet steam flows than those in our previous studies, using the new facility to investigate and evaluate the effects of the static pressure.

Commentary by Dr. Valentin Fuster
2017;():V004T04A029. doi:10.1115/PVP2017-65207.

In steam generators and other heat exchangers, there are a lot of tube bundles subjected to two-phase cross-flow. The fluctuating pressure on tube bundle caused by turbulence can induce structural vibration. The experimental data from a U-tube bundle of steam generator in air-water flow loop are analyzed in this work. The different upper bounds of buffeting force are used to calculate the turbulence buffeting response of U-tubes, and the calculation results are compared with the experimental results. The upper bounds of buffeting force include one upper bound based on single-phase flow, and two upper bounds based on two-phase flow. It is shown that the upper bound based on single-phase flow seriously underestimated the turbulence excitation, the calculated vibration response is much less than the experimental measurement. On the other hand, the vibration response results calculated with the upper bounds based on two-phase flow are closer to the measured results under most circumstances.

Commentary by Dr. Valentin Fuster
2017;():V004T04A030. doi:10.1115/PVP2017-65240.

When flexible pipes are subjected to internal flow, the pipes lose stability by flutter and divergence in increasing the fluid velocity. In addition, they also lose stability when they are subjected to external annular axial flow. In this paper, the pipe is subjected to internal flow and external flow at the same time. The dynamic stability of a double wall pipe structure system subjected to an internal flow and an external flow simultaneously is thought to be one of the important pipe structures for the development of a piping system in the field of ocean mining, and in the field of fluid energy generation, and so on. In this paper, the pipe structures are assumed to be composed of the cantilevered elastic tube structure. For the analysis of the internal flow, the conventional inviscid stability analysis method is applied. For the analysis of the external annular axial flow, both the viscous solution using the Navier-Stokes equation of motion and the ideal fluid solution which viscous influence are added to are applied. Changing the flow direction and the fluid velocity as for the internal flow and the external flow, the dynamic stability of the double wall pipes is investigated and discussed. Moreover, changing the flow rate and the density of a fluid and a structure, these effects on the stability of double wall pipes are investigated.

Commentary by Dr. Valentin Fuster
2017;():V004T04A031. doi:10.1115/PVP2017-65244.

Flow-induced acoustic resonances in piping with closed side branches or T-junctions are one of the causes of severe structural vibrations, which sometimes cause fatigue damage to piping and components in a power plant and many engineering applications. In this paper, on the basis of the results of steam flow experiments and calculations, the effects of the liquid phase on the flow-induced acoustic resonance at closed side branches in the steam flow piping of BWRs are described, and some suggestions for the steam piping design of BWRs are also given. The liquid phase in a steam flow forms droplets or liquid film, which may affect the amplitude, frequency and critical Strouhal number of the resonance. From the results of wet steam experiments and CFD calculations, we have found that in some cases the wetness of the steam flow may decrease the resonant amplitude and change the frequency owing to the interaction of the vortex generation or damping by the existence of the liquid film and droplets. Therefore, for the wet steam piping design of BWR, some suggestions for taking these effects into consideration, under actual BWR steam conditions are described.

Commentary by Dr. Valentin Fuster
2017;():V004T04A032. doi:10.1115/PVP2017-65272.

The problem of fretting-wear damage between a vibrating structure and its supports is discussed in this paper. Typical components of concern are piping systems and pipe-supports, multispan heat exchanger tubes and tube supports, and nuclear fuel bundles and fuel channels.

Fretting-wear damage is related to the dynamic interaction between a structure and its supports. This interaction is conveniently formulated in terms of a parameter called “work rate” to predict fretting-wear damage. Work rate is simply the integral of contact force over sliding distance per unit time.

Fretting-wear damage may be investigated from an energy point of view. It is essentially the mechanical energy or power dissipated through contact forces and sliding that causes fretting-wear damage. Development of a simple formulation that relates tube vibration response and fretting-wear damage is reviewed in this paper. Some new practical examples and simple calculations are discussed.

Topics: Wear , Vibration , Damage
Commentary by Dr. Valentin Fuster
2017;():V004T04A033. doi:10.1115/PVP2017-65298.

Self-sustaining oscillations of flow over ducted cavities and corrugated pipes proved to be a potential source of tonal noise and possible failure in industrial applications. Most of the recent studies focused on the flow over a single cavity to simplify the problem and establish basic understanding of the phenomenon. This paper investigates experimentally the flow over multiple cavities, specifically two and three-cavity configurations, and the results are compared with those of a single cavity. Two different categories of experiments were performed in this study. The first category of measurements quantified the aeroacoustic source of various cavity configurations as a function of Strouhal numbers and acoustic velocity of the resonant pipe mode. The cavities are situated at the acoustic pressure node of a piping system which was sufficiently long to maintain the acoustic velocity fairly constant along the test section. The second category involved self-excited oscillations where the cavities were tested in a short piping system. The flow velocity was gradually increased and the acoustic pressure amplitude and frequency were measured and the lock-in ranges for different shear layer modes were identified. A semi-empirical model is then developed to use the measured source to predict the self-excited oscillation amplitude.

Commentary by Dr. Valentin Fuster
2017;():V004T04A034. doi:10.1115/PVP2017-65325.

To ensure the long-term safe operation of newly constructed Tainter gates, methods of analysis and design criteria are needed in the design stage to assure the dynamic stability of any new Tainter gate. For this purpose, the present study provides a detailed procedure for the dynamic design of Tainter gates that can be applied to preliminary designs by gate engineers to assure the dynamic stability of their gate designs. The dynamic stability of the gate can be determined using the natural vibration characteristics ascertained by finite element method (FEM) analysis, reasonable values of actual structure damping actually measured by the field vibration tests, and theoretical analysis of the coupled-mode self-excited vibration that has been previously established by authors. The procedure and the important points of each step are detailed in an example determination of the dynamic stability of a practical Tainter gate.

Commentary by Dr. Valentin Fuster
2017;():V004T04A035. doi:10.1115/PVP2017-65335.

This paper presents experiments and an analysis on self-excited vibration of a plate supported by air pressure in a floating conveying machine. In this study, the instability conditions are examined by theoretical analysis in consideration of the effect of compressibility of air in a chamber. The system’s characteristic equation is derived from the plate motion coupled with equations of the gap flow between the plate and the chamber surface. The vibration characteristics and the instability conditions of the self-excited vibration are examined through experiments. The stability of the plate is affected by an air flow rate, a mass of the plate, a spring stiffness of the plate. We clarified those influences on the instability conditions of the self-excited vibration. The unsteady fluid force acting on the plate (bottom surface) is investigated by measuring the unsteady pressure. The local work done by the unsteady fluid force is also clarified. Lastly, the instability mechanism and important parameters of the self-excited vibration are discussed based on the theoretical model and experimental results.

Commentary by Dr. Valentin Fuster
2017;():V004T04A036. doi:10.1115/PVP2017-65342.

Flow-induced vibrations of tubes in two-phase heat exchangers are a concern for the nuclear industry. EDF has developed a numerical tool, which allows one to evaluate safety margins and thereafter to optimize the exchanger maintenance policy.

The software is based on a semi analytical model of fluid-dynamic forces and dimensionless fluid force coefficients which need to be evaluated by experiment. A test rig was presented in previous PVP conferences with the aim of assessing parallel triangular tube arrangement submitted to a two-phase vertical cross-flow: a kernel of nine flexible tubes is set in the middle of a rigid bundle. These tubes vibrate as solid bodies (in translation) both in the lift and drag directions.

This paper presents some extended physical analysis applied to some selected points of the aforementioned experiment series: the response modes are identified by means of operational modal analysis (i.e. under unmeasured flow excitation) and presented in terms of frequency, damping and mode shapes.

Among all the modes theoretically possible in the bundle, it was found that some of them have a higher response depending on the flow velocity and the void fraction. Mode shapes allow to argue if lock-in is present and to clarify the role of lift and drag forces close to the fluidelastic instability.

Commentary by Dr. Valentin Fuster
2017;():V004T04A037. doi:10.1115/PVP2017-65349.

Since steam generators are used, many experiments have been performed to understand the different phenomena appearing in them. One of the main issues to reproduce a similar flow on a reduced-scale experiment is the choice of the two-phase mixture. Air/water and boiling freon are among the most used two-phase flow mixtures. In the present document, a finite-volume CFD code dedicated to multi-phase flows based on a two-fluid approach (extended to n) is used to compare two mixtures numerically. Thanks to two experiments, an air/water horizontal channel and a freon/freon inclined tube bundle, a first part highlights the validity of the two-phase flow modeling. Then, based on inlet superficial velocities from both experiments, a numerical experiment is performed by using each mixture on the other experiment. Void fraction profiles and two-phase flow regimes are compared in order to exhibit the behaviors of both mixtures in a steam generator like tube bundle. Results turn out to provide useful information (void fraction profiles mainly) about mixture properties, and near-wall loads in the vicinity of cylinders are investigated.

Commentary by Dr. Valentin Fuster
2017;():V004T04A038. doi:10.1115/PVP2017-65352.

Efficient modelling and accurate knowledge of the mechanical behaviour of the reactor core are needed to estimate the effects of seismic excitation on a nuclear power plant. The fuel assemblies (in the reactor core) are subjected to an axial water flow which modifies their dynamical behaviour. Several fluid-structure models simulating the response of the core to a seismic load has been developed in recent years; most of them require high computational costs. The work which is presented here is a first step in order to simplify the fluid forces modelling, and thus to be able to catch the main features of the mechanical behaviour of reactor core with low computational costs. The main assumption made in this work is to consider the fluid flow as an inviscid potential flow. Thus, the flow can be described only using one scalar function (velocity potential) instead of a vector field and strongly simplifies the fluid mechanics equations, avoiding the necessity to solve Navier-Stokes equations. The pressure distribution around a cylinder is first solved in Fourier space for different values of the parameters (wavenumber, confinement size).

The method is applied to a simple geometry (cylinder in an axial flow with a variable confinement) in order to test its effectiveness. The empirical model is then compared to simulations and reference works in literature. The configuration with large confinement has been solved, and results were in agreement with Slender Body Theory. The dependency on the confinement size strongly depends on the wavenumber, but in any case added mass increases as the confinement size decreases. Finally, future perspectives to extend the model to a group of cylinders and to improve the model are discussed (i.e. add viscosity to the model).

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2017;():V004T04A039. doi:10.1115/PVP2017-65402.

A full scale pipework system, typical of oil and gas installations located on the sea floor, was subjected to vibration tests in both dry and submerged conditions. The frequency range examined covered 10 Hz to 500 Hz. The objective of the tests was to provide experimental data so that computer simulations could be developed and validated. The method used to determine the vibration properties was that of an experimental modal analysis using an impact hammer. The hammer was modified for underwater use. In dry conditions the damping was found to be very small (damping ratio less than 0.0002) despite the construction being typical. When submerged the effect of the surrounding water was significant. The changes in the natural frequencies from the dry case to the wet case occurred in such a complex manner that it was not possible to identify a simple shift between wet and dry vibration modes. It was necessary to include appropriate added mass coefficients in the computer simulation for both the pipe and the support system. The effect of the surrounding water on the damping was measured but found to be insignificant. It was concluded that immersion in water does not add significant damping to oil and gas pipework.

Topics: Damping , Pipes , Water
Commentary by Dr. Valentin Fuster
2017;():V004T04A040. doi:10.1115/PVP2017-65405.

This paper investigates experimentally the mechanism of tone generation from flow over a simplified model of a perforated plate. To simplify the geometry to two dimensions, a perforated plate is modeled by a series of rectangular slats with an adjustable gap width between them. This apparatus is tested at various angles of incidence and flow velocities, to identify the conditions favorable to the production of tonal noise.

The results of this research are presented in two main parts. First, the acoustic response of the test plates is documented by means of microphone measurements. It is found that for an angle of incidence between 5 and 30 degrees and a flow velocity of 10 to 30 m/s, tonal noise is produced. Outside of this range of angles, the produced sound is broadband. In the second part, phase-locked particle image velocimetry (PIV) is used to study the flow field. It is found that vortices form in the free shear layer of the gaps between the slats. These vortices impinge on the side of the downstream slat and are then ejected through the gap to the backside of the plate. As these vortices leave the edge of the downstream slat, counter rotating vortices are shed in sympathy with the incident vortices. Vortex pairs are therefore periodically shed which are thought to be the cause of tone generation.

Commentary by Dr. Valentin Fuster
2017;():V004T04A041. doi:10.1115/PVP2017-65431.

This paper presents a flutter analysis of a slender web in a cross air flow. In the flutter analysis, a Doublet-point method (DPM) [1] based on an unsteady lifting surface theory is used to calculate the unsteady fluid force acting on the sheet surface. The equation of motion of the web with tension is derived by using the finite element method (FEM). Flutter velocity, frequency and mode are examined through the root locus of the flutter determinant of the system with changing flow velocity of air. In this study, these flutter characteristics derived by flutter analysis are compared with wind-tunnel experiments. The influence of tension of the web on flutter velocity, frequency and mode is clarified.

As tension of the web becomes higher, the flutter velocity and corresponding frequency increase. In any tension, coupled mode flutter of bending and torsional modes occurs. Then, local work done by the fluid force around the upstream end of the web is positive. On the other hand, near the downstream end of the web, the local work is negative.

Commentary by Dr. Valentin Fuster
2017;():V004T04A042. doi:10.1115/PVP2017-65511.

In the nuclear industry, flow-induced vibrations of Steam Generator (SG) tubes are extensively studied for safety justifications and for optimizing the maintenance policies of these components. This paper presents a hybrid testing approach that reproduces the coupled fluid-elastic forces acting on the SG-tubes, by means of an active control system. By modal control strategy and precise pole placement, it is possible to recreate in dry conditions the dynamics of the tube in the turbulent secondary flow. Since the quality of pole placement strongly depends on the model accuracy, this paper also presents the identification method of the instrumented tube.

Topics: Fluids , Testing
Commentary by Dr. Valentin Fuster
2017;():V004T04A043. doi:10.1115/PVP2017-65525.

Hydrophobic surfaces, enabling flow slip past a solid boundary, can be effective for suppressing flow unsteadiness, as well as for heat transfer enhancement; both are important for heat exchanger applications. In the present work, a computational investigation of forced convection heat transfer in cross-flow past a hydrophobic circular cylinder is performed at a Reynolds number value of 300, for which flow past a non-hydrophobic cylinder is three-dimensional. Here, the cylinder surface is maintained at a constant temperature, whereas a Prandtl number of unity is considered. Surface hydrophobicity is modelled based on the Navier model. In a first step, slip conditions are implemented on the entire cylinder surface (full slip), for a nondimensional slip length b* = b/D = 0.20, b being the slip length and D the cylinder diameter. This results in a suppression of flow unsteadiness, as well as in a simultaneous heat transfer enhancement; the latter is quantified by the increase of the mean Nusselt number. Next, in order to reduce the extent of the hydrophobic region, and thus the associated cost, a partial slip setup is considered. This setup consists of alternating hydrophobic and non-hydrophobic strips along the spanwise direction, the width of which is selected considering the spanwise wavelength, λz, of three-dimensional flow. Further, following recent studies of the authors on two-dimensional flow, a non-hydrophobic region is considered around the average rear stagnation point (in the circumferential direction), for all hydrophobic strips. It is shown that the present setup can result in values of mean Nusselt number comparable to those attained with full slip. Overall, the present results illustrate that a proper implementation of partial hydrophobicity on the cylinder surface, along the circumferential and the spanwise direction, results in a suppression of wake unsteadiness and fluctuating forces, as well as in a simultaneous enhancement of heat transfer rates.

Commentary by Dr. Valentin Fuster
2017;():V004T04A044. doi:10.1115/PVP2017-65529.

Flow-induced vibration analysis of the San Onofre Nuclear Generating Station (SONGS) Replacement Steam Generators is made using non-proprietary public data for these steam generators on the Nuclear Regulatory Commission public web site, www.NRC.com. The analysis uses the methodology of Appendix N Section III of the ASME Boiler and Pressure Vessel Code, Subarticle N-1300 Flow-Induced Vibration of Tubes and Tube Banks. First the tube geometry is assembled and overall flow and performance parameters are developed at 100% design flow, then analysis is made to determine the flow velocity in the gap between tubes and tube natural frequencies and mode shapes. Finally, the mass damping and reduced velocity for tubes on the U bend are assembled and plotted on the ASME code Figure N-11331-4 fluid elastic stability diagram.

Commentary by Dr. Valentin Fuster
2017;():V004T04A045. doi:10.1115/PVP2017-65545.

Flow-Induced Vibration (FIV) caused by turbulent flow inside a pipe could lead to fatigue failure with shell mode vibration. Our previous study investigated the excitation source of the FIV for tee junctions experimentally to understand the FIV mechanism and provided Power Spectral Density (PSD) profiles of pressure fluctuation. In the present study, experiments were performed with more extensive measurement points for both 90- and 45-degree tees to understand a more detailed mechanism. PSD plots were provided, featuring different pressure fluctuation characteristics at each measurement point among both angle tees. It also emerged that the PSD level declined with increasing distance from the impingement point. Unsteady Computational Fluid Dynamics (CFD) simulations with the Large Eddy Simulation (LES) model were also performed to understand the turbulent structure for the tee junctions. The frequency characteristic of the simulated pressure fluctuation effectively matched those of the experiments at each measurement point, which implies that CFD simulation with an LES model could reveal reasonable predictions of the FIV excitation source for tee junctions. Simulation results showed that the relatively large vortex shed from the branch pipe impinged periodically on the main pipe bottom and the large vortex was dissipated downstream. These vortex behaviors would be the main mechanism generating the FIV excitation source.

Commentary by Dr. Valentin Fuster
2017;():V004T04A046. doi:10.1115/PVP2017-65565.

According to the results of conventional wind tunnel tests on rectangular cross sections with side ratios of B/D = 2–8 (B: along-wind length (m), D: cross-wind length (m)), motion-induced vortex excitation was confirmed. The generation of motion-induced vortex excitation is considered to be caused by the unification of separated vortices from the leading edge and secondary vortices at the trailing edge [1]. Spring-supported test for B/D = 1.18 was conducted in a closed circuit wind tunnel (cross section: 1.8 m high×0.9 m wide) at Kyushu Institute of Technology. Vibrations were confirmed in the neighborhoods of reduced wind speeds Vr = V/fD = 2 and Vr = 8 (V: wind speed (m/s), f: natural frequency (Hz)). Because the reduced wind speed in motion-induced vortex excitation is calculated as Vr = 1.67×B/D = 1.67×1.18 = 2.0 [1], vibrations around Vr = 2 were considered to be motion-induced vortex excitation. According to the smoke flow visualization result for B/D = 1.18 which was carried out by the authors, no secondary vortices at the trailing edge were formed, although separated vortices from the leading edge were formed at the time of oscillation at the onset wind speed of motion-induced vortex excitation, where aerodynamic vibrations considered to be motion-induced vortex excitation were confirmed. It was suggested that motion-induced vortex excitation might possibly occur in the range of low wind speeds, even in the case of side ratios where secondary vortices at trailing edge were not confirmed.

In this study, smoke flow visualizations were performed for ratios of B/D = 0.5–2.0 in order to find out the relation between side ratios of rectangular cross sections and secondary vortices at trailing edge in motion-induced vortex excitation. The smoke flow visualizations around the model during oscillating condition were conducted in a small-sized wind tunnel at Kyushu Institute of Technology. Experimental Reynolds number was Re = VD/v = 1.6×103. For the forced-oscillating amplitude η, the non-dimensional double amplitudes were set as 2η/D = 0.02–0.15. Spring-supported tests were also carried out in order to obtain the response characteristics of the models.

Commentary by Dr. Valentin Fuster
2017;():V004T04A047. doi:10.1115/PVP2017-65585.

A cylinder array vibrating in a fluid exhibits multiple coupled frequencies and coupled mode shapes. For a square array of four cylinders with a pitch-to-diameter ratio of 1.25, one cylinder was excited. In order to obtain the coupled modes of the whole array without the influence of exciters, a new method named noncontact-measurement image processing system was used in this paper. This system can guarantee the consistency of the frequencies quantificationally. Due to noncontact-measurement, the flow field around cylinder arrays will not be disturbed and can be measured precisely. Experimental results show that if we want to obtain all the coupled mode shapes, the consistency of cylinder frequencies (within 2%) must be insured and the vibrated cylinder should be chosen with larger amplitude in the coupled mode shapes. Distinct coupled frequencies correspond to symmetric mode shapes, while repeated coupled frequencies correspond to asymmetric ones. Besides, the acoustic fluid-solid interaction numerical method was used to calculate the vibration process of multi objects in still water. The numerical simulation results were in good agreement with experimental ones. It is found that for the same number of cylinders, the bandwidth of coupled frequencies increases with the decrease of pitch-to-diameter ratio, and that for the same pitch-to-diameter ratio, the bandwidth of coupled frequencies increases with the increase of the number of cylinders. Detuning of the cylinder frequencies leads to changes of coupled frequencies and coupled mode shapes.

Commentary by Dr. Valentin Fuster
2017;():V004T04A048. doi:10.1115/PVP2017-65664.

Damping is known to be a major parameter in the seismic design of nuclear facilities. Of special interest is the case of fuel assemblies in PWR plants, which, unlike other components, are submitted to axial flows: it has been known since the late 80s that their frequency response to lateral excitations was largely dependent on the flow velocity, and the issue raised by this observation is to determine a consistent fluid force model which could be used in seismic design.

In the scientific literature, the standard model of fluid forces exerted upon an oscillating slender body was originally derived by Lighthill, and it involves a lift coefficient which, up to a reference frame shift, describes the force generated by a small angle of inclination of the body axis against the flow direction. Recent works by Divaret et al. have provided a value of this lift coefficient equal to 0.11 for a single cylinder, and to 0.18 for a square array of 5 by 8 cylinders, the Reynolds numbers being in the range of 104.

Sticking to the idea that the damping stems from the local angle of inclination of the structure against the flow direction, the present study revisits recent tests performed in the Hermes test rig of CEA Cadarache, where a fuel assembly was submitted to incipient flow velocities varying from 1.5 to 5m.s−1, and to a lateral force exerted upon the middle grid, generating displacements in the ranges of a few mm and of a few Hz. Under the assumption that the fuel assembly behaves in an approximately linear manner and that it undergoes harmonic deformations close to its first natural mode shape, the dissipative fluid force can be expressed by an adequate combination of the hydraulic cylinder force and of the structure displacements. A lift coefficient equal to 0.3–0.4 is obtained with this procedure, which stands for the overall fuel bundle, rods and grids included.

Topics: Fluids , Fuels , Damping
Commentary by Dr. Valentin Fuster
2017;():V004T04A049. doi:10.1115/PVP2017-65692.

Hydrogen as one of energy sources is attracting attentions because of CO2 free combustion that can deaccelerate global warming. Recently, hydrogen enriched combustion technology for gas turbine combustors is developing, in which hydrogen is added to natural gas. However, hydrogen-rich combustion has different combustion characteristics from conventional natural gas combustion. In particular, such variety of combustion characteristics may lead to combustion oscillation, which may cause fatigue breaking of structural elements due to resonance with components. Combustion oscillation is mainly induced by thermo-acoustics interaction. Therefore, it is necessary to investigate characteristics of hydrogen-enriched combustion sufficiently. To understand combustion characteristics of enriched hydrogen mixture, combustion experiments were performed for various ratios of hydrogen in the fuel mixture. In this study, a mock-up combustor of a micro gas turbine combustor is used, where a radial swirler is installed to mix fuel and air and stabilize the flame. To grasp the characteristics of combustion oscillation, pressure fluctuation was detected by a pressure sensor installed at the bottom of the combustor. It is found that larger hydrogen ratio in the fuel mixture extends the range of large pressure fluctuation region expressed by the root-mean-square value. Succeedingly, more detail oscillation characteristics were examined by FFT analysis. In the case of natural gas 100%, the oscillation of around 350 Hz was detected. On the other hand, in the case of the hydrogen-contained fuel mixture, two kinds of oscillating frequencies around 200 and 400 Hz were detected. To examine the cause of the difference among these three oscillating frequencies, a simplified stepped tube model with closed- and open-end is considered. For further investigation, acoustic boundary conditions were measured by acoustic impedance method. Moreover, to obtain the representative flame positions and temperature in the combustor, CFD calculations were performed, and the measured acoustic impedance was combined with the CFD results. Then, parametric studies with various thermo-pressure interaction index were performed to obtain the effect of thermo-pressure interaction index on natural frequencies and gains using the Nyquist plot. As a result, it was found that the self-excited oscillation limit is sensitive to the value of thermo-pressure interaction index.

Commentary by Dr. Valentin Fuster
2017;():V004T04A050. doi:10.1115/PVP2017-65708.

In degraded situations of heat-exchangers, tubes may become loosely supported while subjected to intense crossflow which generates both turbulent and fluid-elastic forces. The vibro-impacting regimes that result have been studied by the authors during these last few years, based on analytical experiments and numerical simulations. Taking advantage of this material, the paper aims at showing some dynamic effects that have been observed and drawing lessons concerning the vibration of tubes under cross-flow when they are linearly unstable.

If the fluid-elastic damping drops until the total damping becomes negative when the flow reduced velocity increases, a non-linear gap-system escapes from instability by reinforcing the sequence of impacts and its apparent frequency. On the other hand, the turbulent excitation is characterized by broadband PSDs that decrease with frequency. Thus the vibro-impacting response of the tubes results from a competition between the turbulent and fluid-elastic forces, according to a process that depends on the gap size. The fluid-elastic coupling forces may be either stabilizing (positive damping) or destabilizing (negative one), and, in a more amazing way, the random forces may be dissipative.

The paper illustrates the previous points from the tested experimental configuration which was mainly 1-DOF. Dimensionless results are given for this configuration. Extensions to more realistic tubes are discussed from numerical simulations of a straight beam with three loosely supports. The starting point of simulations is though experiments where the fluid-elastic forces would act, but not the turbulent ones, which would produce limit cycles in the phase space. Turbulence is then considered as perturbation of limit cycles, and as shown below by notably introducing a dimensionless “gap-turbulence” parameter, smaller the gap sizes are, larger the relative weight of turbulence is. The Rice frequency and the mean impact force are indicators of this relative weight and the competition between the fluid-forces.

From this general understanding, and using preliminary results with the beam model, a few guidelines are finally evoked for determining allowable gaps sizes in degraded situations. But a lot of work has to be done with more sophisticated models to concretize these ideas.

Commentary by Dr. Valentin Fuster
2017;():V004T04A051. doi:10.1115/PVP2017-65725.

ASTRID is a project for an industrial prototype of a 600 MWe sodium cooled Fast Reactor, led by CEA. An important program is in progress for the development and the validation of numerical tools for the simulation of the dynamic mechanical behavior of the Fast Reactor cores, with both experimental and numerical parts. The cores are constituted of Fuel Assemblies (of FA) and Neutronic Shields (or NS) immersed in the primary coolant (sodium), which circulates inside the Fluid Assemblies. The FA and the NS are slender structures, which may be considered as beams, form a mechanical point of view.

The analysis of the dynamic behavior of tubes bundles immersed in a dense fluid is a major challenge in the nuclear industry (reactor cores, steam generators). In some cases, the excitation is given by the fluid flow, with a complex behavior which may lead to instabilities. The paper only considers the case of an external excitation (earthquake or shock). The fluid leads to two main effects: “inertial effects” with lower vibration frequencies and “dissipative effects” with a higher damping.

In the general case the fluid has to be described by the Navier-Stokes equations. It is possible to use the Euler linear equations in the case of vibrations of the tubes in a globally stagnant fluid. In all cases the modeling of the system could lead to huge numerical problems if each tube is described explicitly. Homogenization technics allow to limit the size of the problem.

Homogenization methods taking into account the Euler equations for the fluid have been developed, and widely used for analyses of the dynamic behavior of reactor cores. Only the inertial effects are theoretically described but the dissipative effects may be roughly taken into account by using a Rayleigh damping.

The paper presents an improvement of the method, allowing a better description of the dissipative effects, with a more general form of the expression of the forces exchanged between the fluid and the tubes. The theoretical basis of the numerical model are presented, as well as illustrations of the interest of the method: a better physical description is obtained for the dynamic behavior of the tubes bundle, particularly in the case of interactions with free fluid, without tubes.

Commentary by Dr. Valentin Fuster
2017;():V004T04A052. doi:10.1115/PVP2017-65727.

ASTRID is a project for an industrial prototype of a 600 MWe sodium cooled Fast Reactor, led by CEA. An important program is in progress for the development and the validation of numerical tools for the simulation of the dynamic mechanical behavior of the Fast Reactor cores, with both experimental and numerical parts. The cores are constituted of Fuel Assemblies (of FA) and Neutronic Shields (or NS) immersed in the primary coolant (sodium), which circulates inside the Fluid Assemblies. The FA and the NS are slender structures, which may be considered as beams, form a mechanical point of view.

The dynamic behavior of this system has to be understood, for design and safety studies. Two main movements have to be considered: global horizontal movements under a seismic excitation, and opening of the core. The fluid leads to complex interactions between the structures in the whole core. The dynamic behavior of the core is also strongly influenced by contacts between the beams and by the sodium, which limit the relative displacements. Numerical methods and models are built to describe and simulate this dynamic behavior. The validation of the numerical tools is based on the results of different experimental programs, already performed or in progress.

The paper is mainly devoted to the modeling of the Fluid Structure Interaction phenomena in the Fast Reactor cores.

Tubes bundles immersed in a dense fluid are very common in the nuclear industry (reactor cores and steam generators). In the case of an external excitation (earthquake or shock) the presence of the fluid leads to “inertial effects” with lower natural frequencies, and “dissipative effects”, with higher damping. The geometry of a tubes bundle is complex, which may lead to very huge sizes for the numerical models. Many works have been made during the last decades to develop homogenization, in order to simplify the problem.

Theoretical analyses are presented on different simplifications and assumptions which can be made in the homogenization approach. The accuracy of the different assumptions depends of the conditions of the system: fluid flow or fluid at rest, small or large displacements of the structure.

In the general case, it is theoretically necessary to consider the Navier Stokes equations: the fluid flow is fully nonlinear. Models have been developed during the last years, based on the Euler linear equations, corresponding to a fluid at rest, with small displacements of the structure. Only the inertial effects are theoretically described but the dissipative effects may be taken into account by using a Rayleigh damping.

Different theoretical analyses show that, even in the case of a nonlinear fluid flow, the linear potential flow models may be used as linear equivalent models. In the cases with an important head loss in the fluid flow through the tubes, the fluid movement is mainly driven by the important forces exchanged with the structure and by the pressure gradient. The global equations of the system are close to the equations used for porous media, like the Darcy equations.

An important condition to get a relevant model is to describe globally the energy balance in the system. The energy given to the fluid by the solid correspond to a variation of kinetic energy in the fluid and to energy dissipation in the fluid. Attention will be paid to the cases where the tubes bundle is in interaction with free fluid, without tubes. The global equation of the system has to be accurate for the tubes bundle and for the free fluid also.

Commentary by Dr. Valentin Fuster
2017;():V004T04A053. doi:10.1115/PVP2017-65732.

In this work, the acoustic effects of horizontal air-water flow through an orifice are investigated experimentally. Single phase flow (air) and two-phase flow (air and water) tests are performed for two sets of orifices. One set of straight edged and one set of upstream rounded orifices. For each set, the diameters of the orifices were 2, 5, and 10mm, with a thickness of 5 mm. The two-phase flow is generated by injecting water at a rate of 0 to 40 g/s to air in a pipe with diameter of 25 mm. The air rate is fixed in the range from 5.8 to 14 g/s, where the upstream pressure varies from 1.5 to 4 bar at ambient temperature. Unsteady pressure fluctuations are recorded at two upstream and two downstream position.

The valve noise standard NEN-EN-IEC (60534-8-3, 2011) for dry gas is assessed by means of experimental data in dry conditions at fixed air mass flow rate. Predictions of sound power spectra by means of the standard are found to be more accurate compared to those obtained following Reethof & Ward (1986), also in conditions of a choked orifice.

In case of multiphase flow already at very low liquid fractions of much less than 1%, the standard is no longer valid. The frequency spectrum is no longer determined by the jet noise but starts to be dominated by low frequency general multiphase flow. The Strouhal number based on the jet conditions is an order lower than Sr = 0.2 indicating process variations rather than jet noise. Furthermore, at choking conditions the further expansion which occurs in single phase flow is likely different at multiphase flow. For non-choked flow, the standard can be adapted using multiphase mixture properties. This does lead to a good prediction. However at choked conditions, this method fails.

Commentary by Dr. Valentin Fuster
2017;():V004T04A054. doi:10.1115/PVP2017-65767.

The purpose of this study is to investigate the vibration mechanism of a spring-loaded valve placed in a “push-to-open” configuration in a piping system. A non-linear theoretical model of the valve vibration is developed to describe the interaction mechanisms between the unsteady flow through the valve, the acoustic field in the piping system, and the oscillation of the valve plate. The aim of this phenomenological model is to better understand the main system parameters causing the valve vibration. The model relies on a one-dimensional unsteady Bernoulli representation of the flow and a single degree of freedom model of the valve plate motion with impact conditions at the valve seat and lift limiter. Impact forces are determined through the means of a pseudo-force method. The model is cast in state-space form and solved using a fourth-order Runge-Kutta stencil. The predicted limit cycle amplitudes follow the same trends as experimental findings over the opening range of the valve. Modal characteristics are also consistent with experimental data.

Commentary by Dr. Valentin Fuster
2017;():V004T04A055. doi:10.1115/PVP2017-65812.

The paper describes the use of Fluid-Structure-Interaction (FSI) to evaluate turning vane blades in an inlet distributor with two-phase gas/liquid flow. A fatigue failure mechanism attributed to flow induced vibration (FIV) resulting from vortex shedding and un-steadiness in the flow field is evaluated. This failure mechanism can be missed in a complex flow field that is assumed to be steady. Natural frequency of the turning vanes was found to be well within the flow induced forcing frequency range, which led to structural failure of the vanes.

Commentary by Dr. Valentin Fuster
2017;():V004T04A056. doi:10.1115/PVP2017-65898.

The integrity of tube bundles is very important especially when dealing with high-risk applications such as nuclear steam generators. A major issue to system integrity is the flow-induced vibration (FIV). FIV is manifested through several mechanisms including the most severe mechanism; fluidelastic instability (FEI). Tube vibration can be constrained by using tube supports. However, clearances between the tube and their support are required to allow for thermal expansion and for other manufacturing considerations. The clearance between tubes may allow frequent impact and friction between tube and support. This in turn may cause fatigue and wear at support and potential for catastrophic tube failure.

This study aims to investigate the dynamics of loosely supported tube array subjected to cross-flow. The work is performed experimentally in an open-loop wind tunnel to address this issue. A loosely-supported single flexible tube in both triangle and square arrays subjected to cross-flow with a pitch-to-diameter ratio of 1.5 and 1.733, respectively were considered. The effect of the flow approach angle, as well as the support clearance on the tube response, are investigated. In addition, the parameters that affect tube wear such as impact force level are presented.

Commentary by Dr. Valentin Fuster
2017;():V004T04A057. doi:10.1115/PVP2017-65901.

The shut-down of the San Onofre Nuclear Generating Station (SONGS) has been attributed to damaging streamwise Fluidelastic Instability (FEI) of the steam generator tubes, a phenomenon which has traditionally been assumed not to occur. This has generated a significant research effort to better understand this phenomenon and to develop appropriate design criteria for its prevention. Most current design codes are based on Connors criterion for FEI which neglects both streamwise FEI and the effects of tube array pattern and pitch ratio. It is becoming clear that array geometry and pitch ratio are important determining factors in FEI, especially in the streamwise direction.

This paper presents an extension of the theory of Lever and Weaver to consider arrays of flexible fluid-coupled tubes which are free to become unstable in both the transverse and streamwise directions. This simplified modelling approach has the advantages of being very tractable for numerical parametric studies and having no need for experimental data input. Previous research by the authors has shown that the predictions of this model agree very well with the available experiments for parallel triangular arrays for both transverse and streamwise FEI. In this paper, the results of such studies are presented for the both transverse and streamwise FEI for square inline and normal triangular arrays and compared with the authors’ previous results for parallel triangular arrays. It is shown that FEI is strongly influenced by array geometry, especially for small pitch ratio arrays operating at low values of the mass damping parameter. The results show good agreement with the available experimental data.

Commentary by Dr. Valentin Fuster
2017;():V004T04A058. doi:10.1115/PVP2017-65913.

This paper investigates the phase characteristics of vortex shedding from tube banks on acoustic resonance. We measured the time variation of a phase between surface pressures related to the lift force on a tube and acoustic pressure on a side wall related to the acoustic particle velocity when acoustic resonance occurred in in-line tube banks. The measured tube was installed at the second rows in the tube banks. As the peak level of spectrum of surface pressure fluctuations increased, the coherence between vortex shedding and wall acoustic pressure in the tube banks also increased. The phase delay between the lift force and acoustic pressure on the side wall was calculated by using a proposed modeling method. In addition, we discuss the verification of the synchronization feedback for a coupling condition between a sound field and wake oscillator.

Commentary by Dr. Valentin Fuster
2017;():V004T04A059. doi:10.1115/PVP2017-65929.

Acoustically induced vibration (AIV) is a vibration of piping systems caused by the acoustic loading generated mainly from pressure reducing devices. Recently, the capacities of the pressure reducing systems have been increased and some of the piping systems which are susceptible to acoustic fatigue, such as in flare and depressuring system. Demands on the development of reasonable design method for AIV is increasing. In this paper, the mechanisms of the fatigue failure of branch connection due to AIV were intensively studied. Firstly, the mechanism of the stress concentration was discussed. branch vibration caused by the shell mode vibration was assessed using several branch connection models, massless rigid model, fixed rigid model, and beam model. Next, the relationship between shell-vibration and stress concentrations is studied and re-organized based on acoustic vibration theories. Finally, the risk of the fatigue failure of the branch connection due to acoustic loading was discussed.

Commentary by Dr. Valentin Fuster
2017;():V004T04A060. doi:10.1115/PVP2017-65933.

The current work studies air-water flow through a ½-inch flow restricting orifice installed in a 1-inch pipe. Investigation of two phase flow downstream the orifice and its effects on vibration of the piping structure have been carried out. Several flow regimes from bubbly to stratified-wavy flow have been analyzed to evaluate the effects of flow pattern, phase redistribution, bubble frequency, and liquid flow rate on the vibration of the structure. The liquid velocity fields have been obtained using Particle Image Velocimetry (PIV) along with post processing algorithm for phase discrimination. Proximity sensors have been used to capture the pipe response in two orthogonal directions. Also, a capacitance sensor was used to measure the two-phase void fraction. The results show that the magnitude and nature of vibrations of the piping structure is largely affected by the frequency and size of the bubbles upstream, vortex creation by pressure fluctuation downstream, liquid flow rate, and the flow pattern upstream. Slug flow and stratified flow patterns induced significant vibrations in the examined structure. The location of the transition region of slug flow on flow pattern maps, play important role in the dynamic response of the structure to the flow.

Commentary by Dr. Valentin Fuster
2017;():V004T04A061. doi:10.1115/PVP2017-65967.

This research analyzes the interaction between fibers and the air jets that are used to accelerate them in fiber processing industries. Typically, supersonic flow is used to achieve sufficiently high thread speeds. However, this flow contains shocks and expansions, resulting in large longitudinal variations in force on the thin and flexible thread. Consequently, a complex fluid-structure interaction (FSI) occurs between the supersonic air flow and the thread.

In this research, the fluid-structure interaction between a supersonic air flow and a thread is studied numerically using three-dimensional simulations. The thread is represented by a smooth and flexible cylindrical body. The displacement of the thread is calculated for a given traction on its surface using a finite element structural dynamics code. The compressible flow around the thread is calculated using a finite volume computational fluid dynamics (CFD) code, using the arbitrary Lagrangian-Eulerian (ALE) framework to account for the thread deformation.

In these partitioned simulations, the kinematic and dynamic equilibrium conditions on the fluid-structure interface are satisfied using a coupling algorithm. Two coupling algorithms are compared and the influence of numerical parameters is investigated. The fluid-structure interaction simulations reveal transversal running waves in the thread. By comparing the speed of these waves with the propagation speed of the shock waves in the tube, it can be concluded that these phenomena are not related.

Commentary by Dr. Valentin Fuster
2017;():V004T04A062. doi:10.1115/PVP2017-66068.

The tubes in the U-bend region of a recirculating type of nuclear steam generator are subjected to cross-flow of a two-phase mixture of steam and water. There is a concern that these tubes may experience flow-induced vibration, including the damaging effects of fluidelastic instability.

This paper presents an update and results from a series of flow-induced vibration experiments performed by Canadian Nuclear Laboratories for the Electric Power Research Institute (EPRI) using the Multi-Span U-Bend test rig. In the present experiments, the main focus was to investigate fluidelastic instability of the U-tubes subjected to a cross-flow of air. The tube bundle is made of 22 U-tubes of 0.5 in (12.7 mm) diameter, arranged in a rotated triangular configuration with a pitch-over-diameter ratio of 1.5. The test rig could be equipped with variable clearance flat bar supports at two different locations to investigate a variety of tube and support configurations.

The primary purpose of the overall project is to study the effect of flat bar supports on ‘in plane’ (‘streamwise’) instability in a U-tube bundle with realistic tube-to-support clearances or preloads, and eventually in two-phase flow conditions. Initially, the test rig was designed for tests in air-flow using an industrial air blower. Tests with two-phase Freon refrigerant (R-134a) will follow.

This paper describes the test rig, experimental setup, and the challenges presented by simulating an accurate representation of current steam generator designs. Results from the first series of tests in air flow are described.

Topics: Air flow
Commentary by Dr. Valentin Fuster
2017;():V004T04A063. doi:10.1115/PVP2017-66158.

The flow induced vibration occurs frequently in a steam generator in the nuclear power plant. The large-scale steam generator has a large number of tube supports whose cell has rhombus-type shape, and there is a tiny clearance between tube and its support grid. The damping is very complex because of non-linearity and randomness.

The experiment for damping was performed to investigate it with a number of 13 support spans both in air and water environment. The lower part of multi-span fixture was excited by root-mean-square random force with the range of 1∼10 newton to get the frequency response function. The half-power bandwidth method was applied to obtain the damping ratio. The sensitivity of a number of spans was investigated in the range of 9 ∼ 13. In addition, the damping was reviewed from a comparison with Pettigrew [1∼4] and ASME B&PV Code [5].

Topics: Heat , Damping , Water
Commentary by Dr. Valentin Fuster
2017;():V004T04A064. doi:10.1115/PVP2017-66166.

An analytical model was developed by Sim to calculate the two-phase damping ratio for upward two-phase flow perpendicular to horizontal tube bundles. To verify the model, the present experiment is performed with a typical normal square array of cylinders subjected to the two-phase flow of air-water in the tube bundles. The diameter of cylinder is 18mm and the pitch ratio to diameter is 1.35. Using a pressure transducer and data acquisition system, pressure loss along the flow direction in the tube bundles is measured to evaluate the two-phase Euler number and the two-phase friction multiplier. The drag force along the flow direction on a tube is measured to calculate the drag coefficient and the two-phase damping ratio. The experimental results of the two-phase damping ratios are compared with the analytical results given by Sim’s model for homogeneous two-phase flow. It was found that, as increasing the mass flux, the drag force and the drag coefficients given by experimental test are close to the results calculated by the homogeneous model. As a result, the damping ratio can be evaluated by the homogeneous model for bubbly flow of sufficiently large mass flux.

Commentary by Dr. Valentin Fuster

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