3rd Joint US-European Fluids Engineering Summer Meeting

2010;():1-6. doi:10.1115/FEDSM-ICNMM2010-30134.

Pressure waves created in liquid mercury pulsed spallation targets have been shown to create cavitation damage to the target container. One way to mitigate such damage would be to absorb the pressure pulse energy into a dispersed population of small bubbles, however, creating such a population in mercury is difficult due to the high surface tension and particularly the non-wetting behavior of mercury on gas-injection hardware. If the larger injected gas bubbles can be broken down into small bubbles after they are introduced to the flow, then the material interface problem is avoided. Research at the Oak Ridge National Labarotory is underway to develop a technique that has shown potential to provide an adequate population of small-enough bubbles to a flowing spallation target. This technique involves gas injection at an orifice of a geometry that is optimized to the turbulence intensity and pressure distribution of the flow, while avoiding coalescence of gas at injection sites. The most successful geometry thus far can be described as a square-toothed orifice having a 2.5 bar pressure drop in the mercury flow of 8 L/s for one of the target inlet legs. High-speed video and high-resolution photography have been used to quantify the bubble population on the surface of the mercury downstream of the gas injection site. Also, computational fluid dynamics has been used to optimize the dimensions of the toothed orifice based on a RANS computed mean flow including turbulent energies such that the turbulent dissipation and pressure field are best suited for turbulent break-up of the gas bubbles.

Topics: Turbulence , Bubbles
Commentary by Dr. Valentin Fuster
2010;():7-11. doi:10.1115/FEDSM-ICNMM2010-30350.

It is well known that reentrant jet motion in periodic cloud cavitation means upward flow from the cavity closure area of cloud cavitation. However the mechanism of reentrant motion seems to remain unsolved clearly. In the present study some experiments were conducted about the mechanism of reentrant motion in a fixed type cavity for a convergent-divergent nozzle. High speed video observation and image analysis based on the frame difference method were made about unsteady cloud cavitation with a periodic structure of cavitation cloud shedding. As a result, the main points are experimentally found as follows; 1) a typical pattern of reentrant motions can be caused by the pressure wave propagation from the collapse of cavitation cloud shed downstream and 2) the frame difference method is very useful in an image analysis for high speed video observation of cavitating flow because the trajectory of pressure waves can be clearly visualized by the method.

Commentary by Dr. Valentin Fuster
2010;():13-17. doi:10.1115/FEDSM-ICNMM2010-30395.

For gas turbines burning liquid fuels, improving fuel spray and combustion characteristics are of paramount importance to reduce emission of pollutants, improve combustor efficiency and adapt to a range of alternative fuels. Effervescent atomization technique, which involves the bubbling of an atomizing gas through aerator holes into the liquid fuel stream, has the potential to give the required spray quality for gas turbine combustion. Bubbling of the liquid stream is presently used in a wide range of other applications as well such as spray drying, waste-water treatment, chemical plants, food processing and bio- and nuclear-reactors. In order to optimize control of the required aeration quality and thus the resulting spray quality over a wide range of operating conditions, it is important that the dynamics of bubble formation, detachment and downstream transport are well understood under these circumstances. The paper reports on an experimental study conducted to investigate the dynamics of gas bubbles in terms of bubble detachment frequency when injected from an orifice that is subjected to a liquid cross-flow. The experiments were conducted over a range of gas and liquid flow rates and at various orientations of the liquid channel. Analyses presented here are based on shadowgraph images of two-phase flow, acquired using a high speed camera and a low intensity light source. An image processing algorithm was developed for the detection and characterization of the bubble dynamics. Results show that bubble detachment frequency is a function of both liquid cross-flow rate and the gas-to-liquid flow rate ratio.

Commentary by Dr. Valentin Fuster
2010;():19-27. doi:10.1115/FEDSM-ICNMM2010-30435.

Cavitation of a hydrofoil is observed in detail by using a high speed video camera. A paint removal test is also carried out in order to evaluate cavitation aggressiveness for erosion. 2D hydrofoil profile is Clark Y 11.7% and its angle of attack is seven degrees. Cavitation number is σ = 1.08. The experimental results are compared with cavitation CFD. Numerous features of unsteady cavitation are observed such as cyclic fluctuation of the sheet cavity, existence of the glassy cones on a sheet cavity, generation of the cloud cavitation from the sheet cavity and the isolated bubbles traveling over the suction surface of the blade. The isolated traveling bubbles and their collapses are thought to be one of the main causes of the severe paint removals. The isolated traveling bubbles are derived from the flowing cavitation nucleus or from abrupt onset at the leading edge of the blade. For computing these complicated phenomena, combination of grid scale bubbles (GSB) and sub grid scale bubble model (SGSB) are proposed. GSB shall be computed by using the computational scheme for the free surface with phase change model. SGSB can be computed with conventional cavitation model. The breakup of GSB generates SGSB, and the coalescence of SGSB makes GSB. Upper limit of void fraction of SGSB is estimated in the range of five or ten percent from the simple speculation of the structure of packed spheres. The two types of cavitation bubble inception model are also discussed based on the generation of the isolated bubbles observed in the experiments. To verify the proposed concepts of cavitation model, a traveling air bubble over a hydrofoil is computed by using the free surface flow scheme of Volume of Fluid (VOF) approach. Cavitation on the hydrofoil is also computed by VOF approach with boiling model concerning the heat transfer. Both the computed results show qualitatively similar characteristics of the bubble dynamics to those in experimental results.

Topics: Cavitation , Hydrofoil
Commentary by Dr. Valentin Fuster
2010;():29-37. doi:10.1115/FEDSM-ICNMM2010-30504.

The presence of cavitation and turbulence in a diesel injector nozzle has significant effect on the subsequent spray characteristics. However, the mechanism of the cavitating flow and its effect on the subsequent spray is unclear because of the complexities of the nozzle flow, such as the cavitation phenomena and turbulence. A flow visualization experiment system with a transparent scaled-up vertical multi-hole injector nozzle tip was setup for getting the experimental data to make a comparison to validate the calculated results from the three dimensional numerical simulation of cavitating flow in the nozzle with mixture multi-phase cavitating flow model and good qualitative agreement was seen between the two sets of data. The critical conditions for cavitation inception were derived as well as the relationship between the discharge coefficient and non-dimensional cavitation parameter. After wards, the testified numerical models were used to analyze the effects of injection pressure, back pressure, cavitation parameter, Reynolds number, injector needle lift and needle eccentricity on the cavitating flow inside the nozzle. Combined with visual experimental results, numerical simulation results can clearly reveal the three-dimensional nature of the nozzle flow and the location and shape of the cavitation induced vapor distribution, which can help understand the nozzle flow better and eventually put forward the optimization ideas of diesel injectors.

Commentary by Dr. Valentin Fuster
2010;():39-48. doi:10.1115/FEDSM-ICNMM2010-30658.

An inertial bubble collapsing near a solid boundary generates a fast impulsive micro jet directed towards the boundary. The jet impact on the solid boundary can cause pitting, and this effect has been taken advantage of in surgeries such as when micro-bubbles are driven ultrasonically to cavitate in tissue and produce jets that are believed to induce the surgical effect. In this experimental investigation we are interested in the jetting from single cavitation bubbles near boundaries. By introducing a through hole in the boundary beneath a single laser-induced bubble it is hypothesized that the forming jet upon bubble implosion will proceed to penetrate through the hole to the other side and that it may be utilized in useful application such as precise surgeries. We study the cases of a bubble in an infinite medium, near a blank solid boundary, and above a hole in a solid boundary. We find in the case of the hole the unexpected formation of a counter jet that is directed away from the hole and into the bubble. These findings are contrasted to similar counter jetting behaviors from bubbles near boundaries with viscous and elastic properties.

Topics: Lasers , Bubbles
Commentary by Dr. Valentin Fuster
2010;():49-55. doi:10.1115/FEDSM-ICNMM2010-30970.

This work aims at analyzing and realizing a horn-type sonochemical reactor which can be operated in a very low ultrasonic power density but results in a large volume of cavitation zones. The sonoreactor contains three main components, namely a Langevin-type piezoelectric transducer (20 kHz), a metal horn, and a circular cylindrical sonicated cell filled with tap water. In order to diminish the generation of cavitation bubbles near the horn-tip, an enlarged cone-shaped horn is designed to reduce the ultrasonic intensity at the irradiating surface and to get better distribution of energy in the sonicated cell. It is demonstrated both numerically and experimentally that the cell geometry and the horn position have prominent effects on the pressure distribution of the ultrasound in the cell. With appropriate choices of these parameters, the whole reactor works at a resonant state. Several acoustic resonance modes observed in the simulation are realized experimentally and used for generating a large volume of cavitation field.

Commentary by Dr. Valentin Fuster
2010;():57-64. doi:10.1115/FEDSM-ICNMM2010-30984.

A study has been carried out at the Saint Anthony Falls Laboratory (SAFL) to investigate various aspects of the flow physics of a supercavitating vehicle. For the experimental work presented here, artificial supercavitation behind a sharp-edged disk was investigated for various model configurations. Results regarding supercavity shape, closure, and ventilation requirements versus Froude number are presented. Conducting experiments in water tunnels introduces blockage effects that are not present in nature. As a result, effects related to flow choking are also discussed. Various methods for computing ventilated cavitation number, including direct measurement of pressure, Laser Doppler Velocimetry, and use of previous numerical results, were compared. Results obtained are similar in character to previous results from various authors, but differ significantly in measured values. Supercavitation parameters, especially the minimum obtainable cavitation number are strongly affected by tunnel blockage.

Commentary by Dr. Valentin Fuster
2010;():65-71. doi:10.1115/FEDSM-ICNMM2010-31222.

The comparison of experimental data and results obtained from four global models — homogeneous, Dukler, Martinelli and Chisholm, used to evaluate the two-phase flow pressure drop in circular 90° horizontal elbows — is presented in this paper. An experimental investigation was carried out using three galvanized steel 90° horizontal elbows (E1, E2, E3) with internal diameters of 26.5, 41.2 and 52.5 mm, and curvature radii of 194.0, 264.0 and 326.6 mm, respectively. According to the experimental results, the model proposed by Chisholm best fitted them, presenting for each elbow an average error of E1 = 18.27%, E2 = 28.40% and E3 = 42.10%. Based on experimental results two correlations were developed. The first one is the classical Chisholm model modified to obtain better results in a wider range of conditions; it was adjusted by a dimensionless relationship which is a function of the homogeneous volumetric fraction and the Dean number. As a result, the predictions using modified Chisholm model were improved presenting an average error of 8.66%. The second developed correlation is based on the entire two-phase mass flow taken as liquid and adjusted by the homogeneous volumetric fraction ratio. The results show that this last correlation is easier and accurate than the adjusted Chisholm model, presenting an average error of 7.75%. Therefore, this correlation is recommended for two-phase pressure drop evaluation in horizontal elbows.

Topics: Two-phase flow
Commentary by Dr. Valentin Fuster
2010;():73-79. doi:10.1115/FEDSM-ICNMM2010-30527.

Utilizing low temperature differences from solar vacuum tube collectors or waste heat in the range 70–200 °C seems to be the most promising and commercial interesting field of applications for thermoacoustic systems. Recently a novel 4-stage “self matching” traveling wave engine is developed and tested. Beside the low acoustic loss and compactness, due to traveling wave feedback, all components per stage are identical which is beneficial from (mass) production point of view. Based on this concept a 100 kWT thermoacoustic power (TAP) generator is under construction. This project is carried out in the framework of phase two of the Dutch SBIR program. The 100 kWT TAP will be installed at a paper manufacturing plant in the Netherlands for converting part of the flue gas at 150°C from the paper drying process into electricity. Emphasis in this project is on production and cost aspects lowering the investment per kWe to a level competitive to ORC’s. After successful completion of this pilot, commercialization and delivery of 100kW to 1 MW thermoacoustic power generators for industrial waste heat recovery and as add-on for CHP systems is planned to begin in 2012. The same concept of the 4-stage traveling wave engine is also implemented in an atmospheric pressure operated thermoacoustic cooking device for developing countries which generate beside hot water up to 50 W electricity. Details, ongoing work and experimental results of these projects will be presented.

Topics: Waves , Generators , Travel
Commentary by Dr. Valentin Fuster
2010;():81-89. doi:10.1115/FEDSM-ICNMM2010-30639.

Several analytical or numerical models available in the litterature allow to predict the onset of thermoacoustic engines [5, 6, 11]. However, most of these models rely on strong assumptions concerning for instance the stack geometry or the shape of the temperature field in the thermoacoustic core. The purpose of the work presented here is to put forward a new method allowing the prediction of the onset of the thermoacoustic instability while being exempted from a certain number of these assumptions. This method consists in measuring the transfer matrix of the thermoacoustic core by means of an appropriate experimental setup developed in our laboratory. The results are then introduced into an analytical modeling leading to the prediction of the onset conditions (in terms of heating power supply and acoustic frequency) of the thermoacoustic instability in an engine of specified geometry (straight duct or closed loop, coupled or not with an acoustic load like a secondary resonator or an electrodynamic alternator). The results of measurements will be presented, and the predictions of the onset obtained from these measurements will be compared with those actually observed in a standing-wave thermoacoustic engine.

Commentary by Dr. Valentin Fuster
2010;():91-96. doi:10.1115/FEDSM-ICNMM2010-30672.

Thermoacoustic refrigerators produce a cooling power from an acoustic energy. Over the last decades, these devices have been extensively studied since they are environment-friendly, robust and miniaturizable. Despite all these advantages, their commercialization is limited by their low efficiency. One reason for this limitation comes from the complex thermo-fluid process between the stack and the two heat exchangers which is still not sufficiently understood to allow for optimization. In particular, at high acoustic pressure level, vortex shedding can occur behind the stack as highlight by [Berson & al., Heat Mass Trans, 44, 10151023 (2008)]. The created vortex can affect heat transfer between the stack and the heat exchangers and thus, they can reduce the system performance. In this work, aerodynamic and thermal measurements are both conducted in a standing wave thermoacoustic refrigerator allowing investigation of vortex influence on the system performance. The proposed device consists on a resonator operated at frequency of 200 Hz, with hot and cold heat exchangers placed at the stack extremities. The working fluid is air at ambient temperature and atmospheric pressure. The aerodynamic field behind the stack is described using high-speed Particle Image Velocimetry. This technique allows the acoustic velocity field measurement at a frequency up to 3000 Hz. Thermal measurements consist on the acquisition of both the temperature evolution along the stack and the heat fluxes extracted at the cold heat exchanger. These measurements are performed by specific micro-sensors developed by MEMS technology. The combination of these two measurements should be helpful for the further understanding of the heat transfer between the stack and the heat exchangers.

Commentary by Dr. Valentin Fuster
2010;():97-103. doi:10.1115/FEDSM-ICNMM2010-30788.

The improvement of the thermal coupling between the stack of a thermoacoustic refrigerator and the heat exchangers is necessary to achieve high-efficiency and stable operation. Heat transport by the thermoacoustic effect depends on both the velocity and temperature fields. Inside the stack, it can be described by the linear theory of thermoacoustics. However, departures from linear behaviours are expected near the edges of the stack and in the heat-exchangers due to the generation of vorticity and temperature harmonics. The present work focuses on the experimental characterization of temperature harmonics near the edges of a thermoacoustic stack. Experiments are conducted in an 18cm-long resonator operated with air at atmospheric pressure at the resonance frequency of approximately 464Hz. Drive ratios up to 3% are achieved, which corresponds to temperature oscillation amplitudes up to 2.5K. Temperature measurements are performed using a novel procedure recently proposed by Berson et al., Rev. Sci. Instrum. 81, 015102 (2010). The instantaneous temperature is measured with a cold wire operated by a Constant-Current Anemometer (CCA). In addition, we record the output signal of the same wire, under the same flow conditions — which is made possible by the periodicity of the acoustic wave — and operated in the heated mode by a Constant-Voltage Anemometer (CVA). During post-processing, the thermal inertia of the cold wire operated with the CCA is corrected using the CVA signal. This procedure does not require any physical properties of the wire such as the diameter. In addition, it does not require the knowledge of a heat-transfer/velocity relationship for the wire. This is all the most important for thermoacoustic systems since no such relationship is available in oscillating flows. Results validate the generation of temperature harmonics near the stack edges. The spatial distributions of the first and second harmonic amplitudes are compared with a one-dimensional model. The model is an extension of an analytical model from the literature [Gusev et al., J. of Sound and Vibration 235, (2000)] that takes into account axial conduction. Experimental results show an excellent qualitative agreement with the model and demonstrate the importance of axial conduction on the nonlinear thermal field behind the stack.

Commentary by Dr. Valentin Fuster
2010;():105-109. doi:10.1115/FEDSM-ICNMM2010-30798.

We report on an experimental study conducted to study the streaming velocity fields in the vicinity of the stack in a thermoacoustic device. Synchronized Particle Image Velocimetry (PIV) technique was used to measure the two-dimensional streaming velocity fields. The streaming velocity fields were measured at both sides of the porous stack over a range of pressure amplitudes (drive ratios). The results show that the streaming flow structure is significantly different on hot and cold sides of the stack. The hot side of the stack experienced higher magnitudes and higher spatial variability of the streaming velocities compared to the cold side. The difference in the velocity magnitude between the hot and cold sides of the stack showed a significant increase with an increase in the drive ratio.

Commentary by Dr. Valentin Fuster
2010;():111-116. doi:10.1115/FEDSM-ICNMM2010-31150.

Domestic heating contributes to a significant amount of energy usage in the Netherlands. Due to scare energy resources, attention to develop new and efficient technologies is increasing. At ECN, a burner driven heat pump employing thermoacoustic technology is being developed for possible applications in households and offices. The desired temperature lift is from 10 °C to 80 °C. As a first step the heat pump is driven by a linear motor. Measurements and performance analysis of the heat pump are presented in this paper. The heat pump has a coefficient of performance which is the ratio of heat produced to the work input of 1.38 when operating between 10 °C to 80 °C. The performance relative to maximum possible Carnot value is 26.5%.

Topics: Heat pumps
Commentary by Dr. Valentin Fuster
2010;():117-121. doi:10.1115/FEDSM-ICNMM2010-31151.

This paper describes the design of a mechanical resonator for a thermoacoustic Stirling-engine. The engine was previously run with a quarter-wavelength acoustic resonator. The advantage of the mechanical resonator is that it is compact and would dissipate less acoustic power. The mechanical resonator consists of a twin piston-spring assembly moving in opposite phase to cancel vibrations. The system uses flexure springs to suspend the piston in a cylinder leaving a narrow gap between them. The narrow gap acts as a dynamic seal between the fronts and back sides of the piston. Simulation calculations show that the mechanical resonator dissipates 40% less acoustic power than the acoustic one. This will lead to more useful acoustic power output from the thermoacoustic Stirling-engine. In addition, the size of the system is reduced considerably.

Commentary by Dr. Valentin Fuster
2010;():123-130. doi:10.1115/FEDSM-ICNMM2010-31166.

The role of unavoidable space between heat exchangers and stack inside a thermoacoustic standing wave generator is investigated. A two-dimensional Low Mach number viscous and heat conducting flow model of the active thermoacoustic cell, comprising heater and cooler separated by a stack made of parallel conducting plates, is described. Three different models of heat exchangers are implemented and compared. Ideal heat exchangers consist of a fluid zone with imposed temperature. The other two models are made up of stacks of horizontal plates, either with specified wall temperatures, in one model and with constant prescribed heat fluxes in the other. A multiple scale analysis allows for coupling the active thermoacoustic cell model with the flow inside the resonator, obtained as a solution to a linear acoustic formulation. When a large enough temperature difference is applied between the heat exchangers, initial pressure perturbations grow. Different resonant modes are amplified for different configurations, in the same way as in experimental observations.

Commentary by Dr. Valentin Fuster
2010;():131-136. doi:10.1115/FEDSM-ICNMM2010-30123.

Laminar flow is increasingly important area of study as it dominates microscale and milliscale applications in devices such as microvalves, pumps, and turbines and in biomedical applications such as stents and biological flows. Studies of pressure losses in junctions have mostly been focused on turbulent flow conditions that exist in larger scale piping systems. There is a need for laminar flow studies of energy losses in junctions so that engineers can better predict, design, and analyze flow in microscale and other small scale systems. Unlike in the turbulent regime, Reynolds number plays a dominant role in energy losses for laminar flow, so new studies should document the effects of Reynolds number. This paper documents laminar flow experiments in a milliscale junction. This work builds on previous experience of the authors in computational fluids dynamics simulations of junctions. The planar junction under study consists of a circular tubes with two outlets and one inlet. A general technique has been developed to produce computer and physical models of junctions in which the inlet tube size is set, but the outlets are allowed to vary in size and angle relative to the inlet tube. A generalized algorithm has been implemented to create three-dimensional models of the junctions for both computational and experimental studies. The junction test sections for experiments are milled from cast acrylic in two pieces to match three-dimensional computer models. The test sections are placed in a system that provides steady-state flow of water to test sections and has been designed to measure pressure losses and flow rates through the test section.

Commentary by Dr. Valentin Fuster
2010;():137-143. doi:10.1115/FEDSM-ICNMM2010-30183.

As a special nozzle, central-body nozzle has attracted a lot of attention since its concept was proposed in 1980s. With Karman vortex street principle and phase-change theory, cavitation is expected to occur just after the central body. However, turbulent features in water jet flow discharged from central-body nozzle have not been illustrated by sufficient experimental data when water is ejected into ambient air. Under jet pressures of 11MPa, 13MPa and 17MPa, free water jet discharged from a central-body nozzle was experimentally studied. Phase Doppler particle anemometry (PDPA) was applied to measure non-intrusively the flow fields. Four traverse sections were selected for data visualization and representative description of the flow features. Control volumes at center and rim of the four sections were monitored for recording of every validated single droplet passing through. Experiment results indicate that pressure increase influences maximum velocity significantly. Obvious statistical characteristics exist at jet center and jet rim. The statistical feature of the droplet distribution varies slightly with pressure increase. Turbulent fluctuation is proved to be in reasonable relation to droplet behavior and droplet diameter distribution.

Topics: Nozzles , Water
Commentary by Dr. Valentin Fuster
2010;():145-148. doi:10.1115/FEDSM-ICNMM2010-30299.

The resistance loss of sprinkler irrigation system is an important parameter, it has significance and practical value for studying its device characteristics. In this study, according to the selected materials and setting modes, both the pressure drop along the pipe and the local resistance loss were calculated out. The theoretical device characteristics of one sprinkler irrigation system was analyzed. A calculating schema has been carried out in order to apply the method to any sprinkling system. Experimental study was carried out for this system. Water distribution maps for the sprinklers were drawn using Matrix Laboratory (MATLAB). The hydraulic characteristics were as follow, 7.2, 3.0, 10.0 mm/h was the average, minimum, and maximum irrigated intensity, respectively. It supplied theoretical foundation for the reasonable application of sprinkler irrigation system for the future.

Commentary by Dr. Valentin Fuster
2010;():149-158. doi:10.1115/FEDSM-ICNMM2010-30304.

An experimental study was conducted to quantify the flow characteristics of wall jets pertinent to trailing edge cooling of turbine blades. A high-resolution stereoscopic PIV system was used to conduct detailed flow field measurements to quantitatively visualize the evolution of the unsteady vortex and turbulent flow structures in cooling wall jet streams and to quantify the dynamic mixing process between the cooling wall jet streams and the main stream flows. The detailed flow field measurements are correlated with the adiabatic cooling effectiveness maps measured by using pressure sensitive paint (PSP) technique to elucidate underlying physics in order to improve cooling effectiveness to protect the critical portions of turbine blades from the harsh ambient conditions.

Commentary by Dr. Valentin Fuster
2010;():159-167. doi:10.1115/FEDSM-ICNMM2010-30713.

Pneumatic pressure probes are well-mature measuring devices to characterize both pressure and velocity fields for external and internal flows. The measuring range of a particular probe is significantly influenced by important factors, like its geometry, the separation angle between the holes, the holes tapping or even flow conditions like separation and stagnation points or the local Reynolds number. Ideally, every pressure probe must be specifically designed for the particular application where it is needed. However, this procedure requires a detailed calibration of the probe for the whole expected range of velocities and incidences. This implies an important cost in both economic terms and operating times. Thus, the definition of an accurate numerical model for the design and calibration of pressure probes at different flow conditions is particularly desirable for these purposes. The first step towards the establishment of this useful methodology is the development of a reliable model to predict numerically the probe measuring characteristics. Thus, in this paper a numerical 3-D model is presented to characterize the calibration of a three-hole pneumatic pressure probe. In particular, a trapezoidal geometry with a 60 degree angle between the holes is considered here. The simulation of the flow incidence is carried out using the commercial code FLUENT, analyzing the influence of different mesh densities and turbulence models. The complete set of numerical cases includes different flow velocities and several yaw angles. The numerical results have been validated using experimental results obtained in a calibration facility, focusing on the definition of a numerical tool for the design and calibration of three-hole pneumatic probes under incompressible flow conditions.

Commentary by Dr. Valentin Fuster
2010;():169-178. doi:10.1115/FEDSM-ICNMM2010-30740.

Stratified flows are frequently observed in environmental and oceanic applications, which often involve interaction of momentum and scalar flux. In this study, Particle Image Velocimetry and Planar Laser Induced Fluorescence are applied to simultaneously measure the velocity and density fields of a turbulent jet discharged horizontally into an environment with density difference, for studying the mixing and entrainment process in stratified flows. The data are analyzed to gain understanding of the physical mechanism of vertical mixing (mixing along gravity direction) and horizontal mixing (mixing along horizontal direction) introduced by the turbulent jet flows. The dataset also provides a test platform for mixing models used in stratified flow simulations.

Commentary by Dr. Valentin Fuster
2010;():179-184. doi:10.1115/FEDSM-ICNMM2010-30747.

Primary blast injury, caused by exposure to the primary pressure wave emitted from explosive ordnance, is a common trauma associated with modern warfare activities. The central nervous system is particularly vulnerable to primary blast injury, which is responsible for many of the war related casualties and mortalities. An ex vivo model system is developed to introduce a blast wave, generated from a shock tube, directly to spinal cord tissue sample. A high-speed shadowgraphy is utilized to visualize the development of the blast wave and its interaction with the tissue samples. The surface deformation of the tissues is also measured for the analysis of internal stress and possible damage occurred in the tissue sample. Understanding the temporal development of the blast-tissue interaction provides valuable input for characterizing and modeling blast-induced neurotrauma. Particularly, tracking the sample surface deformation over time provides realistic boundary conditions for numerically simulating the injury and understanding the temporal development of stress.

Commentary by Dr. Valentin Fuster
2010;():185-192. doi:10.1115/FEDSM-ICNMM2010-30748.

Respiratory gaseous flow measurement is one of an unsteady gas flow measurement and becoming very important. It has a wide field of application, for example, a measurement of lung function, an evaluation of respiratory gas exchange, a grasp of medical condition and so on. Especially, the evaluation of the absolute quantity and the analysis of the breathing waveform pattern are very important in the respiratory gaseous flow measurement. However, the dynamic characteristics of the respiratory gaseous flow meter has not been quantitatively measured and evaluated in the actual unsteady flows. There is substantial literature dealing with the measurement of unsteady gas flow. Most of these studies generated unsteady mass flows by using piston cylinders. Clearly, in these studies, substantial efforts must have been required in order to minimize the sensitivity dependence of density fluctuation on pressure and temperature variations. On the other hand, the dynamic characteristic evaluation of the gaseous flow meter which reproduced the sinusoidal waveform with only a single frequency component in the measurement frequency band was typically enough. However, the respiratory airflow waveform with the various frequency components and the shapes is complicated. Moreover, we already know that the respiratory waveform pattern changes by a state of health and activities. To solve these problems, this paper deals with the development of unsteady gas flow generator for the various breathing waveform reproduction. At first, we carry out the survey on the respiratory gaseous flow. Based on the research background and the above mentioned survey, we develop and introduce the unsteady gas flow generator which can generate the various respiratory flows. And we show the effectiveness of the developed unsteady gas flow generator. Moreover, we conduct the performance evaluation of the developed unsteady gas flow generator and the uncertainty analysis.

Topics: Gas flow , Generators
Commentary by Dr. Valentin Fuster
2010;():193-201. doi:10.1115/FEDSM-ICNMM2010-30813.

Microscopic digital Holographic PIV is used to measure the 3D velocity distributions in the roughness sublayer of a turbulent boundary layer over a rough wall. The sample volume extends from the surface, including the space between the tightly packed, 0.45 mm high, pyramidal roughness elements, up to about 5 roughness heights away from the wall. To facilitate observations though a rough surface, experiments are performed in a facility containing fluid that has the same optical refractive index as the acrylic rough walls. Magnified in line holograms are recorded on a 4864×3248 pixel camera at a resolution of 0.67μm/pixel. The flow field is seeded with 2μm silver coated glass particles, which are injected upstream of the same volume. A multiple-step particle tracking procedure is used for matching the particle pairs. In recently obtained data, we have typically matched ∼5000 particle images per hologram pair. The resulting unstructured 3D vectors are projected onto a uniform grid with spacing of 60 μm in all three directions in a 3.2×1.8×1.8 mm sample volume. The paper provides sample data showing that the flow in the roughness sublayer is dominated by slightly inclined, quasi-streamwise vortices whose coherence is particularly evident close to the top of the roughness elements.

Commentary by Dr. Valentin Fuster
2010;():203-209. doi:10.1115/FEDSM-ICNMM2010-31028.

Efforts are underway in the Surface and Microanalysis Science Division at the National Institute of Standards and Technology to study trace aerodynamic sampling of contraband materials (explosives or narcotics) in non-contact trace detection systems. Trace detection systems are designed to screen people, personal items, and cargo for particles that have contaminated surfaces. In a typical implementation of people screening, a human subject walks into a confined space where they are interrogated by a series of pulsed air jets and are screened for contraband materials by a chemical analyzer. The screening process requires particle and vapor removal, transport, collection, desorption, and detection. Aerodynamic sampling is the critical front-end process for effective detection. In this paper, a number of visualization techniques are employed to study non-contact aerodynamic sampling in detail. Particle lift-off and removal is visualized using high-speed videography, transport of air and particles by laser light scattering, and desorption surface heating and cooling patterns by infrared thermography. These tools are used to identify sampling inefficiencies and may be used to study next-generation screening approaches for aerodynamic sampling of particles and vapors.

Commentary by Dr. Valentin Fuster
2010;():211-215. doi:10.1115/FEDSM-ICNMM2010-31136.

There appears fluttering phenomena in a hard disk drive system with high-speed disks rotating inside a closed space, leading to degrade of reading and writing performance. The precise pressure distribution on the disk may improve the performance, but there has been no report because it is very hard to measure the surface pressure using conventional techniques, such as pressure taps. While pressure sensitive paint (PSP) seems to be suitable for the pressure measurement on the disk, we have to compensate its highly temperature-sensitive characteristics of PSP, because the temperature distribution on the disk is not assumed to be uniform. We employed PySO3 H based PSP, which has small temperature sensitivity, and have obtained the pressure distribution on the disk rotated at various speeds (10000–20000 rpm) successfully. The result showed that the pressure is higher at the disk outside than at the center, and forms a concentric circle distribution. Moreover, we found that the pressure difference between the inner and outer region of the disk increases as a square of disk rotation speed.

Topics: Pressure , Paints , Disks
Commentary by Dr. Valentin Fuster
2010;():217-224. doi:10.1115/FEDSM-ICNMM2010-31198.

Hot-wire anemometry is an established technique for velocity measurements in turbulent flows. Calibration of hot-wire probes is challenging due to the nonlinear relationship between the probe output voltage and the velocity, and the sensitivity to the temperature difference between the heated wire and the ambient flow. A triple-wire probe contains three mutually orthogonal wires that permit the three components of the local instantaneous velocity vector to be measured simultaneously. Calibration data reduction methods for multi-wire probes, based on variable-angle calibration techniques, may include curve-fits and direct-interpolation schemes. In the present study, a novel calibration data reduction method for a triple-wire probe is reported which uses an artificial neural network. Such a method has been successfully applied by other researchers for the calibration of seven-hole pressure probes. For the triple-wire probe, the neural network is used to produce a calibration relation between the three probe output voltages and the three components of the local velocity vector. Variable-angle calibration data were obtained for a triple-wire probe for velocity magnitudes from 5 to 40 m/s, yaw angles from −35° to +35° , and roll angles from 0° to 345° . A three-layer perceptron feed-forward network, using a Levenberg-Marquardt training algorithm, was applied to the calibration data, to map the mean voltages to the mean velocity components. The network was tested using an independent data set. The present results yielded standard errors of approximately ±0.38 m/s, ±0.25 m/s and ±0.26 m/s in the magnitudes of the streamwise, vertical, and cross-flow velocity components, respectively. The results showed that the present neural network model is not significantly sensitive to the size of the calibration data set, suggesting it may be a more convenient calibration data reduction method compared to other methods.

Commentary by Dr. Valentin Fuster
2010;():225-231. doi:10.1115/FEDSM-ICNMM2010-31290.

The accuracy in measuring flow of fluids such as gas and oil has a great importance for the Algerian economy. The flows of fluids in non-standard conditions, presence of disturbances, in which there are flow meters in pipes, make a very important error. International standards ISO 5167 and AGA3 stipulate that the meter is installed in a fully developed flow. This article describes a numerical investigation of development and establishment of flows in the presence of a double bend 90° in perpendicular planes as a perturbation. The software used was code Fluent where different turbulence models are tested to better simulate and view the effectiveness of models in the description of the flow of fluid compared to flow behaviour cited in the standards and the experimental results. The numerical experimentation is done with air in a pipe of 100mm diameter at a Reynolds number 105 . The numerical analysis is based on solving Navier-Stokes equation system with several turbulent models, k-ε, k-ω, RSM and its variants.

Commentary by Dr. Valentin Fuster
2010;():233-238. doi:10.1115/FEDSM-ICNMM2010-31291.

This numerical study is a comparative study of the development and establishment of turbulent flows through three flow conditioners namely Laws perforated plate, the Etoile and the tube bundle. They are installed in a circular pipe with a disturbance generated by a 90° double bend out of plane which causes a very strong swirl of the fluid. The analysis is done with the code Fluent in which the Navier-Stokes equations describe a three-dimensional incompressible flow with the Reynolds stress model (RSM) as a closure system. This article focuses on the effectiveness of the three packers to produce the condition of fully developed velocity profile. The results are compared to references profiles cited in the literature and experimental results. The flow is simulated with air at Reynolds number of 105 in 100mm pipe diameter. The velocity profiles are compared with the profile obtained by the universal law of power 1/7th .

Commentary by Dr. Valentin Fuster
2010;():239-244. doi:10.1115/FEDSM-ICNMM2010-31313.

The droplet sizes and velocities contained in vessel generated spray are difficult to quantify. This paper describes three different methods to quantify velocity and size distributions from high speed video of spray from a planing boat. These methods include feature tracking, displacement tracking and video inversion. For the feature tracking method, the images were preprocessed using contrast limited adaptive histogram equalization, and then converted to binary images with a specific intensity cutoff level. Image statistics were then generated from this image, including droplet area and effective diameter. These images were processed using commercial PIV software to obtain velocities. For the displacement tracking method, the images were also converted to binary images with a specific intensity cutoff level. Image statistics were again compiled from this binary image. A droplet filter was then applied using a binary erosion image processing technique, where large droplets were removed because the entire droplet may not be in frame, and small droplets were removed because they might not overlap between frames. Droplets were then tracked by comparing the bounding boxes of two droplets between time frames. The video inversion method consisted of the manipulating the original high speed videos from spatial x-y frames in time space to time-y frames in x-space, where the x-axis is longitudinally along the ship and the y axis is vertical to the ship. From this orientation, the speed of the general spray mass could be determined by summing the pixels in time columns for each × frame. Comparisons of droplet size distribution between the feature and displacement tracking method yield qualitatively similar results, with some disagreement likely due to the different threshold levels. The trend of the distribution curve suggests that both methods are unable to resolve the smallest droplet sizes, due to the processing filters applied as well as the field of view of the camera. The three analysis methods compare well in their spray velocity computation, and are also similar to spray speed predictions found in the literature for a given geometry and vessel speed.

Topics: Sprays , Vessels
Commentary by Dr. Valentin Fuster
2010;():245-256. doi:10.1115/FEDSM-ICNMM2010-31314.

Obtaining real-time, in situ slurry concentration measurements during unsteady mixing can provide increased understanding into mixer performance. During recent tests an ultrasonic attenuation sensor was inserted into a mixing vessel to measure the slurry concentration during unsteady mixing in real time during pulse jet mixer operation. These pulse jet mixing tests to suspend noncohesive solids in Newtonian liquid were conducted at three geometric scales. To understand the solids suspension process and resulting solids distribution, the concentration of solids in the cloud was measured at various elevations and radial positions during the pulse jet mixer cycle. In the largest scale vessel, concentration profiles were measured at three radial locations: r = 0, 0.5 and 0.9 R where R is the vessel radius. These radial concentration data are being analyzed to provide a model for predicting concentration as a function of elevation. This paper describes pulse jet mixer operation, provides a description of the concentration probe, and presents transient concentration data obtained at three radial positions: in the vessel center (O R), midway between the center and the wall (0.5 R) and near the vessel wall (0.9 R) through out the pulse to provide insight into pulse jet mixer performance.

Topics: Pulsejets
Commentary by Dr. Valentin Fuster
2010;():257-263. doi:10.1115/FEDSM-ICNMM2010-30828.

Two-phase flows including particle-particle collisions and two-way coupling in a turbulent duct flow were simulated using a direct simulation approach. The direct numerical simulation (DNS) of the Navier-Stokes equation was performed via a pseudospectral method was extended to cover two-way coupling effects. The effect of particles on the flow was included in the analysis via a feedback force that acted on the fluid on the computational grid points. The point particle equation of motion included the Stokes drag, the Saffman lift, and the gravitational forces. Several simulations for different particle relaxation times and particle mass loading were performed, and the effects of the inter-particle collisions and two-way coupling on the particle deposition velocity, fluid and particle fluctuating velocities, particle normal mean velocity, and particle concentration were determined. It was found that when particle-particle collisions were included in the computation, the particle deposition velocity increased. When the particle collision was neglected but the particle-fluid two-way coupling was accounted for, the particle deposition velocity decreased slightly. When both inter-particle collisions and two-way coupling effects were taken into account in the simulations, the particle deposition velocity increased. Comparisons of the present simulation results with the available experimental data and earlier numerical results are also presented.

Commentary by Dr. Valentin Fuster
2010;():265-267. doi:10.1115/FEDSM-ICNMM2010-31043.

A numerical study on the turbulence and vorticity of local scour underneath an offshore pipeline placed on a non-cohesive sandy seabed and forced by a steady flow current is presented. The numerical model solves the Navier-Stokes equations using an innovative Level Set technique. The model predicts the behavior of the movable sediments through both drift and lift force components. Mean and turbulent flow quantities were extracted by temporal averaging. Results on the distribution and evolution of turbulent kinetic energy and vorticity will be illustrated at the conference.

Commentary by Dr. Valentin Fuster
2010;():269-274. doi:10.1115/FEDSM-ICNMM2010-30137.

The modeling of particle deposition and transport in pipes is one of the most challenging problems in multiphase flow, because the underlying physics is multi-faceted and complex, including turbulence of the carrier phase, particle-turbulence interaction, particle-wall interactions, particle-particle interactions, two-way and four-way couplings, particle agglomeration, deposition and re-suspension. We will discuss these issues and present new routes for the modeling of particle collision stress. Practical examples like black powder deposition and transport in gas pipelines will be presented and discussed. The model employed is based on dense-particle formulation accounting for particle-turbulence interaction, particle-wall interactions, particle-particle interactions via a collision stress. The model solves the governing equations of the fluid phase using a continuum model and those of the particle phase using a Lagrangian model. Inter-particle interactions for dense particle flows with high volume fractions (from 1% to close packing ∼60%) have been accounted for by mapping particle properties to an Eulerian grid and then mapping back computed stress tensors to particle positions. Turbulence within the continuum gas field was simulated using the V-LES (Very Large-Eddy Simulation) and full LES, which provides sufficient flow unsteadiness needed to disperse the particles and move the deposited bed.

Commentary by Dr. Valentin Fuster
2010;():275-282. doi:10.1115/FEDSM-ICNMM2010-30369.

Fluidized bed technology can be used for pyrolysis and gasification of solid fuel particles such as biomass, which is important to industry because of its potential as an alternative for petroleum-based fuels. To efficiently utilize a fluidized bed reactor it is necessary, among other factors, to investigate the mixing and segregation behavior of the fuel particles with the bed material. In order to characterize the material distribution, a technique to visualize the biomass inside a fluidized bed reactor has been developed using X-ray computed tomography (CT) scans. This paper presents an image analysis procedure that can be used to quantify and characterize the local mixing and segregation in a 3D fluidized bed.

Topics: Fluidized beds
Commentary by Dr. Valentin Fuster
2010;():283-291. doi:10.1115/FEDSM-ICNMM2010-30789.

In this article, air entrainment as a result of an impinging round water jet and a wall-jet was experimentally studied by means of videometry and image processing methods and also by means of a measurement technique based on a wire-mesh sensor. Therefore, two different experimental setups were utilized. For the first setup, a series of experiments at different conditions was performed and evaluated for both round jets and wall-jets. Jet lengths ranged between 0.01 and 0.2 m and jet exit velocities between 0.9 and 3.5 m/s. Image processing algorithms were applied to extract information about jet penetration depth, width of the bubble plume and bubble size distribution. The second facility was used to create a falling film in a square pipe (5 cm × 5 cm). Downstream of the impact point, a wire-mesh sensor was used to measure the gas entrainment characteristics at one axial location. Video image processing was also used in this experiment to gather more qualitative information about the gas entrainment process. Video images are compared with the images obtained by the wire-mesh sensor showing good qualitative agreement. The induction trumpet and a thin sheet of gas that forms around the jet and penetrates into the pool causing the entrainment were clearly identified. Results indicate that the gas void fraction increases and the bubble size decreases as the superficial liquid velocity increases.

Topics: Jets
Commentary by Dr. Valentin Fuster
2010;():293-302. doi:10.1115/FEDSM-ICNMM2010-31126.

In order to safe design and optimize performance of some industrial systems, it’s often needed to categorize two-phase flow into different regimes. In each flow regime, flow conditions have similar geometric and hydrodynamic characteristics. Traditionally, flow regime identification was carried out by flow visualization or instrumental indicators. In this research3 kind of neural networks have been used to predict system characteristic and flow regime, and results of them were compared: radial basis function neural networks, self organized and Multilayer perceptrons (supervised) neural networks. The data bank contains experimental pressure signalfor a wide range of operational conditions in which upward two phase air/water flows pass to through a vertical pipe of 5cm diameter under adiabatic condition. Two methods of signal processing were applied to these pressure signals, one is FFT (Fast Fourier Transform) analysis and the other is PDF (Probability Density Function) joint with wavelet denoising. In this work, from signals of 15 fast response pressure transducers, 2 have been selected to be used as feed of neural networks. The results show that obtained flow regimes are in good agreement with experimental data and observation.

Commentary by Dr. Valentin Fuster
2010;():303-313. doi:10.1115/FEDSM-ICNMM2010-31127.

In order to safe design and optimize performance of industrial systems which work under two phase flow conditions, it’s often needed to categorize flow into different regimes. In present work the experiments of two phase flow were done in a large scale test facility with length of 6m and 5cm diameter. Four main flow regimes were observed in vertical air-water two phase flows at moderate superficial velocities of gas and water: Bubbly, Slug, Churn and Annular. Some image processing techniques were used to extract information from each picture. This information include number of bubbles or objects, area, perimeter, height and width of objects (second phase). Also a texture feature extraction procedure was applied to images of different regimes. Some features which were adequate for regime identification were extracted such as Contrast, Energy, Entropy and etc. To identify flow regimes a fuzzy interface was introduced using characteristic of second phase in picture. Also an Adaptive Neuro Fuzzy (ANFIS) was used to identify flow patterns using textural features of images. The experimental results show that these methods can accurately identify the flow patterns in a vertical pipe.

Commentary by Dr. Valentin Fuster

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