bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability Analysis of Cassie-Baxter State Under Pressure Driven Flow
Authors: Kim Tae Jin , Hidrovo Carlos H.
Published: March 2011

The Cassie-Baxter state is a phenomenon in which a liquid rests on top of a textured surface with a gas layer trapped underneath the liquid layer. This gas layer introduces an effective shear free boundary that induces slip at the liquid-gas interface, allowing for friction reduction in liquid channel flows. Multiple studies have shown that different surface configurations result in different friction reduction characteristics, and most work is aimed at controlling the roughness factor and its shape in order to achieve an increased slip flow. This paper investigates the effects that different texturing geometries have on the stability of the Cassie state under pressurized microchannel flow conditions. To test the stability effects associated with the pressurized microchannel flow conditions, microfluidic channels with microstructures on the side walls were designed and fabricated. The microstructures were designed to induce the Cassie state with a liquid-air interface forming between the texturing trenches. The air trapped within the microstructure is treated as an ideal gas, with the compressibility induced pressure rise acting as a restrictive force against the Wenzel wetting transition. The model was validated against experimental flow data obtained using microchannel samples with microtextured boundaries. The microchannels were fabricated in PDMS (poly-dimethylsiloxane) using soft lithography and were baked on a hot plate to ensure the hydrophobicity of the microtexture. Pressure versus flow rate data was obtained using a constant gravitational pressure head setup and a flow meter. The liquid-gas interface layer in the microchannel was visualized using bright field microscopy that allowed measurement of the liquid penetration depth into the microtexturing throughout the microhannel. The experimental results indicate that air trapped in the pockets created by micro-cavity structures prevented the liquid layer from completely filling the void. As expected, the pressure drop in the micro-cavity textured channel showed a considerable decrease compared to that in the flat surfaced channel. These results also suggest that micro-cavities can maintain the Cassie state of a liquid meniscus, resting on top of the surface, in larger pressure ranges than open spaced micro-pillars arrays.
Performance and Flow Conditions of Contra-Rotating Small-Sized Axial Fan
Authors: Shigemitsu Toru , Fukutomi Junichiro , Okabe Yuki , Iuchi Kazuhiro
Published: March 2011

Small-sized axial fans are used as air cooler for electric equipments. But there is a strong demand for higher power of fans according to the increase of quantity of heat from electric devices. Therefore, higher rotational speed design is conducted although, it causes the deterioration of efficiency and the increase of noise. Then the adoption of contra-rotating rotors for the small-sized axial fan is proposed for the improvement of performance. In the present paper, the performance curves of the contra-rotating small-sized axial fan with 100mm diameter are shown and the velocity distributions for a designed flow rate at the inlet and the outlet of each front and rear rotor are clarified with experimental results. Furthermore, the flow conditions between front and rear rotors of the contra-rotating small-sized axial fan are investigated by numerical analysis results and higher performance design of the contra-rotating small-sized axial fan is discussed.
Numerical Investigation of Laminar Flow and Heat Transfer in Micro-Cylinder-Groups
Authors: Guan Ning , Liu Zhigang , Takei Masahiro , Zhang Chengwu
Published: March 2011

A numerical study on flow and heat transfer of de-ionized water over in-line and staggered micro-cylinder-groups had been performed with Reynolds number varying in the range from 0 to 150. A 3-D incompressible numerical model was employed to investigate the vortex distributions and the influences of the vortexes on the flow and heat transfer characteristics at low Re numbers in micro-cylinder-groups with different geometrical parameters, including micro-cylinder diameters (100μm, 250μm and 500μm), ratios of pitch to micro-cylinder diameter (1.5 2 and 2.5) and ratios of micro-cylinder height to diameter (0.5, 1, 1.5 and 2), etc. The vortex distributions, the flow and temperature fields, and the relationships among them were investigated by solving the numerical model with the finite volume method. It was found that the vortex number became larger with the increase of pitch ratio, and the change of flow rate distribution affected the heat transfer characteristics apparently. The appearance of vortexes in micro-cylinder-group increased the differential pressure resistance; as a result the total flow resistance in micro-cylinder-groups correspondingly increased. Meanwhile, the local heat transfer coefficients nearby the locations of vortexes greatly increased due to the boundary layer separation, which further enhanced the heat transfer in micro-cylinder-groups. The new correlations which could predict Nusselt number of de-ionized water over micro-cylinders with Re number varying from 0–150 had been proposed considering the differential pressure resistance and the natural convection based on numerical calculations in this paper.
Analysis of Cavitation Surge Considering the Acoustic Effect of the Inlet Line in a Rocket Engine Turbopump
Authors: Nanri Hideaki , Tani Naoki , Kannan Hiroki , Yoshida Yoshiki
Published: March 2011

In a liquid rocket engine, cavitation in an inducer of a turbopump sometimes causes instability phenomena when the inducer is operated at low inlet pressure. Cavitation surge (auto-oscillation), one such instability phenomenon, has been discussed mainly based on an inertia model assuming incompressible flow. By using this model, the frequency of the cavitation surge decreases as the inlet pressure decreases. However, we obtained interesting experimental results in which the cavitation surge frequency varied disconnectedly. Therefore, we considered the factor of fluid compression employed one-dimensional analysis applying an acoustic model, combining the inlet pipe with the sonic velocity of liquid oxygen. Consequently, the analytical results qualitatively corresponded with the experimental results. In addition, an actual liquid rocket propulsion system is usually equipped with a Pogo suppression device (PSD), which is a kind of accumulator with a hydraulic compliance, upstream of a liquid oxidizer turbopump. We modified the analytical model to include the effect of this PSD and compared the analytical results with the experimental results. It was found that the frequency of cavitation surge basically became the Helmholtz frequency, defined by the cavitation compliance and the length of pipe between the PSD and the turbopump. And when the frequency of cavitation surge coincided with one of the acoustic resonance frequencies of the inlet pipe, the cavitation surge was strongly excited.
A Closed Loop Approach to Simulate the Unsteady Three-Dimensional Flow in an Axial Flow Pump
Authors: Benra Friedrich-Karl , Dohmen Hans Josef
Published: March 2011

A new method is put forward to model the flow in a highly loaded axial flow pump. A directional loss model is utilized to model the function of a valve behind the pump stator vanes. A periodic boundary condition between the inlet and the outlet of the pump is applied to model a closed loop. Thus no flow specification in either the inlet or outlet of the pump is required; also it is not necessary to give the turbulence level. By this method no pressure level inside the flow domain is given by a boundary condition. To avoid numerical instability the pressure level has to be given at least at one grid point. A given constant pressure somewhere in the loop domain is physically invalid, especially at stall condition of the pump. This is avoided by introducing a reservoir with a constant pressure boundary condition that is nearly decoupled from the pressure field inside the main pump loop by a huge flow resistance. Consequently this method can avoid specifying non-physical stationary boundary conditions at the inlet and the outlet for transient simulations. The new model can predict the mass flow fluctuations in the pump. These fluctuations are not very strong at stable operating conditions but increase in part load or stalled flow conditions. The transient numerical results obtained by the new approach are compared with those obtained by the conventional simulation with stationary boundary conditions (constant total pressure at the inlet and fixed mass flow at the outlet) and also with results of experimental investigations performed by Kosyna and Stark. The different flow structures inside the blade passages of the pump are described and compared in detail for part load, overload and design point as well as for stalled flow conditions.
Research on Two-Phase Flow Instability in Evaporator of Refrigeration System
Authors: Liang Nan , Tian Changqing , Shao Shuangquan
Published: March 2011

As one kind of fluid machinery related to the two-phase flow, the refrigeration system encounters more problems of instability. It is essential to ensure the stability of the refrigeration systems for the operation and efficiency. This paper presents the experimental investigation on the static and dynamic instability in an evaporator of refrigeration system. The static instability experiments showed that the oscillatory period and swing of the mixture-vapor transition point by observation with a camera through the transparent quartz glass tube at the outlet of the evaporator. The pressure drop versus mass flow rate curves of refrigerant two phase flow in the evaporator were obtained with a negative slope region in addition to two positive slope regions, thus making the flow rate a multi-valued function of the pressure drop. For dynamic instabilities in the evaporation process, three types of oscillations (density wave type, pressure drop type and thermal type) were observed at different mass flow rates and heat fluxes, which can be represented in the pressure drop versus mass flow rate curves. For the dynamic instabilities, density wave oscillations happen when the heat flux is high with the constant mass flow rate. Thermal oscillations happen when the heat flux is correspondingly low with constant mass flow rate. Though the refrigeration system do not have special tank, the accumulator and receiver provide enough compressible volume to induce the pressure drop oscillations. The representation and characteristic of each oscillation type were also analyzed in the paper.
Experimental Analysis of a Two Phase Air-Water Flow in a Tube of Small Size Diameter Under Various Inlet Conditions
Authors: Zeguai S. , Chikh S. , Rahli O. , Tadrist L.
Published: March 2011

An experimental apparatus is setup to analyze a two phase air-water upward flow in a vertical tube with an inner diameter of 3 mm. Air is axially injected through a microduct of 260 μm inner diameter. Various inlet conditions for air pressure and water flow rate are tested covering a wide range of superficial velocities JL = 0.221 to 0.312 m/s and JG = 0.061 to 0.083 m/s for a given position of air injection (x = 8cm). A fast camera with 250 fps is used to visualize different flow regimes. Experiments showed that the flow type is very sensitive to inlet conditions and several flow regimes were observed namely: the bubbly flow, the slug flow and the annular flow.
Justification of Non-Dimensional Schemes and Identification of Parameters on Modelling Electrokinetic Transport Phenomena in Microchannels
Authors: Shao Zhanjie , Schneider Gerry , Ren Carolyn
Published: March 2011

The electrokinetic transport phenomena are to be numerically studied based on cross-linked microchannel networks, which have been commonly employed for on-chip capillary electrophoresis applications. Applied potential field, flow field and concentration field should be solved to predict the species transport process under electrokinetic flows. Together with the well-designed channel geometry, a detailed physical model was firstly formulated through a series of governing equations and corresponding boundary/initial conditions, which was briefly re-presented from our previous publications. The emphasis of current work was to justify the simplest non-dimensional scheme and identify the most beneficial parameters so that an effective and simplified non-dimensional model was developed for numerical studies.
Numerical Simulations of the Intra-Aneurysmal Vortex Shedding in Induced Mouse Abdominal Aortic Aneurysms
Authors: Ford Matthew D. , Piomelli Ugo , Cao Richard Y. , Funk Colin D. , Pollard Andrew
Published: March 2011

A feature of particular interest observed in vivo in murine abdominal aortic aneurysm (AAA) is the presence of a vortex shed from the proximal edge of the abdominal aortic aneurysm. It is unclear whether the occurrence of the shed vortex is due to the periodic nature of the flow-rate waveform, to geometric features, or to the compliant nature of the vessel tissue. Numerical simulations were performed on 3D semi-idealized and 2D axi-symmetric models of the abdominal aortic aneurysm at a mean Reynolds number of 63 and a Womersley number of 2 (for unsteady inflow conditions). The numerical results from the 3D model showed good agreement with the flows visualized by Doppler Ultrasound with respect to the development of the observed shed vortex. The idealized axi-symmetric models under steady flow conditions showed no signs of vortex shedding. Under unsteady inflow conditions, however, shear-layer roll-up occurred near the peak systolic velocity. The presence of a proximal lip was found to lead to vortex separation (from the wall) earlier in the cardiac cycle, and the presence of the proximal narrowing led to the earliest vortex separation. The sensitivity to the inflow waveform shape also showed that the presence of the shedding, in the model with proximal narrowing, disappeared when the peak-to-mean velocity ratio was reduced by approximately half. Therefore, we conclude that the observed intra-aneurysmal vortex shedding is a shear-layer rollup phenomenon; however, the geometry can act to enhance further the observed vortex shedding.
Coherent Structures in the Near Wake of a Blunt Trailing Edge Profiled Body
Authors: Doddipatla Lakshmana Sampat , Naghib-Lahouthi Arash , Hangan Horia , Siddiqui Kamran
Published: March 2011

Wake flows behind two dimensional bodies are mainly dominated by two coherent structures, namely the Karman-Benard vortices and the streamwise vortices, also referred to as rolls and ribs respectively. The three dimensional wake instabilities lead to distinct instability modes (mode-A, mode-B and mode-C or mode-S) depending on the flow Reynolds number and geometric shape. The present investigation explores the mechanism in which the flow transitions to three dimensionality in the near wake of a profiled leading edge and blunt trailing edge body. A combination of Planar Laser Induced Fluorescence visualizations and Particle Image Velcoimetry measurements are conducted in the Reynolds numbers ranging from 250 to 550. The results indicate that three instability modes (mode-A, mode-B and mode-C) appear in the wake transition to three dimensionality, and their order of appearance does not occur through the traditional route as observed in circular cylinder flows. It is found that mode-C instability with a spanwise spacing of 2.4D dominates the near wake development.
Effect of Nanoparticles on the Spreading of Bubble Triple Line on Top of a Stainless Steel Substrate Nozzle
Authors: Vafaei Saeid , Wen Dongsheng
Published: March 2011

The purpose of this study is to investigate the effect of gold nanofluid on the formation of gas bubbles on top of a stainless steel substrate plate nozzle. The experiment reveals a unique phenomenon of enhanced pinning of the triple line of gold nanofluids for bubbles forming on the substrate plate, i.e the gold nanoparticles are found to prevent the spreading of the triple line during the bubble formation. Different to the liquid droplet measurement, the bubble contact angle is found to be slightly larger for formation of bubbles inside gold nanofluids. It is also observed that bubbles develop earlier inside the nanofluids with reduced bubble departure volume and increased bubble formation frequency. The shape of the bubble is found to be in good agreement with predictions of the Laplace-Young equation under the low gas flow rates inside water. Such a good agreement is also observed for bubbles forming inside nanofluids except a few characteristic points. The variation of solid surface tensions and the resultant force balances at the triple line are believed to be responsible for the modified dynamics of the triple line inside gold nanofluids and subsequent bubble formation.
Effect of Non Uniform Out-of-Plane Illumination and Shear Rate on the Accuracy of nPIV Velocity Measurements
Authors: Khader Rana G. , Sadr Reza
Published: March 2011

Nano-particle image velocimetry (nPIV) uses evanescent-wave illumination to measure two velocity components, U and V, tangent to the wall in a region with thickness of order of hundred nano meters. In this region the illumination intensity decays exponentially with distance normal to the wall, z, and hence tracers closer to the wall have “brighter” and “bigger” images than those that are further away, i.e. at larger z. Moreover fluid velocity varies in this region with z and hence tracers at different distance from the wall move at different speeds. Furthermore, Brownian displacement of particle tracers in this region is comparable to the displacement due to the fluid convection. The variation in the displacement of particle images in this region, with different brightness and velocities, can bias the near-wall velocities obtained using standard correlation based PIV method. Artificial nPIV images of nano particle in a flow field with linear out of plane velocity profile were used in this work to investigate the impact of these issues upon the accuracy of nPIV data. Uniform and Gaussian random distribution noise were added to the images to simulate electronic noise and shot noise, respectively. The artificial images were obtained and processed for various experimental parameters to incorporate different illumination profile and shear rates. The results demonstrate that non-uniform illumination affects the bias in the estimated tracer velocity for the shear flow. Non-uniform intensity also affects the bias due to Brownian diffusion; however, correction for Brownian diffusion can reduce this bias error.
Uncertainty Quantification of Turbulence Model Coefficients via Latin Hypercube Sampling Method
Authors: Dunn Matthew C. , Shotorban Babak , Frendi Abdelkader
Published: March 2011

This paper is concerned with the propagation of uncertainties in the values of turbulence model coefficients and parameters in turbulent flows. These coefficients and parameters are determined from experiments performed on elementary flows and they are subject to uncertainty. The widely used k–ε turbulence model is considered. It consists of model transport equations for the turbulence kinetic energy and rate of turbulent dissipation. Both equations involve various model coefficients about which adequate knowledge is assumed known in the form of probability density functions. The study is carried out for the flow over a 2D backward-facing step configuration. The Latin Hypercube Sampling method is employed for the uncertainty quantification purposes as it requires a smaller number of samples compared to the conventional Monte-Carlo method. The mean values are reported for the flow output parameters of interest along with their associated uncertainties. The results show that model coefficient variability has significant effects on the streamwise velocity component in the recirculation region near the reattachment point and turbulence intensity along the free shear layer. The reattachment point location, pressure, and wall shear are also significantly affected.
A Molecular Dynamics Study on the Absorption of Ions Into Carbon Nanotubes
Authors: Cannon James J. , Tang Dai , Kim Daejoong
Published: March 2011

The ability of ions to be selectively absorbed into nano-pores is important for a wide range of industrial and biological processes. To make practical use of such ionic selectivity, a molecular-level understanding of the dynamics is required, so that conditions can be tailored and optimised as required. This is especially the case when the ions being separated are very similar. We have therefore conducted molecular dynamics simulations in order to study the selective absorption of sodium and potassium ions into a nano-scale pore. We show how ion hydration and the presence of a negative ion lead to alternative selectivity depending on the size of the pore.
Liquid Squeeze Film Damping in Microsystems Applications
Authors: Huang Shujuan , Borca-Tasciuc Diana-Andra , Tichy John
Published: March 2011

Squeeze film damping (SFD) in microscale systems employing plates parallel to a substrate and operating in a liquid environment is theoretically investigated. Previous analytical or numerical studies of SFD in microsystems are mainly focused on devices working in a compressible fluid, such as air [1–3], where fluid inertia is negligible. However, liquid inertia appears to significantly affect the dynamic response of a microsystem operating in liquid environment. This paper outlines a theoretical framework that takes into account the inertia effects on dynamic response and illustrates this effect presenting the magnification factor for media with high and low density.
Numerical Calculation of Triangle Circulation Drip Irrigation Emitters
Authors: Wang Xinkun , Li Junhong
Published: March 2011

Based on the hydraulic characteristics of triangle circulation, sudden-expansion pipe and sudden contraction pipe, Construct a drip irrigation emitter with strong turbulent flow, large over-current cross-section and strong anti-clogging ability. Triangular circulation flow path emitters for the key structural parameters for the factors, application of computer numerical fluid dynamics CFD software FLUENT6.3, simulate triangular circulation flow channel structure, analyze the influence of various structural parameters on hydraulic performance of emitters, flow index, flow rate and the anti-clogging ability. The results show that increase unit cusp, unit chamfer, the flow index increased, but the effect to varying degrees; inlet dimension increase, flow index reduced; flow channel depth and flow channel width increases, the discharge increases. Increase unit cusp, unit chamfered, can improve the anti-clogging performance of emitters. Based on the above results, to design an emitter structure with good performance, and its numerical simulation analysis, the flow index, flow rate and anti-clogging ability have met the requirements of drip irrigation. Provide a theoretical basis for the triangle circulation emitter structure design and quantitative analysis. The research has a positive meaning for energy conservation.
Experimental Studies of Nozzle/Diffuser Micropumps Using Enhancement Structure
Authors: Yang Kai-Shing , Chao Tzu-Feng , Chen Ing Youn , Wang Chi-Chuan
Published: March 2011

This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design subject to enhanced structures. A total of three micropumps, including two enhancement structurs having two-fin or obstacle structure and one conventional micro nozzle/diffuser design. It is found that the pressure drops across the designed micro nozzles/diffusers are increased considerably when the obstacle or fin structure are added. The resultant maximum flow rates are 42.08 mm3/s and 50.15 mm3/s for conventional micro nozzle/diffuser and added two-fin structure in micro nozzle/diffuser operated at a frequency of 350 Hz. It is found that the mass flowrate for two-fin design surpasses that of conventional one when the frequency is below 400 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a least flowrate throughout the test range.
Experiments on the Stability of Taylor-Couette Flow With Different Radial Temperature Gradient
Authors: Liu Dong , Kim Hyoung-Bum
Published: March 2011

The effect of the temperature gradient and the presence of slits in the outer cylinders involved in creating a Taylor-Couette flow was investigated by measuring the velocity field inside the gap simultaneously. The slits were azimuthally located along the inner wall of outer cylinder and the number of slits was 18. The results showed that the buoyant force due to the temperature gradient clearly generated the helical flow when the rotating Reynolds number is small. For the plain model, the transition to turbulent Taylor vortex flow is not affected by the temperature gradient considered in this study. In addition, the transition process of 18-slit model was accelerated due to the slit wall. As the temperature gradient became larger, the critical Reynolds number of the transition process decreased.
Micro-PIV and CFD Studies Show Non-Uniform Wall Shear Stress Distributions Over Endothelial Cells
Authors: Dol Sharul S. , Salek M. Mehdi , Viegas Kayla D. , Rinker Kristina D. , Martinuzzi Robert J.
Published: March 2011

Wall shear stress acting on arterial walls is an important hemodynamic force determining vessel health. A parallel-plate flow chamber with a 127 μm-thick flow channel is used as an in vitro system to study the fluid mechanics environment. It is essential to know how well this flow chamber performs in emulating physiologic flow regimes especially when cultured cells are present. Hence, the objectives of this work are to computationally and experimentally study the characteristic of the flow chamber in providing a defined flow regime and shear stress to cultured cells and to map wall shear stress distributions in the presence of an endothelial cell layer. Experiments and modeling were performed for the nominal wall shear stresses of 2 and 10 dyn/cm2. Without endothelial cells, the flow field is uniform over 95% of the chamber cross-section and the surfaces are exposed to the target stress level. Using PIV velocity data, the endothelial cell surfaces were re-constructed and flow over these surfaces was then simulated via FLUENT. Once endothelial cells are introduced, local shear variations are large and the velocity profiles are no longer uniform. Due to the velocity distribution between peaks and valleys, the local wall shear stresses range between 47–164% of the nominal values. This study demonstrates the non-uniform shear stress distribution over the cells is non-negligible especially in small vessels or where blockage is important.
Multi Objective Shape Optimization of a Low Specific Speed Impeller for a Rocket Engine Turbopump
Authors: Tani Naoki , Shimiya Noriyuki , Yoshida Yoshiki , Yamanishi Nobuhiro
Published: March 2011

A rocket engine should be small and low weight, a turbopump for a rocket engine must be smaller and have higher rotation speed than the conventional pumps. However, to achieve high thrust, pump discharge pressure must be high enough. As a result, a low specific speed impeller is often chosen for a rocket engine impeller. Generally speaking, efficiency of such a low specific speed impeller is lower since blade loading becomes high and large scale secondary flow will likely occur especially around the trailing edge. Therefore, to clarify the high efficiency shape, multi objective optimization of low specific speed impeller was carried out in the present study. The optimized result showed that there is a strong tradeoff between head and efficiency, and this tendency is not influenced by the flow rate. This means that performance dependency by a flow rate may be small by such a low specific speed impeller. Shape comparison between efficiency and head optimum results showed that not only outlet blade angle but also inlet blade angle are important for high efficiency impeller. By modifying these two blade angles, blade loading distribution is changed and blockage by secondary flow region is changed. As a result, for the high head impeller, large scales blockage occurs at the trailing edge, however, for the efficiency optimum result, blockage near the trailing edge becomes smaller.
Computation of an Unsteady Orifice Flow in a Circular Pipe From Wall Pressure Measurements Based on Measurement-Integrated Simulation Using a Turbulent Model
Authors: Nakao Mitsuhiro , Kawashima Kenji , Kagawa Toshiharu
Published: March 2011

Visualizing the state of real turbulent flow is important in many applications such as safe operation and fault diagnosis in plant or pipeline. Two approaches to this purpose exist: experimental measurement and numerical simulation. In experimental measurement, reliability of the result at measured point is easy to evaluate. However, information of the whole flow field is difficult to obtain. On the other hand, numerical simulation easily obtains any information of the flow field. However, the reliability of the result strongly depends on the numerical model and boundary condition and/or the initial condition. In general, the more precise results are needed, the heavier computation load we spend. None of these approaches is superior, and combination methods of them are subjected to extensive research. Above all, we particularly paid attention to measurement-integrated (MI) simulation proposed by Hayase et al. MI simulation can expect to reduce computational load. We have applied MI simulation to unsteady oscillatory airflows passing through an orifice. In our previous study, a standard k-ε model was used for MI simulation. Estimation error remained due to inadequate consideration of the feedback law. In our latest study, the feedback law was decided considering an effect of computation grid on CFD of contracted flow. As a result, wall pressures near the orifice plate and axial velocities on vena contracta estimated with MI simulation showed good agreement with that of measurement. In the present paper, we deal with visualization of unsteady oscillatory airflows passing through an orifice from wall pressure measurement based on MI simulation using a turbulent model. The former studies have used measured inlet flow rate which is unknown in many actual case. Compared with the flow rate measurement, wall pressure measurement is simple. Therefore, we consider MI simulation using only wall pressure are of practical use. The developed MI simulation was performed with unsteady flow rate with the frequency up to 10 Hz. Computation results obtained with the developed MI simulation using coarse computation grid is compared with experimental results. It is confirmed that flow field obtained with the developed MI simulation is close to that of experiment.
Flow Structure of Supersonic Jet From a Straight Micro-Tube
Authors: Hong Chungpyo , Yoshida Yasuhiro , Asako Yutaka , Suzuki Koichi
Published: March 2011

In the case of micro-channels, the boundary layer is formed on the walls and it plays a role of a wall of a converging and diverging nozzle. Then, the outlet Mach number is beyond unity even if the tube is straight. Therefore, in the present study, the successive incident and reflected shock waves on underexpanded state jet from a straight micro-tube whose diameter ranges from 150μm to 500μm were visualized by Schlieren and Shadowgragh methods. The stagnation pressure ranges from 597 to 963 kPa. The flow characteristics on supersonic jet at the micro-tube outlet were also obtained. Also, it is confirmed that Mach number at a straight micro-tube outlet is beyond unity since the shock wave generates from the needle and the Mach number fluctuates in the jet. The experimental correlation for the distance from the micro-tube outlet to the Mach disk as a function of the ratio of stagnation pressure to ambient back pressure was proposed and compared with available correlations in literature.
Experimental Study of Blade Rigidity Effects on the Global and the Local Performances of a Thick Blades Axial-Flow Fan
Authors: Nouri Hussain , Ravelet Florent , Sarraf Christophe , Bakir Farid
Published: March 2011

An experimental investigation on the aerodynamic performances of thick blades axial-flow fans was carried out in this study. Two fans are considered, the first one is rotomoulded (in plastic) and the second one is milled (in aluminium). Both have exactly the same shape, except that the rotomoulded fan has hollow blades. They were designed from an existing fan (manufactured by plastic injection process) used in the cooling system of an automotive vehicle power unit. As far as shape is concerned, the only difference between the two first fans and the traditional injected fan is the blade thickness, whereas as far as rigidity is concerned, the only difference between the rotomoulded and the milled fans is the ability of the rotomoulded fan to be deformed easier than the milled fan. The aim of this study is to determine on the one hand the influence of the blade thickness and on the other hand the way the deformation of the hollow blades may affect the global and the local performances. The global performances of the fans were measured in a test bench designed according to the ISO 5801 standards. The curve of the aerodynamics characteristics (pressure head versus flow rate) and of the global efficiency are slightly lower for the roto-moulded fan. The wall pressure fluctuations were also investigated for three flow rates: one corresponding to the maximum efficiencies of both fans and the two others corresponding to an under-flow and an over-flow rate. The power spectral density (PSD) levels, are between six and nine times higher for the roto-moulded fan at nominal flow rate. At partial flow rate, however, the PSD levels are close for both fans.
The Effect of Oscillatory Boat-Tail Flaps on the Near Wake of a Prototypical Heavy Vehicle
Authors: Johansson Martin , Sheridan John , Fredriksson Sverker
Published: March 2011

Due to the increased interest in the aerodynamics of truck / trailer combinations as a means to reduce their fuel consumption, experiments have been conducted on a model of a trailer to study possible changes in its near wake arising form attached aft-flaps. The model was fitted with vertical flaps on both sides of the back plate, which were set to oscillate at given frequencies and amplitudes. Experiments were carried out using the PIV system in a Water Channel at Monash University. The oscillation of the flaps perturbed the flow in such a way that the shear layer was significantly changed and the roll up of vortices was moved further upstream, causing a rise in the rate of entrainment and a shorter reattachment length. It also had the effect of a reduced wake size and higher velocities within the wake. These are all effects that are consistent with an increase in the base pressure at the back plate of the trailer and therefore also a reduced drag coefficient, since they are strongly connected.
Numerical Study on Rotor-Stator Interaction of a Supersonic Reaction Turbine for a Liquid Rocket Engine
Authors: Kawatsu Kaname , Tani Naoki , Yamanishi Nobuhiro
Published: March 2011

For an open cycle liquid rocket engine, such as the expander bleed cycle, the mass flow rate of turbine driving gas should be small, especially to improve rocket engine performance. However, work output must be high as possible. As a result, pressure ratio of the turbine becomes high, and Mach number at both nozzle exit and rotor inlet becomes supersonic. As a result, strong shock wave interaction can be generated between nozzle exit and rotor inlet, and this interaction affects the turbine aerodynamic performance. However, this rotor-stator interaction of supersonic turbine has not yet been clarified. Therefore, as the first step, it is important to clarify the structure of the flow field and to evaluate the accuracy of CFD method as practical engineering tool for liquid rocket engine design. In the present study, quasi 3-D RANS simulations were applied to the NACA supersonic turbine and the numerical results were compared with the experimental ones to evaluate numerical methodology. Turbulence models and rotor/stator interface modeling method were compared, and their impacts to the turbine aerodynamic performance estimation were evaluated. In addition to these points, the flow field between nozzle and rotor region and the turbine efficiency were investigated. The present results clarify some features of rotor-stator interaction. The shock wave, which is generated near the nozzle exit caused by encounter of nozzle exit flow, reflects at the neighbor nozzle wall and affects the rotor region. At the same time, the shock wave from the rotor leading edge impinges the nozzle cascade, and these shocks interact with each other. The present results showed that Mach number at nozzle outlet becomes different due to each turbulence and rotor/stator interface models. This difference of Mach number influences the shape of detached shock wave at the leading edge of rotor blade, and changes the entire rotor region flow field such as static pressure profile of rotor region. Thus, turbine efficiency may be influenced by these different features of flow field.
The Visualization Study of Contact Line Characteristics in Triangle Wetting Region of Rectangle Microgroove
Authors: Wang Tao , Hu Xuegong , Guo Chaohong , Tang Dawei
Published: March 2011

The paper acquires the images of triple phase contact line along the axial direction in microgroove triangle wetting region by the direct visualization means. The contact line images are processed by computer software, and the change law for triple phase contact line is obtained. The contact line angle between contact line and axial direction along microgroove is obtained by analyzing contact line. Results indicate that the length of contact line and the thickness of liquid film on side wall of microgroove reduce along with heat flux addition; but increase as the tilt angle increases. The contact line angle increases with the increase of tilt angle and the decrease of heat flux. In addition, changes of the length of contact line, the thickness of liquid film and the contact line angle for ethanol case is more significant than that for distilled water.
The Pressure Drop and Dynamic Contact Angle of Motion of Triple-Lines in Hydrophobic Microchannels
Authors: Yu Dongin , Choi Chiwoong , Kim Moohwan
Published: March 2011

At two-phase flow in microchannels, slug flow regime is different for wettability of surface. A slug in a hydrophilic microchannel has liquid film. However, a slug in a hydrophobic microchannel has no liquid film instead, the slug has triple-lines and makes higher pressure drop due to the motion of the triple-line. In previous researches, pressure drop of triple-line is depended of dynamic contact angle, channel diameter and fluid property. And, dynamic contact angle is depended of static contact angle, superficial velocity and fluid property. In order to understand the pressure drop of motion of triple-lines, pressure drop of slug with triple-lines in case of various diameters (0.546, 0.763, 1.018, 1.555, 2.075 mm), various fluids (D.I.water, D.I.water-1, 5, 10% ethanol mixture) and various superficial velocity (j = 0.01∼0.4 m/s) was measured. Dynamic contact angle was calculated from relation of the pressure drop of slug with triple-lines. Comparing with previous dynamic contact angle correlations, previous correlation underestimated dynamic contact angle in the region of this study. (10−4≤Ca≤10−3, 10−2≤We≤10−1, 68°≤θS≤110°)
Evaluating CFD Modeling of a Thermal Storage Unit
Authors: Bonadies Monica F. , Ho Son H. , Kapat J. S.
Published: March 2011

When collecting the energy of the sun for domestic use, there are several options, which include photovoltaic cells and evacuated tube collectors. Arrays of evacuated tube collectors are used to heat water for domestic applications, supplementing the use of a typical hot water heater, while photovoltaic cells transform the sun’s radiation into electricity. The benefit of the tube collectors is that they supplement an appliance that uses a fairly large amount of electricity when compared to others in an average home. However, the collectors cannot operate during the night time and produce more hot water than needed at their peak operation point. A thermal storage unit can be used to even out the conversion of energy throughout the day to solve this problem. This study proposes a system using paraffin wax to store thermal energy collected during the day by melting the wax. The system makes use of a finned heat exchanger, with paraffin wax on the shell side, and glycol on the tube side as the heat transfer fluid. It also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax used in the study is quite effective at storing thermal energy, it has the problem of low conductivity. So, fins are added to the storage and extraction loops to increase the wax’s thermal conductivity. The fins not only help to melt the wax more quickly but also act as nucleation sites when the wax freezes. Once all the wax is melted, energy can be exchanged from it to heat water. When creating such a unit, it is useful to have simulation tools to guide its design. One such tool is FLUENT, which will be used in this study to create a simulation of part of the unit. The simulation will be compared to experimental data from a prototype unit and evaluated based upon its strengths and weaknesses.
Flow and Heat Transfer of Micro-Tube Bank
Authors: Koizumi Yasuo , Katsuta Atsushi , Ohtake Hiroyasu
Published: March 2011

Heat transfer and flow behavior in a mini-tube bank was examined. The tube bank was simulated with wires of 1 mm diameter. The wires were arranged in the 5×5 in-line array and the 5×5 staggered array with the arranging pitch = 3. Experiments were performed in the range of the tube Reynolds number Re = 4 ∼ 3,500. Numerical analyses were also performed with the commercial CFD code of STAR-CD. The heat transfer coefficient of the tube of the first row was well expressed with the existing heat transfer correlations. In the case of the in-line array, unlike usual sized tube banks, the measured heat transfer coefficients of the tubes after the second row were lower than those of the first row and the difference between those increased as the Reynolds number was increased. At approximately Reynolds number ≃ 50, the difference turned to decrease; the heat transfer coefficients initiate to recover to the first row value. Then, the heat transfer coefficient in the rear row became larger at approximately Re ≃ 1,000 than that of the first row. In the case of the staggered array, the decrease in the heat transfer coefficient in the rear row was smaller than that in the case of the in-line array. The recovery of the heat transfer coefficient to the first row value started at a little bit lower Reynolds number and it exceeded the first row value at approximately Re ≃ 700. The flow visualization results and also the STAR-CD analytical results indicated that when the Reynolds number was low, the wake region of the preceding tube was stagnant. This flow stagnation caused the heat transfer deterioration in the front part of the rear tube, which resulted in the lower heat transfer coefficient of the rear tube than that of the first row. As the Reynolds number was increased, the flow state in the wake region changed from the stagnant condition to the more disturbed condition by periodical shedding of the Karman vortex. This change caused the recovery of the heat transfer in the front region of the rear tube, which resulted in the recovery of the heat transfer coefficient of the rear tube.
A Numerical 3-D Model of a Trapezoidal Three-Hole Pneumatic Pressure Probe for Incompressible Flow
Authors: Argu¨elles Di´az Katia Mari´a , Ferna´ndez Oro Jesu´s Manuel , Blanco Marigorta Eduardo , Barrio Perotti Rau´l
Published: March 2011

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.
Experimental Investigations of Transfer Phenomena in a Confined Plane Turbulent Impinging Water Jet
Authors: Koched Amine , Pavageau Michel , Aloui Fethi
Published: March 2011

In this study, we are interested in the hydrodynamics of impinging plane jets. Plane jets are widely used in ambience separation in HVAC, fire safety, food process engineering, cooling of electronic components etc. Despite their important industrial applications, plane jets have not been studied as extensively as axisymmetric jets. Plane jet exhibit different kind of instabilities stemming from either streamlines with strong curvature in the impingement region or inflection points in the transverse profile of the streamwise component velocity in the lateral mixing layers. Previous works in the GEPEA laboratory were performed on these flows. These works and the majority of the studies reported in the literature deal with turbulent air jets in various configurations. Very little studies have been done on water impinging jets. Taking into account the fact that the viscosity of water is smaller than air, at the same Reynolds number, it is easier to detect phenomena such as vortices. Phenomena can be observed at lower velocities making it possible to record signals with standard frequency bandwidths. This makes it easier also to do a Lagrangian tracking of vortices. We specially focused our study on the impinging zone of the jet. The dynamics of the impinging zone has not formed the subject of numerous studies. There were no studies that characterize the vortices at the impinging region of water jets in terms of size, center positions, vortex intensity, eccentricity, statistical distribution and interaction with the impinging wall. Consequently, a confined water plane jet impinging a flat plate was studied using PIV (Particle Image Velocimetry). We used POD decomposition for filtering PIV data. Then, we applied the λ2 criterion to the recorded velocity fields to detect and characterize the vortices at the impingement. A statistical analysis was then performed. A wide range of Reynolds numbers is considered: 3000, 6000, 11000 and 16000. The corresponding results will be presented.