bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of Characteristics of Gas-Liquid Two-Phase Flows in a Rectangular Microchannel With Return Bends
Authors: Kawahara Akimaro , Sadatomi Michio , Matsuo Hideki , Shimokawa Satoshi
Published: June 2012

Gas-liquid two-phase flows in a horizontal rectangular microchannel with return bends have been investigated. The width and the depth of the microchannel are 240 μm and 230 μm, respectively. T-junction type gas-liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, pure ethanol, 49wt% ethanol aqueous solution and HFE7200 were used as the test liquids, while nitrogen gas as the test gas. The flow pattern, the bubble velocity, the bubble length and the liquid slug length were measured, and the void fraction was determined as the ratio of the gas superficial velocity to the bubble velocity. The bubble velocity at a downstream position from the bend is faster than that at an upstream position, and thus the void fraction is smaller at a downstream position. The bubble velocity data were well correlated with the well-known drift flux model with Kawahara et al.’s distribution parameter correlation. The bubble length data at the upstream and the downstream positions are also correlated with the scaling law proposed by Garstecki et al., irrespective of the test liquids. The liquid slug length data are correlated with an exponential function of the void fraction. The ratio of the bubble length to the bubble pitch is also well correlated with a linear function of the homogeneous void fraction.
Numerical Study of Bubble Growth on a Micro-Finned Surface
Authors: Lee Woorim , Son Gihun
Published: June 2012

Bubble growth on a micro-finned surface, which can be used in enhancing boiling heat transfer, is numerically investigated by solving the conservation equations of mass, momentum, and energy. The bubble deformation or the liquid-vapor interface is determined by the sharp-interface level-set method, which is modified to include the effect of phase change and to treat the contact angle and the evaporative heat flux from the liquid microlayer on an immersed solid surface of a microfin. The numerical method is applied to clarify bubble growth and heat transfer characteristics on a surface including fin and cavity during nucleate boiling which have not been provided from the previous experimental studies. The effects of single fin, fin-cavity distance, and fin-fin spacing on the bubble dynamics are investigated. The micro-fin is found to affect the activation of cavity. The fin-cavity configuration is found to determine the bubble formation in a cavity. The vapor removal rate is also observed to significantly depend on the fin-fin spacing.
Investigation on Oscillating Behavior of Laser-Induced Cavitation Bubble Near Wall in Room Temperature Water and Cryogenic Liquid
Authors: Yamaguchi Tomoya , Oshida Yuki , Soon Wong Teck , Maeno Kazuo , Ota Masanori
Published: June 2012

Cavitation is bubbling phenomena caused by local pressure reduction or strong energy concentration in liquids. Cavitation is widely observed in fluid machinery and it causes vibration, noise, erosion, and loss of efficiency. In addition, cavitation occurs in laser treatment and causes the ruin of human tissues. There are a lot of researches on cavitation phenomena in room temperature liquids. It is known that laser-induced bubble generates pressure waves, and the bubble oscillates and collapses eventually. Recently, it is reported that cavitation occurs in cryogenic liquid such as rocket propellant. However, there are few researches about cavitation in cryogenic liquid. Generally, the thermo-physical properties such as surface tension and viscosity of cryogenic liquid are different from those of room temperature water, and cavitation in cryogenic liquid occurs near boiling temperature (low subcooling region). In our experiment, single bubble was generated by focused laser, and its behavior and pressure wave were visualized. The focusing point of the laser was also settled near a wall in the liquids by designated distance. Interacting behavior of laser-induced bubble with the wall was also investigated. We chose liquid nitrogen (LN2) as cryogenic liquid for safety. LN2 was pressurized in order to increase the degree of subcooling. These experiments show difference of oscillation periods between bubble in water at room temperature and in liquid nitrogen. In addition, there is difference of oscillating behavior of LN2 bubbles in low and high subcooling regions. We discuss influence of the degree of subcooling and stand-off distance. It is found that shapes of deformation and oscillation period of bubble are influenced by these parameters.
Numerical Simulation of Gas-Liquid Slug Flow Along Vertical Pipes Using the Slug Tracking Model
Authors: Pachas N. Alex , Perea M. Ce´sar , Morales Rigoberto E. M. , Cozin Cristiane , Rosa Eugeˆnio E. , Mazza Ricardo A.
Published: June 2012

The intermittent gas-liquid flow, or slug flow, in vertical tubes occurs over a wide range of gas and liquid flow rates, with many applications, such as oil industry. Predicting the properties of this kind of flow is important to design properly pumps, risers and other components involved. In the present work, vertical upward slug flow is studied through a one-dimensional and lagrangian frame referenced model called slug tracking. In this model, the mass and the momentum balance equations are applied in control volumes constituted by the gas bubble and the liquid slug, which are propagated along the pipe. The flow intermittency is reproduced through the conditions at the entrance of the pipe, which are analyzed in statistical terms. These entrance conditions are given by a sequence of flow properties for each unit cell. The objective of the present work is to simulate the slug flow and its intermittency through the slug tracking model. The numerical results are compared with experimental data obtained by 2PFG/FEM/UNICAMP for air-water flow and good agreement is observed.
Developing Tracer Particles for X-Ray Particle Tracking Velocimetry
Authors: Drake Joshua B. , Kenney Andrea L. , Morgan Timothy B. , Heindel Theodore J.
Published: June 2012

X-ray imaging, as a noninvasive flow visualization technique, has been shown to be a useful method for observing and characterizing multiphase flows. One type of X-ray flow visualization technique, called X-ray Particle Tracking Velocimetry (XPTV), tracks an X-ray attenuating particle in an opaque fluid flow. A significant challenge with XPTV is identifying tracer particles with the desired fluid flow characteristics (e.g., small and neutrally buoyant) but yet differentially attenuate X-rays, which is based primarily on density differences. This paper describes the manufacturing of XPTV tracer particles that satisfy specific particle characteristics including high X-ray attenuation, uniform shape, specified effective density, and desired diameter. An example use of these particles as an intruder particle in a fluidized bed (to simulate biomass injection) is then demonstrated using X-ray stereographic imaging to determine intruder particle position as a function of time in a three-dimensional opaque system.
Experimental Investigation of Turbulence Transition of Natural Convection Boundary Layer Along a Vertical Plate in Water With Sub-Millimeter-Bubble Injection
Authors: Kitagawa Atsuhide , Endo Hiroki , Hagiwara Yoshimichi
Published: June 2012

This paper presents an experimental investigation of the turbulence transition of the natural convection boundary layer along a vertical plate in water with sub-millimeter-bubble injection. In this study, we focus on the relationship between the bubble injection position L and the turbulence transition of the boundary layer. Temperature and velocity measurements show that sub-millimeter-bubble injection for L = 1.6 mm suppresses the turbulence transition of the natural convection boundary layer, while that for L = 3.6 mm enhances the turbulence transition of the boundary layer. For L = 1.6 mm, the appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section is restricted near the wall, though the peak value of the liquid velocity fluctuation is high. In contrast, in the case of L = 3.6 mm, the relatively large liquid velocity fluctuation induced by bubbles at the upstream unheated section distributes widely over the laminar boundary layer width. Therefore, we expect that the turbulence transition of the natural convection boundary layer for the case with bubble injection depends on the magnitude and appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section.
Effect of Frequency Analysis Algorithms on Velocity Data Using Ultrasonic Doppler Method
Authors: Sakagami Ryosuke , Murakawa Hideki , Sugimoto Katsumi , Takenaka Nobuyuki
Published: June 2012

An ultrasonic velocity profile (UVP) method can measure instantaneous one-dimensional velocity profile along the measuring line. It is non-intrusive technique, and it can be easily applied for existing facilities. The UVP method utilizes the ultrasonic Doppler frequency reflected on tracer particles for obtaining the velocity. For analyzing the Doppler frequency, there are some algorithms such as Fast Fourier Transform (FFT), auto-correlation, and so on. However, the influence of the algorithms on measuring velocity profile has not been investigated. In this study, difference of the algorithms for velocity measurements was investigated using a developed system. It consists of an ultrasonic pulser-receiver which transmits and receives ultrasonic waves, a high-speed digitizer which records wave data, and a PC which analyzes and calculates velocity. FFT and auto-correlation method were used for the calculation. In case of the FFT, there are 2 methods to calculate the Doppler frequency. One is a method to calculate the frequency of maximum value in the power spectral density (PSD), and the other is a method to calculate the average value of the frequency in the PSD. Velocity distributions were compared to a laser Doppler velocimeter (LDV), and the appropriate algorithms were chosen depending on the flow conditions. From the results, it was clarified that the auto-correlation method could reduce the number of ultrasonic repetition for the measurement.
Large-Eddy Simulation on a Pulverized Coal Combustion Furnace With a Complex Burner
Authors: Watanabe Hiroaki , Tanno Kenji , Kurose Ryoichi , Komori Satoru
Published: June 2012

Large-eddy simulation (LES) is applied to a pulverized coal combustion field in a combustion test furnace with a practical advanced low NOx burner called CI-α burner, and its validity is investigated by comparing with the experiment. The motion of coal particles is calculated by the Lagrangian method with a parcel model. In the coal combustion modeling, three chemical processes are considered, namely devolatilization, char combustion and gaseous combustion. The direct closure SSFRRM (scale similarity filtered reaction rate model) is used as a turbulent combustion model. The results show that a swirling recirculation flow is formed in the central region close to the burner and its size and strength dynamically change with time. The predicted distributions of gaseous temperature and oxygen are in general agreement with the experiment.
Study on the Particle Behavior in a Screw Conveyer System by the Discrete Element Method
Authors: Shigeto Yusuke , Sakai Mikio , Mizutani Shin , Koshizuka Seiichi , Matsusaka Shuji
Published: June 2012

Large amount of particles are used in the industrial systems. Numerical analyses of these systems are expected to reduce designing cost. However the numerical analysis of powder is not used practically, because it requires high calculation cost which grows up with the number of particles. Besides, there are memory consumption problem which is required for calculation space. In this paper, the parallel simulation techniques of the Discrete Element Method (DEM) on multi-core processors are described. In the present study, it is shown that the algorithm enables all the processes of the DEM to be executed parallel. Moreover, a new algorithm which makes the memory space usage effectively and accelerates the calculation speed is proposed for multi-thread parallel computing of the DEM. In the present study, the memory space usage is shown to be reduced drastically by introducing this algorithm. In addition, the coarse grain model which emulates original particles with less calculation particles is applied in order to reduce calculation cost. For the practical usage of the DEM in industries, the simulation is performed in a large-scale powder system which possesses a complicated drive unit. In the current study, it is shown that the large scale DEM simulation of practical systems is enabled to be executed by our proposing algorithms.
3D Numerical Simulation of Polydisperse Pressurized Gas-Solid Fluidized Bed
Authors: Fede P. , Simonin O. , Ghouila I.
Published: June 2012

Three dimensional unsteady numerical simulations of dense pressurized polydisperse fluidized bed have been carried out. The geometry is a medium-scale industrial pilot for ethylene polymerization. The numerical simulation have been performed with a polydisperse collision model. The consistency of the polydisperse model predictions with the monodisperse ones is shown. The results show that the pressure distribution and the mean vertical gas velocity are not modified by polydispersion of the solid phase. In contrast, the solid particle species are not identically distributed in the fluidized bed indicating the presence of particle segregation.
Kinetic Approach for Solid Inertial Particle Deposition in Turbulent Near-Wall Region Flow Lattice Boltzmann Based Numerical Resolution
Authors: Diounou E. , Fede P. , Fournier R. , Blanco S. , Simonin O.
Published: June 2012

The purpose of the paper is the deposition on the wall of inertial solid particles suspended in turbulent flow. The modeling of such a system is based on a statistical description using a Probability Density Function. In the PDF transport equation, an original model proposed Aguinaga et al. (2009) is used to close the term representing the fluid-particle interactions. The resulting kinetic equation may be difficult to solve especially in the case of the particle response time is smaller than the integral time scale of the turbulence. In the present paper, the Lattice Boltzmann Method is used in order to overcome such numerical problems. The accuracy of the method and its ability to solve the two-phase kinetic equation is analyzed in the simple case of inertial particles in homogeneous isotropic turbulence for which Lagrangian random walk simulation results are available. The results from LBM are in accordance with the random walk simulations.
Influence of Wall Pressure on Power Reduction of Plug Conveying by Ultrasonic
Authors: Koike Yoshimitsu , Kofu Kenji , Ochi Mitsuaki
Published: June 2012

Ultrasonic was applied to the horizontal line in plug conveyance of granular particles, and pressure drop was measured. As the result of this experimental research, pressure loss became smaller linearly with increasing of ultrasonic amplitude regardless of kinds of particles and flow conditions. Therefore, ultrasonic can cut the frictional resistance between packed particles and wall surface. Additionally the pressure sensor sheets were stuck inside the pipe at every 45 degree at a certain pipe cross section. Therefore 8 sheets were used. Measured values showed that the pressure against the pipe wall by particle bed have distribution in peripheral direction. Additionally pressure to the wall become smallest at the top of pipe cross section, and largest at the bottom. Then, the ratios of each pressure to the average value for all positions were calculated, and pressure distribution in peripheral direction was made clear. The ratio approached 1.0 as particle velocity in the packed bed become faster, and this means that pressure difference at each position become smaller. When particle velocity become faster, particles were pushed hard and the powder pressure against the pipe wall dominates for all pressure which works to the wall. This pressure is considered to affect the almost same value in all directions. Therefore the ratios approach 1.0. Additionally, the relationship of average pressure to the pipe wall and pressure loss reduction effect by ultrasonic was investigated. As a result, the pressure loss could not be reduced by ultrasonic as pressure of particle bed against the pipe wall become larger. This was assumed that it’s hard to float particles as particle pressure to the wall become larger and a lot of particles contact with pipe surface.
Effects of Reynolds Number on Drag Reduction in T-Junction Pipes due to Small Obstacles
Authors: Ando Toshitake , Shakouchi Toshihiko , Suzuki Yoshitaka , Tsujimoto Koichi
Published: June 2012

T-junction pipes are used to distribute one flow into two flows or join two flows into one flow. Separated vortex flow regions near the corners of junctions are caused in these types of flows. They reduce the effective cross-sectional area of the pipe flows and then create large flow resistance or drag. The corners of junctions are generally rounded to avoid flow separation and reduce flow resistance. We tried to reduce the flow resistance of counter-flow T-junction pipes in which two flows in the opposite direction entered the junction, mixed, and then vertically flowed out together by using a simple method. We mounted two small weir-shaped obstacles on the walls of the two upstream pipes by the side around the junction corners, which is a new way we propose of controlling flow separation. The pressure distribution along the pipes was measured and the drag of the T-junction pipe was estimated. Additionally, the effects of the Reynolds number on the flow resistance and its rate of reduction by mounting small obstacles were clarified. The two major results we obtained were: (1) the flow resistance of T-junctions could be reduced by about a maximum of 30% by mounting small obstacles at heights of 0.30 D and 0.47 D (D: pipe diameter) from the upstream of the corner. We also found (2) the rate of reduction in flow resistance increased with decreasing Reynolds numbers between 5–10 × 104, but this decreased rapidly between 2.5–5 × 104.
Active Control of Flow Separation Over a NACA0024 Airfoil by DBD Plasma Actuator and FBG Sensor
Authors: Walker Seth , Segawa Takehiko , Jukes Timothy , Furutani Hirohide , Iki Norihiko , Takekawa Shinya
Published: June 2012

Dielectric barrier discharge plasma actuators (DBD-PA) and fiber Bragg grating flow sensors (FBG-FS) have been investigated for active control of flow separation around a NACA0024 airfoil. Tangential jets were produced in the vicinity of the DBD-PA slightly aft of the leading edge of the airfoil. The flow separation control ability was evaluated at a low Reynolds number, Re = 5.0×104, in an open-circuit wind tunnel. Analysis of instantaneous and time-averaged velocity distributions around the airfoil was achieved using a particle image velocimetry (PIV) system. The flow conditions induced by the DBD-PA to suppress the flow separation were found for angles of attack of α = 8°, 12°, and 16°. When unaided by the DBD-PA system, flow separations from NACA0024 airfoil are suppressed significantly for certain Reynolds numbers and angles of attack. FBG-FS attached a chord-wise cantilever near the trailing edge of the airfoil was used to measure strain fluctuations for its feasibility to detect flow separation in real time and construct feedback control system with DBD-PA. In this study, it was found that standard deviations of strain fluctuations increase obviously in cases of flow conditions at which the flow around NACA0024 airfoil separates.
Investigation on Circular and Chevron Impinging Jets by IR Thermography and Time-Resolved Tomographic PIV
Authors: Violato D. , Ianiro A. , Cardone G. , Scarano F.
Published: June 2012

This paper describes an experimental investigation on impinging jets issued through a circular and a chevron nozzle by means of IR thermography and time-resolved tomographic particle image velocimetry. Measurements are performed at Reynolds number of 5000, based on the flow rate and on the nozzle diameter D. Time-resolved tomographic experiments are performed at kilo-hertz repetition rate in a tailored water jet facility where a plate is placed 4 D off the nozzle. The measurements are done at the nozzle exit in a cylindrical volume with a diameter of 4.5 D and an axial length of 4 D. Time-averaged heat transfer measurements of the convective heat transfer on the plate are performed by means of IR thermography with the heated-thin-foil heat transfer sensor for nozzle-to-plate distances ranging from 2 D to 10 D. Heat transfer measurements are shown in terms of Nusselt number (Nu) maps. The instantaneous three-dimensional organization of coherent structures is described for both the configurations. The heat transfer performances of the chevron jet are compared with those of the circular one, analyzing the influence of the nozzle-to-plate distance on the distribution of Nu. The most relevant issues for the chevron jet and the relation between the flow organization and the heat transfer are investigated.
Anomalous Decrease in Flow Rates of Micro-Orifice Flows of Water With Increasing the Elapsed Time After Inception of the Flow
Authors: Hasegawa Tomiichi , Ushida Akiomi , Uchiyama Hiroshige , Narumi Takatsne
Published: June 2012

Flow rates under some pressure differentials were measured for the flow of water through micro-orifices. It was found that the flow rate decreases as the time has elapsed after inception of the flow under a constant pressure differential and that the decrease in flow rates is larger for city water than for pure water, and the gold orifice provides a less decrease with time in flow rates than the nickel orifice, although the flow rate at the incipient stage of the experimental run is lower in the gold orifice than in the nickel orifice.
Rheo-Optic Stress Measurement Between Small Bubbles Under Pressure-Oscillating Field
Authors: Iwata Shuichi , Aritake Ayumi , Mori Hideki , Takahashi Tsutomu
Published: June 2012

The rapid elimination of tiny gas bubbles from highly viscous and/or viscoelastic fluids remains a significant challenge. We have proposed a new and effective method (Pressure Oscillating Defoaming; “POD” for short) for increasing the bubble rising velocity in such fluids based on application of an oscillating pressure. A bubble undergoes alternating expansion and contraction under pressure-oscillation field, where the oscillating flow decreases quadratically with the distance from the surface, giving biaxial elongational flow and shear flow in the vicinity of the bubble surface. When two air bubbles are set vertically in a quartz cell filled with shear-thinning fluids, the lower-bubble rises faster than the upper bubble under pressure-oscillating field generated by pressing a rubber sheet at the bottom of the cell. However, this mechanism of the behavior has not been fully understood. By using liquid with a good photoelastic correlation and rheological property similar to the UCM model, we try measuring the stress in the vicinity of small bubbles under pressure-oscillating field. In the case of single bubble, the stress profile measured by the birefringence flow technique was good agreement with stress profile calculated by unsteady state finite element analysis. In this study, retardation profiles were measured near and between the bubbles. In the closer bubble case, the higher retardation profile was observed between the bubbles under pressure-oscillating field.
Influence of Acoustic Pressure and Flexural Vibration on Friction Reduction Effect by Ultrasonic
Authors: Nakayama Takahiro , Ochi Mitsuaki , Kofu Kenji
Published: June 2012

The influence of acoustic pressure and flexural vibration on friction reduction effect by ultrasonic has been investigated in this study. Then the relationship between friction reduction of particles by each effect and the particle density has been shown by using of ultrasonic. Additionally, the maximum of particle density which could receive the friction reduction effect by the acoustic pressure has been expressed. In short, when the particle density was large, the influence of friction reduction effect by the acoustic pressure decreased and the influence of friction reduction effect by the flexural vibration grew. In this study, two plates were set parallel to each other. The dried particles were scattered on the lower plate, and the ultrasonic was applied. Then, the entire equipment setup was tilted slowly until the scattered particles began to move, and the friction coefficient was measured. Then, influence of acoustic pressure and flexural vibration were evaluated. In order to evaluate the reduction effect by acoustic pressure and flexural vibration, firstly, the distribution of acoustic pressure between reflection plate and the vibration plate have been measured. As a result, it was clarified that acoustic pressure distribution became the almost same whether ultrasonic was applied for the upper or lower plate, and the reflection plate vibrated little. Therefore it was possible to divide the influence of acoustic pressure and flexural vibration on the friction reduction.
Three-Dimensional Wing Design Towards the Future Mars Airplane
Authors: Kojima Ryoji , Lee Donghi , Tatsukawa Tomoaki , Nonomura Taku , Oyama Akira , Fujii Kozo
Published: June 2012

The effects of aspect ratio and Reynolds number on aerodynamic characteristics of three-dimensional rectangular wing at low Reynolds number of 103 to 105, are investigated with Reynolds-averaged Navier-Stokes solver with the Baldwin-Lomax model. Present results show that lift coefficient decreases drastically at lower aspect ratio than 4. Besides, the much larger viscous drag coefficient is obtained at the lower Reynolds number, especially lower than 104. In order to focus on designing practical wings, the particular cases under the condition of fixed wing-surface area and fixed main stream velocity are conducted. The results show that there is trade-off between the decrease in viscous drag coefficient with increasing Reynolds number and the increase in lift coefficient with increasing aspect ratio. At the lower Reynolds number condition, as the former effect is stronger than the latter one, maximum lift-to-drag ratio is obtained at lower aspect ratio.
Aerodynamic Performance Evaluation of 3D Aircraft Configurations by Turbulence Models
Authors: Kwak Einkeun , Park Sang-il , Lee Namhun , Lee Seungsoo
Published: June 2012

Numerical simulations of 3D aircraft configurations are performed in order to understand the effects that turbulence models have on the aerodynamic characteristics of an aircraft. An in-house CFD code that solves 3D RANS equations and 2-equation turbulence model equations is used for the study. The code applies Roe’s approximated Riemann solver and an AF-ADI scheme. Furthermore van Leer’s MUSCL extrapolation with van Albada’s limiter is adopted. Various versions of Menter’s k-omega SST turbulence models as well as Coakley’s q-omega model are incorporated into the CFD code. Menter’s k-omega SST models include the standard model, the 2003 model, the model incorporating the vorticity source term, and the model containing controlled decay. Turbulent flows over a wing are simulated in order to validate the turbulence models contained in the CFD code. The results from these simulations are then compared to computational results of the 3rd AIAA CFD Drag Prediction Workshop. Moreover, numerical simulations of the DLR-F6 wing-body and wing-body-nacelle-pylon configurations are conducted and compared to computational results of the 2nd AIAA CFD Drag Prediction Workshop. Especially, the aerodynamic characteristics as well as flow features with respect to the turbulence models are scrutinized. The results obtained from each simulation incorporating Menter’s k-omega SST turbulence model variations are compared with one another.
Effects of Cylinder Diameters, Reynolds Number and Distance Between Two-Tandem Cylinders on the Wake Profile and Turbulence Intensity
Authors: Rahimzadeh Amin , Malek Jafarian Majid , Khoshnevis Amir
Published: June 2012

A series of experimental and numerical investigations on two tandem cylinders wake have been studied. The velocity profile and turbulence intensity have been acquired by a single one dimensional Hot Wire anemometer. The two cylinders were mounted in a tandem manner in the horizontal mid plane of the working section. The effect of the upstream cylinder diameter, Reynolds number and the distance between the cylinders on the wake profile and turbulence intensity on the downstream cylinder was investigated, while the Reynolds number ranged between 1.5× ⟦10⟧ ^4 ∼ 3× ⟦10⟧ ^4. The upstream cylinder diameter (d) was 10, 20 and 25 mm, while the downstream cylinder diameter (D) was 25 mm, corresponding to d/D ranging from 0.4 ∼1.0. The spacing ratio L/d (where L is the distance between the upstream cylinder center and the leading stagnation point of the downstream cylinder) was 2 and 5.5, covering different flow regimes. Observations indicate that two symmetric turbulence intensity peak will occur at mean velocity gradient area. Turbulence area will increase in width for both L/d = 2 and 5.5 as increasing distance from the cylinder (x/D) and decreasing free stream velocity. But totally the range of the turbulence area for L/d = 5.5 is greater than L/d = 2. The wake profiles show that the velocity defect increases as increasing upstream cylinder’s diameter for L/d = 5.5. While this order cannot be accessed for L/d = 2. It is observed that sudden and unusual velocity defect happened for L/d = 2 and d/D = 0.8 cases, which means that the most velocity defect is running on. Also, numerical solution results of velocity profile have been compared with the mentioned experimental results at station 4 and velocity of 10 m/s for both L/d = 2 and 5.5. Results shows a little difference because of using one-dimensional Hot-wire.
Comparison Study of RANS CFD Simulations for Subsonic Civil Aircraft Configurations
Authors: Murayama Mitsuhiro , Yamamoto Kazuomi , Tanaka Kentaro
Published: June 2012

In this paper, the numerical prediction capability of the force predictions in the current computing environments, applicability of turbulence model, and future need of the development are discussed using computational results for geometries used in the series of the AIAA CFD workshop. Computations are conducted for two aircraft wing-body configurations at each cruise Mach condition and a simplified high-lift wing-body configuration with deployed high-lift devices of flap and slat at landing condition using both a multi-block structured grid solver, UPACS, and an unstructured grid solver, TAS-code, which are widely used in Aviation Program Group of Japan Aerospace Exploration Agency. It is shown that to improve eddy-viscosity model or apply more complex model like Reynolds-Stress-Transport models is required for reliable prediction of aerodynamic forces with corner flow separation, which can cause a problem on the consistent prediction of flow fields, and the application of the non-linear stress model could predict the corner flow separation reasonably. For the computations of high-lift configurations, it is shown that although computational results with 30 million grid points show engineeringly reasonable solutions, further increase of grid resolution and high geometric fidelity will be still important to increase the reliability of computational results with improving the fidelity of physical modeling.
Computational Study of Flow Characteristics of Thick and Thin Airfoil With Implicit Large-Eddy Simulation at Low Reynolds Number
Authors: Kojima Ryoji , Nonomura Taku , Oyama Akira , Fujii Kozo
Published: June 2012

The flow fields around NACA0012 and NACA0002 at Reynolds number of 23,000, and their aerodynamic characteristics are analyzed. Computations are conducted with implicit large-eddy simulation solver and Reynolds-averaged-Navier-Stokes solver. Around this Reynolds number, the flow over an airfoil separates, transits and reattaches, resulting in generation of a laminar separation bubble at angle of attack in the range of certain degrees. Over a NACA0012 airfoil a separation point moves toward its leading edge with increasing angle of attack, and a separated flow may transit to create a short bubble. On the other hand, over a NACA0002 airfoil a separation point is kept at its leading edge, and a separated flow may transit to create a long bubble. Moreover, there appears nonlinearity in lift curve for NACA0012 airfoil, but does not appear in that for NACA0002 in spite of existence of a laminar separation bubble.
Numerical Analyses on Electric Double Layer and Electroosmotic Flow in Nano-Scale Parallel Plates
Authors: Kumamaru Hiroshige , Kobayashi Hikari , Itoh Kazuhiro , Shimogonya Yuji
Published: June 2012

Numerical analyses, both molecular dynamics (MD) analyses and continuous fluid analyses (by the finite difference method), have been performed on electric double layer and electroosmotic flow in nano-scale parallel plates. For a channel width of 8.2 nm, the MD analyses shows that the electric double layer covers whole channel while the continuous fluid analyses indicates that the electric double layer is formed only in the regions near the walls. For a channel width of 20.6 nm, both the MD analyses and the continuous fluid analyses show that the electric double layer appears only in the regions near the walls. It becomes obvious from the MD analyses that the thickness of electric double layer becomes large when the electric field is tilted from the direction of wall surface. By the continuous fluid analyses, the electroosmotic flow velocity is estimated to be 2.5 mm/s and 3.6 mm/s for channel widths of 8.2 nm and 20.6 nm, respectively.
Circulation of a Vortex Ring Generated by Pulsating Jet Flow
Authors: Akagi Fujio , Kawabata Keisuke , Ando Youichi , Yamaguchi Sumio , Furukawa Masato
Published: June 2012

In order to use vortex rings for mass and momentum transport, the relationship between the circulation of vortex rings generated continuously by a pulsating jet flow, which are referred to herein as ‘cyclic vortex rings’, and the conditions of jet flow are investigated experimentally. The results indicate that the formation time at which the cyclic vortex rings reach maximum circulation, i.e., become optimal vortex rings, can be estimated using the concept proposed by Gharib et al. Based on their concept, the optimal vortex rings are formed when the translation velocity of the vortex core becomes equal to the convection velocity of the shear layer of the jet. The translation velocity of vortex rings and the convection velocity of shear layers can be estimated by empirical equations under pulsating jet conditions. Therefore, the formation time of the optimal vortex rings can be estimated from the pulsating jet conditions, when the circulation of vortex rings stop growing and the vortex rings start to become disconnected from the shear layers. The circulation of vortex rings estimated using this method is in good agreement with the measurement data within the measurement error. There exists a specific Strouhal number at which the circulation of the vortex rings reaches the maximum value. This Strouhal number is expected to be the optimal generating condition of cyclic vortex rings for using transportation.
An Appropriate Metric to Switch on a Turbulence Model for Rayleigh-Taylor Instability Driven Mixing
Authors: Rollin Bertrand , Andrews Malcolm J.
Published: June 2012

Rayleigh-Taylor (RT) instability occurs at a perturbed interface between fluids of different densities, when the lighter fluid is accelerated into the heavier fluid (∇p · ∇ρ < 0, where p is pressure, and ρ is density). In time, as the two fluids seek to reduce their combined potential energy, the mixing becomes turbulent. This fundamental instability is observed, and plays a key role, in numerous natural phenomena, e.g. supernovae explosions, and in engineering applications, e.g. Inertial Confinement Fusion (ICF). The importance of initial condition (ICs) effects on the growth and mixing of Rayleigh-Taylor instability open an opportunity for “design” of RT turbulence for engineering, and question our current predictive capability. Indeed, commonly used turbulence models used for engineering applications are tuned for fully developed turbulence, whereas RT instability is a dynamic process that evolves toward turbulence under the influence of ICs. Therefore, our efforts aim at defining a procedure for properly accounting for initial conditions in variable density (RT) turbulence models. Our strategy is to have a model for the “early” evolution of the RT instability that will produce the initial conditions for the turbulence model. We already dispose of a modal model to evolve the RT mixing layer starting from almost any initial conditions. The present work is a first look at determining an appropriate metric for switching from the modal model to a variable density turbulence model.
The Effect of Design Parameters on the Performance of Flagellar Propeller at Low Reynolds Number
Authors: Jeon Hyejin , Kim Yoon-Cheol , Goh Eun , Yim Dongwook , Jin Songwan , Yoo Jung Yul
Published: June 2012

To drive a small object which swims in low Reynolds number situation, we need a new type of propeller which is optimized for low Reynolds number usage since the flow at low Reynolds numbers is dominated by viscous force instead of inertia force. Propeller in a shape of bacterial flagellum can be a strong candidate for propeller of small swimming object. In this paper, we visualized velocity field induced by flagellar shaped propeller using stereoscopic particle image velocimetry. We also have experimentally evaluated the effect of pitch and rotational speeds on the performance of flagellar shaped propeller inspired by flagellum of E.coli using macroscopic model. Silicone oil whose viscosity is 100 times larger than water is used as working fluid to make low Reynolds number situation using macroscopic model. Thrust, torque and velocity were measured as a function of pitch and rotational speed, and efficiency was calculated using measured results. We found that the maximum efficiency of flagellar propeller reaches where the pitch angle is about 40°. However, the effect of rotational speed on the efficiency is relatively smaller than that of pitch. And the flow pattern behind the rotating propeller was altered by pitch of the propeller.
Coalescence Phenomena of Liquid Droplets in a Circular Tube
Authors: Muraoka Masahiro , Ogawa Naoki , Matsuda Hiroshi , Kasuya Jin , Komiya Kenji , Ueno Ichiro , Mizoguchi Hiroshi
Published: June 2012

The authors focus on coalescence of liquid droplets with/without suspended particles in a vertical tube filled with a quiescent fluid. Motion of single or two droplets moving upward due to buoyancy force in the tube and coalescence time (i.e., period between instances when the relative velocity of two droplets becomes zero after their apparent contact and when the coalescence takes place) are examined. Effects of suspended particles in the droplets on their coalescence and induced flow in coalescing droplets are also investigated by using image processing.
Large Scale Flow Pulsation in Square Arrayed Rod Bundles
Authors: Kim Taehwan , Jung Hee-Yoon , Bae Ji-Yeul , Kim Kyung Min , Cho Hyung Hee , Shin Chang Hwan , In Wang-Kee
Published: June 2012

Turbulent air flow through subchannels is simulated to obtain characteristic frequency on large scale flow pulsation in square arrayed 6-rod bundles. The turbulent mixing between the subchannels is dominated by the large scale flow pulsation phenomenon. Therefore, it is crucial to understand flow patterns in the subchannels for the design of reliable nuclear reactor elements. Numerical simulation using a CFD code is performed by solving the unsteady equations with large eddy simulation (LES) model. For the numerical simulation, two kinds of grid including two subchannels are created. Models with two different P/D ratios (P/D = 1.08, 1.35) are simulated. P/D = 1.08 case is our main concern to investigate flows in tight lattice rod buddle. The shortened channel length with periodic boundary condition is employed to reduce the computing time. The geometry of the simulated model is based on the 3-D experimental equipment. The velocity fluctuation data in the stream-wise and cross directions are obtained from the simulated model to find the characteristic frequency. The large scale flow pulsations are explained by time history of velocity, Fast Fourier Transform analysis and cross-correlation analysis. The fluctuation frequency at the center of gap and 20 mm away from that point are obtained. The characteristic frequency is 87.5 Hz through the entire monitoring points. Results demonstrate that the periodic flow pulsations occur with small pitch-to-diameter ratio (P/D = 1.08) and the vortices trains are highly synchronized. There is no large scale pulsation in P/D = 1.35 case.
Vortex Structure of a Vortex Ring Over a Butterfly Wing and its Dynamic Behavior
Authors: Fuchiwaki Masaki , Kuroki Taichi , Tanaka Kazuhiro , Tabata Takahide
Published: June 2012

Micro-Air-Vehicles (MAVs) that mimic the flight mechanisms of insects have been attracting significant attention in recent years. These technologies are developed with the aim of lifesavings in the area with the risk of secondary disasters, maintenance works for constructions such as bridges, information collection on planet searches, monitoring of security risks for the purpose of security means. A number of researchers have attempted to develop small flap flying objects and MAV with various actuators and devices. However, these robots were not practical. One of the reasons for this is that the flying mechanism of insects has not yet been clarified sufficiently. We have clarified that a couple of large-scale vortex is formed over the wing. The purpose of the present study is to clarify the dynamic behavior and the detailed structure of the vortices of the flapping butterfly wing, and we carried out the PIV measurement around the flapping butterfly wing. The vortex ring develops over the wings when the wings flap downward to the bottom dead position and then passes through the butterfly completely and grows until reaching the wake at the bottom dead position. The vortex ring develops over the wing while growing from the leading edge toward the trailing edge. The maximum vorticity of the vortex ring over the wing moves from the leading edge to the trailing edge with the downward flapping. On the other hand, the vorticity of the LEV decays with downward flapping.
Fluid-Structure Interactive Simulation Using a Virtual Flux Method
Authors: Morinishi Koji , Fukui Tomohiro
Published: June 2012

This paper describes the resent development of a virtual flux method for simulating fluid-structure interaction problems. The virtual flux method is one of the sharp interface Cartesian grid methods. The numerical flux across the interface is replaced with the virtual flux so that proper interface conditions must be satisfied there. In this study, the virtual flux method is applied to numerical flow simulations about reciprocating engines. The compressible Navier-Stokes equations are coupled with the equation of motion of the piston, connecting rod, and crank system. Intake and exhaust valves are lifted up and down according with the crank angle in the intake and exhaust strokes. Instead of modeling the complex fuel combustion process, a proper amount of energy is added to the Navier-Stokes equation at the beginning of each expansion stroke, to retain the four stroke engine cycle at a constant revolution rate. Initially the engine is started by starter motor force, which is added for a few seconds. The engine comes to work at the revolution rate intended after some initial transition cycles. With designing the intake and exhaust valve lift properly, intake mass and revolution rate are improved by several percent. It is confirmed that the virtual flux method is easily applicable to the simulation of fluid-structure interaction problems.
Turbulent Mixing With Density Differences
Authors: Derksen Jos
Published: June 2012

Homogenization of initially segregated and stably stratified systems consisting of two miscible liquids with different density and the same kinematic viscosity in an agitated tank was studied computationally. Reynolds numbers were in the range of 3,000 to 12,000 so that it was possible to solve the flow equations without explicitly modeling turbulence. The Richardson number that characterizes buoyancy was varied between 0 and 1. The stratification clearly lengthens the homogenization process. Two flow regimes could be identified. At low Richardson numbers large, three-dimensional flow structures dominate mixing, as is the case in non-buoyant systems. At high Richardson numbers the interface between the two liquids largely stays intact. It rises due to turbulent erosion, gradually drawing down and mixing up the lighter liquid.
Fluid Flow and Entropy Generation Characteristics Analysis of the Channel Geometry in X-Shaped Micro-Channels
Authors: Tu Shu-Min , Torii Shuichi , Shih Yang-Cheng
Published: June 2012

The micro-channels and micro-devices have been widely used in mechanical engineering applications. A number of investigations have been conducted to better design the fluid flow and heat transfer in micro-channel, particularly as it pertains to applications involving the thermal control of electronic devices. The analysis of entropy generation mechanism is very important to optimize the second-law performance of these energy conversion devices in micro-scale. The main concern of this paper is to investigate the channel geometry effect on the mixing performances in the X-shaped micro-channels. Seven different tested channels consisting of shrunk-channel, normal-channel and magnified-channel, made of acrylic fabric with the width ranging from 0.7 to 1.3 mm, are considered. As the working fluid, water, is injected to microchannel at different mass flow rate, over a wide range of flow condition, 0.52 < Re < 718, have been discussed. Numerical simulation of the entropy generation, temperature gradient, velocity vector, and pressure drop has deliberated with experiment. Through the evaluations of the overall entropy generation in the whole flow domain, the results show that; as the Reynolds number below 136.68, magnified-channels have the lower entropy generation and best mixture of performance; above 136.68, in the smallest the channel geometry, the transition form early from laminar flow, the unsteady flow is an advantage for mixing in the limited mixing area, therefore, they generated the best mixing performance. It is clear that the channel geometry plays an important role on the mixing performances in the X-shaped micro-channels.