bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Induced Crystallization of Colloidal Dispersion
Authors: Toga Shinji , Narumi Takatsune
Published: June 2012

In this study, we have examined a crystallization effect of colloidal dispersion induced by the various elongational flows. Extremely strong electrostatic repulsion makes a crystal structure called ‘colloid crystal’. A colloid crystal has hundreds of nano-meters in grating scale and it reflects the visible light due to the Bragg diffraction. It has the potential to become different photonic devices such as an inexpensive photonic device and a planar laser source, but it requires the evolution of the process of making a single-crystal with external stimulus. The methods using flow operation described in this study are expected to the crystallization action of a colloidal dispersion. In the experiment, 2 types of the flow have been examined. The flows have a contraction or an expansion part between two parallel plates separated by 0.1 mm gap and it cause deformations of a contraction or an extension for the colloid. We have evaluated the crystallization effects by a spectroscopic observation of visible-lights reflection on the flow region. As a result, while expansion flows have no crystallization effect, contraction flows have shown it.
Numerical Assessment of Turbulent Models at a Critical Regime on Unstructured Meshes
Authors: You Ju Yeol , Kwon Oh Joon
Published: June 2012

The main objective of the present study is to investigate the performance of different turbulent models for the flow simulation around a circular cylinder at a critical Reynolds number regime (Re = 8.5×105, Tu = 0.7%). To simulate the various flow features such as laminar-turbulent transition inside the boundary layer and the unsteady vortex shedding in the wake region, a hybrid RANS/LES model (SAS model) and a correlation-based transition model (γ - Reθ model) were used and the feasibilities of them for the flow simulation at a critical Reynolds number regime were demonstrated. A vertex-centered finite-volume method was adopted to discretize the incompressible Navier-Stokes equations and an unstructured mesh technique was used to discretize the computational domain. The inviscid fluxes were evaluated by using 2nd-order Roe’s FDS and the viscous fluxes were computed based on central differencing. A dual-time stepping method and the Gauss-Seidel iteration were used for unsteady time integration. To reduce the computational costs, the parallelization strategy using METIS and MPI libraries was adopted. The unsteady characteristics and time-averaged quantities of the flow fields were compared between the turbulent models. The numerical results have been also compared with experimental data. At the critical regime, turbulent models have showed quite different results due to the different abilities of each model to predict various flow features such as laminar-turbulent transition, unsteady vortex shedding.
Application of a High-Resolution Compact Finite Difference Method to Computational Aeroacoustics of Compressible Flows
Authors: Zuo Zhifeng , Maekawa Hiroshi
Published: June 2012

WCNS is an efficient high-resolution nonlinear scheme for solving flow-fields including discontinuity. In the present paper, a two-dimensional, unsteady, compressible flow field produced by the interaction between a strong planar shock wave and a strong vortex are simulated numerically using WCNS. The simulation shows the effects of the vortex on a planar shock and the production of acoustic waves by the shock-vortex interaction. At the early times of interaction, the shock wave is perturbed by the vortex and a precursor is produced; with the shock wave emerges from the vortex flow field, a Mach structure was generated and the secondary acoustic wave was formed by the interaction of the reflected shock (MR2) with the precursor. Both components of acoustic wave (the precursor and the second sound wave) propagate radially and have a quadrupolar nature. By this simulation, the ability of WCNS for computational aeroacoustic problems is confirmed.
Experimental Study of Blowing Direction Effects of DBD Plasma Actuator on Separation Control of Flow Around an Airfoil
Authors: Nonomura Taku , Sekimoto Satoshi , Asada Kengo , Oyama Akira , Fujii Kozo
Published: June 2012

An experimental study of plasma actuator on separation control is conducted. The plasma actuator is used for control of separated flow around NACA0015 airfoil. The Reynolds number based on chord length is set to 60,000 and the angle of attack is set to 12[deg]. The plasma actuator is applied with normal mode and burst mode, where normal mode denotes continuous actuation and burst mode denotes temporary intermittent actuation. Also, actuations for co-flow blowing and counter blowing are conducted. The averaged pressure coefficients of wing surface and velocity fields are measured. For velocity fields, PIV measurement is adopted. Comparing counter and co-flow blowings of plasma actuator, the effects of counter blowing is investigated. Also, for both co-flow and counter blowing cases, we investigate the effects of burst mode. Through the series of experiments, following two types of mechanism for separation control will be discussed. One type is considered to be directly giving momentum in the boundary layer which seems to be more active in co-flow blowing with normal mode. The other type is considered to be enhancement of the mixing, leading to increase in momentum thickness of the boundary layer. The latter mechanism seems to be active in the burst mode with both co-flow and counter blowing.
Supersonic Wind Tunnel Experiment on Aerodynamic Characteristics and Winglets Effects of the Tapered Supersonic Biplane
Authors: Fujisono Takashi , Yamashita Hiroshi , Toyoda Atsushi , Nagai Hiroki , Asai Keisuke , Matsuno Takashi , Kawazoe Hiromitsu , Jeong Shinkyu , Obayashi Shigeru
Published: June 2012

The aerodynamic characteristics and the effects of tip plates of a tapered supersonic biplane wing during the starting process have been investigated through Experimental and Computational Fluid Dynamics (EFD/CFD). Three types of the wing model were used: without tip plate (type-N); with the tip plate which covers only the aft-half of the wing tip (type-A); with the tip plate which covers the entire wing tip (type-B). Experiment was conducted in the supersonic blowdown wind tunnel with 600 mm × 600 mm cross section located at the High-speed Wind Tunnel Facility of Institute of Space and Astronautical Science (ISAS/JAXA). The flow conditions covered from M∞ = 1.5 to 1.9 with increments of 0.1. Pressure-Sensitive Paint was applied to measure pressure distributions on the surface of the wing. CFD simulations were conducted to compare with experiments and to investigate effects of the Mach numbers in detail. The tapered biplane wing without the tip plate was found to start between M∞ = 1.8 and 1.9. The difference of the starting Mach numbers between type-N and type-A was small. On the other hand, the starting Mach number of type-B was about 0.05 higher than that of type-N.
Computational Study of Aerodynamic Characteristics of an Airfoil With DBD Plasma Actuator
Authors: Asada Kengo , Fujii Kozo
Published: June 2012

The relation between aerodynamic characteristics and the effectiveness of separation control with the DBD plasma actuator over the airfoil are discussed. The flow-fields around the NACA0015 airfoil are simulated with implicit large-eddy simulation using compact difference scheme. The normal mode generates moderately separated region over the airfoil and gains lift by negative pressure at the vortex center. The burst mode with nondimensional burst frequency of 1 enhances the vortex shedding from the separation shear layer and avoid the massive separation from the leading edge. However, the lift coefficient oscillate very much, in this case. The burst mode with nondimensional burst frequency of 6 improves the airfoil performance by suppressing the separation region. These facts indicate that the unsteady aerodynamic characteristics must be discussed when the effectiveness of separation control is evaluated.
Lift-Force Measurement for Hovering Flight of Four-Winged Flapping Micro Aerial Vehicle
Authors: Hiraki Koju , Fujii Kazuki
Published: June 2012

In order to establish the measurement technique of the hovering force of a flapping aerial vehicle the indirect method was proposed. In the indirect measurement the flapping vehicle was placed over a horizontal plate with a certain distance, and the force acting on the plate was measured by the loadcell installed beneath the plate. First, this technique was evaluated by the thrust force measurement of a rotating propeller. The indirectly measured force was compared with that of the direct measurement. The averaged values of the two measurements agreed quite well. It is also proved that the standard deviation of the indirect measurement was only 7.6 percent of that of the direct measurement. The measured force was insensitive to the vertical position of the plate below the propeller. When the propeller was inclined toward the horizontal plate, the measured values coincided nearly to the normal component of the force vector. This indirect technique was applied to the flapping aerial vehicle. The standard deviation of the indirect measurement was 10 percent of that of the direct measurement. The highest hovering force was obtained at 80 degrees in attitude angle, which is nearly vertical nose-up attitude. This fact corresponded to that the air stream generated by the flapping motion was at maximum in the direction of 80 degrees. The obtained hovering force was 0.126 N, and this value is quite close to the gravitational force acting on the flapping aerial vehicle, which was 0.131 N.
Effect of Reynolds Number on the Turbulent Flow Structure in the Near-Wall Region of an Impinging Round Jet
Authors: Hammad Khaled J. , Milanovic Ivana
Published: June 2012

Time-Resolved Particle Image Velocimetry was used to study the effect of the Reynolds number on the turbulent flow structure of a submerged water jet impinging normally on a smooth and flat surface. A fully developed turbulent jet and a semi-confined flow configuration ensured properly characterized boundary conditions allowing for straightforward assessment of turbulence models and numerical schemes. The Reynolds number based on jet mean exit velocity was 5,000, 10,030 and 15,050 while the pipe-to-plate separation distance was fixed at two diameters. Turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components while Proper Orthogonal Decomposition (POD) analysis identified the most energetic coherent structures in the stagnation and wall-jet regions.
Influence of a Weak Superposed Centripetal Flow in a Rotor-Stator System for Several Pre-Swirl Ratio
Authors: Abdel Nour Fadi , Debuchy Roger , Bois Ge´rard
Published: June 2012

The present study is devoted to the influence of a superposed centripetal flow q in a rotor-stator cavity with a peripheral opening. In the literature, previous works have already shown that a weak radial inflow has no major influence near the periphery of the cavity, whereas the flow behavior is strongly modified when approaching the axis [1–2]. The challenge of this work is to give a better understanding of this phenomenon. Attention is focused on a rotor-stator system characterized by a small gap ratio G and high Reynolds number Re, so that the flow is divided into two boundary layers separated by a core region, according to the classification by Daily and Nece [3]. Until now, numerous authors obtained analytical solutions for the central core flow behavior: following the analysis performed by Schlichting [4], and assuming that the evolution of the velocity in the Ekman boundary layer corresponds to a 1/7 power law, Poncet et al. [2] proposed an analytical law predicting the evolution of the core swirl ratio K versus a local dimensionless coefficient of flow rate Cqr. Debuchy et al. [5] improved this last solution by taking into account the radial exchange of fluid outside the boundary layers. This last approach was used by Abdel Nour et al. [6] in a rotor-stator cavity without any superposed flow (isolated cavity). They obtained an original analytical law, different from the classical similitary solution proposed by Batchelor [7] in the case of infinite disc, convenient for a cavity with peripheral opening and small pre-swirl ratio. In this paper, the authors present an original analytical law in order to model the central core flow behavior in a rotor-stator cavity subjected to a very weak inflow rate (q → 0). The validity of the solution is tested with the help of: • a new set of experimental data including the radial and tangential mean velocity measured by hot-wire anemometry, • experimental results from the literature.
Flow Entrainment of Gravity Currents
Authors: Mok K. M. , Yeh Harry H. , Ieong K. K. , Hoi K. I.
Published: June 2012

The entrainment of gravity currents advancing over a horizontal bed was studied. A two-dimensional rigid-lid flow model was derived assuming ambient-fluid entrainment to the mixing region being supplied only from the bottom layer of the approaching flow. Two sets of laboratory experiments were carried out using the laser-induced fluorescence (LIF) flow visualization technique. With given parameters such as the total fluid depth, densities of the fluids, height of the gravity current head and its propagation speed, and the denser-fluid flow depth behind the head under the mixing region, our model predicts that the thickness of the front flow layer to be entrained is about 35 percents of the height of the gravity current head. Qualitative examination of the flow structures along various planes in the developed fronts suggests that the actual flow structures at the foremost part of the current head are complex and three dimensional. Entrainment of ambient fluid to the current is through various directions starting at its front, which creates an unstable stratification condition there favorable for the subsequent complex three-dimensional eddy generation and growth leading to the formation of the short-crested billows exhibiting the lobe-and-cleft features in the following flow.
Experimental Investigaton of the Single-Phase Friction Factor and Heat Transfer Inside the Horizontal Internally Micro-Fin Tubes in the Transition Region
Authors: Tam Hou Kuan , Tam Lap Mou , Ghajar Afshin J. , Sun Cheong , Leung Hau Yin
Published: June 2012

To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications. It is commonly understood that the micro-fin enhances heat transfer but at the same time increases the pressure drop as well. In the previous studies, majority of the works were focused on the development of correlations in a particular flow regime, especially in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of pressure drop and heat transfer from laminar to transition and eventually the turbulent regions. Therefore, more in-depth study is necessary. In this study, pressure drop and heat transfer were measured simultaneously in a single test section fitted with several micro-fin tubes and the measured data was compared with the data of a plain tube. There were different fin geometries (fin spiral angle, fin height, and number of fins per cross-sectional area) inside the micro-fin tubes. From the friction factor results, the transition from laminar to turbulent was clearly established and shown to be inlet dependent. The transition friction factor characteristic for the micro-fin tubes was different from that of the plain tube. The transition range for the micro-fin tubes was shown to be much wider than that of the plain tube. From the experimental results, it could also be observed that the increase of fin spiral angle lead to the early transition for the friction factor. For the heating condition, the effect of heating on the friction factor was observed primarily in the lower transition region. From the heat transfer results, the transition from laminar to turbulent was clearly established and shown to be inlet and spiral angle dependent. The larger spiral angle caused the earlier transition and the higher heat transfer inside the micro-fin tube. For all the micro-fin tubes with two inlet types, it can be observed that the efficiency index is larger than one when Reynolds number is larger than 5,000.
The Effect of Inner Surface Roughness and Heating on Friction Factor in Horizontal Micro-Tubes
Authors: Tam Lap Mou , Tam Hou Kuan , Ghajar Afshin J. , Ng Wa San , Wong Ieok Wa , Leong Ka Fu , Wu Choi Keng
Published: June 2012

According to Krishnamoorthy et al. [1], pressure drop measurements for horizontal micro-tubes under isothermal condition have been conducted by various researchers in recent years. From their literature review, it was shown that the friction factor in micro-tubes could unanimously be predicted by using macro-scale theory and that there is a need to investigate certain issues like (a) the effect of micro-tube diameter on the transition Reynolds number range and (b) the effect of the inner surface roughness on the friction factor and transition region. Regarding to the point (a), Ghajar et al. [2] measured the pressure drop for a horizontal mini- and micro-tubes with various diameters in the transition region under isothermal condition. Their experimental results indicated the influence of the tube diameter on the friction factor profile and on the transition Reynolds number range. However, regarding to the point (b), the effect of roughness on friction factor profile and transition was still not fully understood. Moreover, only a few studies have investigated the effect of heating on friction factor in micro-tubes, especially, in the transition region. Therefore, in this study, an experimental setup was built to measure pressure drop for horizontal micro-tubes under the isothermal and uniform wall heat flux boundary conditions. Water was used as the test fluid and the test section was glass and stainless steel micro-tubes with various roughness and diameters. From the measurements, the effect of roughness and heating on friction factor and transition region was clearly observed. For friction factor under isothermal condition, compared to the macro-tube, the micro-tube had a narrower transition region due to the roughness and the decrease in the tube diameter delayed the start of transition. For friction factor under heating condition, the laminar and transition data were different from the isothermal case. Heating also delayed the start of transition. The effect of heating was not seen on the turbulent region. For isothermal and heating boundary conditions, the increase of inner surface roughness induced a narrower transition region.
Relation Between Disturbance of Boundary Layer and Free Stream Turbulence Component
Authors: Norimatsu Ryohei , Takai Shogo , Matsubara Masaharu
Published: June 2012

In a flat plate boundary layer subjected to isotropic free stream turbulence of few percents turbulence intensity, the streaks, which longitudinally elongated regions of high and low streamwise velocity, appear in the boundary layer and then break down to turbulent spots. Experiments and DNS have revealed that the profile of the streamwise fluctuation energy has a peak at the middle of boundary layer and that the disturbance grows in proportion to the streamwise distance from the leading edge. These results were in good agreement with the non-modal theory. Though the theory suggested that in lower free stream turbulent case modal disturbance has chance to develop and breakdown to turbulence, experimental investigations have not clarified the maximum intensity of the free stream turbulence at which the modal disturbance triggers transition. It is know that there are other processes of the boundary transition that start with a short streak breakdown. In this study various types of the free stream turbulence including anisotropic turbulence are scrutinized using turbulence grids and their relation to the disturbance growth in the boundary layer is investigated. The result with hot-wire measurements shows that the spanwise spectra of the free stream turbulence are essential factor for the non-modal growth. This would be a major step for developing prediction method of boundary layer transition.
Computational Modeling of Future Wind Power Installations
Authors: Nemabakhsh Ali , Olinger David , Hussein Islam , Tryggvason Gretar
Published: June 2012

Advanced wind power installations are likely to include floating wind turbines that can be placed far off-shore and airborne wind power generator that can harvest the wind at altitude beyond what tower-based turbines can reach. The feasibility of such installations does, however, depend on the ability to optimize the design to make it economical. Here we describe computational studies of the dynamics of floating wind turbine platform and planned examination of airborne energy generation devices. The computational approach for both systems relies on the use of an immersed boundary method for the moving platforms. For the floating wind turbine the free surface is captured by a level set approach. In addition to capturing the dynamics of a moving turbine tower and a flexible wing, the modeling of the tethers provides new challenges in both cases.
Effects of Swirl on Velocity-Field Uniformity in Disk Shape SOFC Channel Flow
Authors: Tsunoda Kazumi , Aminaka Kazuyuki
Published: June 2012

Swirling flow behavior between two parallel disk shape plates was experimentally investigated with the aid of a particle image velocimetry (PIV). The experiment was performed at low Reynolds numbers (Re < 100) to simulate the practical operation in a disk shape planar-type solid oxide fuel cell (SOFC). To improve flow uniformity, we designed a new channel with circle involute shape current collectors. In the new channel, a swirling flow was generated and its velocity in a core region was kept at nearly constant value toward the channel exit. This trend was observed regardless of flow rates, and hence flow uniformity was achieved over the wide range of Reynolds numbers. This is because a flow passage consisting of two adjacent involute shape current collectors functions as a constat-area channel due to the geometrical property of the circle involute. In order to understand the above mentioned flow behavior, a swirling fluid motion in the channel with the circle involute shape current collector was investigated by using steady state Euler’s equation of motion. We confirmed that the velocity component in the flow direction was dominant compared with that in the other direction and played primary role to maintain a swirling motion through the centripetal acceleration term.
Numerical Simulations of Droplets on the Hydrophobic and Hydrophilic Walls by Lattice Boltzmann Method
Authors: Matsukuma Yosuke , Inoue Gen , Minemoto Masaki
Published: June 2012

Gas-liquid flows in/on porous structures are simulated by using of the two-phase Lattice Boltzmann method (LBM), in which the wetting boundary conditions on solid wall with complex geometry are incorporated. The complex geometry simulating the packed bed is numerically constructed by the discrete element method (DEM). It is confirmed that structure of the simulated packed bed is similar to the actual bed by comparison of wall friction factor. Next the behaviors of droplet on the porous structures are simulated with different wetting properties. For hydrophilic cases, the droplets set on the porous structure at initial stage penetrated into the porous structure as time marching on and spread in the bed. It was shown that the droplet behavior depends on the surface tension and its viscosity. From these numerical simulations, the applicability of LBM to Gas-liquid flows in/on porous structures was confirmed.
Simulation of Hydrogen Production by Using Concentrated Solar Energy Through Thermo-Chemical Water Splitting Process: Part I—Simulation of Zinc Oxide Reduction Reaction
Authors: Khan Furqan Ahmad , Siddiqui Kamran
Published: June 2012

Due to the depleting reserves of fossil fuels and their harmful effects on the environment, there is an urgent need to explore clean alternative energy resources to fulfil the growing energy demand. Sunlight is an abundant source of energy, and its storage and utilization in the form of hydrogen is considered to be effective and cleanest. The present research is focused on the numerical investigation of hydrogen production through thermo-chemical decomposition of water. In this paper we report on the first step reaction which is the endothermic reduction of zinc oxide in a solar reactor/receiver coupled with a parabolic dish type solar energy concentrator. The simulations were conducted in a three-dimensional reactor model using the commercial CFD software FLUENT. The results of parametric study showed an increase in the fractional conversion of zinc oxide with a decrease in the diameter of the zinc oxide particles, while this fractional conversion decreased with a decrease in the zinc oxide mass flow rate. It was also observed that the particle initial temperature has no effect on the fractional conversion of the zinc oxide. The outlet temperature of the fluid mixture was also not influenced significantly by the zinc oxide fractional conversation and remained over 1800K for the entire fractional conversion range of zinc oxide.
Performance Test of Electricity Generation Utilizing Vortex Induced Vibration
Authors: Iiyoshi Ryota , Kamijyo Masahiro , Yamada Shuichi , Koide Mizuyasu , Takahashi Tsutomu , Shirakashi Masataka
Published: June 2012

The Karman vortex induced vibration (KVIV) is observed over a wide range of conditions, and has been regarded as a negative phenomenon until now since it has caused many accidents. Therefore a lot of researches have been conducted to predict and to avoid it. Recently, however, KVIV is regarded as a process to convert energy of natural flows into mechanical energy, and techniques for electricity generation utilizing it are proposed. The electric power of this method is smaller than that of wind and water turbine generations, but this method has possibility to become a smaller and more maintenance-free apparatus than rotary machines. In earlier works, we found that the trailing vortex shed periodically from a cruciform two-circular-cylinder system, and that it induces a cross flow vibration on the upstream cylinder (TVIV) over a wide velocity range, which becomes broader by replacing the downstream cylinder by a strip-plate. Because of this character, an electricity generator utilizing TVIV should be effectively applied to rivers of which velocity usually varies largely. The purpose of this work is to develop a technique to generate electricity utilizing TVIV in water flow. Experiments using a water tunnel and an open-surface water channel are conducted to know conditions of the maximum electric power and to test the performance in a river. The optimum gap-to-diameter ratio is 0.22 since the cylinder vibration amplitude is largest. The optimum resistance of the circuit is the value which makes the virtual damping due to electricity generation nearly equal to the structure damping. The performance test in the water channel shows that the open surface and the turbulence in flow have little influences on the cylinder vibration amplitude and the synchronization velocity range of KVIV. However, TVIV is not observed, maybe because of the large aspect ratio.
Study of the Regenerator Porosity Influence on Gamma Type Stirling Engine Performances
Authors: Gheith Ramla , Aloui Fethi , Tazerout Mohand , Ben Nasrallah Sassi
Published: June 2012

In this study, a Gamma type Stirling engine with compressed air as working fluid have been experimented. This engine operates at a maximum charge pressure of 10 bars. It runs at a maximum speed of 600 rpm, and can provide 500 W of brake power on the shaft. This Stirling engine setup was equipped with thermocouples and pressure sensors in order to measure the instantaneous temperatures and pressures. The regenerator is the key element of the Stirling engine. Its geometrical and physical properties influence directly on the engine performances. Our experimental study was focused on the regenerator, and especially on its porosity and its constituting material. We have initially made our experiments for 4 different regenerators material’s constituted of: Stainless steel, Copper, Aluminum and Monel 400. Secondly, we have experimented 3 regenerators made of copper with different porosities. From the obtained results, we conclude that the regenerator have an important role to enhance heat exchanges and to improve the Stirling engine performances. Indeed, these performances are also function of the porosity of each material constituting the regenerator, which conditions the quality and the quantity of heat exchanged in the Stirling engine.
CFD Prediction for an Overpressure Buildup Phenomenon of SRI Hydrogen Test in an Open Space
Authors: Kang Hyung Seok , Kim Sang Baik , Kim Min-Hwan , No Hee Cheon
Published: June 2012

A computational fluid dynamics (CFD) calculation for a hydrogen explosion test with a complicated obstacle tube geometry of pitch 21.3mm and diameter 99.1mm at a stoichiometric condition was performed to establish a CFD analysis method for a hypothetical hydrogen explosion accident between a very high temperature reactor (VHTR) and a hydrogen production facility. We developed a spark ignition model to simulate high ignition energy of 40J induced by an electric device for 2 ms in the hydrogen explosion based on an energy conservation law. We performed a sensitivity calculation by varying a constant value of the eddy dissipation model (EDM), a time step size, and a cell length size around the obstacle tube to evaluate an effect of each factor on the flame propagation and overpressure buildup phenomenon. The CFD results of the flame front time of arrival (TOA) and overpressure were compared with those of the test data. The comparison results showed that the spark ignition model with a radius of 6 cm, a pressure of 105.7 kPa, a temperature of 1000 K, a turbulent mixing time of 2 ms, and an assumption of the 10% product mass fraction can reasonably initiate the hydrogen flame propagation in the CFD calculation. As for the CFD analysis method, the EDM constants of A = 10 and B = 0.8, the time step size of 0.01 ms, the cell length of 1 cm around the obstacle tube predicted the measured flame front TOA and peak overpressure with an error range of about 27.8% and 53.3%, respectively. Therefore, it is known that the CFD analysis with the EDM may be used as an accurate evaluation tool to provide the 3-dimesnional information of the flame front TOA and overpressure buildup phenomenon if the CFD analysis method is properly chosen.
An Investigation on the Issues in Modelling of Free Flight in Animal Locomotion
Authors: Maeda Masateru , Nakata Toshiyuki , Liu Hao
Published: June 2012

Aiming at establishing an effective computational framework to accurately predict free-flying dynamics and aerodynamics we here present a comprehensive investigation on some issues associated with the modelling of free flight. Free flight modelling/simulation is essential for some types of flights e.g. falling leaves or auto-rotating seeds for plants; unsteady manoeuvres such as take-off, turning, or landing for animals. In addition to acquiring the deeper understanding of the flight biomechanics of those natural organisms, revealing the sophisticated aerodynamic force generation mechanisms employed by them may be useful in designing man-made flying-machines such as rotary or flapping micro air vehicles (MAVs). The simulations have been conducted using the coupling of computational fluid dynamics (CFD) and rigid body dynamics, thus achieving the free flight. The flow field is computed with a three-dimensional unsteady incompressible Navier-Stokes solver using pseudo-compressibility and overset gird technique. The aerodynamic forces acting on the flyer are calculated by integrating the forces on the surfaces. Similarly, the aerodynamic torque around the flyer’s centre of mass is obtained. The forces and moments are then introduced into a six degrees-of-freedom rigid body dynamics solver which utilises unit quaternions for attitude description in order to avoid singular attitude. Results are presented of a single body model and some insect-like multi-body models with flapping wings, which point to the importance of free-flight modelling in systematic analyses of flying aerodynamics and manoeuvrability. Furthermore, a comprehensive investigation indicates that the framework is capable to predict the aerodynamic performance of free-flying or even free-swimming animals in an intermediate range of Reynolds numbers (< 105).
PIV Measured Hemodynamic Study With Several Stents in a Cerebral Aneurysm Model for New Stent Design
Authors: Yu Chang-Ho , Shida Shuya , Matsumoto Kaoru , Ohta Makoto
Published: June 2012

The rupture of an aneurysm can lead to a subarachnoid hemorrhage, which can be a fatal illness in a large percentage of cases. In treating aneurysm intravascular stenting is thought as a better option compared with clipping treatment. The treatment of cerebral aneurysm with a porous stent also has recently proposed as a minimally invasive way to prevent rupture and favor coagulation mechanism inside aneurysm. The analysis of a flow pattern with and without stent in cerebral aneurysm model was performed with Particle Image Velocimetry (PIV). Three types of stents were used in the present investigation and the porosity of stent 1, 2 and 3 is 80%, 74% and 64%, respectively. Reduced velocity, smaller average vorticity magnitude and different flow patterns inside aneurysm are observed when the three different proposed stents are used. The stent 3 (Porosity = 64%) decrease the magnitude of the velocity by 98% and change the flow pattern in the aneurysm. Our results show that not only the role of the porosity but also the stent shape affect the flow and help us understand the characteristic of stent designs.
Mixing of Matter by a Rising Bubble Near a Wall
Authors: Koso Toru , Iwashita Hiroyuki , Usuki Fumihiko
Published: June 2012

The turbulent mixing of liquid mass caused by an air bubble rising near a wall in a still liquid in a pipe is investigated experimentally using a photochromic dye. A part of the liquid is activated by UV light and subjected to the fluid motion caused by a zigzag rising bubble of which Reynolds number is 214. The visualized mixing patterns showed that the dye is mixed by vortex motions in the bubble wake that is similar to the case of a bubble rising in the center of the pipe. The concentration distributions were deduced from the dye images using Lambert-Beer’s law and the turbulent diffusion coefficient (TDC) was evaluated from the temporal changes in the mass dispersion. The TDCs showed that a near-wall bubble generates stronger mixing than for a bubble in the center of the pipe. This stronger mixing can be attributed to the large-scale vortices observed for a near-wall bubble, which remains active for a longer time due to the lack of oppositely rotating vortices and mixes more fluids.
Study on Conditional Statistics in Two-Dimensional Liquid Jet With the Second-Order Chemical Reaction
Authors: Watanabe Tomoaki , Yasuhara Hiroki , Sakai Yasuhiko , Kubo Takashi , Nagata Kouji , Terashima Osamu
Published: June 2012

It is important in engineering to elucidate the mechanism of a chemical reaction in turbulent flow. But there are still few studies on reacting turbulent flow in a liquid phase. In this study, the two-dimensional liquid jet with the second-order reaction (A+B←R) is investigated. The concentrations of the species R and the conserved scalar (which is the concentration of other species independent of the above chemical reaction) are measured simultaneously by the optical fiber probe based on light absorbtion spectroscopic method. The concentrations of species A and B are obtained from the conserved scalar theory. Regarding the velocity field, the streamwise velocity is measured by the hot-film anemometer. The moment closure methods are often used for the prediction of turbulent flow. But it is difficult to apply it to the reacting turbulent flow because of the high non-linearity of the reaction rate terms. It is commonly known that the values of concentrations depend strongly on the mixture fraction (which is a conserved scalar) defined as the normalized concentration of the species which is independent of reaction. Hence, Conditional moment closure (CMC) methods are useful for the prediction of the turbulent flow with chemical reactions. In this study, conditional scalar statistics are investigated by using the conditional moment closure methods and experimental data. It is shown that the conditional averages of concentration of reactant and product species approach the equilibrium limit (which correspond to the limiting case of the fast chemical reaction) in the downstream direction and the value of the conditional scalar (mixture fraction) dissipation decreases and its distribution varies in the downstream direction and comes to show the local minimum value near the point η = ξS (which is the stoichiometric value of the mixture fraction).
Impact of Air System Bleeding on Aircraft Engine Performance
Authors: Zhao Bin , Li Shaobin , Li Qiushi , Zhou Sheng
Published: June 2012

Air system bleeding is indispensable to aircraft engines despite its negative impact on the engine thrust and the fuel consumption. However, the compressor performance can be improved if the bleeding design is optimized. The model in this paper is a one-dimensional engine model based on air system bleeding. The relation between the compressor performance and the engine thermodynamic cycle caused by bleeding is analyzed to explore the potential of air system bleeding in improving compressor and engine performance. The results show that if bleeding brings an increase the pressure ratio of compressor, the negative impact on engine specific fuel consumption can be inhibited. If the efficiency of compressor is increased after bleeding, the negative impact on engine thrust can be alleviated. With proper bleeding flow rates, if both the pressure ratio and the efficiency increase at the same time, the negative impact on the engine performance can be eliminated.
Hydraulic Disturbance Method to Reduce the Pressure Fluctuation in Francis Turbine Draft Tube
Authors: Li Zhangchao , Chang Jinshi , Ji Xingying , Liu Wanjiang , Xin Zhe
Published: June 2012

For a Francis turbine, when operating at partial flow rate the fixed-pitch runner shows a strong swirl at the runner outlet which induces a helical vortex (so-called vortex rope) in the draft tube. The precessing vortex rope causes severe pressure fluctuation which effects the steady and secure operating of the turbine. Three-dimensional unsteady turbulent flow simulation with RNG k-ε turbulence model of complete flow passage of a model Francis turbine at partial discharge is performed. To verify the simulation, the model turbine is tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China. An ideal result of the simulation is obtained. The simulation predicts the shape of the helical vortex successfully in the draft tube cone, and the predicted values of the pressure fluctuation frequencies and amplitudes agree well with the test data. The hydraulic disturbance method is introduced, i.e., injecting water with velocity circulation from the runner cone to reduce the pressure fluctuation in Francis turbine draft tube. The injected water with velocity circulation destroys the forming mechanism of vortex rope and eliminates the pressure fluctuation accordingly. The flow in the turbine with water injection is simulated, and it is indicated that with appropriate flow rate and velocity circulation water injection the pressure fluctuation in the draft tube is reduced effectively.
The Influence of Major Diameter Solid Particle on the Double-Channel Pump Performance
Authors: Li Yi , Cui Qiaoling , Zhu Zuchao , He Zhaohui , Cui Baoling
Published: June 2012

Based on mixture model, the numerical simulation of solid-liquid two-phase flow in a double channel pump (Specific speed ns = 81) was carried out. The effects of particle diameter, particle volume fraction and flow rate on solid volume concentration distribution, relative velocity distribution and abrasion characteristics were studied. The results reveal that in the impeller, more particles concentrate at the nut of the shaft end and the edge of the impeller outlet. So those regions are worn seriously. The abrasive types are sliding wear on the impeller outlet edge and impact wear on the nut respectively. In the wall of the volute, the concentrated areas of particles move round the anticlockwise direction when the mixture flow rate is larger. The reason is the mixture velocity is larger as the flow rate increases, and meanwhile the centrifugal force and gravity force are invariable. So the particles move round the impeller rotational direction consequently. In the volute, particles concentrate on the tongue and wall region, especially on the sections I, II, V and VII. So the areas are easily worn out. The abrasive type is the heavy sliding wear in the volute wall. Numerical simulation results are consistent with the actual situation. It follows that the calculating method is feasible.
An Optimization Method for Centrifugal Compressor Design Using the Surrogate Management Framework
Authors: Ha Kyoung Ku , Kang Shin Hyoung
Published: June 2012

A variety of centrifugal compressors are used in various fields of industry these days. The design requirements are more complicated, and it is difficult to determine the optimal design point of a centrifugal compressor. The aim of this study was to propose an efficient optimization method for centrifugal compressors considering the impeller, the vaneless diffuser, and the overhung type volute. The optimization was performed using the surrogate management framework (SMF). The design parameters were the impeller exit radius, the exit blade angle, and the flow coefficient. Sample points in the design space were selected according to the Design of Experiments (DoE) theory. The CFD simulations were executed on the impeller and the diffuser at every sampled point. The volutes were described using a one-dimensional but reliable theory to reduce the simulation time. An approximation model based on the Kriging method was constructed using this dataset. Then, an optimal design point that minimized the objective function was determined in a substitute design space using the pattern search method because of its efficiency and rigorous convergence. The optimization process, underlying methods, and results are described in this paper.
Study of High Efficiency and Low Noise Centrifugal Fan
Authors: Nagae Takanori , Zheng Zhiming
Published: June 2012

Centrifugal fans are widely used for air-conditioning equipments. Demands for air conditioners have arisen for quiet considered the indoor condition, energy saving associated with the global warming, and which have made it important to develop high performance fans. In order to meet these demands, the technology was studied to reduce leakage flow for a high efficiency and low noise centrifugal fan in this research. Leakage flow occurs when a part of air discharged from fans flows into a gap between the bellmouth and the shroud. This flow is the pressure loss because it returns from the high-pressure side to the low-pressure side wastefully. Furthermore, the high noise is caused, because this leakage air can’t flow along the shroud. As the final successful technology to reduce leakage flow, we have developed for the new bellmouth with multiple ribs on the bellmouth plane placed opposite fan. By using the new bellmouth, the centrifugal fan’s motor input for the 4-direction ceiling-embedded cassette type air conditioner has been reduced by 5.4[%] and the noise by 1.2[dBA]. We analyzed the leakage flow structures of the centrifugal fan were analyzed by the numerical simulation and LDV measurements. It has been found that the leakage flow is decreased, since the partial leakage flows back in an opposite direction to a gap between the bellmouth and the shroud. Additionally, it has been found that the turbulence intensity of outlet airflow is decreased by the new bellmouth.
Study on P-Q Curves of Cooling Fans for Thermal Design of Electronic Equipment (Effects of Opening Position of Obstructions Near a Fan)
Authors: Fukue Takashi , Ishizuka Masaru , Hatakeyama Tomoyuki , Nakagawa Shinji , Koizumi Katsuhiro
Published: June 2012

This study describes an operation pressure and supplies flow rate of an axial cooling fan installed in high-density packaging electronic equipment. Fan performance is generally defined by their P-Q curve, specifically, a relationship between fan pressure rise (ΔP) and flow rate (Q). A compact cooling fan often operates in a high-density mounting device, which may decrease the fan performance. In this study, we focus on an obstruction near a fan, which is electronic components such as PCBs, capacitors and heat sinks, as one of a factor which decreases fan performance. We installed a perforated plate which simulated the above components near a fan and measured the P-Q curve. To investigate a relationship between a fan performance decrease and an opening position near the fan, a part of the perforated plate was closed. Closed position was changed and explored an opening condition which caused the dominant fan performance decrease. From experiments, it was found that the fan performance was decreased when flow passage in front of a fan was blocked by an obstruction. Especially, when flow passage in front of a fan hub was blocked, a dominantly reduction of fan pressure was caused. An obstruction rear a fan has no effect on a fan performance curve itself. In addition, opening conditions in front of a fan tip had a little influence on a fan pressure characteristic when there was no obstruction in front of a hub.
Unsteady and Three-Dimensional Flow Mechanism of Spike-Type Stall Inception in an Axial Flow Compressor Rotor
Authors: Kikuta Hiroaki , Iwakiri Ken-ichiro , Furukawa Masato , Yamada Kazutoyo , Gunjishima Satoshi , Okada Goki
Published: June 2012

The unsteady behaviors and three-dimensional flow structure of the spike-type stall inception in an axial flow compressor rotor have been investigated by experimental and numerical analyses. In order to capture the transient phenomena of spike-type stall inception experimentally, “SFMT (Simultaneous Field Measurement Technique)”, by which instantaneous pressure distributions on the casing wall were acquired, was developed. By applying this technique, the unsteady flow pattern on the casing wall was extracted for each phase of development process of the stall inception. The details of three-dimensional flow structure in the stall inception process were revealed by the numerical analysis using a detached-eddy simulation (DES). At the stall inception, the characteristic patterns of the casing wall pressure distributions are observed in the experimental results: the low pressure regions moving in the circumferential direction and the variations of the low pressure regions at the leading edge. Considering the results of DES, these patterns are made by the vortices fragmented from the deformed tip leakage vortex or the tornado-type separation vortex and also are made by the tornado-type separation vortex itself, as well. The vortical flow structures have been elucidated. These vortices actually result from the leading edge separation at the blade tip. Therefore, it has been found that spike-type stall inception is dominated by the leading edge separation at the rotor blade tip.