PIV measurements in a turbulent wall jet over a backward-facing step in a three-dimensional,non-confined channel

A study of a turbulent wall jet over a backward-facing step is especially of interest because it shows a rich phenomenon flow and a mechanism to alter the flow characteristics downstream of the step. However, studies on this flow configuration are rare. In this paper, we considered this flow configuration in a non-confined channel as the specific engineering applications of electrical rotating machines and alternator that can be found in modern wind generators of the power production industry and automobile engines. The turbulent wall jet over a backward-facing step in a non-confined wind tunnel had the jet Reynolds number of 24,100 and the step Reynolds number of 11,900. Particle image velocity (PIV) and stereoscopic PIV measurements were performed along the central plane and several cross-stream planes. Numerical simulation of the test configuration was conducted by solving the three-dimensional Reynolds Averaged Navier–Stokes (RANS) equations with the second-order closure Reynolds stress model (RSM). The mean flow fields and second-order statistical moments from the RSM simulation were compared to results that were obtained through the PIV and stereo-PIV experiments. The mean reattachment length obtained from the current configuration was much shorter than those from the backward-facing step in the plane channel. The stereo-PIV measurements in the cross-stream planes revealed a high three-dimensionality of the flow, a high population of streamwise vortice in the upper region, near the side walls and the corners formed by the side walls and the bottom wall. The obtained results also confirmed the presence of the wall-jet formation on the bottom wall.     

绕后台阶与准方腔耦合流道流场的数值计算与试验研究

用Galerkin有限元法对绕后台阶与准方腔耦合流道流场进行数值模拟。数学模型采用的是流函数—涡量形式的N-S方程,分别计算了雷诺数214.2、428.4和642.6三种情况下流场的结构,计算所得瞬态流场和稳态流场以直观速度矢量图和等值线图的形式给出,并绘出了部分纵截面上速度分布剖面图,分析了雷诺数对流场结构的影响。同时应用近年来发展起来的且获得广泛应用的全场瞬时速度测量技术,即粒子图像测速技术PIV)对流场进行了试验研究,所得速度矢量图与数值计算结果吻合较好。计算和试验结果表明:绕后台阶与准方腔流道流场比较复杂,同时具有后台阶流动和方腔流动的结构特征。研究结果对工程实际具有重要意义。     

Pressure and flow characteristics in a shallow hydrostatic pocket with rounded pocket/land joints

The paper studies the development of the flow and pressure maps in a shallow hydrostatic bearing pocket and, on a comparative basis, discusses the effects of the pocket-to-land exit geometry when it takes different shapes (sharp 90° angle, and rounded with different radii of curvature). The numerical simulation uses a dimensionless formulation of the Navier-Stokes equations written in primitive variables for a body fitted coordinate system, and applied through a collocated grid. The model includes on one hand the coupling between the pocket flow and a finite length feedline flow, and the pocket and the adjacent lands on the other hand. Geometrically, all pockets have the same footprint, same land length, and same capillary feedline. The numerical simulation uses the Reynolds nuber (Re) based on the runner velocity, and the inlet jet strength (F) as dynamic similarity parameters, while the radius of curvature of the pocket/land joint is used as a geometric parameter. The study treats the laminar ranges of the Re number.     

Effects of the velocity waveform of the physiological flow on the hemodynamics in the bifurcated tube

The periodicity of the physiological flow has been the major interest of analytic research in this field up to now. Among the mechanical forces stimulating the biochemical reaction of endothelial cells on the wall, the wall shear stresses show the strongest effect to the biochemical product. The objective of present study is to find the effects of velocity waveform on the wall shear stresses and pressure distribution along the artery and to present some correlation of the velocity waveform with the clinical observations. In order to investigate the complex flow phenomena in the bifurcated tube, constitutive equations, which are suitable to describe the rheological properties of the non-Newtonian fluids, are determined, and pulsatile momemtum equations are solved by the finite volume prediction. The results show that pressure and wall shear stresses are related to the velocity waveform of the physiological flow and the blood viscosity. And the variational tendency of the wall shear stresses along the flow direction is very similar to the applied sinusoidal and physiological velocity waveforms, but the stress values are quite different depending on the local region. Under the sinusoidal velocity waveform, a Newtonian fluid and blood show big differences in velocity, pressure, and wall shear stress as a function of time, but the differences under the physiological velocity waveform are negligibly small.     

Towards DES in CFD-based optimization: The case of a sharp U-bend with/without rotation

Abstract Channels with a sharp U-bend are typical elements of convective cooling systems designed for blades of high-temperature gas turbines. The flow passing a sharp bend is characterized by a high level of pressure loss that is attributed to development of intensive secondary flows, as well as to formation of a large-size separation adjacent to the divider wall. A considerable reduction of pressure losses can be achieved by proper changes in geometry of the walls forming the U-bend, first of all, in the divider wall geometry. The paper presents results of CFD-based optimization of a stationary or orthogonally rotating U-bend of strong curvature. Baseline geometry is same as used by Cheah et al. (1996) for detailed measurements of U-duct isothermal turbulent flow at the Reynolds number of 10⁵, with and without system rotation. The minimization of the total pressure loss in the bend is achieved by means of a hybrid numerical optimization method that uses design of experiments and a metamodel for fast surrogate modeling of the design space. Optimization databases are produced by incompressible fluid unsteady RANS-(k-ω)-SST calculations. Due to optimization of the divider wall profile, the total pressure loss coefficient has been reduced by 47-67 % depending on the rotation number (Ro = 0,±0.2). For the original and the optimized shapes of the bend, simulation is performed also with the eddy-resolving IDDES method validated against the velocity field and pressure measurement data for the original geometry. For optimized shapes, IDDES predicts somewhat lower pressure losses comparing to the RANS model, by 3 to 15 % depending on the rotation number.

     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

Flow instabilities in a graft anastomosis: a study of the instantaneous velocity fields

The major cause of arterial bypass graft failure is intimal hyperplasia. Fluctuating wall shear stresses in the graft, which are associated with disturbed flow, are believed to be important factors in the development and localization of intimal hyperplasia. This study, based upon water as the working fluid, has investigated the flow structure inside a 30 degree Y-junction with different fillet radii at the intersection between the graft and the host artery at various Reynolds numbers and distal outlet segment (DOS) to proximal outlet segment (POS) flow ratios. The structure of the flow has been investigated experimentally using particle image velocimetry (PIV). The two-dimensional instantaneous velocity fields confirm the existence of a very complex flow, especially in the toe and heel regions for the different fillet radii and clearly identify features such as sinks, sources, vortices and strong time dependency.     

Finite Element Numerical Simulation and PIV Measurement of Flow Field inside Metering-in Spool Valve

The finite element method (FEM) and particle image velocimetry (PIV) technique are utilized to get the flow field along the inlet passage, the chamber, the metering port and the outlet passage of spool valve at three different valve openings. For FEM numerical simulation, the stream function ψ -vorticity ω forms of continuity and Navier-Stokes equations are employed and FEM is applied to discrete the equations. Homemade simulation codes are executed to compute the values of stream function and vorticity at each node in the flow domain, then according to the correlation between stream function and velocity components, the velocity vectors of the whole field are calculated. For PIV experiment, pulse Nd: YAG laser is exploited to generate laser beam, cylindrical and spherical lenses are combined each other to produce 1.0 mm thickness laser sheet to illuminate the object plane, Polystyrene spherical particle with diameter of 30-50 μm is seeded in the fluid as a tracing particles, Kodak ES1.0 CCD camera is employed to capture the images of interested, the images are processed with fast Fourier transform (FFT) cross-correlation algorithm and the processing results is displayed. Both results of numerical simulation and PIV experimental show that there are three main areas in the spool valve where vortex is formed.Numerical results also indicate that the valve opening have some effects on the flow structure of the valve. The investigation is helpful for qualitatively analyzing the energy loss, noise generating, steady state flow forces and even designing the geometry structure and flow passage.     

Optimization in PIV algorithm for visualizing vortices in bubble wake

The PIV (Particle image velocimetry) is the most commonly used method for flow field observation because of its high efficiency and non-interference to the flow field. This study aims to clarify the optimal parameter conditions used in the cross-correlation algorithm of PIV for flow fields with vortices. The influence factors on the error of the cross-correlation algorithm are analyzed and discussed using a synthetic flow field, including the seeding conditions, the velocity gradient and vortex size. It is confirmed that the minimum particle number density per interrogation window is about 10, which generally limits the minimum size of the window. For a vortex, when the vortex size is fixed, the velocity gradient corresponding to the characteristic velocity both controls the lower and upper limitation of window size. For a relatively small vortex, generally a window not larger than the vortex size is asked. Then, a strategy to improve the observation based on the existing equipment is discussed and applied to visualize a rising bubble wake based on LIF (Laser-induced fluorescence) images.     

轴流涡轮平面叶栅出口二维速度场的实验研究

将粒子图像测速仪(PIV)和热线风速仪(HWA)应用到低速轴流平面叶栅实验台上.在2个不同进口流量和3个不同叶片高度下,利用热线风速仪测量平面叶栅出口瞬态速度场,并采用激光粒子图像测速仪测量圆柱尾迹和出口速度场的整场信息.研究发现较大的进口速度或较低的叶片高度情况下,叶栅出口二次流流动强烈;同时发现尾迹的存在会破坏均匀流场,导致叶型流动损失的增加.     

Experimental Study of Lubricating Grease Flow inside the Gap of a Labyrinth Seal Using Microparticle Image Velocimetry

In this study, the flow of lubricating greases in a labyrinth seal geometry is studied using microparticle image velocimetry (µPIV). The aim is to evaluate the grease velocity distribution inside the gap of a labyrinth seal and to find a relationship between the grease consistency and the transferred speed from the rotating ring in order to choose the correct grease as a sealing medium. In addition, the grease flow characteristics are important for the understanding of fracture due to grease layer displacement. For these purposes, four greases with different rheological properties were used in µPIV experiments. It was found that the grease consistency plays a crucial role in speed development as well as the grease composition and presence of a slip effect at the grease–rotating wall interface.     

Fluid flow characteristics in the aquaculture tank for a breeding fish

The aquaculture tank is used for breeding fish in sea water which was pumped up to land. The flow characteristics in the aquaculture were investigated with varying the tank geometry and flow rate. The numerical analysis has been employed for calculating the velocity and temperature distributions in a water tank of rectangular type. The finite volume method and SIMPLE algorithm with 3-dimensional standard k-ε turbulence model were used for the numerical analysis. For comparison with experimental results, the PIV system was applied to visualize the flow patterns quantitatively. The numerical results showed good agreements with the experimental results. The mean velocity and temperature versus aquarium depth were represented for various circulating flow rates. Especially, the aquaculture environment is recommended that the aquarium depth has to be d=0.5 m, and this case is the condition of higher velocity and temperature in winter season.     

Lubricating Grease Shear Flow and Boundary Layers in a Concentric Cylinder Configuration

Grease is extensively used to lubricate various machine elements such as rolling bearings, seals, and gears. Understanding the flow dynamics of grease is relevant for the prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. In this study, grease flow is visualized using microparticle image velocimetry (μPIV). The experimental setup includes a concentric cylinder configuration with a rotating shaft to simulate the grease flow in a double restriction seal geometry with two different grease pocket sizes. It is shown that the grease is partially yielded in the large grease pocket geometry and fully yielded in the small grease pocket. For the small grease pocket, it is shown that three distinct grease flow layers are present: a high shear rate region close to the stationary wall, a bulk flow layer, and a high shear rate boundary region near the rotating shaft. The grease shear thinning behavior and its wall slip effects have been identified. The μPIV experimental results have been compared with a numerical model for both the large and small gap size. It is shown that the flow is close to one-dimensional in the center of the small pocket. A one-dimensional analytical model based on the Herschel-Bulkley rheology model has been developed, showing good agreement with the measured velocity profiles in the small grease pocket. Furthermore, wall slip effects and shear banding are observed, where the latter imply that using the assumption of uniform shear in conventional concentric cylinder rheometers may result in erroneous rheological results.     

An experimental study of developing and fully developed flows in a wavy channel by PIV

An experimental study is presented for a flow field in a two dimensional wavy channels by PIV. This flow has two major applications such as a blood flow simulation and the enhancement of heat transfer in a heat exchanger. While the numerical flow visualization results have been limited to the fully developed cases, existing experimental results of this flow were simple qualitative ones by smoke or dye streak test. Therefore, the main purpose of this study is to produce quantitative flow data for fully developed and developing flow regimes by the Correlation Based Correction PIV (CBC PIV) and to conjecture the analogy between flow characteristics and heat transfer enhancement with low pumping power. Another purpose of this paper is to examine the onset position of the transition and the global mixing, which results in transfer enhancement. PIV results on the Fully developed and developing flow in a wavy channel at Re=500, 1000 and 2000 are obtained. For the case Reynolds Number equals 500, the PIV results are compared with the finite difference numerical solution.     

超声空化对细微流道中固体颗粒的运动特性研究

针对细微流道中的多相流调控问题,研究超声空化效应对微流道内固体颗粒运动特性的影响。利用Fluent有限元技术对空化气泡溃灭的过程进行模拟,得在超声声压作用于近壁区的气泡时,气泡凹陷、破壁、溃灭的演变过程,并对其周围流场的速度矢量分布进行研究,仿真结果显示,空化气泡溃灭能够产生射向壁面的高速微射流,其最大速度达到28m/s,进而确定细微流场观测实验的最优参数;利用高速摄像机对细微流道中气泡的演变过程进行观测实验,并与仿真实验结果进行对照,观测实验结果表明,利用超声空化效应能够实现对固体颗粒向流道壁面运动的有效引导,为实现细微流道的流场调控、提高加工精度等问题提供理论支持。     

TWO-DIMENSIONAL PARTICLE IMAGE VELOCIMETRY（PIV） MEASUREMENTS IN A TRANSPARENT CENTRIFUGAL PUMP

A special transparent centrifugal pump is designed. Detailed optical measurements of the flow inside the rotating passages of a five-bladed shroud centrifugal pump impeller have been performed by using two-dimensional particle image velocimetry (PIV). The flow is surveyed at three load conditions qV/Vd = 0.4, qV/qVd = 1.0, qVlqVd = 1.5, respectively. As a result, phase averaged PIV velocity vector maps on three planes between hub and shroud of the impeller are presented. At design load, the mean field of relative velocity is predominantly vane congruent, showing well-behaved flow without separation. The distributions of the relative velocity on different plane along the pump shaft are very different and there is always a low velocity zone near the pressure-side of the blade at both low and design flow rate, but the low-velocity-zone at the low flow rate is much larger than that at the design one. The study demonstrates that the PIV technique is efficient in providing reliable and detailed velocity data o     

Large eddy simulation of flow around a bluff body of vehicle shape

The turbulent flow with wake, reattachment and recirculation is a very important problem that is related to vehicle dynamics and aerodynamics. The Smagorinsky Model (SM), the Dynamics Subgrid Scale Model (DSM), and the Lagrangian Dynamic Subgrid Scale Model (LDSM) are used to predict the three-dimensional flow field around a bluff body model. The Reynolds number used is 45,000 based on the bulk velocity and the height of the bluff body. The fully developed turbulent flow, which is generated by the driver part, is used for the inlet boundary condition. The convective boundary condition is imposed on the outlet boundary condition, and the Spalding wall function is used for the wall boundary condition. We compare the results of each model with the results of the PIV measurement. First of all, the LES predicts flow behavior better than the k-ε turbulence model. When we compare various LES models, the DSM and the LDSM agree with the PIV experimental data better than the SM in the complex flow, with the separation and the reattachment at the upper front part of the bluff body. But in the rear part of the bluff body, the SM agrees with the PIV experimental results better than them. In this case, the SM predicts overall flow behavior better than the DSM and the LDSM.     

Experimental investigation of particle distribution in a flow through a stenosed artery

The particle distribution of a dilute solid-liquid suspension through a stenosed arterial geometry was investigated. Particle image velocimetry (PIV) was used to determine the velocity as well as to acquire the flow images. The light intensity scattered by particles was evaluated to determine the particle distribution. Flow separation exists where the flow emerges from the stenosis throat. From the PIV images, the particle density distribution exhibited differing non-uniform characteristics which vary with flow rate, particle size and particle concentration. At low flow rates, a particle-free layer is formed. As the flow rate is increased, particles accumulate in concentric recirculation orbits within the downstream vortex. Particles with larger size and higher concentration tend to accumulate more towards the center of the vortex.     

随机粗糙微通道内部流动与传质特性

为了研究微通道壁面随机粗糙度对流体流动和传质特性的影响,采用随机排布准则构建具有典型粗糙元类型的随机粗糙微通道壁面,利用有限元方法分析壁面随机粗糙度对流速、压降、流动阻力和传质性能的影响,并给出粗糙微通道内部Poiseuille数和分子传质扩散的近似变化规律。结果表明,流体在粗糙微通道近壁面区域和主流区的流速差异较大,近壁面区域流动分离现象明显;与光滑微通道相比,粗糙微通道内部各位置的压降和Poiseuille数沿着流动方向呈近似线性增大趋势;微通道壁面粗糙度的存在可以强化流体分子的传质扩散速率,但受粗糙度类型和相对粗糙度的影响较大。     

Three-dimensional features of MHD flows turning in a right-angle duct

This study presents a numerical simulation of three-dimensional (3D) Liquid metal (LM) Magnetohydrodynamic (MHD) flows turning in a right-angle duct with a square cross section under a uniform magnetic field applied perpendicular to the plane of the main flow. The 3D features of the flow in a fluid region adjacent to the duct walls of the turning segment are investigated. Cases with different Hartmann numbers and conductance parameters are analyzed using the CFX code. The MHD features of the LM flow are examined in terms of fluid velocity, current density, electric potential, and pressure gradient. The formation of a velocity recirculation is observed in the inner region of the right-angle segment immediately after the turning of the flow because of the inertial force therein, thereby yielding a region of low electric potential with a complicated current distribution. In particular, in the right-angle segment, the axial velocity in the side layer near the outer wall (that is, in the outer side layer) is relatively lower than that in the inflow and outflow channels. In addition, the velocity recirculation region in the right-angle segment decreases and the pressure gradient increases with an increase in conductance parameter.