Numerical simulation of 2-D laminar flow subjected to the Lorentz force effect in a channel with backward-facing step

This paper presents the numerical solutions of a two-dimensional laminar flow over a backward-facing step in the presence of the Lorentz body force. The Navier-Stokes equations in a vorticity-stream function formulation are numerically solved using a uniform grid mesh of 2001 × 51 points. A second-order central difference approximation is used for spatial derivatives. The solutions progress in time with a fourth-order Runge-Kutta method. The unsteady backward-facing step flow solution is computed for Reynolds numbers 100 to 800. The size and genesis of the recirculating regions are dramatically affected by applying the Lorentz force. The results demonstrate that using an appropriate configuration for applying the Lorentz force can make it an essential tool for controlling the flow in channels with a backward-facing step.     

Jet flow characteristics of sinusoidal wavy nozzles

The flow characteristics of jets issued from a sinusoidal nozzle with in-phase and 180° out-of-phase exit configurations were investigated using PIV (particle image velocimetry) and flow visualization techniques. The experiments were carried out at a Reynolds number of about 6300 based on the mean width of the jet nozzle. Compared to a normal rectangular jet, the sinusoidal nozzle jets have smaller velocity deficits as the flow goes downstream. In addition, the turbulence intensity is suppressed in the horizontal center plane. For the case of in-phase wavy nozzle jet, the length of the potential core exhibits small variations along the lateral direction, while the 180° out-of-phase wavy nozzle jet shows large lateral variation in the length of potential core. The turbulent kinetic energy of the 180° out-ofphase nozzle jet also shows sinusoidal variation in the horizontal planes. Large-scale vortices shed from the sinusoidal edge of the nozzle interact strongly and migrate toward the center plane as the flow develops downstream.     

Stereoscopic-PIV measurement of turbulent jets issuing from a sharp-edged circular nozzle with multiple triangular tabs

Three-dimensional (3D) flow structures of a turbulent jet issuing from a sharp-edged circular nozzle having multiple triangular tabs are experimentally studied by employing a stereoscopic-PIV (SPIV) system. Two different sharp-edged jet nozzles having 4 and 8 tabs are investigated at a jet Reynolds number of Re = 10,000. The SPIV measurements are carried out at 5 different cross-sectional planes along the jet direction. Spatial distributions of turbulent statistics including mean velocity, mean vorticity, and turbulent kinetic energy are obtained at each cross-sectional plane. The jet entrainment rate showing the mixing of the jet and ambient fluids is also estimated using the measured 3D velocity field information. As a result, the jet issuing from the nozzle with 4 tabs shows better turbulent mixing effect at further downstream position than the 8 tabs case because of the reduced reciprocal interactions of the streamwise vortices that promote the turbulent dissipation.     

Laser velocimetry of a plane mixing layer with a specific initial condition for study of turbulence characteristics of enhanced growth rate

Mixing layers are sensitive to the mixing angle and turbulence in the primary streams. Although there is extensive available research on this rather basic flow, there are still no suggestions for a clearly best configuration. For example, the combination of a laminar initial boundary layer and a large mixing angle has received little attention. In this work we test a new experimental configuration with large mixing angle and laminar/turbulent initial boundary layer that was not examined experimentally by LDA and PIV. This setup is expected to be a representation of the initial conditions that must result in better mixing. A plane mixing layer with a velocity ratio of 0.6 is produced by rebuilding an open circuit wind tunnel. Extensive calibration tests on velocity profiles and Reynolds stresses established the position of the self similar region. Velocity field measurements with laser Doppler anemometer (LDA) and particle image velocimeter (PIV) showed enhanced mixing layer growth. PIV plots showed the presence of stream-wise and cross stream vortices in the self-similar region without any considerable change in turbulence characteristics to that of reported in the literature. The article presents a combination of different experimental results that give a deeper understanding of this very configuration.     

Turbulence characteristics of wall jets along strong convex surfaces

Mean flow and turbulence characteristics of two-dimensional wall jets along strong convex surfaces are investigated in an experimental manner. Distributions of mean velocities, Reynolds normal and shear stresses and dissipation across the wall jets are measured, then the balances of advection, production and dissipation in the Reynolds stress equations are examined. The jet spreading and the levels of the Reynolds stresses of the strong convex wall jets increase significantly as the wall jets develop along the surfaces. These trends are well correlated with a curvature parameter represented by the ratio of half-width to surface curvature. The magnitudes of the terms in the Reynolds stress equations for the strong convex wall jet are increased significantly compared with those of the plane wall jet. In particular, these appear to be strong centrifugal effects on the production of the turbulent fluctuation normal to the surface. This production, coupled with the production of the shear stress and the advection of streamwise turbulent fluctuation, increase the interactions of mean flow and turbulence.     

An experimental approach to turbulent heat transfer using a symmetric expanded plane channel

The flow in a symmetric expansion plane channel is known to deflect to one side of a channel even at a low Reynolds number due to the Coanda effect. Details of flow structure have been investigated by various authors; however, there have been a few works conducted in the area of on heat transfer. This paper presents experimental results of turbulent heat transfer in separated and reattached flows in a symmetric expansion plane channel. Experiments were conducted using a low-speed open-circuit wind tunnel. The step H was 20 mm high and 200 mm wide, with an expansion ratio of 2.0. The Reynolds number based on the uniform flow velocity at step and H was varied from 5,000 to 35,000, respectively. The mean and turbulent fluctuating velocities were measured using mainly two types of split film probe. A cold single wire probe was used for measuring the mean and turbulent fluctuating temperatures. It was found that the local Nusselt number profile was considerably different on the upper and lower walls due to the Coanda effect, which is was caused by instability between the upper and lower separated shear layers. Empirical formulae for the maximum Nusselt number in the reattachment region are hereby proposed for the upper and lower walls, respectively. The two formulae are well correlated with the previous general formula proposed. The location of maximum Nusselt number is found to be very close to the flow reattachment point. Details of the velocity and temperature fields were clarified and their correlations with the heat transfer characteristics described above investigated. Furthermore, the wavelet transformation methodology was employed to study instantaneous flow and temperature behaviors, which exhibited its usefulness in the study of the present complicated flow and temperature fields.     

Ghost离心叶轮内部湍流流动数值模拟

应用FLUENT软件对Ghost离心叶轮内部湍流流动进行了数值研究,其中湍流模型采用雷诺应力模型(RSM),转子和定子的耦合采用混合平面模型(MP).计算结果与Johnson的试验结果基本一致,表明了RSM能较好的预测离心叶轮内的湍流流动.分析了离心叶轮内部的湍流流动特性,表明边界层内的二次流对低能流体的输运,使叶轮出口形成"尾迹一射流"现象,同时尾迹区的存在造成能量的损失,且尾迹区位置取决于无量纲数Rossby数.     

Numerical analysis of turbulent flow over a backward-facing step using reynolds stress closure model

Reynolds stress turbulence models are adopted and applied for calculating turbulent flow over a backward-facing step. For the diffusion term in the transport equations for the Reynolds stresses, two gradient-type models are employed and compared. In addition, investigations on the modified ∈ equations are carried out. The results of the computations are compared with the extant experimental data. As a consequence, it is concluded that the Reynolds stress models predict the flow field better than the standardk-∈ model in the recirculating region. However, after the reattachment the return to the ordinary turbulent boundary layer is shown to be too slow to predict the flow field irrespective of turbulence models.     

Spatial and temporal structures of turbulent bubble-driven flows in a rectangular water tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a rectangular water tank were investigated. The time-resolved particle image velocimetry (PIV) technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 2 to 4 ℓ/min at 0.5 MPa, and the corresponding range of bubble-based Reynolds number ranged from 6,740 to 13,220. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition (POD) analysis technique. When the flow rate was increased, the main vortex core moved to the side and bottom wall. Locations of peak turbulent kinetic energy regions depended on the bubble Reynolds number. Both spatial and temporal modes were quite different with respect to the flow rates. The first temporal mode was harmonized with the second temporal mode, with small oscillations in the case of the lowest Reynolds number. However, temporal modes oscillate with higher frequencies when the Reynolds number increases. Based on the result of the FFT analysis of each temporal mode, we conjectured that low-frequency oscillation was attributed to the recirculating flow, whereas a higher dominant frequency was related to the vibration of the free surface that interacts with the rising bubbles.     

Recent applications of particle image velocimetry in aerodynamic research

Particle image velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. For the first time the PIV technique allows the recording of a complete velocity field in a plane of the flow within a few microseconds. The PIV technique thereby provides information about unsteady flow fields which is difficult to obtain with other experimental techniques. The short acquisition times and fast availability of data reduce the operational time, and hence cost, in large scale wind tunnels and test facilities.

At DLR a variety of PIV systems for use in industrial wind tunnels has been developed in the past decade. The flexibility of these portable systems is illustrated by presenting several results of recent PIV applications. More recently the original photographic means of PIV image recording has been partially replaced by high resolution electronic imaging which can provide PIV data nearly on-line. Images recorded by either system use the same multiple-pass, cross-correlation analysis software, whose algorithms are briefly described. Several examples of actual applications are given: the flow issuing from a jet nozzle was imaged by a specially developed high-speed video camera at close proximity. A high resolution dual-frame digital camera was applied in the study of helicopter rotor aerodynamics and wake vortex measurements of an airplane model. Further, large image sequences exceeding 100 PIV recordings provided detailed information on the structure of a turbulent boundary layer.     

Particle image velocimetry study of parameter influence for synthetic-jet application

Flow separation control of a circular cylinder using a synthetic jet positioned at the front stagnation point is experimentally investigated by the particle image velocimetry (PIV) technique. The control results for different excitation parameters, including the stroke length, the excitation frequency, and the momentum coefficient, are compared to distill the essential control parameters, and the influence of the cylinder Reynolds number on the control effect is discussed. The separation control mechanism for the present control configuration is also revealed. It is suggested that the effective control ability of the synthetic jet is attributed to the increment of the turbulent kinetic energy (TKE) and the dissipation rate surrounding the circular cylinder. High level of TKE enhances the dynamics of the fluids and thus flow around the leeward surface is endured a considerable vertical acceleration pointing to the centerline from both sides, which is more resistant to flow separation.     

Particle image velocimetry of a flow at a vaulted wall

The assessment of flow along a vaulted wall (with two main finite radii of curvature) is of general interest; in biofluid mechanics, it is of special interest. Unlike the geometry of flows in engineering, flow geometry in nature is often determined by vaulted walls. Specifically the flow adjacent to the wall of blood vessels is particularly interesting since this is where either thrombi are formed or atherosclerosis develops. Current measurement methods have problems assessing the flow along vaulted walls. In contrast with conventional particle image velocimetry (PIV), this new method, called wall PIV, allows the investigation of a flow adjacent to transparent flexible surfaces with two finite radii of curvature. Using an optical method which allows the observation of particles up to a predefined depth enables the visualization solely of the boundary layer flow. This is accomplished by adding a specific dye to the fluid which absorbs the monochromatic light used to illuminate the region of observation. The obtained images can be analysed with the methods of conventional PIV and result in a vector field of the velocities along the wall. With wall PIV, the steady flow adjacent to the vaulted wall of a blood pump was investigated and the resulting velocity field as well as the velocity fluctuations were assessed.     

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.     

直流低速风洞流场特性研究

对一直流低速风洞的试验段流场品质进行实验研究,利用IFA300热膜风速仪测量了风洞试验段的流场,获得试验段两截面中心线上速度和湍流度分布,结果表明,该风洞具有较好的流场均匀性和稳定性,具备了开展横流中的紊动射流实验研究的基本条件。     

Large eddy simulation of flow characteristics in an unconfined slot impinging jet with various nozzle-to-plate distances

To study the detailed flow structure of the unconfined plane impinging jet, a 3-dimensional flow analysis is carried out focused on the flow mechanism with the non-dimensional distance, L/d, as flow parameter by using the LES turbulent technique. The symbols d and L represent the nozzle width and the nozzle-to-plate distance, respectively. Then, the flow structures along both the stream-wise and spanwise direction are investigated. For these purposes, the plane impinging jet with Reynolds number of 11,000 is analyzed. The nozzle width was fixed at 1.5mm, but the nozzle-to-plate distance was varied between 4mm and 24mm. As a result, the plane impinging jet shows different flow patterns with L/d. In conclusion, the plane impinging jet is classified into three types with the non-dimensional variable, L/d. L/d≤4: Stable impinging jet, 4<L/d<11: periodically-stable impinging jet and L/d≥11: Unstable impinging jet.     

Comparative study of a turbulent wall-attaching offset jet and a plane wall jet

The flow field characteristics of a two-dimensional wall-attaching offset jet (WAOJ) are experimentally investigated by comparing with those of a turbulent plane wall jet(PWJ). The mean velocity, the turbulent stresses and triple velocity correlations are measured with a split film probe and anX wire probe. Even with the strong influence of the suction pressure field in the recirculation bubble at the lower corner, it is found that the WAOJ in the wall jet region has a close similarity with the PWJ. Especially, the decay of maximum velocity and the upper jet spread along the maximum velocity line of the WAOJ are virtually the same as those of the PWJ. The mean velocity profile of the WAOJ attains similarity after the jet impingement onto the lower plate. However the profiles of second and third-order moments of fluctuating velocities vary rapidly before the impingement and then relax very slowly to the similarity profiles of the PWJ.     

Taylor-Couette 流与旋转圆柱轴间流体膜润滑与密封

对国内外Taylor-Couette流的形成与发展,旋转圆柱轴间流体膜润滑与密封的数值计算及其相关实验的研究现状进行了分析。分析结果表明,应用雷诺润滑方程求解旋转圆柱轴间流体膜润滑与密封问题有其先天的不足,雷诺润滑方程的推导过程中所采用的假设不尽合理,在雷诺润滑方程的基础上进行惯性力修正和涡粘性修正,或者是人为地对3种流动进行解耦,只能作为一种近似计算。从Taylor-Couette流的基本理论出发,计算旋转圆柱轴间流体膜润滑与密封应是计算其层流问题的发展方向。对于湍流润滑与密封,直接模拟(DNS)和大涡模拟(LES)应是首选方案。     

An experimental study on swirling flow behind a round cylinder in the horizontal circular tube

An experimental study is performed for turbulent swirling flow behind a circular cylinder using 2-D PIV technique. The Reynolds number investigated are 10,000, 15,000, 20,000 and 25,000. The mean velocity vector, time mean axial velocity, turbulence intensity, kinetic energy and Reynolds shear stress behind the cylinder are measured before and behind the round cylinder along the test tube. A comparison is included with non swirl flow behind a circular and square cylinder. The recirculation zones are showed asymmetric profiles.     

Measurements of the flow fields around two square cylinders in a tandem arrangement

The velocities, turbulence intensities, Reynolds shear stresses, and turbulent kinetic energies of the flow fields around two square cylinders in a tandem arrangement were investigated using particle image velocimetry (PIV). The experiments were made for the spacing between the two cylinders ranging from s/D = 0.5 to 10.0 and two Reynolds numbers of 5,300 and 16,000. The results showed that the flow patterns at s/D≤2.0 were drastically different from those at s/D≥2.5 for both Reynolds numbers. The sudden change in the flow patterns depended on the reattachment of the shear layer separated from the upstream cylinder.     

Determining the discharge rate from a submerged oil leak jet using ROV video

With expanded deep sea drilling in the Gulf of Mexico, and possibly the Arctic, it is imperative to have a technology available to quickly and accurately measure the discharge rate from a submerged oil leak jet. This paper describes an approach to measure the discharge rate using video from a Remotely Operated Vehicle (ROV). ROV video can be used to measure the velocity of visible features (turbulent eddies, vortices, entrained particles) on the boundary of an oil leak jet, from which the discharge rate can be estimated. This approach was first developed by the Flow Rate Technical Group (FRTG) Plume Team, of which the authors Savaş and Shaffer were members, during the response to the Deepwater Horizon (DWH) oil leak. Manual tracking of visible features produced the first accurate government estimates of the oil discharge rate from the DWH. However, for this approach to be practical as a routine response tool, software is required that automatically measures the velocity of visible features. To further develop this approach, experiments were conducted to simulate a submerged oil leak jet using a dye-colored water jet in the U.C. Berkeley Tow Tank facility. Jet exit diameters were 10.2 cm and 20.3 cm. With flow rates up to 11 gal/s, Reynolds numbers in the range of the DWH oil leak jets (up to 500,000) were achieved. The dye-colored water jets were recorded with high speed video and radial profiles of velocity were mapped with Laser Doppler Anemometry (LDA). Particle Image Velocimetry (PIV) software was applied to measure the velocity of visible features. The velocities measured with PIV software were in good agreement with the LDA measurements. Finally, the PIV software was applied to ROV video of the DWH oil leak jet. The measured velocities were 10–50% lower than manual measurements of velocity. More research is required to determine the reasons why PIV software produced much lower velocities than manual tracking for the DWH oil leak jet.