Particle image velocimetry measurements of the separated flow behind a rearward facing step

Particle image velocimetry (PIV) was used to measure mean flow and turbulence characteristics in the separated flow behind a rearward facing step. The confidence limits characterising estimates of mean velocity and turbulence intensity obtained by PIV are discussed. The relevance of background turbulence levels and sample size in determining these limits is considered.     

Particle dispersion in a single-sided backward-facing step flow

The paper describes the particle dispersion in a single-sided backward-facing step flow. Particles of well-known sizes in the diameter range from 1 to 70 μm were suspended in an air flow and the particle motion over a step was measured by mean of a laser-Doppler anemometer. Thus, the local and integral flow quantities, i.e. the mean and turbulent velocity data could be measured precisely. In the experiments, monodispersed particle size distributions were used to exclude particle size related information ambiguity, known as triggering effects or size bias. The results of this study show qualitatively and quantitatively the difference in time-averaged particle dynamics for selected particle sizes in a backward-facing step flow. The experiments show, for different sizes, the changes in the particle velocity field in comparison with the velocity field of the continuous phase deduced from the 1 μm particles, and also imply the strong influences which different particle sizes have on flow data evaluation when size effects are not taken into account with particle-related optical measuring techniques.     

Particle image velocimetry with optical flow

 An optical Flow technique based on the use of Dynamic Programming has been applied to Particle Image Velocimetry thus yielding a significant increase in the accuracy and spatial resolution of the velocity field. Results are presented for calibrated synthetic sequences of images and for sequences of real images taken for a thermally driven flow of water with a freezing front. The accuracy remains better than 0.5 pixel/frame for tested two-image sequences and 0.2 pixel/frame for four-image sequences, even with a 10% added noise level and allowing 10% of particles of appear or disappear. A velocity vector is obtained for every pixel of the image. Received: 18 July 1997/Accepted: 5 December 1997     

Particle image velocimetry measurements in complex geometries

 One of the advantages of Particle Image Velocimetry (PIV) is its ability to determine the instantaneous flow field over two- or three-dimensional domains. Yet PIV has had limited application to complex flow passages because of the difficulty in replicating these geometries with optically transparent materials. In this work, we describe a method for overcoming this difficulty using rapid prototyping techniques. As an illustrative example, the technique has been used to characterize flow in a model of the human nasal cavity. Received: 12 January 1999/Accepted: 30 September 1999     

Three-dimensional particle-tracking velocimetry measurement of turbulence statistics and energy budget in a backward-facing step flow

Using a three-dimensional (3-D) particle-tracking velocimeter, detailed turbulent flow measurements were made in a plane channel with a one-sided 50% abrupt expansion, which acted as a backward-facing step. The turbulent channel flow reached a fully developed state well upstream of the step. The Reynolds number based on the upstream centerline velocity and the step height H was 5540. With the mean reattachment point located at 6.51H downstream of the step, the measurement region ranged from −2H upstream to 12H downstream of the step. Various turbulent statistics and the energy budget were calculated from numerous instantaneous vector distributions. As in previous experimental investigations, the Reynolds normal and shear stresses had maximum values upstream of the reattachment. The stress anisotropy tensor revealed a peculiar phenomenon near the reattachment wall, wherein the spanwise normal stress was the largest among the three normal stresses. The triple velocity correlations indicated large values in the separating shear layer, and hence the turbulent diffusion was a major term in the energy budget. Comparison was made between the present results and those of the direct numerical simulation (DNS) of Le et al. (1993), and it was found that the mean and fluctuating velocities, the Reynolds shear stress, and the turbulent energy budget were in excellent agreement, although there was a considerable difference in the inflow conditions.     

Turbulent flow over a rough backward-facing step

This work characterizes the impacts of the realistic roughness due to deposition of foreign materials on the turbulent flows at surface transition from elevated rough-wall to smooth-wall. High resolution PIV measurements were performed in the streamwise-wall-normal (x–y) planes at two different spanwise positions in both smooth and rough backward-facing step flows. The experiment conditions were set at a Reynolds number of 3450 based on the free stream velocity U_∞ and the mean step height h, expansion ratio of 1.01, and the ratio of incoming boundary layer thickness to the step height, δ/h, of 8. The mean flow structures are observed to be modified by the roughness and they illustrate three-dimensional features in rough backward-facing step flows. The mean reattachment length X_r is significantly reduced by the roughness at one PIV measurement position while is slightly increased by the different roughness topography at the other measurement position. The mean velocity profiles at the reattachment point indicate that the studied roughness weakens the perturbation of the step to the incoming turbulent flow. Comparisons of Reynolds normal and shear stresses, productions of normal stresses, quadrant analysis of the instantaneous shear-stress contributing events, and mean spanwise vorticity reveal that the turbulence in the separated shear layer is reduced by the studied roughness. The results also indicate an earlier separation of the turbulent boundary layer over the current rough step, probably due to the adverse pressure gradient produced by the roughness topography even before the step.     

Particle image velocimetry of active flow control on a compressor cascade

AFC (Active Flow Control) experiments have been performed by means of steady and pulsed blowing out of the sidewalls as well as out of the blade’s suction surface in a highly loaded compressor cascade. PIV (Particle Image Velocimetry) was used to evaluate the fully three-dimensional internal flow field and the impact of AFC methods. The aim was to observe the secondary flow structures and flow instabilities by PIV, to tune the AFC device operation parameters. This paper summarizes the different PIV measurements performed at the stator cascade to give an overview of the dominant flow features in the passage flow field and to obtain a detailed view of control mechanisms. In addition, a new vortex detection method is presented, based on a 2D-wavelet which is applicable in two-dimensional velocity data fields.     

Control of backward-facing step flow using a flapping foil

The effect of oscillating a small foil in plunge on the reattachment of a separated shear layer in a two-dimensional backward-facing step flow has been studied using flow visualization and single component laser Doppler velocimetry (LDV) measurements. It has been shown that a jet instead of a wake is generated by the flapping action of the foil. Results indicate that this action induces strong mixing and entrainment when the foil is located within the recirculation flow region, thereby reducing the reattachment length by as much as 70%. Furthermore, it has been shown that the flapping foil is most effective in reducing the size of the separation zone when placed close to the wall and to the step. It is least effective when placed outside the separated shear layer or downstream of the reattachment zone. Received: 26 August 1999 / Accepted: 29 May 2001     

Particle image velocimetry measurements of a shock wave/turbulent boundary layer interaction

Particle image velocimetry is used to investigate the interaction between an incident shock wave and a turbulent boundary layer at Mach 2.1. A particle response assessment establishes the fidelity of the tracer particles. The undisturbed boundary layer is characterized in detail. The mean velocity field of the interaction shows the incident and reflected shock wave pattern, as well as the boundary layer distortion. Significant reversed flow is measured instantaneously, although, on average no reversed flow is observed. The interaction instantaneously exhibits a multi-layered structure, namely, a high-velocity outer region and a low-velocity inner region. Flow turbulence shows the highest intensity in the region beneath the impingement of the incident shock wave. The turbulent fluctuations are found to be highly anisotropic, with the streamwise component dominating. A distinct streamwise-oriented region of relatively large kinematic Reynolds shear stress magnitude appears within the lower half of the redeveloping boundary layer. Boundary layer recovery towards initial equilibrium conditions appears to be a gradual process.     

Visualization of a locally-forced separated flow over a backward-facing step

A laboratory water channel experiment was made of the separated flow over a backward-facing step. The flow was excited by a sinusoidally oscillating jet issuing from a separation line. The slit was connected to a cavity in which water was forced through a rigid pipe by a scotch-yoke system. The Reynolds number based on the step height (H) was fixed at Re _H =1200. The forcing frequency was varied in the range 0.305?St _H ?0.955 at the forcing amplitude A ₀=0.3. Time-averaged flow measurements were made by a LDV system, especially in the recirculating region behind the backward-facing step. To characterize the large-scale vortex evolution due to the local forcing, flow visualizations were performed by a dye tracer method with fluorescent ink. The vortex amalgamation process was captured at the effective forcing frequency (St _H =0.477) for laminar separation. This vortex merging process enhances flow mixing, which leads to the shortening of the reattachment length.     

Use of holography in particle image velocimetry measurements of a swirling flow

 Particle Image Velocimetry (PIV) is now a well established experimental technique to measure two components of the velocity in a planar region of a flow field. This paper shows how its proven capabilities can be further extended by using holographic recording to register the particle displacements. Among other unique characteristics, holography enables the acquisition of multiple images on a single plate, and the recording of three dimensional images. These features are used to circumvent some of the limitations of conventional PIV. Some of these possibilities are demonstrated in this study by applying the technique to a high Reynolds number swirling flow using a lens-less off-axis orthogonal recording geometry. Received: 25 February 1998/ Accepted: 2 September 1998     

Stereoscopic particle image velocimetry measurements of the flow around a surface-mounted block

The advantages of 3D measurement techniques and the accuracy of the backward projection algorithm are discussed. The 3D calibration reconstruction used is based on an analytical relation between real and image co-ordinates. The accuracy of the stereoscopic particle image velocimetry (PIV) system is assessed by taking measurements of the flow in angular displacement configuration with prisms. A comparison is made with 2D PIV measurements and the accuracy of this stereo PIV algorithm is evaluated. By using this 3D measurement technique, the topology and the main 3D features of the flow around a surface-mounted block are investigated.     

Stereoscopic particle image velocimetry measurements of the flow around a Rushton turbine

 The principles of stereoscopic particle image velocimetry (PIV), including distortion compensation, were applied to the turbulent flow in a vessel stirred by a Rushton turbine. An angular offset configuration was used and tilt-axis lens mounts were incorporated in order to satisfy the Scheimpflug condition, significantly reducing the ordinarily large depth of field requirements of such configurations. A distortion compensation procedure, or in situ calibration, was utilized in place of the ray tracing, or mechanical registration, used in previous studies. The calibration procedure was validated using two tests, one a rigid translation of a speckle target, the other the viscous flow between two concentric cylinders. The results of the tests suggest the success with which the distortion compensation procedure may be applied to real fluid flows. Phase-locked instantaneous data were ensemble averaged and interpolated in order to obtain mean 3-D velocity fields on a cylindrical shell enclosing the turbine blade. From these fields, the tip vortex pairs and the radial jet documented in previous studies of mixer flows were easily identified. Received: 5 February 1999/Accepted: 1 December 1999     

A numerical study of flow over a confined backward-facing step

Numerical solutions using the SIMPLE algorithms for laminar flow over a backward-facing step are presented. Five differencing schemes were used: hybrid; quadratic upwind (QUICK); second-order upwind (SOUD); central-differencing and a novel scheme named second-order upwind biased (SOUBD). The SOUBD scheme is shown to be part of a family of schemes which include the central-differencing, SOUD and QUICK schemes for uniform grids. The results of the backward-facing step problem are presented and are compared with other numerical solutions and experimental data to evaluate the accuracy of the differencing schemes. The accuracy of the differencing schemes was ascertained by using uniform grids of various grid densities. The QUICK, SOUBD and SOUD schemes gave very similar accurate results. The hybrid scheme suffered from excessive diffusion except for the finest grids and the central-differencing scheme only converged for the finest grids.     

Particle image velocimetry of a low-Reynolds-number separation bubble

Boundary-layer separation at a backward-facing step of a flat plate has been investigated in a low-turbulence wind tunnel with high-speed Particle Image Velocimetry (PIV). The experiments focused on the flow pattern behind the step at the Reynolds number based on its height of about 1,000. Under quiet environmental and low-Re-number conditions, laminar-flow separation was observed behind the step subjected to a small amount of the shear-layer instability. Examination of natural and controlled velocity perturbations in the separation region indicated that the version of PIV used in the present study is an appropriate tool to deal with linear instability details that is normally done through hot-wire measurements.     

Measuring turbulence energy with PIV in a backward-facing step flow

Turbulence energy is estimated in a backward-facing step flow with three-component (3C, stereo) particle image velocimetry (PIV). Estimates of turbulence energy transport equation for convection, turbulence transport, turbulence production, viscous diffusion, and viscous dissipation in addition to Reynolds stresses are computed directly from PIV data. Almost all the turbulence energy terms in the backward-facing step case can be measured with 3C PIV, except the pressure-transport term, which is obtained by difference of the other turbulence energy terms. The effect of the velocity spatial sampling resolution in derivative estimations is investigated with four two-dimensional PIV measurement sets. This sampling resolution information is used to calibrate the turbulence energies estimated by 3C PIV measurements. The focus of this study is on the separated shear layer of the backward-facing step. The measurements with 3C PIV are carried out in a turbulent water flow at Reynolds number of about 15,000, based on the step height h and the inlet streamwise maximum mean velocity U₀. The expansion ratio (ER) is 1.5. Turbulence energy budget profiles in locations x/h=4, x/h=6, and x/h=10 are compared with DNS data of a turbulent flow. The shapes of profiles agree well with each other. Different ERs between the PIV case (1.5) and the DNS case (1.2) cause higher values for the turbulence energies measured by PIV than the energies by DNS when x/h=10 is approached. PIV results also show that the turbulence energy level in these experiments is generally higher than that of the DNS data.     

A DPIV study of a starting flow downstream of a backward-facing step

 In this paper an experimental investigation of a starting water flow downstream of a backward-facing step is described. The Reynolds number of the asymptotic steady flow is Re≈4300 based on the step height of s=2 cm and the free stream velocity of U=21.4 cm/s. Velocity measurements were performed with video-based DPIV (Digital Particle Image Velocimetry) at a sampling frequency of 25 Hz. The main purpose of this study is to reveal the temporal development of global structures which could not be analyzed with single-point probes. It was found that at initialization of the flow a regular vorticity street is formed, which collapses at a normalized time t ^* =U t/s≈17 due to vorticity interactions. After this the flow is dominated by complicated vorticity roll-up and shedding dynamics in the recirculation region. The starting phase is terminated for t ^* >40. Prior to the collapse of the vorticity street values of 9 times the steady state asymptotic wall normal stress and of twice the steady state negative wall shear stress were observed. The early increasing slope of the reattachment length is constant over a time of approximately t ^* =8. The collapse of the vorticity street and the vorticity interactions thereafter cause fluctuations both in the velocity in the free shear layer and in the reattachment length. The fully developed flow has a dominant frequency corresponding to a Strouhal number St=fs/U≈0.04. Received: 20 September 1996/Accepted: 1 April 1997