Non-oscillating/Oscillating Shear Layer over a Large Deep Cavity at Low-Subsonic Speeds

The flow over a deep cavity at low subsonic velocity is considered in the present paper. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary layer and show the influence of the cavity on the streamwise statistic components just downstream from the cavity. The streamwise mean and fluctuating velocity profiles are affected by the cavity. PIV measurements reveal the presence for ejection-like events responsible of local perturbations of the skewness and the flatness coefficients. Time-resolved PIV technic is also used to characterize phase properties of shear layer oscillating cycle. It is shown that for deep cavity with first Rossiter mode, only one vortical structure is formed at the cavity leading edge. Then, it grows while convecting downstream along the shear layer. A well-defined ejection process begins after the vortex impact near the cavity downstream corner. A cylinder device placed spanwisely near the cavity leading edge eliminates the resonance and highly modifies the behavior of the shear layer flow. In fact, the shear layer could be divided into upper and lower parts with different structure aspects.     

Wall proximity effects on the effectiveness of upstream control rod

Two dimensional flow over a circular cylinder with an upstream control rod of same diameter is simulated in unbound condition and in wall bounded conditions. The cylinders are placed at various heights from the wall and the inter-distance between cylinders is also varied. The control rod is subjected to different rotation rates. It is found that, in unbound condition, rotating the control rod decreases the critical pitch length (S/D_cr) and increases the drag and Strouhal number of the main cylinder. In presence of plane wall, the shielding provided by the separated shear layers from the control rod in cavity regime is deteriorated due to deflection of shear layers which results in higher drag and large fluctuation of lift coefficient. However, in wake impingement regime, the binary vortices from the control rod are weakened due to diffusion of vorticity and hence, the main cylinder experiences a lower drag and small lift fluctuations than that of unbound condition. The critical height of vortex suppression (H/D_cr) is higher in cavity regime than that of wake impingement regime due to the single extended-bluff body like configuration. The rotation of control rod energizes the wall boundary layer and increases the critical height of vortex suppression. Increasing the rotational rate of control rod decreases the drag force and reduces the amplitude of lift fluctuation. Analysis of the wall shear stress distribution reveals that it suffers a sudden drop at moderate height where the normal Karman vortex shedding changes to irregular shedding consisting of single row of negative vortices. Modal structures obtained from dynamic mode decomposition (DMD) reveal that the flow structures behind the main cylinder are suppressed due to wall and the flow is dominated by the wake of control rod.     

Effect of a moving wall on a fully developed, equilibrium turbulent boundary layer

Experiments were conducted in a wind tunnel to assess the effect of a moving wall on a fully developed, equilibrium turbulent boundary layer. Pitot-static and total head probes were used in conjunction with both single- and two-component hot-wire anemometer probes to quantify the effect of wall motion on the boundary layer velocity statistics. A variable speed, seamless belt formed the wind tunnel test section wall. When stationary, the belt was found to possess a fully developed, equilibrium turbulent boundary layer in excellent agreement with archival data. With the tunnel wall moving at the free-stream speed, and at a sufficient distance above the wall, the velocity statistics in the moving-wall boundary layer were found to collapse well when scaled as a self-preserving turbulent wake. The near-wall mean velocity profile of the moving wall was found to exhibit an extended region of linearity compared to canonical turbulent boundary layer and internal flows. This can be attributed to the reduced shear resulting from wall motion and the subsequent reduction in Reynolds stress. Received: 2 June 1999/Accepted: 8 August 2000     

暖季强降雨对多年冻土南界斜坡路基稳定性影响分析

为阐明表面活性剂水溶液的减阻作用,使用LDV对零压梯度的二维湍流平板边界层中的CTAB 表面活性剂水溶液的湍流特性进行了实验研究. 结果表明：与牛顿流体相比,CTAB水溶液边 界层的粘性底层增厚;主流时均速度分布有被层流化的趋势,对数分布域上移;主流方向速 度湍动强度峰值减小,且远离壁面,在靠近边界层中部,出现第2峰值;垂直于主流方向的 速度湍动强度受到了大幅度抑制,雷诺应力沿着边界层厚度方向几乎为零. 结果说明CTAB 水溶液具有减弱湍流湍动各个成分相关度的作用,从而能够使雷诺应力降低、湍流能量生成 项减小最终降低流体的输送动力.     

Vortex reynolds number in turbulent boundary layers

The effects of vortex Reynolds number on the statistics of turbulence in a turbulent boundary layer have been investigated. Vortex Reynolds number is defined as the ratio of circulation around the vortex structure to the fluid viscosity. The vortex structure of the outer region was modeled and a full numerical simulation was then conducted using a high-order spectral method. A unit domain of the outer region of a turbulent boundary layer was assumed to be composed of essentially three elements: a wall, a Blasius mean shear, and an elliptic vortex inclined at 45° to the flow direction. The laminar base-flow Reynolds number is roughly in the same range as that of a turbulent boundary layer based on eddy viscosity, and the vortex-core diameter based on the boundary-layer thickness is nearly the same as the maximum mixing length in a turbulent boundary layer. The computational box size, namely, 500, 150, and 250 wall units in the streamwise, surface-normal, and spanwise directions, respectively, is approximately the same as the measured quasi-periodic spacings of the near-wall turbulence-producing events in a turbulent boundary layer. The effects of vortex Reynolds number and the signs of the circulation on the moments of turbulence were examined. The signs mimic the ejection and sweep types of organized motions of a turbulent boundary layer. A vortex Reynolds number of 200 describes the turbulence moments in the outer layer reasonably well.     

A time-resolved hot-wire shear stress probe for turbulent flow: use of laminar flow calibration

A specially-designed rotating rig for producing near Couette flow was used in the calibration of a marginally elevated hot-wire shear stress probe. The probe was then used for measurements in both the turbulent boundary layer and pipe flows. Results showed that the mean wall shear stress can be accurately predicted and the near wall statistical quantities of intensity, skewness and flatness of shear stress fluctuations concurred well with previous works, thereby supporting the notion of a time-resolved shear stress probe for turbulent flows.     

Mechanism of compressor airfoil boundary layer flow control using nanosecond plasma actuation

The flow control effects of nanosecond plasma actuation on the boundary layer flow of a typical compressor controlled diffusion airfoil are investigated using large eddy simulation method. Three types of plasma actuation are designed to control the boundary layer flow, and two mechanisms of compressor airfoil boundary layer flow control using nanosecond plasma actuation have been found. The plasma actuations located within the laminar boundary layer flow can induce a small vortex structure through influencing on the density and pressure of the flow field. As the small vortex structure moves downstream along the blade surface with the main flow, it can suppress the turbulent flow mixing and reduce the total pressure loss. The flow control effect of the small vortex structure is summarized as wall jet effect. Differently, the plasma actuation located within the turbulent boundary layer flow can act on the shear layer flow and induce a large vortex structure. While moving downstream, this large vortex structure can suppress the turbulent flow mixing too.     

Investigation of the heat transfer mechanism in supersonic turbulent boundary layers

A method is described for calculating turbulent Prandtl numbers from Mach number and total temperature profiles in supersonic boundary layers. The calculations are based on boundary layer measurements in the Mach number range from 3.5 to 5. The investigations clearly indicate that in addition to accurate profile measurements reliable values of shear stress and heat flux at the wall must exist, in order to be able to calculate the turbulent Prandtl number in the viscous regime of the boundary layer. For flow conditions with and without heat transfer, the derived turbulent Prandtl numbers indicate that the turbulent transport of heat decreases much faster towards the wall than the turbulent transport of momentum. The results of the analysis show that only the unequivocal qualitative result of increasing turbulent Prandtl numbers in the viscous region of the boundary layer, can be expected. The variation of the turbulent Prandtl number can be described successfully using a simple approximation, based on the mixing length concept, and is applied to the calculation of total temperature distribution using the law of the wall for compressible flow.     

Experimental studies of turbulence in mercury flows with transverse magnetic fields

The differential pressure reading from a static hole pair is utilized for determination of the local wall shear stress. Both the hole diameter and forward-facing angle are varied to test the sensitivity of the device. The static hole pair in tested in a two-dimensional zero pressure gradient turbulent boundary layer on a smooth surface. The calibrating values for the local wall shear is determined from the universal scaling laws for the mean velocity profile in the inner part of the turbulent boundary layer. The static hole pair is found to be sensitive to imperfections in the manufacturing process, and needs an individual calibration in order to make accurate measurements of the local skin friction possible.     

Using digital holographic microscopy for simultaneous measurements of 3D near wall velocity and wall shear stress in a turbulent boundary layer

A digital holographic microscope is used to simultaneously measure the instantaneous 3D flow structure in the inner part of a turbulent boundary layer over a smooth wall, and the spatial distribution of wall shear stresses. The measurements are performed in a fully developed turbulent channel flow within square duct, at a moderately high Reynolds number. The sample volume size is 90 × 145 × 90 wall units, and the spatial resolution of the measurements is 3–8 wall units in streamwise and spanwise directions and one wall unit in the wall-normal direction. The paper describes the data acquisition and analysis procedures, including the particle tracking method and associated method for matching of particle pairs. The uncertainty in velocity is estimated to be better than 1 mm/s, less than 0.05% of the free stream velocity, by comparing the statistics of the normalized velocity divergence to divergence obtained by randomly adding an error of 1 mm/s to the data. Spatial distributions of wall shear stresses are approximated with the least square fit of velocity measurements in the viscous sublayer. Mean flow profiles and statistics of velocity fluctuations agree very well with expectations. Joint probability density distributions of instantaneous spanwise and streamwise wall shear stresses demonstrate the significance of near-wall coherent structures. The near wall 3D flow structures are classified into three groups, the first containing a pair of counter-rotating, quasi streamwise vortices and high streak-like shear stresses; the second group is characterized by multiple streamwise vortices and little variations in wall stress; and the third group has no buffer layer structures.     

Vortex dynamics of a cylinder wake in proximity to a wall

Large-eddy simulations (LES) are used to investigate the modifications of wake dynamics and turbulence characteristics behind a circular cylinder placed near a wall for varying gap-to-diameter (G/D) ratios (where G signifies the gap between the wall and the cylinder, and D the cylinder diameter). The three-dimensional (3-D), time-dependent, incompressible Navier–Stokes equations with a dynamic subgrid-scale model are solved using a symmetry-preserving finite-difference scheme of second-order spatial and temporal accuracy. The immersed boundary (IB) method is employed to impose the no-slip boundary condition on the cylinder surface. Flow visualizations along with turbulence statistics are presented to gain insight into the flow structures that are due to interaction between the shear layers and the approaching boundary layer. Apart from the vortex shedding mechanism, the paper illustrates the physics involving the shear layer transition, stretching, breakdown and turbulence generation, either qualitatively or quantitatively, in the presence of a wall for a Reynolds number of Re=1440 (based on D and the inlet free-stream velocity U_∞).     

Low Reynolds number axisymmetric turbulent boundary layer on a cylinder

In the present study, an axisymmetric turbulent boundary layer growing on a cylinder is investigated experimentally using hot wire anemometry. The combined effects of transverse curvature as well as low Reynolds number on the mean and turbulent flow quantities are studied. The measurements include the mean velocity, turbulence intensity, skewness and flatness factors in addition to wall shear stress. The results are presented separately for the near wall region and the outer region using dimensionless parameters suitable for each case. They are also compared with the results available in the open literature.The present investigation revealed that the mean velocity in near wall region is similar to other simple turbulent flows (flat plate boundary layer, pipe and channel flows); but it differs in the logarithmic and outer regions. Further, for dimensionless moments of higher orders, such as skewness and flatness factors, the main effects of the low Reynolds number and the transverse curvature are present in the near wall region as well as the outer region.     

Simulation of turbulent flow and heat transfer in channels between rod bundles

The turbulent flow and heat transfer in triangular rod bundles are investigated theoretically with CFD code FLUENT. The unsteady Reynolds Stress Model is adopted as turbulence modeling. The wall function is used for near wall boundary layer. The calculation results were in agreement with experimental data. The effects of the Reynolds number and pitch to diameter ratio on the flow and heat transfer in the lattice are significant. The traditional theoretical models could not predict the flow and heat transfer in the lattice. The P/D = 1.03 is a critical point. In this case, the flow and heat transfer in the lattice is the most desirable and most efficient, and the nuclear power could also reach its maximum. The variation of large scale coherent structure with pitch to diameter ratio is consistent with the variation of the Nusselt number with pitch to diameter ratio.     

Quenching of Premixed Flames at Cold Walls: Effects on the Local Flow Field

The presence of a turbulent premixed flame strongly influences the properties of the adjacent velocity boundary layer. This influence is studied here using a generic configuration where at atmospheric pressure turbulent premixed methane/air flames interact with a temperature stabilized wall. The experiment is optimized for well-defined boundary conditions and optical accessibility in the zone where the flame impinges at the wall. Laser based diagnostic methods are used to measure two components of the velocity field by particle image velocimetry simultaneously with the flame front position using laser induced fluorescence of the OH molecule. Two measurement planes are selected that are aligned perpendicularly to the surface of the wall. Based on this data, the flow field near the wall is analyzed by different methodologies using laboratory-fixed and flame-conditioned statistics, a quadrant splitting analysis of the Reynolds stresses and an evaluation of the production term of the turbulent kinetic energy. The results of chemically reactive cases are compared to their corresponding non-reactive flows for otherwise identical inflow conditions. In the zone of flame-wall interactions the boundary layer structure and its turbulence are dominated by the turbulent flame. Important features are that the flame compresses the boundary layer already upstream the location where the flame is finally quenched and that ejection and sweeps are no longer the dominant mechanisms as in non-reactive boundary layers. This experimental data may serve additionally as a database for model development for near wall reactive flows.     

Effect of finite cavity width on flow oscillation in a low-Mach-number cavity flow

The current study is focused on examining the effect of the cavity width and side walls on the self-sustained oscillation in a low Mach number cavity flow with a turbulent boundary layer at separation. An axisymmetric cavity geometry is employed in order to provide a reference condition that is free from any side-wall influence, which is not possible to obtain with a rectangular cavity. The cavity could then be partially filled to form finite-width geometry. The unsteady surface pressure is measured using microphone arrays that are deployed on the cavity floor along the streamwise direction and on the downstream wall along the azimuthal direction. In addition, velocity measurements using two-component Laser Doppler Anemometer are performed simultaneously with the array measurements in different azimuthal planes. The compiled data sets are used to investigate the evolution of the coherent structures generating the pressure oscillation in the cavity using linear stochastic estimation of the velocity field based on the wall-pressure signature on the cavity end wall. The results lead to the discovery of pronounced harmonic pressure oscillations near the cavity’s side walls. These oscillations, which are absent in the axisymmetric cavity, are linked to the establishment of a secondary mean streamwise circulating flow pattern near the side walls and the interaction of this secondary flow with the shear layer above the cavity.     

FLOW STRUCTURE IN THE TURBULENT BOUNDARY LAYER OVER DIRECTIONAL RIBLETS SURFACES

本文采用时间解析的二维粒子图像测速技术,对零压力梯度光滑以及汇聚和发散沟槽表面平板湍流边界层统计特性和流动结构进行了研究.结果表明在垂直于汇聚和发散沟槽表面的对称平面内,相对于光滑壁面,发散沟槽壁面使当地边界层厚度、壁面摩擦阻力、湍流脉动、雷诺应力等明显减小;而汇聚沟槽壁面对湍流边界层特性和流动结构的影响正好相反,汇聚沟槽使壁面流体有远离壁面向上运动的趋势,因而导致边界层厚度增加了约43%;同时,在汇聚沟槽表面情况下流向大尺度相干结构更容易形成,这对减阻是不利的.此外,顺向涡数量在湍流边界层的对数区均存在一个极大值,发散沟槽表面所对应的极大值位置更靠近沟槽壁面,而在汇聚沟槽表面则有远离壁面的趋势,由顺向涡诱导产生的较强的喷射和扫掠运动会在湍流边界层中产生较强的剪切作用,顺向涡数量的减少是发散沟槽壁面当地摩擦阻力降低的主要原因.     

Temporal development of turbulent boundary layers with embedded streamwise vortices

The interaction of streamwise vortices with turbulent boundary layer has been investigated using large-eddy simulation. The initial conditions are a pair of counterrotating Oseen vortices with flow between them directed toward the wall (common-flow-down), superimposed on various instantaneous realizations of a turbulent boundary layer. The time development of the vortices and their interaction with the boundary layer are studied by integrating the filtered Navier-Stokes equations in time. The most important effects of the vortices on the boundary layer are the thinning of the boundary layer between vortices (downwash region) and the thickening of the boundary layer in the upwash region. The vortices first move toward the wall as a result of the self-induced velocity, and then apart from each other because of the image vortices due to the solid wall. The Reynolds stress profiles highlight the highly three-dimensional structure of the turbulent boundary layer modified by the vortices. The presence of significant turbulent activity near the vortex center and in the upwash region suggests that localized instability mechanisms in addition to the convection of turbulent energy by the secondary flow are responsible for this effect. High levels of turbulent kinetic energy and secondary stresses in the vicinity of the vortex center are also observed. The numerical results show good agreement with experimental results.This work was supported by the Office of Naval Research under Grant N00014-89-J-1638. Computer time was supplied by the San Diego Supercomputing Center.     

单一倾斜热线探头在三维湍流场中的应用

研究的目的是确立用单一倾斜热线探头进行三维湍流测试的基础技术．在阐明基本原理，基础方程式的基础上，对比实验结果，证明应用所阐明的方法、用单一倾斜热线探头进行三维湍流测试是切实可行的．对边界层壁面附近的测试结果表明考虑平均速度和变动速度的高次相关项后可以得到更加精确的实验结果．     

Investigation of the flow in a circular cavity using stereo and tomographic particle image velocimetry

The turbulent flow over a circular cavity with an aspect ratio of D/H = 2 is investigated by multi-planar stereoscopic particle image velocimetry and with tomographic particle image velocimetry (PIV). The main aim of the study is the flow topology and the turbulent structure of the asymmetrical flow pattern that forms inside the cavity at these specific conditions. The flow field is measured in the vertical symmetry plane to describe the overall recirculation pattern in the cavity and the turbulent shear layer developing from the separation point. In this specific regime the shear layer fluctuations are recognized as those caused by instabilities together with the effect of the incoming boundary layer turbulence. Additional observations performed at several wall-parallel planes at different height inside the cavity allow to further evaluate the secondary flow circulation generated by this asymmetric regime. The observed flow pattern consists of a steady vortex, occupying the entire cavity volume and placed diagonally inside the cavity such to entrain the external flow from one side, capture it into a circulatory motion and eject it from the opposite side of the cavity. The spatial distribution of the turbulent fluctuations also reveals the same structure. The tomographic PIV measurement returns a visual inspection to the instantaneous three-dimensional structure of the turbulent fluctuations, which at the investigated height exhibit a low level of coherence with slightly elongated vortices in the recirculating flow inside the cavity.