Modification of near-wall turbulence structure in a shear-driven three-dimensional turbulent boundary layer

 Most high Reynolds number flows of engineering interest are three-dimensional in nature. Key features of three-dimensional turbulent boundary layers (3DTBLs) include: non-colateral shear stress and strain rate vectors, and decreasing ratio of the shear stresses to the turbulent kinetic energy with increasing three-dimensionality. These are indicators that the skewing has a significant effect on the structure of turbulence. In order to further investigate the flow physics and turbulence structure of these complex flows, an innovative method for generating a planar shear-driven 3DTBL was developed. A specialized facility incorporating a relatively simple geometry and allowing for varying strengths of crossflow was constructed to facilitate studies where the skewing is decoupled from the confounding effects of streamwise pressure gradient and curvature. On-line planar particle image velocimetry (PIV) measurements and flow visualization results indicate that the experimental configuration generates the desired complex flow, which exhibits typical characteristics associated with 3DTBLs. Furthermore, spanwise shear results in modification of the near-wall turbulence structure. Analysis of near-wall flow visualization photographs revealed a reduction of mean streak length with increasing spanwise shear, while streak spacing remained relatively constant. In the most strongly sheared case, where the belt velocity is twice that of the freestream velocity, the mean streak length was reduced by approximately 50%. Received: 28 October 1997/Accepted: 4 February 1998     

Investigation of turbulence structures in a drag-reduced turbulent channel flow with surfactant additive by stereoscopic particle image velocimetry

In the present study, we employed stereoscopic particle image velocimetry (PIV) to investigate the characteristics of turbulence structures in a drag-reduced turbulent channel flow with addition of surfactant. The tested drag-reducing fluid was a CTAC/NaSal/Water (CTAC: cetyltrimethyl ammonium chloride; NaSal: sodium salicylate) system at 25°C. The weight concentration of CTAC was 30 ppm. Stereoscopic PIV measurement was performed for a water flow (Re=1.1×10⁴) and a CTAC solution flow (Re=1.5×10⁴ with 54% drag reduction) in both the streamwise–spanwise and wall-normal-spanwise planes, respectively. The three-dimensionality of hairpin vortex structures in the near-wall region for wall-bounded turbulent flow was reproduced by conditionally averaging the stereoscopic two-dimensional-three-component velocity fields. A series of wall-normal vortex cores were found to align with the near-wall low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased on average by about one order of magnitude in CTAC solution flow compared with water flow; the spanwise spacing between the near-wall low-speed streaks in the solution flow is increased by about 46%. The streamwise vorticity of the vortex cores appearing in the wall-normal-spanwise plane was also decreased by the use of drag-reducing surfactant additives.     

An experimental study into the effects of streamwise and spanwise acceleration in a turbulent boundary layer

We compare two turbulent boundary layers produced in a low-speed water channel experiment. Both are subjected to an identical streamwise pressure gradient generated via a lateral contraction of the channel, and an additional spanwise pressure gradient is imposed on one of the layers by curving the contraction walls. Despite a relatively high streamwise acceleration, hot-film probe measurements of the mean-velocity distributions show that the Reynolds number increases whilst the coefficient of friction decreases downstream. Visualization of the viscous layers using hydrogen bubbles reveal an increase in the non-dimensional streak spacing in response to the acceleration. Changes in statistical moments of the streamwise velocity near the wall suggest an increased dominance of high-velocity fluctuations. The near-wall streaks and velocity statistics have little sensitivity to the boundary layer three-dimensionality induced by the spanwise pressure gradient, with the boundary-layer crossflow velocity reaching 11 % that of the local freestream velocity.     

Three-dimensional pressure fluctuation fields in the vicinities of cantilevered cylindrical obstacles

The fields of turbulent near-wall pressure fluctuations in the vicinities of cylindrical bodies mounted orthogonal to the surface in a flow are experimentally investigated. The considerable inhomogeneity and three-dimensionality of the pressure fluctuation field in the measurement area is shown. The dependence of the main characteristics of the inhomogeneous pressure fluctuation field on geometric parameters is presented. A weak influence of the boundary layer thickness and the obstacle height, when greater than the cylinder diameter, is demonstrated.     

Turbulent Channel Flow with an Artificial Two-Dimensional Wall Layer

Turbulent flow of an incompressible fluid in a plane channel with parallel walls is considered. The three-dimensional time-dependent Navier-Stokes equations are solved numerically using the spectral finite-difference method. An artificial force which completely suppresses lateral oscillations of the velocity is introduced in the near-wall zone (10 % of the channel half-width in the neighborhood of each wall). Thus, the three-dimensional flow zone, in which turbulent oscillations can develop, is separated from the wall by a fluid layer. It is found that the elimination of three-dimensionality in the neighborhood of the walls leads to a significant reduction in the drag. However, complete laminarization does not occur. The flow in the stream core remains turbulent and can be interpreted as a turbulent flow in a channel with walls located on the boundary of the two-dimensional layer and traveling at the local mean-flow velocity. The oscillations developing inside the two-dimensional layer, which have significant amplitude, distort the flow only in the adjacent zone. Beyond this zone the distributions of the mean characteristics and the structure of instantaneous fields completely correspond to ordinary turbulent flow in a channel with rigid walls. The results obtained confirm the hypothesis of the unimportance of the no-slip boundary conditions for the fluctuating velocity component in the mechanism of onset and self-maintenance of turbulence in wall flows.     

An experimental study of the effects of three-dimensionality on the near wall turbulence structures using flow visualization

The effects of three-dimensionality on the turbulence producing near-wall structures over a rotating disk are examined using hydrogen bubble visualization. Laser Doppler anemometry measurements of the tangential and radial velocities indicate the flow to resemble that of an infinite disk. It was found that the crossflow acts to reduce vertical mixing during the turbulence ejection events, thus reducing the efficiency of the boundary layer in producing shear stress.This research has been sponsored by the Department of Energy Office of Basic Energy Sciences through Grants DE-FG03-86ER13608 and DE-FG03-93ER12309.     

MICRO-SCALE NUMERICAL STUDY OF THE EFFECT OF ERYTHROCYTE MECHANICAL PROPERTIES ON THE NEAR-WALL MOTION OF PLATELET

采用浸入式边界法,模拟了多个红细胞和血小板在毛细血管内流动过程中的相互影响。通过改变红细胞体积比和红细胞的力学特性,分析了红细胞力学特性对血小板在与内皮细胞发生粘附前的动力学行为的影响机理,包括：红细胞对血小板趋壁效应的影响,血小板在流动过程中的变形情况,并从血小板所受垂直壁向合力的角度深入研究了红细胞和血小板之间的相互作用。数值模拟的结果表明,增加血流中的红细胞体积比,减小了血小板和血管壁之间的距离,增大了血小板的变形,血小板所受垂直壁向合力呈现剧烈波动,两者之间的挤压显著加强;而增大红细胞硬度,使得血小板的离壁距离增大。     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE (III)──A STATISTICAL AND DYNAMICAL MODEL OF COHERENT STRUCTURE AND ITS HEAT TRANSFER MECHANISM

Following Tsai & Ma^[1] and Tsai & Liu^[2], a statistical and dynamical near-wall turbulent coherent structural model with separate consideration of two different portions: locally generated and upstream-transported large eddies has been established. With this model, heat transfer in a fully developed open channel in the absence of pressure gradient is numerically simulated. Database of fluctuations of velocity and temperature has also been set. Numerical analysis shows the existence of high-low temperature streak caused by near-wall coherent structure and its swing in the lateral direction. Numerical results are in accordance with the computations and experimental results of other researchers.     

A cellular scale numerical study of the effect of mechanical properties of erythrocytes on the near-wall motion of platelets

The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.     

Three-dimensional features of a Mach 2.1 shock/boundary layer interaction

2D particle image velocimetry was used to study the three-dimensionality of the shock-boundary layer interaction generated by a small 20° compression ramp in a low aspect ratio continuously operated wind tunnel. High-resolution data were taken in four streamwise-wallnormal planes: three planes located in the sidewall boundary layer and one near the tunnel centerline. The incoming boundary layer was found to show three-dimensionality, with significant overshoot in the velocity profiles observed near the sidewall. The size of the wedge influenced the interaction, which was weaker than that observed in the case of a large compression wedge. The flow turning angle was ≈8° near the tunnel centerline and changed significantly across the span. Measurements behind the compression wedge in the centerline plane showed that both velocity and turbulence properties were nearly fully recovered ≈14δ behind the compression corner. The shock angle varied with spanwise position, and a multi-shock structure was observed in the sidewall planes. The size of the interaction decreased in the sidewall boundary layer. Non-monotonic variations in both velocity and turbulence profiles across the sidewall planes suggest the presence of significant spanwise flows, possibly corner vortices.     

The turbulence structure of drag-reduced boundary layer flow

The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel to the wall (x–z plane), for a total of 30 planes across the thickness of the boundary layer. For increasing drag reduction, we found a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of near-wall vortical structures.     

Modes of vortex formation and transition to three-dimensionality in the wake of a freely vibrating cylinder

The present work is aimed to give some insight into the relation between vortex shedding modes and transition to three-dimensionality in the wake of a freely vibrating cylinder by establishing a numerical model and analyzing the relevant results of two- and three-dimensional simulations. The compressible flow past an elastically-mounted cylinder is solved by using the immersed boundary method (IB method). The cylinder is free to vibrate in the transverse direction with zero structure damping. The response of displacement amplitude is studied with the variation of reduced velocity. Whether P+S mode exists in three-dimensional flow and the occurrence of 2P mode is caused by flow transition from two-dimensional to three-dimensional are problems of concern. Both 2P and P+S wake modes are observed in two- and three-dimensional simulations. The numerical results indicate that the flow transition from two-dimensional to three-dimensional is coupled with the cylinder vibration in the synchronization/lock-in regime. The wake formation given by three-dimensional simulations suggests that the P+S mode might exist in reality when the flow is reverted to two-dimensional by vortex induced vibration (VIV) at Re=300–350. When Reynolds number increases to 425, the wake formation undergoes transition to three-dimensionality and 2P mode is observed. The effect of mass ratio on the flow transition to three-dimensionality is studied. The relationship between wake modes and aerodynamic forces is discussed.     

Experimental measurements and computational modeling of the flow field in an idealized human oropharynx

The velocity field in the central sagittal plane of an idealized representation of the human oropharynx (HOP) during steady inspiration, simulating oral inhalation through an inhaler mouthpiece, was measured experimentally using endoscopic particle image velocimetry (PIV). Measurements were made at three flow rates: 15, 30, and 90 L/min, which correspond to a wide range of physiological conditions. Extensive tests were performed to verify the veracity of the PIV data. The flow was also modeled computationally using Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) methods. The PIV data clearly indicate the complex nature of HOP flow, with three-dimensionality and several regions of separation and recirculation evident. Comparison of the experimental and computational results shows that, although the RANS CFD reproduces the basic features of the flow, it does not adequately capture the increased viscous effects at lower Reynolds numbers. The results demonstrate the need for more development and validation of CFD modeling, in particular RANS methods, in these flows.     

Possibility of drag reduction usingd-type roughness

An experiment was carried out in a low-speed wind tunnel to study the turbulence structure of the boundary layer over a two-dimensional square cavity on a flat plate. The main purpose of this investigation is to examine the way a square cavity modifies the near-wall structure of the turbulent boundary layer leading to a possible drag reduction overd-type roughness. The experimental results on pressure coefficient and friction coefficient indicated a small reduction in total drag in this configuration. This seems to be due to the stable vortex flow observed within the cavity which absorbs and reorganizes the incoming turbulence in the cavity, thereby modifying the near-wall turbulence structure of the boundary layer. The resultant turbulence structure was very similar to that over drag-reducing riblets surface.     

Possibility of drag reduction usingd-type roughness

An experiment was carried out in a low-speed wind tunnel to study the turbulence structure of the boundary layer over a two-dimensional square cavity on a flat plate. The main purpose of this investigation is to examine the way a square cavity modifies the near-wall structure of the turbulent boundary layer leading to a possible drag reduction overd-type roughness. The experimental results on pressure coefficient and friction coefficient indicated a small reduction in total drag in this configuration. This seems to be due to the stable vortex flow observed within the cavity which absorbs and reorganizes the incoming turbulence in the cavity, thereby modifying the near-wall turbulence structure of the boundary layer. The resultant turbulence structure was very similar to that over drag-reducing riblets surface.     

Influence of blowing on the anisotropy of the Reynolds stress tensor in a turbulent channel flow

The effects of localized wall blowing through a porous strip are investigated using hot-wire anemometry in a turbulent channel flow. Three blowing magnitudes are studied: σ=0.22, 0.36 and 0.58, where σ is the momentum flux gain ratio and that of the incoming channel flow at three different positions from the spanwise porous strip. The main emphasis of this work was the departure from isotropy of the turbulent flow with localized blowing. The anisotropic invariant map (AIM) for the Reynolds stress tensor revealed that blowing decreased the anisotropy of the turbulent structure in the near-wall region, and a decrease in the longitudinal integral length scale was observed when the blowing rate increased.     

Near-wall flow in a three-dimensional boundary layer on the endwall of a 30° bend

 Turbulence measurements are reported on the three-dimensional turbulent boundary layer along the centerline of the flat endwall in a 30° bend. Profiles of mean velocities and Reynolds stresses were obtained down to y ⁺≈2 for the mean flow and y ⁺≈8 for the turbulent stresses. Mean velocity data collapsed well on a simple law-of-the-wall based on the magnitude of the resultant velocity. The turbulence intensity and turbulent shear stress magnitude both increased with increased three-dimensionality. The ratio of these two quantities, the a ₁ structure parameter, decreased in the central regions of the boundary layer and showed profile similarity for y ⁺<50. The shear stress vector angle lagged behind the velocity gradient vector angle in the outer region of the boundary layer, however there was an indication that the shear stress vector tends to lead the velocity gradient vector close to the wall. Received: 16 July 1996/Accepted: 14 July 1997     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE (III)──A STATISTICAL AND DYNAMICALMODEL OF COHERENT STRUCTURE AND ITSHEAT TRANSFER MECHANISM

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Gravity Effects on Fiber Dynamics in Wall Turbulence

The present study investigated gravity effects on the dynamical behavior of inertial fibers suspended in a vertical channel flow. Direct numerical simulations were performed to obtain the turbulent flow field and the fibers were modelled as prolate spheroidal point particles. For each of the four fiber classes, three different gravity configurations were considered: upward flow with gravity opposing, downward flow with aiding gravity, and channel flow in absence of gravity. Results for the fiber distribution and the translational and rotational fiber motion were reported. In the near-wall region, the presence of gravity resulted in an increased fiber density in the downward flow but a nearly uniform distribution of fibers in upward flow. However, the preferential clustering of fibers in near-wall low-speed streaks was unaffected by gravity. The mean wall-normal or drift velocity of the fibers was higher in the downward flow and lower in the upward flow as compared to the case with no gravity. The suppressed drift velocity in the upward flow resulted in a more uniform fiber distribution throughout the channel in contrast to the near-wall accumulation of fibers in the two other cases. Overall gravity turned out to have negligible effects on some of the statistics of the least inertial fibers whereas the inclusion of gravity had a strong impact for heavier fibers.     

Quantitative visualization of the near-wall structures in a turbulent pipe flow by image correlation velocimetry

Laser-induced fluorescent dye visualization and image correlation velocimetry were employed to delineate near-wall turbulent structures in a pipe flow. The sweeping and ejection events near the wall and the downstream evolution of a large-scale eddy structure rotating in a counter-clockwise direction were clearly reflected in the instantaneous fluctuating velocity fields. This eddy structure was found to form mostly in the logarithmic region and to dominate the flow structures there, while the ejection and sweeping events in the log layer were greatly influenced by the existence of the large-scale eddy structure. Received: 29 January 2001 / Accepted: 22 October 2001