Skin-friction drag reduction in turbulent channel flow based on streamwise shear control

It is known that stretching and intensification of a hairpin vortex by mean shear play an important role to create a hairpin vortex packet, which generates the large Reynolds shear stress associated with skin-friction drag in wall-bounded turbulent flows. In order to suppress the mean shear at the wall for high efficient drag reduction (DR), in the present study, we explore an active flow control concept using streamwise shear control (SSC) at the wall. The longitudinal control surface is periodically spanwise-arranged with no-control surface while varying the structural spacing, and an amplitude parameter for imposing the strength of the actuating streamwise velocity at the wall is introduced to further enhance the skin-friction DR. Significant DR is observed with an increase in the two parameters with an accompanying reduction of the Reynolds stresses and vorticity fluctuations, although a further increase in the parameters amplifies the turbulence activity in the near-wall region. In order to study the direct relationship between turbulent vortical structures and DR under the SSC, temporal evolution with initial eddies extracted by conditional averages for Reynolds-stress-maximizing Q2 events are examined. It is shown that the generation of new vortices is dramatically inhibited with an increase in the parameters throughout the flow, causing fewer vortices to be generated under the control. However, when the structural spacing is sufficiently large, the generation of new vortex is not suppressed over the no-control surface in the near-wall region, resulting in an increase of the second- and fourth-quadrant Reynolds shear stresses. Although strong actuating velocity intensifies the near-wall turbulence, the increase in the turbulence activity is attributed to the generation of counter-clockwise near-wall vortices by the increased vortex transport.     

Anisotropy measurements in the boundary layer over a flat plate with suction

Laser Doppler velocity measurements are carried out in a turbulent boundary layer subjected to concentrated wall suction (through a porous strip). The measurements are taken over a longitudinal distance of 9× the incoming boundary layer thickness ahead of the suction strip. The mean and rms velocity profiles are affected substantially by suction. Two-point measurements show that the streamwise and wall-normal autocorrelations of the streamwise velocity are reduced by suction. It is found that suction alters the redistribution of the turbulent kinetic energy k between its components. Relative to the no-suction case, the longitudinal Reynolds stress contributes more to k than the other two normal Reynolds stresses; in the outer region, its contribution is reduced which suggests structural changes in the boundary layer. This is observed in the anisotropy of the Reynolds stresses, which depart from the non-disturbed boundary layer. With suction, the anisotropy level in the near-wall region appears to be stronger than that of the undisturbed layer. It is argued that the mean shear induced by suction on the flow is responsible for the alteration of the anisotropy. The variation of the anisotropy of the layer will make the development of a turbulence model quite difficult for the flow behind suction. In that respect, a turbulence model will need to reproduce well the effects of suction on the boundary layer, if the model is to capture the effect of suction on the anisotropy of the Reynolds stresses.     

Investigation of particle-laden turbulent pipe flow at high-Reynolds-number using particle image/tracking velocimetry (PIV/PTV)

An experimental investigation of a high Reynolds number flow (Re = 320 000) of a dilute liquid-solid mixture (<1% by volume) was conducted. The turbulent motion of both the liquid phase (water) and particles (0.5, 1, and 2 mm glass beads) was evaluated in an upward pipe flow using a particle image/tracking velocimetry (PIV/PTV) technique. Results show that the Eulerian mean axial velocity of the glass beads is lower than that of the liquid phase in the central region but higher in the near-wall region. Moreover, the presence of the coarse particles has a negligible effect on the turbulence intensity of the liquid phase. Particles show higher streamwise and radial fluctuations than the liquid-phase at the tested conditions. The profiles of particle concentration across the pipe radius show almost constant concentration in the core of the pipe with a decrease towards the near wall region for 0.5 and 1 mm particles. For the 2 mm particles, a nearly linear concentration gradient from centre to the pipe wall is observed. The results presented here provide new information concerning the effect of a dispersed particulate phase on the turbulence modulation of the liquid carrier phase, especially at high Reynolds numbers. The present study also demonstrates how correlations developed to determine if particles cause turbulence attenuation/augmentation are not applicable for solid-liquid flows at high Reynolds numbers. Finally, the importance of particle-fluid slip velocity on fluid phase turbulence modulation is illustrated.     

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.     

Characteristic turbulent structure of a modified drag-reduced surfactant solution flow via dosing water from channel wall

The modification of the near-wall structure is very important for the control of wall turbulence. To ascertain the effect of near-wall modulation on the viscoelastic drag-reduced flow, the modified characteristics of a surfactant solution channel flow were investigated experimentally. The modulation was conducted on the boundary of the channel flow by injecting water from the whole surface of one side of the channel wall. The diffusion process of the injected water was observed by using the planar laser-induced fluorescence technique. The velocity statistics and characteristic structure including the spatial distributions of instantaneous streamwise velocity, swirling strength, and Reynolds shear stress were analyzed based on the velocity vectors acquired in the streamwise wall-normal plane by using the particle imaging velocimetry technique. The results indicated that the disturbance of the injected water was constricted within a finite range very near the dosing wall, and the Reynolds shear stress was increased in this region. However, the eventual drag reduction rate was found to be increased due to a relatively large decrement of viscoelastic shear stress in this near-wall region. Moreover, the flow structure under this modulation presented obvious regional characteristics. In the unstable disturbed region, the mixing of high-speed and low-speed fluids and the motions of ejection and sweep occurred actively. Many clockwise vortex cores were also found to be generated. This characteristic structure was similar to that in the ordinary turbulence of Newtonian fluid. Nevertheless, outside this disturbed region, the structure still maintained the characteristics of the drag-reduced flow with non-Newtonian viscoelastic additives. These results proved that the injected Newtonian fluid associated with the modified stress distribution creates a diverse characteristic structure and subsequent enhanced drag reduction. This investigation can provide the experimental basis for further study of turbulence control.     

Velocity statistics along the stagnation line of an axi- symmetric stagnating turbulent flow

 Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally. A low turbulence, uniform air flow from a nozzle type air supply with an exit diameter of 50 mm stagnates at a wall located 50 mm downstream. A flow velocity is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile in the streamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low near the wall. With the turbulence generator installed, a significant turbulence structure appears near the wall. When the wall is approached, the rms value of velocity fluctuations in the streamwise direction decreases monotonically while the profile of the rms value in the radial direction reaches a maximum near the wall. The increase in the rms value of velocity fluctuations in the radial direction near the wall is attributed to the bi-modal histogram of the fluctuating velocity in the radial direction. Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation point reaches a maximum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram. Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, can be normalized successfully by similarity parameters based on the strain rate and the reference turbulent kinetic energy introduced by Champion and Libby. Received: 7 April 1995/Accepted: 27 September 1996     

The diagnostic plot — a litmus test for wall bounded turbulence data

A diagnostic plot is suggested that can be used to judge wall bounded turbulence data of the mean and the rms of the streamwise velocity in terms of reliability both near the wall, around the maximum in the rms as well as in the outer region. The important feature of the diagnostic plot is that neither the wall position nor the friction velocity needs to be known, since it shows the rms value as a function of the streamwise mean velocity, both normalized with the free stream velocity. One must remember, however, that passing the test is a necessary, but not sufficient condition to prove good data quality.     

The structure of Reynolds stress in the near-wall region of a fully developed turbulent pipe flow

The structure of the Reynolds stress in the near-wall region of a fully developed turbulent pipe flow, at a pipe Reynolds number of 8,923, was investigated. Because the closed circuit tunnel used glycerine as a working fluid, measurements could be readily made inside the viscous sublayer. Two laser Doppler velocimeter (LDV) systems were combined to measure the two point spatial correlation, R ₁₂, between the stream wise and radial velocities in a radial plane of the pipe. The correlation measurements extended over the region from y ⁺ of 2 to 64 in the direction normal to the pipe wall and covered more than 800 wall units in the streamwise direction. Two-dimensional maps of the correlation coefficient were established for six different distances of the streamwise velocity probe from the wall. The use of LDV systems allowed the measurements to be made for small spatial separations of the probes without fear of probe interference effects. A characteristic feature of the correlation contour maps, that maxium correlation arises for small non-zero separation of the probes, may not have been observed had invasive techniques been employed.     

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.     

Direct numerical simulation of a particle-laden flow in a flat plate boundary layer

In this paper, a direct numerical simulation of particle-laden flow in a flat plate boundary layer is performed, using the Eulerian–Lagrangian point-particle approach. This is, as far as we know, the first simulation of a particle-laden spatially-developing turbulent boundary layer with two-way coupling. A local minimum of the particle number density is observed in the close vicinity of the wall. The present simulation results indicate that the inertial particles displace the quasi-streamwise vortices towards the wall, which, in turn, enhance the mean streamwise fluid velocity. As a result, the skin-friction coefficient is increased whereas the boundary layer integral thicknesses are reduced. The presence of particles augments the streamwise fluctuating velocity in the near-wall region but attenuates it in the outer layer. Nevertheless, the wall-normal and spanwise velocity fluctuations are significantly damped, and so is the Reynolds stress. In addition, the combined effect of a reduced energy production and an increased viscous dissipation leads to the attenuation of the turbulent kinetic energy.     

充分发展圆管湍流的实验研究

采用粒子数字图像测速（digital particle image velocimetry,DPIV)和定量流动显示技术（quantitative flow visualization,QFA)对充分发展的圆管湍流进行了研究。测量结果和直接数值模拟（direct numerical simulation,DNS)结果进行了比较，结果表明作者开发的DPIV技术取得了满意的精度。在此基础上对圆管湍流的动力学机理进行了研究，分析了上抛和下扫在湍流生成中的贡献以及流动显示结构内的脉动速度分布，测量结果显示在圆管湍流的近壁区存在横向强脉冲现象和流动显示所能观察到的结构为上抛占主导地位的结构。     

Experimental Investigation on Turbulent Structure of Drag Reducing Channel Flow with Blowing Polymer Solution from the Wall

Turbulent drag reducing flow with blowing polymer solution from the channel wall was investigated experimentally using particle image velocimetry (PIV). Experiments were carried out with varying conditions of blowing polymer solution (e.g. weight concentration of polymer solution). Reynolds number based on the channel height and mean velocity was set to 20000 and 40000. When the polymer solution was blown from the channel wall, streamwise velocity fluctuation little increased, but wall-normal velocity fluctuation, Reynolds shear stress and correlation coefficient decreased significantly only near the blower wall. This behavior corresponds to the decrease of the ejection and sweep in the near-wall region observed by the investigation of instantaneous velocity map. On the contrary, this characteristic behavior was not observed at a position away from the blower wall (y/(H/2) > 0.4) and the scatter plot was almost the same as that of the water flow in this region. These results suggest that there are two regions in the drag reducing flow with blowing polymer solution from the wall; one is a non-Newtonian region which exists near the blower wall, and the other is a Newtonian region at a distance from the wall. The non-Newtonian region plays a key role in the drag reduction by the blowing polymer solution.     

Relaminarization by Steady Modification of the Streamwise Velocity Profile in a Pipe

We show that a rather simple, steady modification of the streamwise velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarizes. Two different devices, a stationary obstacle (inset) and a device which injects fluid through an annular gap close to the wall, are used to control the flow. Both devices modify the streamwise velocity profile such that the flow in the center of the pipe is decelerated and the flow in the near wall region is accelerated. We present measurements with stereoscopic particle image velocimetry to investigate and capture the development of the relaminarizing flow downstream these devices and the specific circumstances responsible for relaminarization. We find total relaminarization up to Reynolds numbers of 6000, where the skin friction in the far downstream distance is reduced by a factor of 3.4 due to relaminarization. In a smooth straight pipe the flow remains completely laminar downstream of the control. Furthermore, we show that transient (temporary) relaminarization in a spatially confined region right downstream the devices occurs also at much higher Reynolds numbers, accompanied by a significant local skin friction drag reduction. The underlying physical mechanism of relaminarization is attributed to a weakening of the near-wall turbulence production cycle.     

PIV measurements of turbulent flows in a 61-pin wire-wrapped hexagonal fuel bundle

The current work experimentally investigates the flow characteristics in the near-wall region of the 61-pin wire-wrapped hexagon fuel bundle via the matched-index-of-refraction technique. Particle image velocimetry (PIV) measurements were taken in the region near the surfaces of the pins, wires and enclosure wall at the Reynolds number of 19,000. From the obtained PIV velocity vector fields, flow statistics such as mean velocity and root-mean-square fluctuating velocity profiles were computed. In addition, spatial-temporal cross-correlations of velocity-velocity and pressure-velocity were analyzed. A strong correlation between the wall fluctuating pressure signal and flow structures was observed. Finally, we applied the POD analysis to the vorticity snapshots obtained in the near-wall region to reveal the dominant flow structures. It was found that the large-scale structures were elongated and aligned with the mean flow direction.     

The influence of temperature on the measurements of Reynolds stresses in shear free turbulence near a wall

 Temperature changes have a significant influence on the measurements of Reynolds stresses in turbulent boundary layers. As compared to the spanwise velocity fluctuations the streamwise turbulence intensity is especially sensitive to temperature deviations. Although this is a general statement its importance is clearly elucidated in a shear-free turbulence near a solid wall, since the mixing due to turbulence production is minimized in this flow. A consequence of temperature influence on hot-wire measurements is that frictional heating from the wall has produced contradictory results in different experiments on shear-free turbulence. In the current paper, measurements of streamwise and spanwise turbulence intensities have been conducted at different wall temperatures, thereby simulating the contradictory results mentioned above. A simple model has been developed showing that the turbulence intensities are affected by both the rms. value of the temperature fluctuations and the correlation between fluctuating temperature and velocity. These correlations are measured and the developed model is used to explain deviations in earlier measurements on shear-free turbulence. Moreover, the individual magnitudes of the two correlations in the temperature correction are estimated and their individual importance is discussed. Received: 17 February 1997 / Accepted: 18 January 1998     

An experimental study of a two-dimensional plane turbulent wall jet

 Laser-Doppler measurements were conducted in a plane turbulent wall jet at a Reynolds number based on inlet velocity, Re ₀, of 9600. The initial development as well as the fully developed flow was studied. Special attention was given to the near-wall region, including the use of small measuring volumes and the application of specific near-wall data corrections, so that wall shear stresses were determined directly from the mean velocity gradient at the wall using only data below y ⁺=4. It was possible to resolve the inner peak in the streamwise turbulence intensity as well as the inner (negative) peak in the shear stress. Limiting values of (u′)⁺ and uv ⁺ were determined. Turbulence data from the outer region of the flow were compared to earlier hot wire measurements and large differences in the normal turbulence intensity and the shear stress were found. These differences can be attributed to high turbulence intensity effects on the hot-wires. Received: 17 October 1996 / Accepted: 8 December 1997     

A bypass wake induced laminar/turbulent transition

The process of laminar to turbulent transition induced by a von Karman vortex street wake, was studied for the case of a flat plate boundary layer. The boundary layer developed under zero pressure gradient conditions. The vortex street was generated by a cylinder positioned in the free stream. An X-type hot-wire probe located in the boundary layer, measured the streamwise and normal to the wall velocity components. The measurements covered two areas; the region of transition onset and development and the region where the wake and the boundary layer merged producing a turbulent flow. The evolution of Reynolds stresses and rms-values of velocity fluctuations along the transition region are presented and discussed. From the profiles of the Reynolds stress and the mean velocity profile, a ‘negative' energy production region along the transition region, was identified. A quadrant splitting analysis was applied to the instantaneous Reynolds stress signals. The contributions of the elementary coherent structures to the total Reynolds stress were evaluated, for several x-positions of the near wall region. Distinct regions in the streamwise and normal to the wall directions were identified during the transition.     

Direct simulation of turbulent flow and heat transfer in a channel. Part I: Smooth walls

Interest in the use of supercomputers for the direct numerical calculation of turbulence prompts the development of efficient numerical techniques so that calculation at higher Reynolds numbers might be made. This paper presents an efficient pseudo-spectral technique, similar to but different from others that have recently appeared, for the calculation of momentum and heat transfer to a constant-property, turbulent fluid in a two-dimensional channel with walls at different, uniform temperature. The code uses no empiricism, although periodic boundary conditions are used for fluctuating quantities in the streamwise and spanwise directions. Calculations were made for a Prandtl number of 0·72 and Reynolds number based on friction velocity and channel half-height of 180 or 2800 based on channel half-height and average velocity. Calculations of mean velocity profile, turbulence intensities, skewness, flatness, Reynolds stress and eddy diffusivity of heat near a wall compare favourably with experimental results. Representative contour plots of the temperature field near the wall and of the spanwise and streamwise two-point velocity correlations are given. Deficiencies are that the calculation requires many hours on a fast computer with a large high-speed memory and that the grid size in each direction for appropriate resolution is approximately proportional to the square of the Reynolds number and to the Prandtl number raised to some power greater than one.     

Direct numerical simulation of particle-laden plane turbulent wall jet and the influence of Stokes number

The distribution and motion of inertial particles in plane turbulent wall jet are investigated using direct numerical simulation, under the assumption of one-way coupling. To our knowledge, this appears to be the first direct numerical simulation of a particle-laden plane turbulent wall jet. It is shown that, in outer part of the wall jet, the behaviour of particles closely resembles that of a free plane jet. Due to the streamwise decay of particle Stokes number, the particle streaks formed in the near wall region of the wall jet are characterized by their intensity variation, which differs significantly from those in the channel flow. The streamwise growth of the particle velocity half-width is approximately equal to that of the fluid velocity half-width and the maximum velocity of particles decays slower than that of fluid due to inertia. The outer scaling can collapse the mean particle velocity in both the inner and outer region for heavier particles. In the buffer region, the particle–fluid velocity difference can be negative or positive depending on the Stokes number since there are two competing effects, namely the memory effect and turbophoresis. In the viscous region, the larger particles are on average faster than fluid and the velocity difference is found to be self-similar depending on outer Stokes number. The near-wall distribution of velocity difference is significantly correlated with the presence of high-momentum particles which are entrained by vortical structures generated in the outer region of the wall jet. These results are useful for environmental and engineering applications.     

An experimental study on turbulence modification in the near-wall boundary layer of a dilute gas-particle channel flow

Turbulence modifications of a dilute gas-particle flow are experimentally investigated in the lower boundary layer of a horizontal channel by means of a simultaneous two-phase PIV measurement technique. The measurements are conducted in the near-wall region with y ⁺?<?250 at Re _τ (based on the wall friction velocity u _τ and half channel height h)?=?430. High spatial resolution and small interrogation window are used to minimize the PIV measurement uncertainty due to the velocity gradient near the wall. Polythene beads with the diameter of 60?μm (d _p ⁺ ?=?1.71, normalized by the fluid kinematic viscosity ν and u _τ) are used as dispersed phase, and three low mass loading ratios (Φ _m ) ranging from 10^?4 to 10^?3 are tested. It is found that the addition of the particles noticeably modifies the mean velocity and turbulent intensities of the gas-phase, as well as the turbulence coherent structures, even at Φ _m ?=?0.025?%. Particle inertia changes the viscous sublayer of the gas turbulence with a smaller thickness and a larger streamwise velocity gradient, which increases the peak value of the streamwise fluctuation velocity ( $ u_{\text{rms}}^{ + } $ ) of the gas-phase with its location shifting to the wall. Particle sedimentation increases the roughness of the bottom wall, which significantly increases the wall-normal fluctuation velocity ( $ v_{\text{rms}}^{ + } $ ) and Reynolds shear stress ( $ - \langle u^{ \prime } v^{\prime } \rangle^{ + } $ ) of the gas-phase in the inner region of the boundary layer (y ⁺?<?10). Under effect of particle–wall collision, the Q2 events (ejections) of the gas-phase are slightly increased by particles, while the Q4 events (sweeps) are obviously decreased. The spatial scale of the coherent structures near the wall shrinks remarkably with the presence of the particles, which may be attributed to the intensified crossing-trajectory effects due to particle saltation near the bottom wall. Meanwhile, the $ v_{\text{rms}}^{ + } $ and $ - \langle u^{ \prime } v^{\prime } \rangle^{ + } $ of the gas-phase are significantly reduced in the outer region of the boundary layer (y ⁺?>?20).