Friction Drag Variation via Spanwise Transversal Surface Waves

The introduction of spanwise velocity is a promising technique to effect the near-wall turbulent flow field to influence friction drag. However, the essential physical mechanism which significantly reduces friction drag has not been understood, yet. It is the objective of this numerical study to improve the fundamental knowledge on the drag reduction mechanism. The investigation is based on spanwise traveling transversal surface waves which are applied to modify the near-wall flow field and to influence friction drag. Two actuation configurations are analyzed in detail. Compared with an unactuated flat plate boundary layer simulation the first wave setup, which represents a low frequency wave at an amplitude larger than the viscous sublayer, leads to a reduced wall-shear stress resulting in friction drag reduction of up to 9%. The second wave setup, which possesses a higher frequency and an amplitude in the range of the viscous sublayer, yields an increase of friction drag of about 8%. Unlike previous investigations which focus on excitation setups to lower friction drag, the comparison of the two wave setups in this study allows to identify the effects which on the one hand, lead to drag reduction and on the other hand, result in drag increase. That is, due to the pronounced differences the major effects determining the friction distribution are more evident. The two key features for drag reduction are the damping of the wall-normal vorticity fluctuations above the entire surface and the decrease of turbulence production. Furthermore, the effect of rearranging streamwise vorticity, which has been stated to be responsible for drag reduction, is found to occur at increasing and decreasing drag, i.e., it is not the effect that lowers the friction drag.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.     

Drag reduction of turbulent channel flows over an anisotropic porous wall with reduced spanwise permeability

The direct numerical simulation(DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.     

上海陆域古河道分布及对工程建设影响研究

采用直接数值模拟方法, 对槽道湍流中展向振动流向传播的波动壁面的流动 控制和减阻问题进行了研究, 讨论了流向参数k_{x}对Stokes层、湍流拟序结构、湍流猝 发事件以及壁面阻力的影响, 并对此类波动壁面的湍流控制和减阻机理进行了讨论. 结果表 明, 当此类波动壁面被用来调制近壁流动时, 仅低频波对湍流流场具有显著影响, 可导致湍 流猝发事件的频率和强度的显著变化; 波数k_{x}的增大对于湍流猝发事件的频率和强度增 减的影响并不同步, 存在一个最优的波数k_{x}, 在其调制下, 固有流场对诱导流场的影响 最弱, 而诱导流场对固有流场的影响显著, 减阻效果最好.     

The effect of streamwise vortices on the turbulence structure of a separating boundary layer

A high Reynolds number flat plate turbulent boundary layer is investigated in a wind-tunnel experiment. The flow is subjected to an adverse pressure gradient which is strong enough to generate a weak separation bubble. This experimental study attempts to shed some new light on separation control by means of streamwise vortices with emphasize on the change in the boundary layer turbulence structure. In the present case, counter-rotating and initially non-equidistant streamwise vortices become and remain equidistant and confined within the boundary layer, contradictory to the prediction by inviscid theory. The viscous diffusion cause the vortices to grow, the swirling velocity component to decrease and the boundary layer to develop towards a two-dimensional state. At the position of the eliminated separation bubble the following changes in the turbulence structure were observed. The anisotropy state in the near-wall region is unchanged, which indicates that it is determined by the presence of the wall rather than the large scale vortices. However, the turbulence in the outer part of the boundary layer becomes overall more isotropic due to an increased wall-normal mixing and a significantly decreased production of streamwise fluctuations. The turbulent kinetic energy is decreased as a consequence of the latter. Despite the complete change in mean flow, the spatial turbulence structure and the anisotropy state, the process of transfer of turbulent kinetic energy to the spanwise fluctuating component seems to be unchanged. Local regions of anisotropy are strongly connected to maxima in the turbulent production. For example, at spanwise positions in between those of symmetry, the spanwise gradient of the streamwise velocity cause significant production of turbulent fluctuations. Transport of turbulence in the spanwise direction occurs in the same direction as the rotation of the vortices.     

Numerical investigation of turbulent channel flow controlled by spatially oscillating spanwise Lorentz force

A formulation of the skin-friction drag related to the Reynolds shear stress in a turbulent channel flow is derived. A direct numerical simulation (DNS) of the turbulent control is performed by imposing the spatially oscillating spanwise Lorentz force. Under the action of the Lorentz force with several proper control parameters, only the periodically well-organized streamwise vortices are finally observed in the near-wall region. The Reynolds shear stress decreases dramatically, especially in the near-wall area, resulting in a drag reduction.     

壁面展向周期振动的槽道湍流减阻机理的研究

利用直接数值模拟研究了带有壁面展向周期振动的槽道湍流.壁面在展向的周期运动使湍流受到抑制,并使壁面摩擦阻力减小.通过对雷诺应力输运方程的分析研究了壁面展向周期振动的减阻机理,进一步揭示了压力变形项在湍流抑制中的关键作用.     

Bypass transition receptivity modes

The prediction of bypass transition remains an important problem in many engineering applications. This is largely because there is no suitable theoretical model for bypass transition and predictions are made using empirical models. This paper presents numerical results for the receptivity of a zero pressure gradient boundary layer subjected to simple freestream waveforms which are the constituent parts of a turbulent flow field. Significant receptivities are only obtained for a minority of freestream waveforms and these lead to two types of flow structure in the boundary layer. The first type of flow structure is essentially two dimensional in nature and consists of two rows of counter-rotating spanwise vortices and is induced by freestream waves of large normal and spanwise wavelength and streamwise wavelengths approximately equal to the boundary layer thickness. The second type of flow structure are the streamwise streaks frequently observed in flow visualisation experiments. These streaks are induced by freestream waves of long streamwise and normal wavelength and spanwise wavelengths in the range of 14.5-46 θ (1.7-5.4δ). The freestream waves can be formed of velocity components in any direction, however the boundary layer is most receptive to fluctuations that lie in a plane perpendicular to the streamwise direction. The overall receptivity to a full spectrum of waves typical of freestream turbulence is considered and is shown to have similar characteristics to those from experiments.     

Predicting Turbulent Spectra in Drag-reduced Flows

In the present work we describe how turbulent skin-friction drag reduction obtained through near-wall turbulence manipulation modifies the spectral content of turbulent fluctuations and Reynolds shear stress with focus on the largest scales. Direct Numerical Simulations (DNS) of turbulent channels up to Re _τ = 1000 are performed in which drag reduction is achieved either via artificially removing wall-normal turbulent fluctuations in the vicinity of the wall or via streamwise-travelling waves of spanwise wall velocity. This near-wall turbulence manipulation is shown to modify turbulent spectra in a broad range of scales throughout the whole channel. Above the buffer layer, the observed changes can be predicted, exploiting the vertical shift of the logarithmic portion of the mean streamwise velocity profile, which is a classic performance measure for wall roughness or drag-reducing riblets. A simple model is developed for predicting the large-scale contribution to turbulent fluctuation and Reynolds shear stress spectra in drag-reduced turbulent channels in which a flow control acts at the wall. Any drag-reducing control that successfully interacts with large scales should deviate from the predictions of the present model, making it a useful benchmark for assessing the capability of a control to affect large scales directly.     

Investigation of turbulence and skin friction modification in particle-laden channel flow using lattice Boltzmann method

Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation (DNS) with a lattice Boltzmann (LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow, the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction.     

On the friction drag reduction mechanism of streamwise wall fluctuations

Understanding how to decrease the friction drag exerted by a fluid on a solid surface is becoming increasingly important to address key societal challenges, such as decreasing the carbon footprint of transport. Well-established techniques are not yet available for friction drag reduction. Direct numerical simulation results obtained by Józsa et al. (2019) previously indicated that a passive compliant wall can decrease friction drag by sustaining the drag reduction mechanism of an active control strategy. The proposed compliant wall is driven by wall shear stress fluctuations and responds with streamwise wall velocity fluctuations. The present study aims to clarify the underlying physical mechanism enabling the drag reduction of these active and passive control techniques. Analysis of turbulence statistics and flow fields reveals that both compliant wall and active control amplify streamwise velocity streaks in the viscous sublayer. By doing so, these control methods counteract dominant spanwise vorticity fluctuations in the near-wall region. The lowered vorticity fluctuations lead to an overall weakening of vortical structures which then mitigates momentum transfer and results in lower friction drag. These results might underpin the further development and practical implementation of these control strategies.     

Turbulent drag reduction by spanwise wall oscillations

In the present work a technique is numerically investigated, which is aimed at reducing the friction drag in turbulent boundary layers and channel flows. A cyclic spanwise oscillation of the wall with a proper frequency and amplitude is imposed, allowing a reduction of the turbulent drag of up to 40%. The present work is based on the numerical simulation of the Navier-Stokes equations in the simple geometry of a plane channel flow. The frequency of the oscillations is kept fixed at the most efficient value determined in previous studies, while the choice of the best value for the amplitude of the oscillations is evaluated not only in terms of friction reduction, but also by taking into consideration the overall energy balance and the power spent for the motion of the wall. The analysis of turbulence statistics allows to shed some light on the way oscillations interact with wall turbulence, as illustrated by visual inspection of some instantaneous flow fields. Finally, a simple explanation is proposed for this interaction, which leads to a rough estimate of the most efficient value for the frequency of the oscillations.     

Transport and deposition of neutral particles in magnetohydrodynamic turbulent channel flows at low magnetic Reynolds numbers

The effect of Lorentz force on particle transport and deposition is studied by using direct numerical simulation of turbulent channel flow of electrically conducting fluids combined with discrete particle simulation of the trajectories of uncharged, spherical particles. The magnetohydrodynamic equations for fluid flows at low magnetic Reynolds numbers are adopted. The particle motion is determined by the drag, added mass, and pressure gradient forces. Results are obtained for flows with particle ensembles of various densities and diameters in the presence of streamwise, wall-normal or spanwise magnetic fields. It is found that the particle dispersion in the wall-normal and spanwise directions is decreased due to the changes of the underlying fluid turbulence by the Lorentz force, while it is increased in the streamwise direction. The particle accumulation in the near-wall region is diminished in the magnetohydrodynamic flows. In addition, the tendency of small inertia particles to concentrate preferentially in the low-speed streaks near the walls is strengthened with increasing Hartmann number. The particle transport by turbophoretic drift and turbulent diffusion is damped by the magnetic field and, consequently, particle deposition is reduced.     

双振子同异步振动主动控制湍流边界层减阻实验研究

本文以镶嵌在平板上沿展向对放的两个压电陶瓷振子为主动控制激励器,自主设计了零质量射流主动控制湍流边界层减阻实验方案.在风洞中开展了双压电振子同步和异步振动主动控制湍流边界层减阻的实验研究,实现了压电振子的周期扰动对湍流边界层多尺度相干结构的干扰和调制,施加控制后减小了壁面摩擦阻力,获得减阻效果.当异步控制100 V, 160 Hz工况时得到最大减阻率为18.54%.小波多尺度分析结果表明,施加控制工况中PZT振子的周期性扰动使得小尺度结构的湍流脉动强度增强,改变了近壁区大尺度和小尺度结构的含能分布,且异步控制工况比同步控制工况的减阻效果好.当双振子振动频率为160 Hz时,流向脉动速度的小波系数PDF曲线呈现出波动特征,尾部变宽显著,近壁湍流脉动更加有序和规则,湍流间歇性减弱.对小尺度脉动进行条件相位平均的结果表明,施加PZT周期扰动后使得大尺度结构破碎成为小尺度结构,小尺度脉动强度增强,实现减阻.随着流向位置离PZT振子越来越远,周期性扰动对相干结构的调制作用逐渐减弱.     

防风网透流风空气动力学特性大涡数值模拟研究

基于有限体积法建立不可压缩粘性流体运动的大涡模拟模型,采用Smagorinsky-Lilly亚格子模型,并引入浸入边界法(IBM)实现无滑移固壁边界条件,对雷诺数30~30000之间防风网透流风进行模拟研究。基于模拟结果,提出蝶型防风网透流风存在4个典型分区结构,流场中存在由蝶型形态引起的大尺度分层剪切流动,加强流体动能耗散。透流风在雷诺数300时发生层流至湍流的转捩,而在雷诺数增长至3000以上时,湍流充分发展,纵向流速脉动强度可达70%。防风网整体空气阻力远大于单个孔口射流阻力的线性叠加,射流间的相互作用以及大尺度的分层剪切结构大大增加流体阻力损失,这为通过优化孔口布置和网板形态来节省材料提供了科学依据。     

Turbulence control in wall-bounded flows by spanwise oscillations

The feasibility of control of wall turbulence by high frequency spanwise oscillations is investigated by direct numerical simulations of a planar turbulent channel flow subjected either to an oscillatory spanwise crossflow or to the spanwise oscillatory motion of one of the channel walls. Periods of oscillation,T _osc. ⁺ =T _osc. u ² /v, ranging from 25 to 500 were studied. For 25<T _osc. ⁺ <200 production of turbulence is suppressed. The most effective suppression of turbulence occurs atT _osc ⁺ =100, for which the overall turbulence production is reduced by 62% compared to the unperturbed channel and sustained turbulent drag reductions of 40% are obtained. The suppression of turbulence is due to a continual shift of the near wall streamwise vortices relative to the wall layer streaks, which in turn leads to a widening, merging and weakening of the wall layer streaks and an overall reduction in the turbulence production. The turbulence suppression mechanism observed in these studies opens up new possibilities for effective control of turbulence in wall-bounded flows.     

Direct numerical simulation of turbulent heat transfer over fully resolved anisotropic porous structures

Lattice Boltzmann direct numerical simulations of turbulent heat transfer over and inside anisotropic porous media are performed. This study considers turbulent plane channel flows whose bottom walls are made from the porous media at the bulk Reynolds number of 2900 with isothermal and conjugate heat transfer wall conditions. Four different porous walls are considered. They are walls with only the wall-normal permeability, with the wall-normal and spanwise permeabilities, with the wall-normal and streamwise permeabilities, and with the isotropic wall-normal, spanwise and streamwise permeabilities. The porosity of the porous walls ranges from 0.6 to 0.8. Discussions on the effects of the anisotropic permeability on turbulent thermal fields are carried out by the instantaneous flow visualizations and the statistical quantities. In particular, temperature fluctuations, turbulent and dispersion heat fluxes are examined both inside and outside the porous walls. Finally, the heat transfer performance is discussed considering the effects of the anisotropic permeability.     

DNS of heat transfer in transitional, accelerated boundary layer flow over a flat plate affected by free-stream fluctuations

Direct numerical simulations (DNS) of flow over and heat transfer from a flat plate affected by free-stream fluctuations were performed. A contoured upper wall was employed to generate a favourable streamwise pressure gradient along a large portion of the flat plate. The free-stream fluctuations originated from a separate LES of isotropic turbulence in a box. In the laminar portions of the accelerating boundary layer flow the formation of streaks was observed to induce an increase in heat transfer by the exchange of hot fluid near the surface of the plate and cold fluid from the free-stream. In the regions where the streamwise pressure gradient was only mildly favourable, intermittent turbulent spots were detected which relaminarised downstream as the streamwise pressure gradient became stronger. The relaminarisation of the turbulent spots was reflected by a slight decrease in the friction coefficient, which converged to its laminar value in the region where the streamwise pressure gradient was strongest.     

高频吹气扰动影响近壁区拟序结构统计特性的实验研究

利用恒温热线风速仪测量了零压力梯度平板上施加由合成射流激发的狭缝周期吹气扰动前后不同流向位置湍流边界层的速度信号, 展开高频吹气扰动影响近壁区湍流结构的统计特性研究. 研究结果表明:高频周期吹气扰动在狭缝下游产生明显的减阻效果. 扰动强度在湍流边界层内的发展沿流向呈衰减趋势, 其与湍流结构的相互作用也相应衰减. 然而, 因高频扰动产生运动的展向涡结构与猝发引起的结构变化尺度相当, 直接影响了近壁区拟序结构产生与发展的统计, 从而使得猝发检测方法VITA 表现出与低频或定常吹气减阻机理相异的现象.      

On the origin of drag increase in varying-phase opposition control

This study investigates the physics underlying the drag increase in a low Reynolds number turbulent channel flow due to varying-phase opposition control by means of direct numerical simulation and modal analysis. The drag increase occurs for an extended region of the parameter space and we consider a controller with a positive phase shift in Fourier domain between sensor measurement and actuator response as a representative example for this regime. Analyses of instantaneous flow fields as well as spatial power spectra show that the structure of drag-increased flows is remarkably different from that of drag-reduced and canonical flows. In particular, the near-wall region is dominated by structures of short streamwise and large spanwise extent. Isolation of a representative control scale shows that these energetic structures can be characterized as spanwise rollers, which induce strong ejection and sweep motions and lead to drag increase. The presence of rollers, and therefore drag increase, in the full nonlinear system correlates well with the presence of an amplified eigenvalue in the eigenspectrum of the linearized Navier–Stokes operator. It is further shown that the scales responsible for drag increase at positive phase shifts are inactive at negative phase shifts and do not contribute to drag reduction. These scales can therefore be excluded from a controller aimed at drag reduction, which relaxes the spatial resolution requirements on the control hardware. The eigenspectrum may be used as a computationally cheap tool to identify such detrimental scales during an early design stage.