基于单个压电振子的湍流边界层主动控制

利用安装在壁面上的单个压电振子周期振荡,采用开环主动控制方案,实现了对平板湍流边界层相干结构猝发的主动控制和壁湍流减阻.根据不同的输入电压幅值和频率,完成了10种工况的实验.在压电振子下游2mm处,用热线风速仪和迷你热线单丝探针,精细测量湍流边界层不同法向位置瞬时流向速度信号的时间序列,分析了在Re_?=2183压电振子振动对湍流边界层平均速度剖面、减阻率和相干结构猝发过程的影响.实验结果表明,施加控制的工况使平均速度剖面对数律层上移,产生减阻效果;压电振子振幅越大,减阻率越高,减阻效果越明显;通过对施加控制前后流向瞬时速度的多尺度湍涡结构脉动动能的尺度分析,当压电振子振动频率与壁湍流能量最大尺度的猝发频率相近时,减阻率达到最大,为25%,说明控制壁湍流能量最大尺度相干结构的猝发是实现壁湍流减阻的关键;通过对比相干结构猝发的流向速度分量条件相位平均波形,发现施加控制的工况中相干结构猝发流向速度分量的波形幅值明显降低,且流向速度在扫掠后期高速阶段迅速衰减,缩短了高速流体的下扫过程,说明压电振子的振动能抑制相干结构的高速流体下扫过程,减弱高速流体与壁面的强烈剪切过程,并使近壁区域相干结构的振幅显著减弱,迁移速度加快,从而减小壁面摩擦阻力.      

柔性壁面湍流减阻机理与多尺度相干结构控制的实验研究

用热膜测速仪以高于对应最小湍流时间尺度的分辨率,精细测量了水槽中刚性壁面和柔性壁面平板湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域的对数律平均速度剖面与壁面摩擦速度、流体粘性系数、壁面摩擦切应力等内尺度物理量的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,通过非线性迭代求解壁面摩擦速度以及湍流边界层壁面摩擦切应力.结果表明柔性壁的湍流边界层速度分布在对数律层有所上移,缓冲层增厚,说明柔性壁面具有一定的减阻作用. 利用湍流多尺度局部平均结构函数的概念和多尺度局部平均结构函数的瞬时强度因子、平坦因子检测多尺度相干结构及其间歇性的方法,提取了湍流边界层多尺度相干结构的条件相位平均波形.对比研究了刚性壁面和柔性壁面平板湍流边界层近壁区域多尺度相干结构的条件相位平均波形及其间歇性的统计特征,分析了柔性壁面具有减阻作用的物理机理.     

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

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.     

雷诺应力各向异性涡黏模型的层析TRPIV测量

利用层析TRPIV测量水洞中平板湍流边界层3D-3C速度场的高分辨率时间序列数据库. 提出了空间局部平均多尺度速度结构函数的新概念, 描述湍流多尺度涡结构的空间拉伸、压缩、剪切变形和旋转. 用空间局部平均多尺度速度结构函数对湍流脉动速度进行了空间多尺度分解. 用空间流向局部平均多尺度速度结构函数, 根据湍流多尺度涡结构在流向的拉伸和压缩物理特征, 提出了新的湍流相干结构条件采样方法, 检测并提取了层析TRPIV数据中相干结构“喷射”和“扫掠”事件中的脉动速度、平均速度变形率、雷诺应力等物理量的空间拓扑形态. 通过研究平均速度变形率各分量与雷诺应力各分量之间的空间相位差异,肯定了壁湍流相干结构雷诺应力各向异性复涡黏模型的合理性.      

壁湍流多尺度相干结构复涡黏模型的实验研究

在湍流相干结构动力学方程中,非相干结构成分对相干结构贡献的雷诺应力的模型为涡黏性模型,即涡黏性系数乘以相干结构平均速度变形率的形式.基于非相干结构成分对相干结构贡献的雷诺应力与相干结构速度变形率之间存在相位差的事实,在理论上提出了非相干结构成分对相干结构贡献的雷诺应力复涡黏性模型的假设.应用热线测速技术,在低速风洞中对湍流边界层非相干结构成分对相干结构贡献的雷诺应力与相干结构法向速度变形率之间的相位关系进行了实验测量.通过分析湍流相干结构猝发过程中非相干结构成分对相干结构贡献的雷诺应力与相干结构速度变形率之间的相位关系,研究了相干结构雷诺应力分量与流向速度法向梯度之间的相位差沿湍流边界层法向的变化规律,肯定了湍流相干结构复涡黏性系数模型的合理性.     

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.     

高分子溶液壁湍流减阻机理的TRPIV实验研究

利用高时间分辨率粒子图像测速技术(TRPIV)对回流式水槽中低浓度高分子溶液壁湍流的减阻机理进行实验研究。通过对比分析高分子溶液和纯水平板湍流边界层在相同来流速度下的平均速度剖面、湍流强度和雷诺应力,发现高分子溶液的壁面摩擦阻力减小了21.77%,并且其缓冲层增厚,按对数律外移,雷诺应力减小;高分子聚合物主要在近壁区起到抑制湍流脉动的作用,而在主流区的作用不太明显。用流向局部平均多尺度速度结构函数和相干结构条件采样方法,检测并对比了高分子溶液和水的壁湍流相干结构“喷射”和“扫掠”事件中的脉动速度、展向涡量、雷诺应力等物理量的二维拓扑形态,发现高分子溶液近壁区相干结构在猝发时的脉动速度减小,涡量受到抑制,雷诺应力明显减小,说明高分子溶液湍流近壁区相干结构“喷射”和“扫掠”的强度变弱,猝发频率降低,动量和能量的输运减弱,揭示出高分子溶液减阻的重要机理。     

纵向沟槽壁面湍流边界层内类开尔文-亥姆霍兹涡结构的流动显示

纵向沟槽壁面的湍流边界层,当沟槽的脊-脊内尺度无量纲展向间距s+ 在一定范围内,与光滑壁面湍流边界层相比,具有减阻效应;并在s+ 约为17 个黏性长度单位时减阻效果达到最优,之后其减阻趋弱,直至增阻;其原因可能是沟槽壁面湍流边界层由于“开尔文-亥姆霍兹” 不稳定性而产生的一种“类开尔文-亥姆霍兹”展向涡结构. 实验采用烟雾流动显示技术,首次在风洞中显示了这种“类开尔文-亥姆霍兹” 展向涡结构,确认了其存在性,并在概念上简单勾勒了其结构模型.      

Streamwise Vortices and Velocity Streaks in a Locally Drag-Reduced Turbulent Boundary Layer

This work aims to understand the changes associated with the near-wall streaky structures in a turbulent boundary layer (TBL) where the local skin-friction drag is substantially reduced. The Reynolds number is R e _?? = 1000 based on the momentum thickness or R e _τ = 440 based on the friction velocity of the uncontrolled flow. The TBL is perturbed via a local surface oscillation produced by an array of spanwise-aligned piezo-ceramic (PZT) actuators and measurements are made in two orthogonal planes using particle image velocimetry (PIV). Data analyses are conducted using the vortex detection, streaky structure identification, spatial correlation and proper orthogonal decomposition (POD) techniques. It is found that the streaky structures are greatly modified in the near-wall region. Firstly, the near-wall streamwise vortices are increased in number and swirling strength but decreased in size, and are associated with greatly altered velocity correlations. Secondly, the velocity streaks grow in number and strength but contract in width and spacing, exhibiting a regular spatial arrangement. Other aspects of the streaky structures are also characterized; they include the spanwise gradient of the longitudinal fluctuating velocity and both streamwise and spanwise integral length scales. The POD analysis indicates that the turbulent kinetic energy of the streaky structures is reduced. When possible, our results are compared with those obtained by other control techniques such as a spanwise-wall oscillation, a spanwise oscillatory Lorentz force and a transverse traveling wave.     

A tentative approach to the direct simulation of drag reduction by polymers

An efficient technique for drag reduction uses dilute solutions of a few p.p.m. of polymers. A possible reduction in drag of up to 80% is achieved. Several experimental observations have been made which tend to indicate that the polymers modify the turbulence structures within the buffer layer. Flow visualisations have shown that the changes consist of a weakening of the strength of the streamwise vortices. Existing literature reveals no attempts of numerical simulation of this phenomenon. In this paper an approach is pursued by using a constitutive equation which relates the elongation viscosity to the local properties of the flow. According to this model this viscosity is large in zones where the amount of strain rate is greater than the amount of vorticity, and is zero when the vorticity exceeds the strain rate. Simulations have been performed in a “minimal channel” to give good resolution with a limited number of grid points. The accuracy of the method is tested by comparison with the results of other techniques. For simulations with polymers, quantitative comparisons cannot be made, but the results reproduce the qualitative outputs of the experiments. The mean streamwise velocity is modified in the buffer layer; the peak of the streamwise turbulent intensity, in wall units, increases and its maximum moves far from the wall; and the vertical turbulent intensity is largely reduced in the wall region. An interesting outcome from both the simulation and the experiments is that the strength of the longitudinal vortices is reduced when the polymers are present.     

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.     

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.     

Coherent structures and riblets

This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed.     

Natural Low-Frequency Riblets

A turbulent flow at modest Reynolds numbers (26,000–32,000, as defined in the text) over flat erodible sandy beds was investigated. The beds were composed of sand grains of uniform size with a diameter smaller than the viscous sublayer of the flow. When a modest sand transport (1.2 × 10-3–8.5 × 10-3 g/s·cm) was present, the near-wall coherent structures of the flow produced a bed form with a streaky longitudinal pattern. This pattern had a lateral spacing of about 100 viscous units and had less drag than a flat wall.The mechanism causing the drag reduction is due to a low-frequency riblet pattern which allows the flow to sustain higher wall shear stresses without additional erosion. That means during the riblet regime the sediment transport remains quasi-constant.The formation of the sand streaks as well as the drag reducing mechanism are discussed.     

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.     

Direct numerical simulation of a turbulent Couette-Poiseuille flow,part 2: Large- and very-large-scale motions

A direct numerical simulation dataset of a fully developed turbulent Couette-Poiseuille flow is analyzed to investigate the spatial organization of streamwise velocity-fluctuating u-structures on large and very large scales. Instantaneous and statistical flow fields show that negative-u structures with a small scale on a stationary bottom wall grow throughout the centerline due to the continuous positive mean shear, and they penetrate to the opposite moving wall. The development of an initial vortical structure related to negative-u structures on the bottom wall into a large-scale hairpin vortex packet with new hairpin vortices, which are created upstream and close to the wall, is consistent with the auto-generation process in a Poiseuille flow (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353–396). Although the initial vortical structure associated with positive-u structures on the top wall also grows toward the bottom wall, the spatial development of the structure is less coherent with weak strength due to the reduced mean shear near the top wall, resulting in less turbulent energy on the top wall. The continuous growth of the structures from a wall to the opposite wall explains the enhanced wall-normal transport of the streamwise turbulent kinetic energy near the centerline. Finally, an inspection of the time-evolving instantaneous fields and conditional averaged flow fields for the streamwise growth of a very long structure near the centerline exhibits that a streamwise concatenation of adjacent large-scale u-structures creates a very-large-scale structure near the channel centerline.     

Velocity statistics and flow structures observed in bypass transition using stereo PTV

It is known from smoke visualizations that in a transitional boundary layer subjected to free-stream turbulence, streaks appear and eventually break down to turbulence after wavy motions. In order to observe the streaky structures directly, a stereo particle-tracking velocimetry system using hydrogen bubbles in a water channel has been developed and validated against laser Doppler velocimetry. Mean flow statistics show good agreement with previous results. With the developed measurement system, the instantaneous spanwise distribution of the streamwise and wall-normal velocities can be measured fast enough to resolve the time development of the streaky structures. Measurements of instantaneous spanwise distributions of the streamwise and wall-normal velocity disturbances show strong negative correlation between the wall-normal and streamwise velocities in the streaks. Published online: 19 November 2002     

On the nature of the organized structures in turbulent near-wall flows

A physical mechanism of onset of large-scale organized structures in turbulent flows along a plane wall which are the cause of intensification of turbulent fluctuations is formulated. The structures take the form of high-speed and low-speed streaks caused by streamwise vortices, i.e., motions in the plane of the transverse cross-section. The streamwise vortices are excited as a result of instability under the action of the anisotropy of the normal components of the Reynolds stress tensor. A model for describing these vortices that gives characteristics in qualitative and quantitative agreement with the experimental data is proposed. In particular, the most probable and mean distances between neighboring vortices are correctly reproduced. The theory makes it possible to explain certain methods of turbulent flow control for the purpose of drag reduction. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 24–30, January–February, 1997. The work was carried out with financial support from the Russian Foundation for Fundamental Research (project No. 96-01-00602).