正弦波沟槽对湍流边界层相干结构影响的TR-PIV实验研究

利用高时间分辨率粒子图像测速(time-resolved particle image velocimetry, TR-PIV)技术,在不同雷诺数下对光滑壁面和二维顺流向、三维正弦波(two/three dimensional, 2D/3D)沟槽壁面湍流边界层流场进行了实验测量,从不同沟槽对湍流边界层相干结构影响的角度分析了其减阻的机理.对比不同壁面的各阶统计量结果发现:沟槽降低了壁面摩擦阻力,存在减阻效果,正弦波沟槽的减阻率增大.运用相关函数、λ_(ci)检测准则等方法提取了不同壁面湍流边界层发卡涡和低速条带等典型相干结构的空间拓扑形态,结果表明:两种沟槽壁面的相干结构在流向和法向上的空间尺度均有不同程度的减小,且相干结构与主流之间的倾角趋于更小,流体在法向上的运动及结构的抬升受到明显抑制,发卡涡诱导喷射和扫掠的能力降低,从而影响了湍流中能量与动量的输运过程及湍流的自维持机制,且相比于2D沟槽, 3D正弦波沟槽作用效果更为明显.在同一雷诺数下,随着距离壁面法向位置的增加,不同壁面湍流边界层低速条带的展向间距都变宽;但同一法向位置处2D/3D沟槽壁面湍流边界层低速条带的间距与光滑壁面的相比更宽,沟槽的存在有效抑制了低速条带在展向上的运动,使得低速条带更稳定.

Influence of sinusoidal riblets on the coherent structures in turbulent boundary layer studied by time-resolved particle image velocimetry

Drag reduction by riblets has drawn the attention of many researchers because of its low production cost and easy maintenance. But due to the fact that the rather low drag reduction riblets can offered, an easy modification to the structure of riblets to improve the performance would be more than necessary. In this work, an investigation of the influences on coherent structure of straight riblets and sinusoidal riblets (s-riblets) in a turbulent boundary layer (TBL) at various Reynolds numbers is carried out experimentally by using the time-resolved particle image velocimetry (TR-PIV). It is found that the skin friction of the turbulent boundary layer is reduced close to the wall, and the logarithmic velocity profile shifts upwards over riblets and s-riblets. The turbulence intensity and Reynolds shear stress are also reduced in the near wall region compared with the scenario of the smooth case, and a better performance on drag reduction is obtained over s-riblets. Coherent structures including hairpin vortex and low speed streaks are extracted over test plates by using the correlation coefficient and \$\lambda_{ci}$\ vortex identification method, to study the mechanism of drag reduction caused by riblets. It is shown that the spatial scale of coherent structure in streamwise and wall-normal direction decrease over riblets and s-riblets to various degrees, the inclination angle between the mainstream and coherent structure also decreases, meaning that the wall-normal movement and upwash motion are suppressed over riblets and s-riblets. Results from the conditional sampling method demonstrate that the induction of ejection and sweep motions by hairpin vortex are inhibited over riblets and hence the exchange of energy and momentum and the self-sustaining mechanism in turbulence are influenced. Furthermore, at the same \$Re_{\tau}$\, the spanwise spacing of low speed streaks turns wider with wall-normal position increasing. At the same \$ y^{+} $\, a larger spacing is seen over riblets and s-riblets, implying that spanwise movement of the streaks is restrained and hence becomes more stable.