超疏水表面对湍流边界层相干结构影响的TRPIV实验研究

湍流边界层中的相干结构是壁面摩擦阻力的主要来源。通过研究超疏水壁面对相干结构的影响，揭示其减阻机理。利用高时间分辨率粒子图像测速技术（TRPIV），分别对流速为0.165m/s的亲水壁面和超疏水壁面平板湍流边界层进行测量，得到了2种壁面瞬时速度矢量场的大样本时间序列。通过对比分析2种壁面的平均速度剖面和湍流度，得到了5.39%的减阻效果。通过二维空间两点相关函数的方法定义并提取相干结构，对比得到超疏水壁面能够有效减小相干结构流向尺度的结论。进一步采用λ_ci准则对发卡涡头进行识别，并以此为条件事件对其周围的脉动速度分布情况进行线性随机估计。结果表明:超疏水壁面能够有效削弱单个发卡涡头的强度，并且能够影响其周围发卡涡包结构的组织形式，整体减弱涡包下方近壁区低速流体质点的流向脉动，从而有效减小壁面摩擦阻力。

TRPIV experimental study of the effect of superhydrophobic surface on the coherent structure of turbulent boundary layer

The existence of coherent structures in the turbulent boundary layer contributes greatly to the skin friction. The investigation of the influence of the superhydrophobic wall on the coherent structure is of great significance in revealing the drag reduction mechanism. The instantaneous velocity vector fields of turbulent boundary layers over hydrophilic and superhydrophobic surfaces were measured using time-resolved Particle Image Velocimetry (TRPIV) at the free-coming velocity of 0.165m/s. The mean velocity profiles and turbulence intensity profiles of the two types of surfaces were compared. And the drag reduction rate of 5.39% was obtained. The coherent structure is extracted by the two-point correlation function. By contrast, it is found that the superhydrophobic surface can effectively reduce the streamwise scale of the coherent structure. And the λ_ci criterion is employed to identify the hairpin vortex head as the conditional event. The linear stochastic estimation of the fluctuating velocity field around the conditional event is performed. The results show that the superhydrophobic surface can effectively weaken the strength of the single hairpin vortex head at the center of the conditional event, and can affect the surrounding hairpin vortex package structures. At the same time, the fluctuating velocity of the flow under the vortex package in the near wall region is weakened as a whole, and thus the skin friction is effectively reduced.