超疏水壁面湍流边界层减阻机理的TRPIV实验

利用高时间分辨率粒子图像测速（TRPIV）技术，开展超疏水壁面材料湍流边界层减阻机理的实验研究.在循环水槽中，对超疏水壁面和亲水壁面湍流边界层瞬时速度矢量场的时间序列进行了实验测量.得到了同一来流速度（0.17m/s）下超疏水壁面和亲水壁面湍流边界层的平均速度、湍流度及雷诺切应力沿法向的分布规律.提出了空间多尺度局部平均涡量的概念，并以此为特征量检测壁湍流发卡涡展向涡头的中心位置.用条件采样及空间相位平均技术提取了不同法向位置发卡涡展向涡头周围流向脉动速度和流线的空间拓扑，对发卡涡展向涡头的俯仰角进行了对比，并从鞍点-焦点动力系统的角度分析了发卡涡展向涡头附近的流线拓扑特征.研究表明：雷诺数约为13500时，相比亲水壁面，超疏水壁面实现了10.1%的减阻.超疏水壁面平均速度明显增大，雷诺切应力减小，流向湍流度减弱，发卡涡展向涡头俯仰角较小，近壁区相干结构的发展受到抑制.

TRPIV experimental investigation of drag-reductionmechanism in turbulent boundary layerover superhydrophobic surfaces

Drag-reduction in turbulent boundary layer (TBL) over superhydrophobic surfaces was investigated by the time-resolved particle image velocimetry (TRPIV). Time series of velocity vector fields in TBL over superhydrophobic surfaces and hydrophilic surfaces were measured in a cyclical water channel. The distributions of mean velocity profile, Reynolds shear stress and turbulence intensity along wall-normal direction were acquired at the same free-stream velocity (0.17m/s). The center of the head of hairpin vortex in wall turbulence was detected with the multi-scale spatial locally-averaged vorticity. Conditional sampling and phase average methods were utilized to extract the spatial topologies of spanwise vortices at different normal positions. Streamwise fluctuation velocity and the pitching angle of spanwise vortex in both cases were compared. The features of streamline topologies around spanwise vortex were analyzed from the saddle-focus dynamical system. Results revealed that a drag reduction of 10.1% is acquired if Reynolds numbers are about 13500. For superhydrophobic surfaces, the average velocity seems to be increased, Reynolds shear stress is decreased, streamwise turbulence intensity is weakened, the pitching angle of spanwise vortex is smaller, and the development of coherent structure near wall region is suppressed.