Experimental study of the mechanism of drag reduction in turbulent boundary layers on the superhydrophobic structured wall with microstructure

The drag reduction mechanism of the wall turbulent boundary layer with superhydrophobic micro-riblets and micro-convex posts is studied experimentally. The instantaneous velocity field in the turbulent boundary layer of the hydrophilic wall, superhydrophobic micro-riblets wall and micro-convex posts wall is measured by high time resolution particle image velocimetry (TRPIV). The frictional shear stress of the three kinds of walls is compared and analyzed. It is found that the superhydrophobic walls achieve drag reduction effect. However, the drag reduction rate of the superhydrophobic micro-riblets wall is higher than that of the superhydrophobic micro-convex posts wall, and the drag reduction rate of the superhydrophobic micro-riblets wall is 13.8%, while the drag reduction rate of the superhydrophobic micro-convex posts wall is 10.2%. Through comparison and analysis on the three kinds of wall corresponding average velocity profile in the turbulent boundary layer, turbulence intensity and Reynolds shear stress profile, it is found that the fluid indeed has sliding speed in the superhydrophobic wall, and in the area of the same normal height at $15 < {y^ + } < 100$, the streamwise turbulence intensity corresponding to the hydrophilic wall, superhydrophobic micro-riblets and micro-convex posts wall decreases successively. At the same time, in the area of the same normal height at $30 < {y^ + } < 80$,the wall-normal turbulence intensity of the superhydrophobic micro-convex posts, hydrophilic wall and superhydrophobic micro-riblets wall decrease successively. Over the whole wall-normal height, the maximum values of Reynolds shear stress on the hydrophilic wall, superhydrophobic micro-convex posts and micro-riblets wall decrease successively. Based on the conditional sampling and phase averaging of the spanwise vortex identified by the ${\Lambda _{{\text{ci}}}}$criterion, it is found that the amplitude of the fourth quadrant event induced by the spanwise vortex of the superhydrophobic micro-riblets wall weakens nearby at ${y^{\text{ + }}} \approx 63$, which leads to the intensity of its sweep events decreasing, and then the drag reduction is realized. In order to further analyze the turbulent fluctuation energy, the instantaneous fluctuation velocity of the whole turbulent boundary layer is summed in time and over the streamwise-normal space by using Proper Orthogonal Decomposition and it is dimensionless to characterize the fluctuation degree of the flow field. The results show that the spanwise slip characteristics of the superhydrophobic micro-convex posts increase the drag and weaken the drag reduction effect caused by the streamwise slip. The streamwise slip characteristics of the superhydrophobic micro-riblets wall can effectively suppress turbulence fluctuation and achieve better drag reduction effects.