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.