TRPIV experimental investigation of drag reduction mechanism in turbulent boundary layer over superhydrophobic-riblet surface

The instantaneous velocity vector fields of turbulent boundary layers over the hydrophilic surface, the superhydrophobic (SH) surface and the superhydrophobic-riblet (SR) surface were measured using Time-Resolved Particle Image Velocimetry(TRPIV). Drag reduction rates of 14.6% and 20.7% for the SH surface and the SR surface respectively were acquired by comparing with the friction coefficient of the hydrophilic surface. By comparing the tendency of the turbulence intensity, it is found that the normal turbulence fluctuation intensity of the hydrophilic surface, the SH surface and the SR surface has no remarkable differences, but the streamwise turbulence fluctuation intensity shows a weakening trend in the region of y⁺ < 150 at the same wall-normal position. By using the spatial filtering method based on Fourier transform, the instantaneous fluctuating velocity field is divided into the large-scale part with the wavelength greater than δ and the small-scale part with the wavelength less than δ. It is found that the inhibitory effect of the SH surface and the SR surface on the streamwise turbulence fluctuation intensity of the large-scale part can reach the wall-normal position of y⁺=150, while the inhibitory effect on the streamwise turbulence fluctuation intensity of the small-scale part can only reach the normal position of y⁺=100. Through the conditional sampling and phase average methods, it is found that at the region of y_ref=0.1δ, compared with the hydrophilic surface, the positive large-scale streamwise fluctuating intensity and the negative wall-normal fluctuating intensity on the SH surface and the SR surface are increasing while the negative large-scale streamwise fluctuating and positive wall-normal fluctuating intensities on the SH surface and the SR surface are decreasing, and there is a gap between the contour with the value of zero and the reference position of the conditional sampling. Comparing the vortical strength of TBL on different wall, it is found that the vortex intensity value of the hydrophilic surface, the SH surface and the SR surface becomes weaker in turn, and hence we can conclude that the SR surface could acquire a higher drag reduction rate than the SH surface, via suppressing the motion of vortices at the near wall region.