Dual-plane stereoscopic PIV measurement of vortical structure in turbulent channel flow on sinusoidal riblet surface

Vortical structure in wall turbulence over a sinusoidal riblet surface is investigated by means of a dual-plane stereoscopic particle image velocimetry (DPS-PIV) measurement. The experiment is made in a channel flow at a friction Reynolds number of 150. The lateral spacing of the adjacent walls of the sinusoidal riblet varies in the streamwise direction and 12% of the drag reduction rate has been confirmed. The DPS-PIV measurement system consists of four high-speed CCD cameras. The laser sheets are provided on streamwise and wall-normal planes and separated 0.5 mm in the spanwise direction to each other. The profiles of the velocity statistics in the flat case show a good agreement with previous data. Since all velocity components can be measured on adjacent laser sheets simultaneously, an instantaneous velocity deformation tensor can be obtained and vortical structures can be identified by a second invariant of the tensor i.e., the Q value. The probability of Q value in the riblet side is almost unchanged from that of the flat side. The analysis of the tracking of vortical structures by using the Q value is performed. As similar to a pathline analysis in a previous study, we confirmed that the riblet surface prevents the vortical structure hitting the wall and vortical structures follow the upward and downward flows induced by the sinusoidal riblet surface