Effect of the angle of attack on the drag performance of shark-inspired riblet surfaces

Rib-like structures observed on the skin of fast-swimming sharks are believed to help them experience lower drags and swim faster than other animals in the ocean. Drawing inspiration from this, researchers have been employing two-dimensional riblet surfaces with various cross-sectional shapes to modulate the experienced drag/lift forces under different dynamic conditions. However, much of the previous research has been focused on fully/partially textured one-sided plates examined in zero angle of attack boundary layers, with limited experimental investigations on the local shear stress distributions. In this talk, we will consider a fully textured, symmetric, slender finite-size riblet plate in its entirety, to investigate the effect of the asymmetry imposed by the change of the angle of attack on local performance of the textures on either side of the plate. We present the results of experiments performed in a water tunnel where simultaneous velocity and load measurements are conducted via high-resolution particle image velocimetry (PIV) and load cell, respectively. Through the PIV measurements, we assess local velocity profiles and shear stress distributions in the boundary layers along the length of the samples, as well as the pressure distribution as a function of both geometry and Reynolds number. We calculate both viscous and form drags and compare the local performance of textures on each side. We employ this analysis to discuss the effectiveness of riblets under various angles of attack to identify and propose localized modifications to texture placement for enhanced/similar drag responses at reduced surface coverage. Our findings can contribute to a more in-depth understanding of the relationship between the performance of riblets and different angles of attack and offer insights for optimizing the riblet coverage for a variety of aerial and marine applications.