An exhaustive review of studies on bio-inspired convergent-divergent riblets

Drawing inspiration from the distinctive textures of shark skin and the feathers and tails of birds in flight, the convergent-divergent surface pattern shows potential in influencing boundary layer structures. This pattern features protrusions aligned obliquely, angled in the streamwise direction, commonly known as riblets. These riblets are noted for their capacity to affect the large-scale and very-large-scale structures that predominate the boundary layer. This study aims to shed light on the impact of convergent-divergent riblets on the boundary layer, especially in the spanwise direction. We present a review of research on vortex generation physics, highlighting helicoidal and rotational motions within and around the riblet valleys. In addition, we explore both experimental and numerical research concerning crucial physical parameters of convergent-divergent riblets, such as yaw angle, wavelength, viscous-scaled riblet height, fetch length, and the transition from riblets to a smooth surface. The potential of these bio-inspired riblets for drag reduction is also examined. Furthermore, this study explores a range of manufacturing techniques for convergent-divergent riblets. Lastly, we examine the potential commercial applications of the convergent-divergent design across various industries.