Direct Numerical Simulation of Turbulent Flow Over Riblets with Thermally-Driven Unstable Stratification

Ribets are micro-groove elements which can reduce skin friction drag. It is unclear how robust these drag-reduction capabilities are when external forces are acting on the fluid. Here, buoyancy forces are introduced by varying the wall Richardson number (Ri_τ = 0,18,60) to create unstable stratification in direct numerical simulations of riblets in a half-channel pressure-driven turbulent flow (Re_τ = 395). It is observed that as Ri_τ increases, drag is increased for a smooth wall. Small riblets experience a baseline increase like that of a smooth wall, while large riblets experience additional drag that is linearly dependent on their groove size. The Fukagata-Iwamoto-Kasagi (FIK) identity is used to decompose how changes in stress contribute to changes in the drag coefficient. It is found that Reynolds stresses are the largest contributor to drag changes. In the cross-stream plane, riblets that experience an additional increase in drag due to Ri_τ have increased Reynolds stresses in their grooves. Additionally, regions of dispersive stresses and vorticity move farther into the groove of large riblets as Ri_τ increases. These results could help better understand the drag reduction mechanisms of riblets across a variety of environments and provide insight into the significance of external forces when designing riblets.