Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy

We investigate the Reynolds analogy over riblets, namely between fractional increase in Stanton number $C_h$ and skin-friction coefficient $C_f$ , relative to a smooth surface. direct numerical simulation data of Endrikat et al. ( Flow Turbul. Combust. vol. 107, 2021, pp. 1–29). The riblet groove shapes are isosceles triangles with tip angles $\alpha = {30}^{\circ }, {60}^{\circ {90}^{\circ }$ trapezoid, rectangle right triangle. viscous-scaled spacing varies $s^+ \approx 10$ $60$ . global is primarily influenced by Kelvin–Helmholtz rollers secondary flows. locally break favourably, i.e. cause larger than These induce negative wall shear stress patches which have no analogue heat fluxes. Secondary flows at riblets’ crests associated local unfavourable breaking analogy, Only triangular riblets trigger strong without appreciable This shape globally preserves from 21$ $33$ However, other weak or non-existent rollers, yet persistent behave similarly rough surfaces. They unfavourably do so greater extent as $s^+$ increases.