Contribution of dispersive stress to skin friction drag in turbulent flow over riblets

We carry out direct numerical simulations (DNSs) of minimal open-channel flow over riblets, which are streamwise-aligned grooves that modify the near-wall flow for drag reduction. Several riblet sizes and cross-sectional geometries are simulated, namely symmetric triangular, asymmetric triangular, blade and trapezoidal. With this unprecedented breadth and detail afforded by the DNS data, we are able to obtain more general insights into the flow physics of riblets. A generalization of the Fukagata–Iwamoto–Kasagi (FIK) identity is used to isolate the different contributions to skin friction drag changes. We show that, in the nonlinear regime of large riblet size, the dispersive contribution is comparable or larger than the turbulent one, representing an important mechanism to the breakdown of drag reduction.