Analysis of underlying physics of drag alteration by riblets using Restricted nonlinear simulations

Riblets (micro-grooves) have been shown capable of altering skin-friction drag in wall-bounded turbulence. Although much is known about the mechanisms underlying drag reduction and its breakdown, the physics associated with the full range of behaviors across a variety of geometries needs further characterization. To this end we exploit the computational advantages of the Restricted nonlinear (RNL) framework to perform simulations over a range of riblet geometries. This reduced order model has been shown to accurately predict salient features such as the roughness function and secondary flow structures. We use the resulting data to analyze stresses and other statistical quantities of the flow field in the vicinity of the riblets in order to investigate the flow structures. We also consider these flows at higher Reynolds numbers via an augmented RNL (ARNL) that captures a wider range of scale interactions than the traditional RNL model through the addition of nonlinear interactions at intermediate streamwise scales. The resulting simulated flow fields are compared against direct numerical simulations of flow over riblets at moderate and high Reynolds numbers.