Effect of straight riblets of the underlying surface on wall bounded flow drag

Abstract

Direct numerical simulations of thin, straight riblets were conducted to investigate the physical mechanisms involved when the inertial effects within the ribs become important. The main parameter that we assess here is the ribs’ height to spacing ratio (a=h/s). In all cases, ‘k’-type behaviour was observed despite the absence of the pressure drag component. This trend also refers to the destruction of the near-wall cycles and was indicated by the shortening of Reynolds stress structures, mimicking the flows over rough walls. Closely-packed ribs with large h/s have a tendency to generate Kelvin–Helmholtz (KH) like structures at the tips, inducing large pressure fluctuations. Conversely, spar..