Published online by Cambridge University Press: 04 July 2016
The paper reports the first computational explorations of the flow in the vicinity of riblets with a two-equation turbulence model. For simplicity fully developed flow in a plane channel is considered though the results should be applicable to boundary-layer flow, as the riblets are confined within the (virtually) constantstress near-wall sublayer. The study of idealised L-shaped (or blade) riblets showed larger levels of drag reduction (up to 22%) than have hitherto been reported. When the usual triangular-profile riblets were considered, however, the maximum drag reduction was reduced to about 10%, in line with experiment.
Although the maximum drag reduction is well predicted, there are serious differences from experiment for the values of h+ at which the optimum performance is achieved. Moreover, the computations suggest that at low Re for h+ < 10, drag increases result. These anomalies highlight two distinct and counteracting weaknesses in the model of turbulence: there is an insufficient sensitivity of the equation to three-dimensional straining in the jow-Reynolds-number region while use of an isotropic viscosity model enforces zero secondary circulation in the vicinity of the riblets.