Riblet Performance Beneath Transitional and Turbulent Boundary Layers at Low Reynolds Numbers.

Several high-resolution scale-resolving simulations are carried out to examine the effect of riblets on the mean and turbulent statistics of a zero-pressure-gradient boundary layer. The Reynolds number is chosen such that the riblets are exposed to both the transitional and turbulent regimes of the boundary layer. This is in contrast to the turbulent channel flow or fully turbulent boundary layers studied in the literature. The boundary layer is subjected to freestream turbulence and roughness tripping. The transition process and the extent of the turbulent regime on the riblets are altered by tripping the boundary layer with an isolated hemispherical roughness element. The influence of the riblets on the transition onset and the viscous drag reduction is demonstrated through time-averaged, phase-averaged, and instantaneous flow quantities. The effect of the V-shaped riblets with both sharp and curved tips (and valleys) is also explored. With riblets, the viscous drag in the turbulent regime is reduced by 4-6% when compared to a smooth surface, and the efficacy of sharp V-shaped riblets is shown to be marginally higher than for the curved riblets. In the transitional flow regime, the coherent structures over riblets are predominantly spanwise oriented. In particular, a separated shear layer forms over the riblet leading edge as the flow encounters an abrupt surface transition from the smooth surface onto the riblets. A leading-edge ramp is shown to effectively minimize the additional spurt in the turbulent kinetic energy and the associated losses incurred due to this abrupt surface change. [ABSTRACT FROM AUTHOR]