High resolution conformal mesh computations for V, U or L groove riblets in laminar and turbulent boundary layers

Abstract

This paper presents results from a series of numerical studies of various geometry riblets in both laminar and turbulent external flows, which complement results already reported by colleagues at IMST, ONERA-CERT, and UMIST, for V and L-groove riblets in laminar internal and external flows. Although these earlier studies suggested that a 2–3% reduction could be achieved in the latter case, questions remained as to the sensitivity of this finding to further grid refinement, particularly as no drag reduction was indicated for fully developed internal flows. New, higher resolution, CRAY 2 V-groove riblet computations, employing both orthogonal cartesian and conformal grids, have tended to confirm an optimum 2.5% drag reduction for riblets scaling on the boundary layer thickness (rather than wall units as for turbulent flow), and computed mean velocity profiles agreed well with LDA experimental data. Conformal mesh computations for semi-circular U, L, and intermediate form, riblets indicated a similar drag reduction in laminar flow, but also revealed important differences in the spanwise distribution of Cf across these riblets compared to the V-groove type. Subsequent turbulent computations for V and L-groove riblets, employing a conformally transformed mixing length, resulted in mean velocity profiles which again compared well with the limited available turbulent flow data. Parametric computations revealed that the maximum drag reduction was considerably overpredicted for the V-groove riblets, but the optimum performance of the L-groove riblets and the variation in drag reduction with h+ were rather better predicted. Any discrepancies could be associated with secondary motions set-up inside the riblets, since these may be responsible for additional momentum transport, and hence higher Cf, but were not accounted for in the modelling.