Control of turbulent transition using fiber surface in flat plate boundary layer

Present paper deals with control of turbulent transition in flat plate boundary layer developed on a fiber surface for the purpose of drag reduction. The previous measurement showed 9% drag reduction as compared with a smooth surface case in lower Reynolds number range because turbulent transition was delayed. Intensive investigation including the streamwise velocity and the spatial correlation measurements was performed in order to reveal transition process and mechanism of the transition delay with the fiber surface, using the trip wire or a turbulance grid'to generate initial disturbances. Hot-wire measurements showed that the fiber surface delays transition in both trip wire and turbulence grid cases. The delay in trip wire case is much later than in the turbulence grid case. The growth of the streamwise velocity fluctuation introduced by the trip wire is dumped by the fiber surface though it does not change in the turbulence grid case. The spatial correlation measurement suggested that there exists streak structures in the transitional boundary layer and that they are suppressed by the fiber surface.