Using the Hot-Film Technique for Velocity Measurements over a Riblet Structured Surface

Abstract

The fact that riblets do reduce shear-stress has been established for more than ten years beyond any reasonable doubt. A plausible explanation is, that the longitudinal ribs rectify the turbulent flow in mean flow direction by hampering the fluctuating cross-flow velocity component w’. In a turbulent boundary layer the cross-flow w’ and the flow component normal to the wall v’ are connected through the low-speed-streaks and burst phenomena. If the cross-flow fluctuation w’ close to the wall can be reduced, the turbulent momentum transfer close to the surface will be reduced as well and consequently, the shear stress $- \rho \cdot (\overline {u'v'} )$ will be decreased.