FLOW ALTERATION AND DRAG REDUCTION BY RIBLETS IN A TURBULENT BOUNDARY-LAYER

Detailed measurements with a miniature single-sensor hot-wire probe of the streamwse velocity field within a riblet groove of dimensionless size h(+) approximate to 14 and s(+) approximate to 28 (based on the r iblet surface average friction velocity) have been made. The wall shea r stress, when integrated over the riblet surface, yields about 4% dra g reduction compared with a smooth surface with the same projected are a. This is largely due to the greatly diminished wall shear stress nea r the bottom of the riblet valley. Four-sensor hot-wire probe measurem ents reveal that riblets significantly reduce the vertical flux of str eamwise momentum within the riblet valley.