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.