Abstract
Turbulent boundary layer structures behind a functional riblet with multiple characteristic scales are investigated by means of a hot-wire technique. The functional riblet employs two characteristic scales of the turbulent boundary layer, where the riblet trapezoidal lower and straight higher combs carrying streamwise grooves reach the buffer and logarithmic layers of the boundary layer, respectively. The turbulent boundary layer profile behind the higher comb shows a friction reduction rate up to 41 %, because the velocity distribution near the wall has a linear profile normal to the wall. Whilst the boundary layer profile behind the lower comb indicates an ordinary friction reduction rate of about 5%. The turbulent boundary layer structures measured far downstream behind the functional riblet indicate how the modified boundary layer develops downstream.
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