Laser Doppler anemometer measurements of turbulent boundary layer over a riblet surface

Laser Doppler anemometer measurements in a turbulent boundary layer downstream of a change from a smooth wall to a V-grooved riblet surface provide some insight into the way the near-wall turbulence structure adjusts to the new boundary condition. Depending on the riblet dimensions, this adjustment results either in drag reduction or drag increase, In the former case (s(+) less than or similar to 25, where s is the distance between neighboring riblet tips and + denotes normalization by wall variables), the turbulence structure is essentially similar to that on a smooth wall. There are major structural differences in the latter case (s(+) greater than or equal to 25). Although near-mall secondary motions occur in both cases, the motions are much weaker when the riblets act to reduce the drag.