LASER-DOPPLER ANEMOMETER MEASUREMENTS OF TURBULENT BOUNDARY-LAYER OVER A RIBLET SURFACE

Laser Doppler anemometer measurements in a turbulent boundary layer do wnstream of a change from a smooth wall to a V-grooved riblet surface provide some insight into the way the near-wall turbulence structure a djusts to the new boundary condition. Depending on the riblet dimensio ns, 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 d istance between neighboring riblet tips and + denotes normalization by wall variables), the turbulence structure is essentially similar to t hat on a smooth wall. There are major structural differences in the la tter case (s(+) greater than or equal to 25). Although near-mall secon dary motions occur in both cases, the motions are much weaker when the riblets act to reduce the drag.