Turbulence structure over a particle roughness

The effect of particle roughness on turbulence structure in a water boundar y layer in an open channel flow is experimentally investigated using a lase r-Doppler velocimeter (LDV). As a wall boundary, a fixed-or a free-particle wall is used against various particle-occupation densities epsilon. The re sults show that for the same epsilon the friction velocity on the fixed-par ticle wall is larger than that on the free-particle wall, and that both fri ction velocities have maxima in the region of 0.1 less than or equal to eps ilon less than or equal to 0.2. The Reynolds stress and turbulence intensit ies outside the roughness sublayer, normalized by the friction velocity, ar e independent of the particle roughness. The u-v quadrant decomposition tec hnique elucidates that ejection events are superior to sweep events in the vicinity of the wall over smooth and free-particle walls, whereas over the fixed-particle wall the sweep events are more dominant in the production of the Reynolds stress than the ejection events. (C) 2001 Elsevier Science Lt d. All rights reserved.