The effects of surface roughness on a spatially-developing turbulent boundary layer (TBL) were investigated by performing direct
numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in
the range Reθ=300~1400. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness
height was k=1.5θin, which corresponds to k/δ=0.045~0.125. To avoid generating a rough wall inflow, which is prohibitively difficult,
a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially-developing characteristics of the
rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value and a
self-preserving form of the turbulent stress was obtained. Introduction of the roughness elements affected the turbulent stress not only
in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent stress in the outer layer,
the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple
products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on
vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.
Issue Date
2006-08-23
Language
ENG
Citation
The Fourth National Congress on Fluids Engineering