DOI: 10.1615/TSFP6
STRUCTURES OF THE TURBULENT BOUNDARY LAYER OVER A ROD-ROUGHENED WALL
SINOPSIS
Turbulent coherent structures near a rod-roughened wall are scrutinized by analyzing instantaneous flow fields obtained from direct numerical simulations (DNSs) of a turbulent boundary layer (TBL). The roughness elements used are periodically arranged two-dimensional spanwise rods, and the roughness height is k/δ =0.05 where δ is the boundary layer thickness. The Reynolds number based on the momentum thickness is varied in the range Reθ= 300~1400. The effect of surface roughness is examined by comparing the characteristics of the TBLs over smooth and rough walls. The roughness has little effect on the vorticity fluctuations in the outer layer. Velocity-pressure gradient tensors of the transport equation for the Reynolds stresses and quadrant analysis disclose that the redistribution of turbulent kinetic energy of the rough wall is similar to that of the smooth wall, and that the introduction of rod roughness has little effect on the relative contributions of ejection and sweep motions in the outer layer. Finally, we demonstrate the existence of coherent structures in the instantaneous flow field over the rod-roughened surface to elucidate the modifications of the near-wall vortical structure.