The structure of turbulence in a rod-roughened channel

This article is a sequel to the recent paper by Ashrafian et al. [Ashrafian, A., Andersson, H.I., Manhart, M., 2004. DNS of turbulent flow in a rod-roughened channel, International Journal of Heat and Fluid Flow 25, 373–383] on direct numerical simulation of a turbulent flow in a rod-roughened channel. Higher-order statistics are compared and the effects of surface roughness on the relatively large scales of the turbulent flow are investigated. It is observed that roughness tends to increase the intensity of the vorticity fluctuations in the roughness sublayer. In the outer layer, however, the r.m.s. of the vorticity fluctuations are unaffected. The Reynolds stress anisotropy invariant maps for the smooth and rough cases clearly showed that the states of near-wall turbulence for the two cases were substantially different whereas in the regions away from the wall the two cases exhibited close similarities. Differences in the turbulent transport process between the two cases were shown through examination of the third-order moments of the velocity fluctuations and the turbulent kinetic energy budgets. It was observed that wall-ward transport of the kinetic energy is substantially increased very close to the wall while the away-from-the-wall transport of kinetic energy is relatively reduced at the edge of the roughness sublayer. The length scales and flow dynamics in the roughness sublayer were also investigated. It was found that turbulence statistics and structure outside the roughness sublayer are hardly affected by the condition at the walls. These observations lent support to the classical wall similarity hypothesis.