Guidelines for modeling a 2D rough wall channel flow

The effects of 2-D roughness elements on the Reynolds stress anisotropy tensor, b(ij) and of the energy dissipation rate anisotropy tensor d(ij) of a turbulent channel flow are investigated using data obtained from direct numerical simulations (DNSs). The roughness elements consist of transverse square rods of size k, placed on one wall of the channel only. While k is kept constant ( k) h = 0(1, h is the half-width of the channel), the spacing w between the rods is varied from w) k = 1 to w) k = 59. The results show that the variation in w) k can dramatically change the structure of the wall region flow. The modeling of the near-wall region needs to reflect the structural changes caused by the variation in w) k. On the basis of the Reynolds stress budgets, attention needs to be given to the turbulent energy and pressure diffusion terms while local isotropy may be a reasonable approximation for the energy dissipation rate, especially over a range of w) k for which the drag is near its maximum.