Direct Numerical Simulation of Turbulent Mixing in a Rough-Wall Flow.

摘要

Rough-wall turbulent flows are more common in engineering application than smooth-wall turbulent flows. Modification of mean flow and turbulence property have been established by enormous experiments. However, details of mechanism of rough-wall turbulence has been understood little because spatial resolution is limited in experiments. Meanwhile, Direct Numerical Simulation (DNS) of a rough-wall turbulent flows which is capable of giving high resolution data requires prohibitively heavy computer power and accordingly, no other DNS data are available than those published by the present authors (Miyake et al., 1999). Our first DNS considered sandgrain roughness whose effect was implemented by profile drag based on Stokes drag and could successfully reproduce experimentally established rough-wall turbulent flow such as downward shift of straight line of logarithmic mean velocity distribution and vanishing of viscous sublayer. It was confirmed that the layer adjacent to the wall up to several tens in wall unit where smooth-wall turbulence exhibits autonomous property independent on the layer above it, is taken over by the layer having property of logarithmic layer, in rough-wall turbulence. While quasi-streamwise vortices play major role to generate high turbulent shear stress in this near-wall layer in smooth-wall turbulent flow, roughness destroy this vortical system and consequently, different mixing system which replaces the role of quasi- streamwise vortices should be found in rough-wall layer. Present work intends to investigate the turbulent mixing in the layer close to the wall of rough-wall turbulence by a DNS of more sound numerical conditions, i.e., without using any model for roughness element.