RANS Study of Flows over Surface across Roughness Transition

Abstract

Reynolds-averaged Navier-Stokes (RANS) modeling approach is carried out in this study to investigate the transition process while air is flowing over wavy surface across a change in roughness. Surface roughness is measured by drag coefficient Cd and roughness length z0. The relation between roughness parameters and terrain feature (amplitude-to-wavelength ratio 2a/) is examined through which the roughness of different sinusoidal wavy surfaces is quantified. Flows over homogeneous wavy surfaces are characterized first. Variations of wind profiles are observed at different streamwise positions, the maximum and minimum Reynolds stress appear at trough and crest, respectively, due to recirculating flows. It is shown that the current RANS modeling results compare favourably well with the waterchannel experiment from Hudson et al. (1993). The effect of roughness change on flow structure is elaborated in this study. Flow transition process is illustrated through the adjustment of mean flows and turbulent momentum fluxes. Three smooth-rough simulations with different roughness parameters are conducted in which the downstream-to-upstreamroughness-length-ratio z0,2/z0,1 is equal to 3.98, 6.46 and 10.20. Computational fluid dynamics (CFD) modeling results show that after surface transition, both the turbulent kinetic energy (TKE) and the vertical momentum flux (u’’w’’) increase with increasing downstream surface roughness z0,2. However, a reduction in vertical momentum flux is observed near the surface transition position. Moreover, the development of a new roughness sublayer (RSL) and a new inertial sublayer (ISL), which are induced by roughness change, could be clearly observed. In view of the surface roughness changes on flows, the depth of two ISLs and internal boundary layer (IBL) are defined quantitatively, finally the conceptual model of “two-layer” flow structure is proposed accordingly.