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RANS Study of Flows across an Abrupt Change in Surface Roughness
Author(s): Wenye Li, Chun-Ho Liu, Wai Chi Cheng
Keywords: Atmospheric flows, Computational fluid dynamics (CFD), Internal boundary layer (IBL), Natural topography, Surface-roughness change, Turbulence characteristics
Flows across an abrupt change in surface roughness lead to the development of an internal boundary layer (IBL). In this paper, the effect of surface discontinuity on the structure of flow and turbulence is unveiled by the Reynolds-averaged Navier-Stokes (RANS) turbulence model. Three configurations of smooth-to-rough transition, which are fabricated by sinusoidal wavy surfaces, are examined to contrast the flow adjustment. After the change in (increasing) surface roughness, the flows decelerate and the downward momentum flux (-\overline{u''w''}) increases to overcome the increasing drag. The changes in friction velocity (u_{\tau,2}/u_{tau,1}) and roughness length (z_{0,2}/z_{0,1}) follow the conventional power law. The developments of roughness sublayer (RSL) and inertial sublayer (ISL), which characterize the flows adjustment, are clearly observed. The flow structure after the roughness transition is also defined quantitatively, through which the interaction among IBL, RSL and ISL is elucidated. The growth of IBL and ISL signifies that the influence from the upstream (smoother) surface is being weakened while the flows are developing in equilibrium with the downstream (rougher) surface. Finally, the winds over complex terrain (Hong Kong Island) are modelled to demonstrate the sea-land effect on atmospheric flows. The results show that the flow dynamics and structure over natural topography are consistent with those over idealised surfaces.