11/4/2022 0 Comments Eddy viscosity ratio external flowemployed the Wilcox wall boundary condition for ω to introduce roughness effect for fully developed turbulence. proposed a hypersonic cross-flow transition criterion considering surface roughness, yet no roughness modeling for fully developed turbulence was presented. However, to capture the laminar-turbulent transition process which is influenced by roughness effect, additional modeling techniques must be supplemented. This equivalent sand grain approach is successful in predicting fully developed turbulence. The increment is determined by the equivalent sand grain height. The basic idea is to mimic roughness effect by increasing the turbulent eddy viscosity or turbulent energy in the wall region. The correlations proposed by Dirling and Grabow and White established the general paradigm of wall roughness modeling. The correlations consider a variety of real roughness shapes. This approach links the real surface to a sand grain surface by converting the real roughness height to the equivalent sand grain height with the help of empirical correlations. To account for the effect of roughness, the “equivalent sand grain” approach is commonly employed. The wall presented in the computational domain is smooth, and the velocity on the wall is also zero. Thus, it is assumed that the roughness-element size in any direction is small compared with the boundary layer thickness so that, above the roughnesses, the flow is averaged over numerous roughness elements that exact location of which is not accounted for. It is unrealistic to set up a graphical CAD model with every roughness-element for a CFD simulation. Therefore, it is necessary to account for roughness in CFD simulations. EDDY VISCOSITY RATIO EXTERNAL FLOW SKINAfter transition, turbulence on a rough surface is enhanced compared with that on a smooth one, leading to higher skin frictions and heat transfer rates. Wall roughness significantly affects laminar-turbulence transition process. Neither natural walls nor man-made walls are perfectly smooth. Fluids in nature and engineering applications often interact with solid walls. Computational Fluid Dynamics (CFD) is widely used as a predictive tool for fluid motions.
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