Abstract
Wall roughness significantly influences both laminar-turbulent transition process and fully developed turbulence. A wall roughness extension for the KDO turbulence/transition model is developed. The roughness effect is introduced via the modification of the k and νt boundary conditions. The wall is considered to be lifted to a higher position. The difference between the original position and the higher position, named as equivalent roughness height, is linked to the actual roughness height. The ratio between the two heights is determined by reasoning. With such a roughness extension, the predictions of the KDO RANS model agree well with the measurements of turbulent boundary layer with a sand grain surface, while the KDO transition model yields accurate cross-flow transition predictions of flow past a 6:1 spheroid.
Highlights
Computational Fluid Dynamics (CFD) is widely used as a predictive tool for fluid motions
It is necessary to account for roughness in CFD simulations
To capture the laminar-turbulent transition process which is influenced by roughness effect, additional modeling techniques must be supplemented
Summary
Computational Fluid Dynamics (CFD) is widely used as a predictive tool for fluid motions. Wall roughness significantly affects laminar-turbulence transition process. To capture the laminar-turbulent transition process which is influenced by roughness effect, additional modeling techniques must be supplemented. Xiang et al [5] proposed a hypersonic cross-flow transition criterion considering surface roughness, yet no roughness modeling for fully developed turbulence was presented. Liu et al [6] employed the Wilcox wall boundary condition for ω [7] to introduce roughness effect for fully developed turbulence. A typical roughness correction imposed on the KDO model could potentially reflect the roughness effects on the transition process. The basic idea is that, according to experimental data [2], the log-law still holds in a turbulent boundary layer with wall roughness. The present work employs the idea of “equivalent sand grain approach”, but the empirical coefficient is determined by reasoning instead of the shifted log-law. The KDO transition model [9] (KDO-tran) with this roughness correction is termed as KDOR-tran
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