In a previous study, we developed a one-equation transition model for the bypass and laminar separation bubble (LSB)-induced transition based on local variables. In this paper, distributed surface roughness effects are taken into account by constructing a new transport equation for the roughness amplification factor Ar. Modified criteria taking account of Ar are proposed to describe the roughness effects on the bypass and LSB-induced transitions. Moreover, to predict the flow properties in the laminar–turbulent region more accurately, a modified boundary condition for rough surfaces is employed. The calculations show that, overall, the rough wall promotes the bypass transition and reduces the size, or even causes the disappearance, of the LSBs. Good agreement of the numerical results from the proposed model with the experimental data indicates that the present roughness correction formula is reasonable and accurate.
It is known that boundary layer transition and turbulent separation flow after transition can be influenced significantly by surface roughness. Because the traditional hybrid RANS/LES method cannot predict the boundary layer transition, and the RANS-based transition model cannot accurately simulate the massively separated flow, the present study sought to build an effective modeling strategy for the laminar, roughness-induced-transition and attached turbulence/massively separated flows that couples the Very-Large-Eddy-Simulation model and a transition model considering roughness effects. This new hybrid model was examined in the cases of separated flat plate and rough cylinder. Our analysis shows that the new hybrid model operates in these transitional separated flows over smooth and rough walls. Compared with the results of other classical methods, the present results are more consistent with the measured data. Furthermore, the `drag crisis' phenomenon of the cylinder is accurately simulated by the present model.
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