Transition Model Extension for Roughness Effects

Depending on the manufacturing process and the operating conditions, airfoil surface may show many different roughness characteristics, together with a significant influence at the boundary layer development. In this paper, only the influence at the laminar-turbulent transition is considered and modeled by an extension of the γ-ReΘ transition model of Langtry and Menter. Essentially, an additional transport equation for a roughness amplification scalar is used to modify the transition onset and development, as already presented by Dassler et al. in 2012. In the meantime, the calibrated functional relationship for Argr has been released for the publication for the first time in this paper. In addition, the model performance will be demonstrated and discussed on test cases with increasing complexity, including turbomachinery cascades and rigs. As an input to the model, the equivalent sand-grain roughness is required. In this way, the versatile roughness characteristics of the investigated surface are reduced to only one parameter. The model has been implemented into the CFD software package TRACE of DLR Institute of Propulsion Technology. Only steady flow test cases have been investigated and validated. The transition intermittency is coupled to the two-equation turbulence model of Wilcox. In this model, the roughness influence at the fully turbulent boundary layers is also captured by the variation of the boundary condition for the specific turbulence dissipation rate ω.