Numerical Study on the Effect of Limiting Eddy Viscosity on Flow Separation Prediction for Airfoils in Wind Energy Applications

Pressure driven flow separation is an important category of flows and many aerodynamic devices are required to operate on the verge of boundary layer separation to maximize efficiency. Flow fields in wind engineering are inherently complex and are characterized by frequent occurrence of separated flows. Current one- and two-equation turbulence models do not capture all the relevant physics associated with flow separation and, hence, can result in erroneous predictions. These models are based on the Boussinesq assumption that relates the apparent turbulent shearing stresses to the rate of mean strain through an apparent scalar turbulent or “eddy viscosity”. A known drawback of this assumption is the inability to account for rapid changes in length scale which hence results in prediction of higher eddy viscosity. In this study the effect of limiting eddy viscosity in a k-ω SST model in CFX solver is studied and applied to predict flow over wind turbine airfoils like S825 and S827. For a certain clipping ratio range a significant improvement in separation prediction was observed when compared to the baseline turbulence model. The evolution of eddy viscosity and its impact on the macro scale flow structures is also discussed.