Aerodynamic Nonlinearities and Dynamic Stability in Transitional Airfoil Flows

This article presents an investigation on boundary layer transition induced aerodynamic nonlinearities, the dynamic stability of an airfoil in the vicinity of such nonlinearities, and the subsequent occurrence of limit cycle oscillations. To study such transitional nonlinear aerodynamics, the well-known experimental LCO case MP77 for the NLR7301 is considered. Although the LCO found for MP77 is not ascribed to boundary layer transition effects, it occurs in the vicinity of a strong nonlinearity in the moment curve that is most likely connected to shock-induced and transitional effects. A similar nonlinearity is created using a static model deformation. Steady results are then computed for the original and deformed airfoil with free and prescribed transition locations. For the target lift coefficient of the wind tunnel experiment, coupled unsteady CFD-CSM computations are performed. Although early results indicated an amplitude limiting effect in the vicinity of the aerodynamic nonlinearity, further investigations are not able to reproduce this behavior once the temporal discretization is improved. Based on this research, the working hypothesis that a transitional aerodynamic nonlinearity is providing the amplitude limiting effect for MP77 cannot be confirmed