Airfoil Thickness Effects on Dynamic Stall Onset

Abstract

<p>Large eddy simulations are used to investigate the effects of airfoil geometry, particularly thickness, on inception of dynamic stall. The investigation is performed for three airfoils from the NACA family at Re_c = 2 x 10⁵. Three symmetric airfoils are studied with thickness-to-chord ratios of 9%, 12%, and 15%. A constant-rate pitch-up motion about the airfoil quarter-chord point is used to study dynamic stall. A static simulation is first carried out with each airfoil set at alpha = 4 degrees. Results of the static simulations are compared with XFOIL predictions as a sanity check. Good code-to-code agreement is observed for aerodynamic pressure- and skin friction coefficient distributions. A ramp function is used to smoothly increase the pitch rate from zero to the desired value and then held fixed. Dynamic simulations are carried out until the angle of attack goes past the lift stall point. Unsteady aerodynamic loads are compared with the corresponding static values. In all cases, dynamic stall onset occurs immediately following the bursting of the laminar separation bubble. However, investigation of the reverse flow region on the suction surface shows tremendous differences between the different airfoils, with the thickest airfoil showing a very large reverse flow region. These observations suggests that the mechanism of stall onset can change from 'LSB burst' to trailing edge separation as airfoil thickness is further increased.</p>