Effects of Compressibility on Dynamic-Stall Onset Using Large-Eddy Simulation

Abstract

Large-eddy simulations of the onset of dynamic stall for a NACA 0012 airfoil, pitching at a constant rate, are performed for a chord Reynolds number of . Using four simulations, spanning , the effects of compressibility are examined for constant Reynolds number. The interplay between the laminar separation bubble (LSB), separation within the turbulent boundary layer, and compressibility effects is elucidated. It is shown that increased compressibility results in earlier dynamic-stall onset related to bursting of the leading-edge LSB. Because of this earlier stall onset, which occurs at the leading edge, the role of the turbulent boundary-layer separation in dynamic stall is diminished. It is hypothesized that fundamental changes in the transition mechanism within the LSB, due to compressibility, are the cause of earlier leading-edge breakdown. At a region of supersonic flow develops above the leading-edge LSB. A unique system of compression and expansion structures is observed and documented. It is shown that this system couples with the inherent unsteadiness of the LSB, resulting in a brief period of high-amplitude coherent flow oscillations, before the development of the dynamic-stall vortex (DSV). At this condition, an elliptical shape of the DSV is observed due to the presence of supersonic flow above and below the developing vortex.