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The onset of unsteady separation and dynamic stall vortex formation over a constant-rate pitching airfoil is analyzed by means of high-fidelity large-eddy simulations. The flowfields are computed by employing a previously developed and extensively validated high-fidelity implicit large-eddy simulation approach based on high-order compact schemes. A NACA 0012 airfoil section is considered at a freestream Mach number of and chord-based Reynolds numbers of . The wing is pitched about its quarter-chord axis at a nominally constant nondimensional rate of from a small initial incidence to an angle of attack beyond the onset of dynamic stall. The unsteady boundary-layer behavior that precedes the dynamic stall vortex formation is described in detail. It is found that the process is characterized by the presence of a laminar separation bubble that contracts with increasing angle of attack as leading-edge suction builds up. Beyond a critical incidence, the laminar separation bubble breaks down and rapid suction collapse ensues. Abrupt turbulent separation follows, allowing the turbulent boundary-layer vorticity to coalesce into a coherent dynamic stall vortex. The remaining turbulent boundary-layer vorticity rolls up into a shear-layer vortex that imparts a much weaker signature on the surface pressure. Maximum surface pressure fluctuations of very high frequency are observed near the leading edge just before laminar separation bubble bursting. The fluctuation level drops significantly as the shear layer moves away from the airfoil surface.
Visbal et al. (Wed,) studied this question.