A vorticity-based criterion to characterise leading edge dynamic stall onset

Abstract

We propose a more conservative, physically-intuitive criterion, namely, the boundary enstrophy flux , to characterise leading-edge-type dynamic stall onset in incompressible flows. Our results are based on wall-resolved large-eddy simulations of pitching aerofoils, with fine spatial and temporal resolution around stall onset. We observe that reaches a maximum within the stall onset regime identified. By decomposing the contribution to from the flow field, we find that the dominant contribution arises from the laminar leading edge region, due to the combined effect of large clockwise vorticity and favourable pressure gradient. A relatively small contribution originates from the transitional/turbulent laminar separation bubble (LSB) region, due to LSB-induced counter-clockwise vorticity and adverse pressure gradient. This results in being nearly independent of the integration length as long as the region very close to the leading edge is included. This characteristic of yields a major advantage in that the effect of partial or complete inclusion of the noisy LSB region can be filtered out, without changing the peak location in time significantly. Next, we analytically relate to the net wall shear and show that its critical value corresponds to the instant of maximum net shear prevailing at the wall. Finally, we have also compared with the leading edge suction parameter (Ramesh et al., J. Fluid Mech., vol. 751, 2014, pp. 500-538) and find that the former reaches its maximum value between and of rotation earlier.