Multiple bifurcations of the flow over stalled airfoils when changing the Reynolds number

Abstract

In the present study, the sudden changes of the flow field past stalled airfoils for small variations of the Reynolds number are investigated numerically. A vortex particle method has been used for the simulations in a two-dimensional framework. The most critical configurations found with this solver are verified through the comparison with the solution given by a mesh-based finite volume solver. The airfoils considered are the NACA0010 and a narrow ellipse with the same thickness. The angle of attack is fixed to $\unicode[STIX]{x1D6FC}=30^{\circ }$ for which complex dynamics of the flow can take place in the different viscous regimes inspected. The Reynolds number ranges between $Re=100$ and $Re=3000$ and, within this interval, numerous bifurcations of the solution are observed in terms of mean lift and drag coefficients, Strouhal number and downstream wake. An analysis of these bifurcations is provided and links are made between the wake structures observed. On this base the flow patterns can be classified in different modes similarly to the analysis by Kurtulus (Intl J. Micro Air Vehicles, vol. 7(3), 2015, pp. 301–326; vol. 8(2), 2016, pp. 109–139). A discussion of the vortical evolution of the flow in the vicinity of the suction side of the airfoil is also provided.