Experiments on the unsteady massive separation over an aerofoil

We have experimentally investigated an unsteady separating flow over a NACA-0012 aerofoil. Both the surface pressure measurements and the whole flow field measurements using the time-resolved particle image velocimetry (TR-PIV) technique were carried out at different angles of attack for a chord-based Reynolds number, Rec = 5 x 104. The instantaneous flow fields were measured at the stall angle of attack (alpha = 10 degrees) and at different post-stall angles of attack (alpha = 11 degrees, 12 degrees, 13 degrees) using the TR-PIV system. Although the mean flow data, as expected, show that the flow is fully/massively separated for alpha > 10 degrees over the suction surface of the aerofoil, the time sequence of the instantaneous flow fields reveals that the massive separation is intermittent in nature. A small separated region grows with time, eventually leading to the massive flow separation and then the massively separated region decays with time leading to a small separated region. In this process, the vortex shedding from the separated shear layer is also observed. We find that an interplay exists between the temporal growth of the separated shear layer and the vortex shedding during the intermittent massive separation. The mechanism for the intermittent massive flow separation has been explained using the time sequence of the instantaneous flow fields. This has further been supported using the proper orthogonal decomposition analysis. Furthermore, the spatiotemporal stability analysis of the local velocity profile shows that when the small separated region goes to a fully separated state, the velocity profiles become absolutely unstable from a convectively unstable state, and vice versa. In addition, these results are found to be consistent with the absolute instability criteria proposed in Alam and Sandham [J. Fluid Mech. 410, 1 (2000)] and Avanci et al. [Phys. Fluids 31, 014103 (2019)].