Experimental investigation of vortex shedding of an airfoil at post-stall incidences

Abstract

The harmonic vortex shedding from airfoil happens in various incidences. It has noticeable effects on the structure design and aerodynamic performance. In this paper, wind tunnel tests were conducted on a stationary NACA4412 airfoil at angles of attack ranging from $0°$ to $90°$ and Re number between $Re=9.8\times {10}^4$ and $Re=1.5\times {10}^5$ to examine the vortex shedding frequency and Strouhal number. The wake dynamics at post-stall incidences were investigated by surface pressure, wake flow velocity measurement, and smoke flow visualization. Three phases of the wake dynamics were observed with increasing the incidences beyond the stall: (i) the tiny vortices are shed from the airfoil’s suction side with scattered frequencies, (ii) the shear layer is separated from the LE, rolls up over the airfoil’s suction side and forms the harmonic vortex street, and (iii) the separation point moves from the airfoil’s suction side to the pressure side and leads to the vortex shedding like the bluff bodies. Frequency analysis of aerodynamic loads shows that the flow field’s low-frequency feature has a substantial effect normal to the surface while the vortex street unsteadiness impacts both perpendicular and parallel to the surface. The base pressure coefficient increases suddenly by the vortex street onset in the region (ii) and reaches about 0.4. The universal Strouhal number of 0.18 that is independent of Reynolds number was captured for the angles of attack well beyond the stall. Flow visualization shows that the vortex street establishes longitudinally closer to the airfoil at lower freestream velocity compared to the upper one.