Measurement of Three-Dimensional Stall Cells on a Two-Dimensional NACA0015 Airfoil

This work aims to identify and quantify the flowfield associated with stall cells on a two-dimensional NACA0015 airfoil with aspect ratio using oil flow visualization and stereoscopic particle image velocimetry. The three-dimensional structures were explored parametrically using oil flow visualization from to 20 deg and to . A critical Reynolds number was observed, above which stall cells form, and its value increased with angle of attack. Stereoscopic particle image velocimetry measurements were taken to quantify the stall cell and correlate the flowfield over the surface to oil patterns on the surface, including skin friction. Next, partial and full span trips were placed at 10% of the chord to artificially induce stall cells. The flowfields and associated vorticity fields for the various cases were measured. It was found that, for the baseline case, where flow is fully separated over the majority of the span, there are large concentrations of turbulent kinetic energy (mainly streamwise and spanwise normal stresses). When a stall cell was induced, the turbulent kinetic energy magnitude was increased in its vicinity; however, it was significantly reduced outboard of the stall cell where the flow was attached.