Investigation of flow over an oscillating airfoil

TLDR

The study investigates how boundary‑layer transition, separation, reattachment, and relaminarization points evolve spatially and temporally across a range of oscillation frequencies and amplitudes before, during, and after stall. Using closely spaced hot‑film sensor arrays, surface‑pressure integration, and smoke‑flow visualizations on an oscillating NACA 0012 airfoil, the authors characterized unsteady boundary‑layer behavior, leading‑edge vortex dynamics, laminar‑separation‑bubble effects, and stall mechanisms, and correlated these with aerodynamic loads and pitching moments. The results provide detailed insight into the unsteady boundary‑layer events and the stalling mechanisms operative at different stages of dynamic stall.

Abstract

The characteristics of the unsteady boundary layer and stall events occurring on an oscillating NACA 0012 airfoil were investigated by using closely spaced multiple hot-film sensor arrays at . Aerodynamic forces and pitching moments, integrated from surface pressure measurements, and smoke-flow visualizations were also obtained to supplement the hot-film measurements. Special attention was focused on the behaviour of the spatial-temporal progression of the locations of the boundary-layer transition and separation, and reattachment and relaminarization points, compared to the static values, for a range of oscillation frequency and amplitude both prior to, during and after the stall. The initiation, growth and rearward convection of a leading-edge vortex, and the role of the laminar separation bubble leading to the dynamic stall, as well as the mechanisms responsible for the stall events observed at different test conditions were also characterized. The hot-film measurements were also correlated with the aerodynamic load and pitching moment results to quantify the values of lift increment and stall angle delay as a result of the observed boundary layer and stall events. The results reported here provide an insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process.