Study of Burst Wakes in a Multi-Element Airfoil Flowfield

Abstract

High-lift multi-element airfoils, such as those used on large transport aircraft during takeoff and landing, can generate strong adverse pressure gradients that, while the surface flow is attached, can cause off-the-surface separation in the wake: so-called wake bursting. The sudden expansion and thickening of the separated wakes has been shown to decrease lift and increase drag. Wake bursting was experimentally studied over a three-element high-lift airfoil, and unsteady velocity measurements were taken with a split-film probe. The tests were performed in the University of Illinois at Urbana–Champaign low-speed low-turbulence subsonic wind tunnel on a multi-element airfoil with a chord length of 1.35 ft (0.411 m) and a model span of 2.8 ft (0.85 m). Results for a Reynolds number of indicate that wake bursting was observed for the wake of the main element and the first flap. A methodology was developed to numerically define the core of each wake both upstream and downstream of the burst point. Data show that the local flowfield angle in the wake core does not significantly change relative to the flowfield outside the wake core. Unsteady results indicate that the velocity fluctuations within the burst-wake region are dominated by turbulence in the shear layers between the wakes with less turbulence observed in the wake cores. These turbulent fluctuations were largest in the shear layers and were observed to spread into the wake cores.