Plunging wake analysis of an airfoil equipped with a Gurney flap

Abstract

Experimental measurements were conducted on a plunging Eppler 361 Gurney flapped airfoil to study wake structure and dynamic stall phenomenon in the wake. The heights of Gurney flap were 2.6 and 3.3% chord. All oscillation data were taken at the plunging amplitude of 6 cm and Reynolds number 1.5 × 10⁵. Special attention was focused on the temporal progressions of the plunging wake for the range of initial angle of attack (0° and 12°) in prior and post-stall flow conditions. The velocity in the wake was measured by hot-wire anemometry. Surface pressure measurements, as supplementary data, were also carried out to look into the link between the boundary-layer flow and the shedding vortical flow. It was found that the hysteresis is detected between the plunging wake in the upstroke and down-stroke. The shape and width of the wake hysteresis loops strongly depend on the initial angle of attack and vertical positions of the sensor. At prior static-stall angle of attack, positive camber effects of flapped airfoil shifted wake profiles downward and more velocity deficits were detected. In the post-stall conditions the hysteresis loop widths for lower-than-centerline vertical positions were remarkably more than counterpart upper positions. The energetic dynamic stall vortex shedding was found to be main responsible for large hysteresis and velocity deficit at these positions. Furthermore, the extent and strength of the stalled wake or flow separation were found to increase for the flapped case which results in a significant increase in the hysteresis loop widths.