A visualization study of tip vortex roll-up of an oscillating wing

Abstract

A flow visualization study was made to examine qualitatively the unsteady wing-tip vortex roll-up of an oscillating wing. A rectangular wing of aspect ratio 4 with NACA 0012 airfoil cross-section was mounted on one side of the wind tunnel wall, and the wing was pitched sinusoidally about the quarter chord. Both the mean incidence and the amplitude of oscillation were set at 15deg;. The reduced frequency of the oscillation was 0.09. Visualized flow patterns both in streamwise and in cross-sectional planes with smoke wire and light sheet arrangement are presented. The vortical flow pattern at various angles of attack clearly demonstrates the periodic vortex roll-up behavior. When the wing is in pitch-up motion, the flow remains attached to the wing, whereas the flow becomes separated during the pitch-down motion. The visualized vortex cores during the pitch-up motion are thus well defined and regular. On the contrary, the vortex cores during the pitch-down motion are of irregular shape, reflecting the roll-up from the separated flow. With reference to the tip-vortex strength (i.e., the circulation) data, the size of the visualized vortex core is seen to increase with the strength of the vortex for both pitch-up and pitch-down cases. Copyright © 1999 by Begell House, Inc.