Robert Bristol
Ph.D. candidate, Mechanical Engineering
U.C. Berkeley
The trailing vortices from a small rectangular airfoil were studied in a towing tank using a particle imaging velocimetry technique. The measurements were made in a Eularian frame, that is, the measurement plane remained fixed while the wing swept by. Chord-based Reynolds numbers ranged from 1.3 x 104 to 6.7 x 104 and circulation-based Reynolds numbers from 2.3 x 103 to 1.7 x 104. The flow was interrogated at distances from 0.2 chord to 60 chords aft of the wing. Data were analyzed for both velocity and velocity-gradient fields perpendicular to the path of travel of the wing. Much of the roll-up of the vortices occurred on the airfoil itself; the vortices were essentially formed within one span. Unsteady strands of oppositely-signed vorticity were observed in the rolling-up vortex sheet immediately aft of the wing. Fluctuating structures such as these have rarely been seen before as most studies have employed point measurements, therefore obtaining a time-averaged picture of the overall flow field. Using a PIV technique, this study made instantaneous measurements in a plane and thus was able to capture unsteady effects. The Lamb-Oseen distribution was found to be a fair fit to the rolled-up vortex structure, although the reader is cautioned that the actual vortices were not quite azimuthally symmetric.