An experimental investigation into the wake vortex roll-up of a rectangular airfoil

R. Bristol
Master's thesis 1999.

The roll-up process of trailing vortices from small rectangular airfoils was 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 $1.3 \cdot 10^4$ to $6.7 \cdot 10^4$ and circulation-based Reynolds numbers from $2.3 \cdot 10^3$ to $1.7 \cdot 10^4$. 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 process occurred on the airfoil itself; the vortices were completely rolled-up 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 essentially 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 vortices, although the reader is cautioned the actual vortices were not quite azimuthally symmetric.