Vortex breakdown over a pitching delta wing

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Vortex breakdown over a pitching delta wing

Gursul, I. / Yang, H.

In order to study the effect of pressure gradient on vortex breakdown, unsteady pressure measurements on a pitching delta wing were carried out. It was shown that the pressure fluctuations induced by the helical mode instability of vortex breakdown can be used to quantify the phase lag between the wing motion and breakdown location. Variation of the standard deviation of pressure fluctuations from the phase-averaged value is shown to be a useful tool to detect the vortex breakdown. With the help of time-averaged pressure measurements with and without forced breakdown, it was shown that the pressure along the centerline (y/s = 0) (where the effect of the vortex is negligible) is representative of the external pressure field for the leading-edge vortices. Variation of the phase-averaged pressure along the wing centerline as a function of time shows that there is a phase delay between the wing motion and the pressure field. This phase delay is close to the phase delay of the movement of breakdown location. The phase delay of pressure increases with increasing reduced frequency and is not sensitive to Reynolds number variation. Similar phase lags for pressure were found for a smaller angle-of-attack range for which the breakdown is absent. This confirms that the observed time delays in breakdown location is related to the variation of the external pressure gradient generated by the wing. The phase lag of the external flow field is due to the shedding of vorticity from the wing. It should also be noted that the swirl level, which is the other important parameter for vortex breakdown, was not measured in this study. However, the pressure measurements suggest that the external pressure gradient plays a major role.