Investigation of support interference in high-angle-of-attack testing

An investigation was undertaken to understand support interference in high-angle-of-attack testing with particular emphasis on premature vortex breakdown induced by struts. In static experiments efforts concentrated on the support geometry and its location with respect to leading-edge vortices generated by delta wings. Extensive flow visualization studies show that vortex breakdown induced by a dummy support might move over the wing depending on the angle of attack and the location of the support. As the lateral distance between the vortex axis and support is varied, static hysteresis of vortex breakdown location was observed, which will have very important implications on force measurements in wind-tunnel testing. The results also suggest that support interference is more important for slender wings. To separate the effects of time-dependent vortex strength and support interference, the model was kept stationary, and a dummy support was oscillated in the spanwise direction in the oscillatory experiments. It was observed that vortex breakdown location oscillates with large amplitudes at low frequencies, but does not show any response at high frequencies, indicating that the frequency response is similar to that of a low-pass filter. Variation of phase-averaged breakdown location showed hysteresis loops and time lags, which are larger for a thin flat plate than a circular cylinder. The results suggest that support interference problems are more complex in transient experiments.