Buffeting Flows over a Low-Sweep Delta Wing

An experimental study was conducted with the aim of understanding the unsteady vortex flows and buffeting response of a nonslender delta wing with 50-deg leading-edge sweep angle. Particle image velocimetry and laser Doppler velocimetry measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. It was found that there is a profound effect of Reynolds number on the structure of vortical flows. The breakdown of the leading-edge vortices is delayed significantly, and the vortices form more inboard at low Reynolds numbers. The secondary vortex effectively splits the primary vortex into two separate concentrations of vorticity, resulting in a dual vortex structure at small incidences. This dual vortex structure diminishes, and a single primary vortex is observed at higher incidences. At higher Reynolds numbers (on the order of 3 × 10 4) the flow approaches an asymptotic state, with further increases in the Reynolds number resulting in only small variations in the location of vortex core and breakdown. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with increasing incidences. However, the maximum buffeting occurs prior to the stall, after the vortex breakdown has reached the apex of the wing. The largest velocity fluctuations near the wing surface are observed along the reattachment line. Hence, the shear-layer reattachment, rather than the vortex breakdown phenomenon, is the most important source of increasing buffet in the prestall region as incidence is increased. The velocity fluctuations in the reattachment region have similar dominant frequencies as slender wings in spite of the differences in the physical nature of the flow. With further increase in incidence, the shear-layer reattachment becomes impossible, resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing-tip acceleration.