Unsteady Vortex Flows and Buffeting of a Low Sweep Delta Wing

Abstract

An experimental study was conducted to understand the unsteady vortex flows and buffeting response of a nonslender delta wing with 50° leading edge sweep angle. Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with large unsteadiness with increasing incidence. 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 pre-stall region as incidence is increased. Velocity fluctuations near the wing surface in the reattachment region exhibit ‘dual-peak’ frequency spectra. With further increase in incidence, the shear layer reattachment fails resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing tip acceleration.