Flow Structure on a Delta Wing of Low Sweep Angle

摘要

The instantaneous and averaged flow structure past a delta wing of low sweep angle is investigated using a technique of high-image-density particle image velocimetry. Emphasis is on crossflow planes, where vortex breakdown and stall occur, and the identification of buffeting mechanisms in these regions. At all values of angle of attack up to the fully stalled condition, the averaged vorticity layer exhibits an elongated form; the classical (single) large-scale concentration of vorticity within the leading-edge vortex of a highly swept wing is not present. At low angle of attack α, this elongated, averaged layer can exhibit, however, well-defined concentrations of vorticity. These elongated vorticity layers are accompanied by narrow recirculation zones adjacent to the wing surface. Furthermore, the averaged streamline topology exhibits, at lower α, a saddle point located slightly outboard of the leading edge, in contrast to a saddle point located on the plane of the symmetry of a highly swept wing. Patterns of velocity fluctuation and Reynolds stress show peaks that are generally coincident with large values of averaged vorticity, which indicates that they arise from unsteady events in regions of high shear. Well-defined concentrations of instantaneous vorticity can be identified at all values of angle of attack α. At low α individual concentrations retain their identity, but at moderate and high α larger-scale clusters of instantaneous vorticity occur. In turn, these patterns of vorticity are in accord with the time-averaged spectra of the fluctuating velocity; the predominant peaks of such spectra take on lower values in regions where larger-scale clusters of vorticity appear. Control in the form of a small amplitude perturbation of the wing, at a frequency corresponding to the subharmonic of the spectral component in the initial region of development of the separated layer, can restabilize the time-averaged patterns of streamline topology and vorticity, such that they resemble those occurring at lower angle of attack α.