Experiments on the unsteady nature of vortex breakdown over delta wings

Abstract

 Vortex breakdown location over delta wings is not steady and exhibits fluctuations along the axis of the vortices. Experiments on the nature and source of these fluctuations were carried out. Spectral analysis and other statistical concepts were used to quantify the unsteady behaviour of vortex breakdown location obtained from flow visualization. The fluctuations consist of quasi-periodic oscillations and high-frequency low amplitude displacements. The quasi-periodic oscillations are due to an interaction between the vortices, which cause the antisymmetric motion of breakdown locations for left and right vortices. The oscillations are larger and more coherent as the time-averaged breakdown locations get closer to each other as angle of attack or sweep angle is varied. The frequency of this organized motion is much smaller than the frequency of any other known instabilities. On the other hand, the most probable frequency for the high-frequency small-amplitude fluctuations of breakdown location is in the same range as the frequency of Kelvin–Helmholtz instability of the separated shear layer. A mechanism for the interaction between the vortices causing the oscillations of breakdown location was proposed. When a splitter plate was placed in the symmetry plane of the wing, the large amplitude quasi-periodic oscillations of breakdown location were suppressed.