Leading edge vortex and shear layer instabilities

Abstract

The stability of a leading edge vortex was studied using available flow visualization and velocity data. The development of a vortex can be envisioned to compose of three possible stages. During the initial stage, the shear layer and its associated vorticity immediately downstream from the apex coalesce to form a primary vortex core. During this stage the vortex flow resembles a forced vortex. When the primary core is sufficiently developed, the secondary vortex becomes a prominent perturbation along the leading edge. The induced cross flow instability leads to the formation of stable, co-rotating, streamwise vortex filaments in the separated shear layer. A second stage of development occurs where the straining field of the main vortex amplifies the secondary instability near the saddle point in-between a co-rotating shear layer vortex pair. This leads eventually to the formation of a secondary vortex core consisting of a series of vortices with an opposite sense of rotation to the primary core. A vortex system as such violates the circulation distribution criterion for helical flow stability, and the vortex becomes centrifugally unstable. If the growth rate of the instability is sufficiently fast compared with the convection rate, a third stage of development occurs over the wing. The expected result is an expansion of the vortex cross-section or vortex breakdown, with the circulation eventually re-establishing a stable distribution.