Vortices incident upon a leading edge: instantaneous pressure fields

Abstract

A mixing-layer flow formed by merging of high- and low-speed streams leads to successive generation of vortices of the same sense that impinge upon a leading edge. Distortion of each incident vortex, secondary-vortex shedding, and 'sweeping’ of flow about the tip of the edge are related to the instantaneous pressure fields via simultaneous flow visualization and pressure measurement. The instantaneous pressure fields are interpreted as downstream travelling waves along the upper and lower surfaces of the edge; in turn, these wavelike pressure variations are linked to the visualized vortex patterns adjacent to each surface.

Near the tip of the edge, where rapid flow distortion occurs, the pressure fields are non-wavelike; on the lower surface of the tip, negligible streamwise phase variations of fluctuating pressure are associated with secondary shedding there, while on the upper surface there is a phase jump. This jump can be as large as π when the incident vortex impinges directly upon, or passes just below, the tip of the edge. Downstream of this near-tip region, the wavelike pressure fields show short and long wavelengths on the lower and upper surfaces respectively. These wavelengths, in turn, differ substantially from the wavelength of the incident-vortex instability.

Irrespective of the transverse location of the incident vortex with respect to the leading edge, maximum pressure amplitude always occurs at the tip of the edge; it takes on its largest value when the scale of secondary shedding from the tip of the edge is most pronounced. Moreover, the fact that the net force on the edge scales with tip-pressure amplitude underscores the crucial role of the local flow distortions in the tip region.