On the impulsive blocking of a vortex–jet

Abstract

In this paper we consider the flow field within and around a vortex as it ‘collides’ with a thin plate at a right angle to its axis of rotation. We show that based solely on inviscid flow theory, vorticity in the core of the vortex is redistributed significantly. The main cause of this redistribution is the presence of axial flow within the vortex; we call this vortical structure which contains axial flow a vortex–jet. In this work we show that when the axial velocity within the vortex is toward the plate, vorticity is redistributed radially outward from the core resulting in a significant reduction in the axial vorticity there; the vortex is said to ‘bulge’ reflecting an increase in the nominal vortex core radius. A by-product of this interaction is that the suction peak amplitude caused by the presence of the vortex rapidly decreases and the pressure soon returns to a quasi-steady distribution. On the other hand, when the axial velocity within the vortex is directed away from the surface, the suction peak persists and the vortex core radius decreases. The numerical results were validated by comparison with an analytical solution for a sinusoidal vortex jet. Analytical solutions were also derived for the initial and final states of a pure jet; the numerical results are strongly supported by the analysis. In addition, all of these results are consistent with experiments, and their relevance to the interaction between a tip vortex and a helicopter airframe is also discussed.