ON THE IMPULSIVE BLOCKING OF A VORTEX-JET

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 rotat ion. We show that based solely on inviscid flow theory, vorticity in t he core of the vortex is redistributed significantly. The main cause o f this redistribution is the presence of axial flow within the vortex; we call this vortical structure which contains axial flow a vortex-je t. 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 nom inal vortex core radius. A by-product of this interaction is that the suction peak amplitude caused by the presence of the vortex rapidly de creases and the pressure soon returns to a quasi-steady distribution. On the other hand, when the axial velocity within the vortex is direct ed away from the surface, the suction peak persists and the vortex cor e radius decreases. The numerical results were validated by comparison with an analytical solution for a sinusoidal vortex jet. Analytical s olutions 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 th eir relevance to the interaction between a tip vortex and a helicopter airframe is also discussed.