A constrained vortex model with relevance to helicopter vortex ring state. Southampton, UK, University of Southampton, 14pp.
(Aerodynamics and Flight Mechanics Internal Report Series, AFM 05/05).
A planar model of a lifting surface descending into its own wake is constructed with the aim of demonstrating some underlying mechanisms of the ‘vortex ring’ state that may be entered
by a rotary wing aircraft during a vertical descent. The model uses line vortices that are periodically released from a point in space and then allowed to evolve in a constrained manner. For low descent velocities the model reproduces a hover state, where the wake vortices move downwards relative to the lifting plane. A critical descent speed is reached after which the model produces a quasi-periodic shedding of vortex agglomerations. This state is reached via a Hopf bifurcation of the steady state and persists until another critical descent velocity after which a steady windmill brake state is possible, in which vortices travel upwards in a regular manner. Unconstrained simulations reveal a more chaotic vortex pattern, but frequency analysis reveals an underlying structure similar to that shown for the constrained model. Besides offering a qualitative understanding of possible mechanisms of vortex ring state, the analysis suggests some dimensionless parameters that collapse the
Actions (login required)