The Vortex Dynamics of the Rotor Vortex Ring Phenomenon

High-resolution computational simulations of the vortical wake of a rotor operating both near to and within the vortex ring state have been conducted using Brown’s Vorticity Transport Model. The non-linear vortex kinematics of the wake, once its geometry has diverged significantly beyond where a linear-stability analysis can yield much insight, are exposed using three-dimensional visualisations of the simulated flowfield. In order to reveal the vortex dynamics that underly the highly unsteady flow within the vortex ring state, a single-bladed rotor, that was decoupled aerodynamically from the surrounding velocity field so that it would act merely as a source of trailed vorticity, was modelled. In spite of the elimination of aerodynamic feedback between the rotor and the flow field the large-scale collapse of the hover wake into the vortex ring state wake is shown still to occur. A significant change in the dominant dynamics of the wake as it swaps from the tubular form that is characteristic of hover or very low speed descent into the toroidal topology of the vortex ring state, is identified. Initial vortex ‘pairing’ leads to re-orientation of vortex filaments parallel to the axis of the rotor. This phenomenon plays an important role in regulating the downwash that the rotor can produce and thus in precipitating the onset of the vortex ring state. The behaviour of the wake in the high-speed vortex ring state that is approached from the windmill brake state is also investigated. The kinematics and structure of the wake when it lies above the rotor is found to be subtly different from that when it lies predominantly below. Considerable and persistent coherence of the vortical structure of the wake when in the vortex ring state is revealed, as are the mechanisms that lead to both small-scale and large-scale wake breakdown events.