The fluid dynamics of the vortex ring state.

Context. Fluid mechanics, wakes, vortex dynamics, rotor wakes

Brief description: Vortex wakes trailed from rotors (helicopters or wind turbine) can collapse into an unstable vortex ring state under certain conditions of imposed rotor relative velocity. The phenomenon is dangerous for any rotorcraft to be in, and pilot strategies exist for avoiding it or flying out of it, but it is particularly hazardous for side-by-side rotor configurations where dangerous rolling moments will occur. The problem may occur in floating wind turbines, where the unsteady loading on the long turbine blades may cause unexpected aeroelastic phenomena and degrade the performance of the turbine. Additionally, rotor driven reverse thrusters may go into a vortex ring state, and the unsteady forces will not produce the expected or required braking effect. The emphasis of the project is upon the understanding of the basic mechanism that leads to the collapse of the rotor wake into the vortex ring state.

Aims and objectives: develop description of development of vortex ring state. Run rotor driven and jet driven flows in the wind tunnel, and take detailed measurements of the collapse of the wake and generation of the vortex ring state. Develop theoretical model and perform stability analysis of the flow field.

Novelty: the research will consider how a prototype of the mean flow will collapse into a vortex ring state like flow. The principle aim is to understand whether the phenomenon is driven by the trailing vortices, or is it the mean flow that is unstable.

Alignment: the project is in the area of fundamental and applied fluid mechanics. It will use National Wind Tunnel Facility resource.

Potential applications and benefits: the direct application is to helicopter and wind turbine aerodynamics, but more will be known about the stability of vortex driven flows.