Vortex Ring State

The tunnel fan induces an upward velocity of about the same magnitude as the rotor-induced velocity. The rotor operates in a doughnut-shaped mass of revolving air, which goes down through the rotor and up around the outside of the tips. This is the vortex ring state. Since no definite and continuous wake exists, the momentum concepts cannot be used. In the extreme case, when the rate of descent is equal to the induced velocity, there is no net mass flow through the rotor, and the momentum equations would indicate that no thrust could be developed. In reality, however, the rotor does have a significant thrust capability even in this condition. The classical boundaries of the vortex ring state are from hover to a rate of descent equal to twice the hover-induced velocity.

Windmill Brake State

If the tunnel upflow is increased to the point where it is higher than the rotor – induced velocity, both the local flow at the rotor and the flow in the remote field are up; there is a well-behaved wake above the rotor, and the momentum concepts can again be used. This condition is known as the windmill brake state, and its governing equation is: