Modelling rotor wakes in ground effect

Numerical prediction of the geometry of the rotor wake and its effect on the performance of the helicopter, when in ground effect, remains a challenge. This is because certain experimentally observed features of the rotor flow field, such as the formation of the characteristic "ground vortex" and the dynamics of its interaction with the remainder of the rotor flow, require extremely long-term calculations, in computational terms, to capture. The development of the rotor now field in ground effect is studied using a computational model which, through its vorticity conserving properties, is ideally suited to capturing vortical features in the flow that take a large number of rotor revolutions to develop. Computations confirm experimental observations that the geometry of the rotor wake undergoes a transition through a set of qualitatively different flow states as the helicopter's forward speed is increased. Transition between states is mediated by what appears to be a convective instability of the vortex sheet generated on the ground plane by the rotor. Certain characteristic features in the rotor thrust and power, and in the variation of control angles with forward speed, can be traced back to the dynamics of the vortical structures produced by the growth of this instability.