Vorticity fields in the wake generated by rotating blades are calculated using a time-marching free-wake method without a non-physical initial condition and the far wake model. The simulated wake geometries in the radial and axial directions agree well the experimental data for a two-bladed rotor for the overall regions of the wake. This agreement can be obtained slowly increasing the rotational speed of rotor blade, instead of an impulsively rotating blade. The unsteady generation process of the wake is calculated for a single rotor with two blades.
The computed free-wake geometry of a single rotor in hover is represented by the three wake regions: well-defined tip vortex region, intermediate pairing region, and initially generated wake bundle. The wake structure near the rotor and the airloads m hover are converged after 10 revolutions of the rotor from the initial start The intermediate pairing region is very important in predictions of unsteady loadings and the noise in certain conditions. This region can be simulated after 20-30 revolutions of the rotor.
Two additional cases are calculated, where the wake plays an important roles; coaxial rotors in hover and forward flight and wakes coupled with rotor dynamics in descent flight.
In case of coaxial rotors, the wake geometries, generating from the upper rotor and lower rotors, are compared with those for four-bladed single rotor case. It can be also found the optimum distance between the upper and lower rotors gives a maximum loadings for coaxial rotors.
The time-marching free-wake method is coupled with nonlinear rotor motions for fully articulated blades with hinge-offsets. The aerodynamics and rotor motions are calculated directly without the iterative method of the trim and motion equations used in linear rotor motion analysis. The transition from the hovering to vertical descent predicted. In vertical descent flight of 45 % of the induced velocity on the disk in hover, a lower oscillating frequency of blade airloads is observed due to the pairing motion of the wake.
