Computational methods for wake modeling and blade airload determination in hover and forward flight

Abstract

Two methods for calculating the wake geometry and blade loads for a hovering helicopter rotor are presented. One approach assumes that the wake trailed from the rotor rolls up into discrete vortices according to the Betz criterion for conservation of momentum. A simplified model is used to represent the vortex system. These vortices are then tracked according to the cortex transport law as they move under the influence of the velocities induced by the complete wake and rotor. Induced velocities are determined from the kinematic relationships (Biot-Savart law). The other approach incorporates the same simplified free wake model of the rotor in a finite difference calculation of the flow field. A variation of the “cloud-in-cell” technique, modified to eliminate self-induced velocity errors for curved vortex filaments, is used. Calculations showing the effect of vortex core size and the number of vortex filaments representing the wake are presented. For large numbers of vortices it is seen that the wake geometry fails to converge. However, only a few vortices are needed to adequately represent the wake. Comparisons with experimental results are presented for both methods. The paper concludes with a brief discussion of a recently developed method for computing blade airloads in forward flight.