Model for rotor tip vortex-airframe interaction. II - Comparison with experiment

Vortex-surface collision - 3-D core flow effects

“…However, these computations were limited to the initial stages of the interaction due to the singular nature of the boundary layer equations for this flow. Affes et al 3 computed the vortex trajectories in the absence of axial flow and for a fixed core radius. Lee et al 12 postulate that the collision process is essentially inviscid and characterized by vorticity redistribution by convection causing the reduction of the suction peak on the top of the airframe seen in experiments within 1 ms (12° of rotor azimuth at 2100 rpm).…”

“…Affes et al 3 estimated the constants A,B,C,D from the experimental vortex trajectory at points as far as possible from the airframe (approximately 5 a c ). This linear model is applied here.…”

“…The trajectories of the tip-vortex have been calculated assuming potential flow outside the vortex 1,2,3 and experimentally documented 4 8,9 .…”

“…The cutoff parameter, related to the core radius a c by the expression µ = a c e -3/4 , is based on producing the correct propagation speed for a ring vortex 18 . A fixed-core radius rotor-tip vortex model of this type has been applied to calculate the vortex trajectory in the potential flow surrounding a cylindrical airframe 3 . The results indicate that when the vortex is more than one core radius away from the airframe, the modified vortex model given above is capable of predicting the experimental vortex trajectory and the pressure on the top of the airframe with reasonable accuracy.…”

Unsteady separation in vortex-induced boundary layers

“…A simplified depiction of this latter problem is provided in figure 1. The complicated unsteady and three-dimensional vortex wake is responsible for many peculiar aerodynamic features of a helicopter in flight [11][12][13]. First, the tip-vortex will interact with the rotor blades, a phenomenon referred to as blade/vortex interaction (BVI).…”