1. The tip vortex completely overpowered the shear layer, going right on through the separated flow region with little modification of strength, structure or trajectory. Thus, there was little interaction effect.One did not need to know a lot about the details of the shear layer to predict such a flow, then: a simple vortex sheet could have modeled the shear layer, for all that it mattered. There was really no need for Navier-Stokes calculations.
2. Is this result general? Consider this: The relative strengths of the vorticity in the shear layer and the tip vortex are what determine their interaction. Helicopter designers don't usually design sharp baksteps into airframes. The vorticity of the shear layer, for a given extenal flow speed, can be no stronger than that in a shear layer coming off a sharp backstep. How about the flow speed? Here the crucial parameter is the Advance Ratio. At high speeds, where the shear layer gets stronger, the advance ratio is large: the wake gets swept back above the airframe, and never hits the fuselage. At low advance ratio where the interaction is strong, the shear layer is even weaker on practical configurations than in our test case. So our result is general.
This means that including flow-separation regions in rotor/airframe interactions is feasible if one can get a reasonable estimate of where to attach vortex sheets.