Mahalingam, R., Komerath, N.M., "Rotor Tip Vortex/Airframe Collision Features". AIAA Paper 96-2013, June 1996.

This paper describes progress towards understanding the basic test case of the interaction between a vortex dominated wake from a 2-bladed rotor and a solid cylindrical surface. Prior work had taken the prediction capabilityin this test case to a level where the trajectories of the tip-vortex and the associated surface pressure were predictable until the actual collision between the vortex and the airframe. The substantial presence of the core axial velocity is shown from direct velociyt measurements. The inboard vortex sheet in seen to roll-up into a discrete circulatory region. The tip-vortex as well as the rolled up inboard vortex structure have strong velocities along the axis and directed towards the blade from which they were generated. Interior structure is seen in the tip-vortex, as previously seen by Thompson et al. This is evidence that the tip-vortex forms due to the rolling up of discrete sheets of vorticity. During the collision the axial velocity in the tip-vortex core filament on the retreating blade side persists even after the flow has stagnated on the advancing blade side; further evidence to the blade wake genesis of the core axial flow. The levels of suction on the RBS are much higher than those seen on top of the airframe.