"Rotor Tip Vortex / Airframe Collision Effects on the Vortex and Airframe".

Mahalingam, R., Komerath, N.M.

AIAA Paper 96-2013, 27th AIAA Fluid Dynamics  Conference, New Orleans, LA, June 1996.

This paper describes progress towards understanding the interaction between a vortex-dominated wake from a two-bladed rotor,  and a solid cylinder surface. Prior work had taken this problem to the level where the flow was predictable except for the details of the collision between the tip vortex and the surface. The substantial role of the core axial velocity, postulated from pressure data on the airframe, is now confirmed  by direct velocity measurements. Both the tip vortex core and the inboard vortex sheet, with opposite vorticity,  have substantial  wake-like velocities:  further evidence that the axial velocity is due to the no-slip condition.  The vortex shows interior structure: Outside the core,  there are  multiple thin regions of jet-like velocity,  attributed to the induced effect of the rolling-up vortex sheets.   This appears to resolve the contradiction between Euler results (jet-like axial velocity) and experiments (wake-like core).   During the collision,   the axial velocity at the top of the airframe persists even after the flow has stagnated on the Advancing Blade Side,  further evidence of  the blade-wake genesis of this flow. Suction peaks on the RBS stay for a long duration.  The levels of suction observed are far above what can be explained using potential flow theory alone.

Complete paper in pdf format