"Summary of the Interaction of a Rotor Wake with a Circular Cylinder".

J.M. Kim, N.M. Komerath.

AIAA Journal Vol. 33, No. 3, March 1995, p. 470-478.

Abstract: To compute the aerodynamics of a rotorcraft in low-speed flight, the interaction of the strong vortices in the rotor wake with the airframe must be modeled. Using a hemisphere-cylinder airframe and a 2-bladed rotor for reference, the various phenomena encountered during such interactions are summarized, combining previous results on various configurations with recent experimental results. Differences between the interaction at the front and aft portions of the wake are discussed. The pre-collision phase conforms to expectations from potential flow, and includes distortion of the vortex trajectory determined by the sense of rotation of the vortex. The collision phase involves complex boundary layer interactions. The axial velocity in the vortex core causes substantial asymmetry, and influences the surface pressure distribution on the airframe side under the advancing rotor blade, where the axial flow stagnates. The post-interaction vortex is much weaker, but still contains some swirl energy. Where flow separation occurs due to airframe shapes, the interaction is not modified significantly, because the vortex dominates the interaction with separated shear layers for parameter values of practical interest. Areas of remaining uncertainty are discussed.