Vortex-surface collision - 3-D core flow effects

Abstract

The interaction of a strong vortex with a curved surface is a basic problem in fluid mechanics, whose solution is crucial to predicting the flow around a rotorcraft. In past work, the initial stages of the interaction between a rotor tip-vortex and a cylindrical airframe were measured and computed using potential flow concepts followed by boundary layer calculations. This paper presents measurements of the lateral velocity component on the top of the airframe during the vortex interaction. These data are used in the prediction of airframe loading in a time frame far beyond that reported in previous work. The axial velocity in the core of the vortex is substantial and is directed back towards the rotor blade. The vortex core circulation scales with rotor rpm. We show here that the axial velocity plays a major role in the final stages of the collision process. Contours of lateral velocity correspond closely to contours of lateral vorticity from previous work. Core axial velocity during the collision is presented as a function of vortex age. The region of high axial velocity flattens and appears to break in two at the final stages of collision. In the computations, the tip- vortex is modeled as a family of helical vortices, whose strength and initial position are chosen based on experimentally measured core axial velocities. The analytical predictions are compared with experimentally measured vortex propagation and surface pressure. NOMENCLATURE Some symbols have multiple meanings. The usage of these symbols should be clear from the context.