Model for Rotor Tip Vortex-Airframe Interaction Part 3: Viscous Flow on Airframe

Abstract

The behavior of vortex systems in the vicinity of solid surfaces is a matter of intense interest in rotorcraft aerodynamics, as well as in many other areas of fluid dynamics. We consider the viscous flow on a simplified model of a helicopter airframe due to a helicopter rotor tip vortex both experimentally and computationally. As the tip vortex approaches the airframe, the computational results predict the genesis of a region just upstream of the main vortex, characterized by reversed flow and rapid growth in size. The experiments clearly show evidence of such a region under the tip vortex in the region predicted by the computations and at roughly the same time. The secondary vorticity field in the computations is of a sign opposite to the vorticity associated with the tip vortex. Results for the streamline patterns and vorticity field during the genesis of the secondary eddy are presented and compared with experimental flow visualization results.

Normal Vortex Interaction with a Circular Cylinder

“…A third objective is to examine the topological form of the separated secondary vortex structures as they are advected away from the cylinder and interact with the primary vortex. This study complements that of Affes et al, 15 which focuses on the initial ejection of vorticity from the cylinder boundary layer.…”

“…11 Computations of vortex-cylinder interaction using an extended formulation of vortex lament theory that accounts for core radius variation due to axial stretching are reported by Marshall and Yalamanchili. 12 Computations of the boundary-layerresponse on a cylinder in the vicinity of a vortex are reported by Affes et al 13 and Xiao et al, 14 where the vortex-induced pressure gradient tangent to the body is obtainedby the lament theory predictions.Comparisons of boundary-layercomputationswith experimental results are given by Affes et al 15 These computations indicate that the vortex-induced velocity eld induces ejection of a tongue-like region of vorticity along the cylinder leading edge. These studies solve the boundary-layerequations instead of the full Navier-Stokes equations, and the computations must be stopped at the point of boundary-layerseparation.…”

Simulation of normal vortex–cylinder interaction in a viscous fluid

“…The boundary-layer response therefore seems not to be strongly dependent on Reynolds-number variation in this range. Affes et al (1998) compare results of boundary-layer computations and experimental flow visualization for rotor vortex impact on a cylinder. Krishnamoorthy et al (1999) present flow-visualization results for the evolution of the secondary vorticity after it is ejected from the cylinder, and for the interaction of the secondary vorticity with the primary vortex.…”

A theory of vortex-surface collisions

“…There is some experimental evidence that the large streamwise pressure gradients in the interaction region may induce the development of secondary suction peak in the pressure (Affes et al 1998) on the advancing side for a short period of time prior to the collision process itself; the initiation of the suction peak may be predicted using interacting boundary layer theory (Xiao et al 1997). However, the spikes seem to be relatively small in amplitude and so for the reasons cited just above, it appears that the influence of viscosity on the collision process may be neglected to leading order.…”

“…The analysis and computations have been performed at The Ohio State University and the experiments have been performed at the Georgia Institute of Technology. While we have done a significant amount of work on the viscous flow under the vortex (Affes et al 1994, Xiao et al 1997, Affes et al 1998, we show that the collision may be described, to leading order by inviscid flow theory.…”