| The flow field generated by a helicopter in flight is extremely complex and it has been recognized that interactions between different components can significantly affect helicopter performance. In particular, the effects of the interaction between the rotor wake, which consists of a highly three-dimensional helical vortex sheet and a high-strength tip-vortex, and the helicopter fuselage are extremely difficult to predict and pose a challenging problem for researchers and designers in the rotorcraft area. In the present work, the unsteady interaction of the rotor tip-vortex with the fuselage and the problem of the three-dimensional boundary-layer so generated on the fuselage underneath the vortex are investigated.;In the first phase of the present work, a simplified model for the interaction of a rotor-tip-vortex with a fuselage or airframe is developed using three-dimensional potential flow analysis. The tip-vortex is idealized as a single three-dimensional vortex filament and the fuselage is modeled by an infinitely long circular cylinder. The Biot-Savart law is employed to describe the flow induced by the vortex and the flow is assumed to be inviscid and irrotational outside the core of the vortex. The present analytical/numerical results for both the vortex trajectory and the pressure distribution on the airframe are in substantial agreement with experimental results prior to impact of the vortex with the airframe. The numerical calculations indicate that a large adverse pressure gradient develops under the vortex on the fuselage causing a rapid drop in the pressure there.;The second phase of the present work focuses on the problem of the unsteady three-dimensional boundary-layer flow induced by the vortex filament moving above the fuselage. Three types of external flow in which the vortex is embedded are considered. These flows are respectively a stagnant medium, a symmetric mean flow and an asymmetric mean flow, the latter of which corresponds to the experimental conditions described above. In each case, the computed results show the development of a variety of complex three-dimensional boundary-layer separation phenomena. In all cases, the boundary-layer solutions show the formation of a secondary eddy which grows in time; this situation is expected to lead to an eruption of boundary-layer fluid into the free stream. The secondary eddy initially starts from a bubble-like shape of swirling flow and then evolves into a more complex structure; in particular, in the symmetric mean flow case, the secondary eddy evolves and grows in time and resembles the initiation of a horseshoe type vortex. |
