Structure of the near wake of a rotor in forward flight and its effect on surface interactions

The near wake of a helicopter rotor has a significant role in the aerodynamics, structural dynamics and aeroacoustics of the helicopter. Experiments on a two-bladed rotor in axial flight demonstrate excellent periodicity and reveal a vortex pairing phenomenon. A quantitative demonstration of periodicity is performed in the wake of a rotor in forward flight. Velocity measurements made at four azimuthal locations in the rotor wake in forward flight indicate strong wake-like axial velocities in the vortex core, comparable to the core circumferential velocity at all azimuthal stations and vortex ages. On the advancing blade side, the inboard wake rolls up into a discrete structure with vorticity close to 40% of the tip-vortex but of opposite sign, and with wake-like axial velocity. Measurements near the blade tip indicate the presence of a strong spanwise flow during the vortex formation process. The evolution of core axial velocity suggests a periodic shedding pattern of rolled-up filaments at the blade tip. It is argued that the decay in core velocities is not related to turbulence in the vortex. The circulation contained in the tip-vortex is only about 40% of the peak bound circulation on the rotor blade, computed using the standard assumptions of blade-element theory. Empirical correlation of data from the literature over a wide range of test conditions confirms this finding. This indicates a need to rethink standard methods of blade-element analysis. The collision of the rotor wake with an airframe reveals the effects of the wake structure. Core circumferential velocity is associated with suction on the airframe surface prior to collision. As the vortex gets cut on top of the airframe and convects down the sides, large suction peaks occur on the retreating blade side where the core axial velocity is directed away from the surface. A milder stagnation occurs on the advancing blade side where the core axial velocity is pointed towards the surface. The inboard sheet rolls up into a concentrated structure above the airframe and causes strong suction peaks.