Formation and evolution of tip vortices of an isolated rotor in forward flight

The most important aspects of the flow field are the blade tip vortices. The proximity and strength of these tip vortices to other blades and the airframe can have major effects on the aerodynamics and aeroacoustics of the vehicle. Realistic models are required to accurately predict these effects.; There is considerable uncertainty in existing vortex prediction models, due to limited knowledge about the vortex structure. This uncertainty relates to rotor vortex decay rates and whether “vortex jitter” is a fundamental fluid dynamic phenomenon. Before answering questions about vortex decay, uncertainty due to “jitter” must be removed.; Another important aspect of the tip vortex flow field involves the physics of the vortex formation process. Changes to the blade tip can affect vortex properties, potentially decreasing the strength of vortex interactions.; Laser velocirnetry was performed on the flow field surrounding a rotor in forward flight to examine the vortex formation and evolution of the near wake tip vortex properties. In the vortex formation process, two mechanisms are dominant: (1) fluid spilling over the blade tip, and (2) a structure forming above and inboard of the blade tip. These two flows evolve into the circumferential velocity and the axial velocity deficit observed in the vortex core. The resulting flow features are not concentric. The nascent vortical structure on the blade upper surface is elliptical. The circulation of this structure grows until the trailing edge and accounts for the full bound circulation. The proximity to the previous blade's tip vortex causes strong interactions with the nascent structure in the wake immediately downstream. Within 40 degrees of rotor azimuth, the remaining tip vortex circulation decreases to roughly 50% of the bound circulation.; This work on the near wake completes the first measurements of near wake vortex properties in forward flight covering nearly one and a half rotor revolutions. After removing uncertainty due to vortex jitter, the vortex remains strong over the measured interval.; Analysis of the validity of using two-dimensional models to estimate velocity error due to particle spinout was performed. Comparison with experimental data shows that errors predicted in current models are overly conservative.