Measurements of the trailing vortex formation, structure, and evolution in the wake of a hovering rotor

High-resolution velocity field measurements and flow visualization images were acquired in the flow field near the tip of a rotor blade operating in hover. Using three-component laser Doppler velocimetry (LDV), the measurements documented the trailing vortex formation, initial structure, and the viscous evolution of the core. The test conditions covered a range of wake ages from as young as one degree, up to about one rotor revolution. For each wake age, vortex core properties were estimated from the velocity field measurements. The test conditions also included different tip shapes including rectangular, tapered, swept, and a subwing tip.; Preliminary measurements were used to refine the technique by understanding the sources of uncertainty and the spatial resolution requirements. This task included developing a new procedure for three-component laser Doppler Velocimetry (LDV) alignment using a laser beam profiler. A detailed uncertainty analysis of the LDV measurement technique was conducted and applied to helicopter rotor blade tip vortex measurements. Finally, spatial resolution requirements were formally established to enable a more accurate reconstruction of the velocity field associated with blade tip vortices. The accuracy of the LDV technique was further verified by an independent comparison with preliminary phase-resolved stereoscopic particle image velocimetry (PIV) measurements. The vortex velocity profiles were compared with three-dimensional LDV measurements using the same rotor test conditions and seeding medium. The various challenges of using PIV versus LDV to study the formation and evolution of helicopter tip vortices were also studied.; A set of benchmark test cases was acquired using LDV for each of the tip shapes. The measurements were supported by detailed flow visualization. The high spatial resolution obtained with LDV has shown that the tip vortex core radius can be less than 3% chord at early wake-ages, but grows asymptotically as it ages. A significant axial velocity deficit existed in the vortex core that was of the order of the peak swirl velocity at early wake-ages, but which quickly diminished as the vortex aged. Using stability analysis combined with flow visualization, the results suggest that the inner core of the vortex is mostly laminar at the vortex Reynolds numbers tested in this experiment. The evidence suggests that the entire tip vortex structure is neither fully laminar or fully turbulent, but is instead in a continuous state of dynamic evolution with a region of relatively slow laminar diffusion and a region of accelerated turbulent diffusion. It is suggested that the variation of peak swirl velocity is the result of the competing influences of an inviscid roll-up process and viscous diffusion within the vortex. The primary effects of the tip shape modification were a change in the convection speed and direction of the vortex core trajectory and a change in the magnitude of the peak swirl velocity.