Design of Propellers with Passive Mitigation of Coherent Tip Vortex Roll-up

A propeller design method has been developed that passively mitigates the formation of coherent tip vortex structures in the near-field of the rotor wake. Using the blade root bending moment coefficient as a surrogate variable, gradients in circulation in the radial direction are avoided in a constrained optimization problem. An additional propeller was created with an alternative optimization scheme to directly penalize gradients in shed circulation radially using a logarithmic function as a comparison. A series of wind tunnel experiments are utilized to verify design predictions and diagnose topological characteristics of the propeller vortex wakes for a conventional power-optimized design and a vortex-attenuated design. Each set of blades designs were optimized for the same specified thrust coefficient, freestream velocity, and design RPM. Thrust and torque data were taken using a test stand to verify that the design performance characteristics were achieved. Phase-locked stereoscopic particle image velocimetry data were acquired across a range of wake phase angles for the power optimized and vortex-attenuated propellers, with a single phase taken for the alternative wake optimized design. The power optimized wake showed typical coherent tip vortex rollup behavior resulting in a strong double helix wake, this is in direct contrast to the vortex-attenuated propeller which featured a distributed sheet of wake vorticity arranged in a conical spiral pattern. Additionally, the wake optimized propeller wake was shown to dissipate faster than the power optimized case. The baseline power optimized design behaved as expected with a uniform axial flow distribution, whereas the wake optimized designs both had significantly increased axial velocities near the center of rotation. This observation is due to the increased inboard induced axial and tangential velocities, promoting an accelerated swirl around the center of rotation. The interaction of the power optimized and vortex-attenuated wakes with a downstream body was also analyzed with stereo particle image velocimetry. Significant differences were observed, with large reductions in flow variability with the vortex-attenuated wake at the measurement plane analyzed.