Design of Rotary Wings with Passive Mitigation of Coherent Tip Vortex Roll-Up

A rotor blade design method is presented that aims to passively mitigate the formation of coherent tip vortex structures in the near-field of the rotor wake. The method leverages fundamentals principles of Helmholtz’s theorems, where the strength of the shed vortex wake is proportional to radial gradients in the blade loading distribution. By avoiding strong radial gradients in the blade loading distribution commonly found in conventional rotary wing designs, the near-field roll-up of the wake vortex can be mitigated. A pair of two-bladed rotor designs were produced using a finite-vortex rotary lifting line framework coupled to a constrained power optimization problem. A baseline rotor was designed based on a conventional, power-optimized approach, alongside a rotor configured with an additional blade-root bending moment constraint to produce a wake-optimized blade load design. Both rotors were configured with the same, lightly-loaded thrust coefficient, and resulting thrust and torque performance for both rotors was verified using a static thrust stand. Phase-averaged stereoscopic particle image velocimetry data were acquired for the two-bladed rotor system in a hover condition. The wake-optimized rotor blade was observed to produce a wake characterized by conical vortex sheets of significantly lower peak vorticity than the compact, helical tip vortices observed for the baseline power-optimized design. As expected, the baseline design was found to produce a relatively uniform axial flow distribution, whereas the reduced tip vortex design exhibited increased axial velocities near the root. This induced flow profile resulted in a radial distortion of the flow structures produced by blade passages, including the shed vortex sheet. The flow structures shed by the wake-optimized design were also found to decay at a faster rate than that for the baseline design.