Impulsive Loading Noise of a Lift-Offset Coaxial Rotor in High-Speed Forward Flight

This study was conducted to investigate the impulsive acoustic characteristics of a lift-offset coaxial rotor in high-speed forward flight, as well as the impact of wake-grid resolution and turbulence modeling on acoustic prediction. High-fidelity computational fluid dynamics/computational structural dynamics loose-coupling simulations were performed for air-load prediction using the high-fidelity rotorcraft aeromechanics simulation software Helios, and impulsive loading noise was simulated using the acoustic prediction software PSU-WOPWOP at eight microphones positioned below the lower rotor plane. No significant discrepancy in air-load and acoustic predictions was found between the three turbulence modeling cases [Spalart–Allmaras (SA)–Reynolds-averaged Navier–Stokes, SA–detached-eddy simulation, and SA–delayed-detached-eddy simulation] with Δfinest=10%Ctip. It was found that the Δfinest=10%Ctip case showed a good convergence of impulsive loading noise at most microphones in terms of the wake-grid resolution. Isolated rotor simulations were also carried out to quantify the relative importance of the two impulsive loading-noise sources of a lift-offset coaxial rotor: self-blade–vortex interaction (BVI) and blade-crossover events. Contours of induced inflow ratio on the rotor disks revealed higher induced velocity for the lift-offset coaxial rotor than the isolated rotors. The higher induced flow ratio reduced the distribution of sectional normal force, strength of self-BVIs, and BVI noise of the lift-offset coaxial rotor. Even with the reduced BVI noise, the lift-offset coaxial rotor showed a dramatically higher impulsive sound-pressure level than the isolated rotors, especially at high speed, as a consequence of mainly blade-crossover events.