Aeroacoustic analysis of a lift-offset coaxial rotor using high-fidelity CFD/CSD loose coupling simulation

This paper presents the aeroacoustic analysis of a lift-offset coaxial rotor in high-speed forward flight using the high-fidelity computational fluid dynamics/computational structural dynamics (CFD/CSD) loose coupling software Helios. Acoustic simulations are performed using the software PSU-WOPWOP at eight microphones positioned below the coaxial rotor. The total power of the three speed cases—100, 150, and 200 kt—is validated against flight-test data and shows good agreement. A series of parametric studies is also conducted to investigate the effect of lift offset, flight speed, and rotor-to-rotor separation distance on acoustics of the coaxial rotor. Strong blade-crossover and self-blade–vortex interaction events of the coaxial rotor, which are major sources of loading noise, are captured via high-fidelity CFD simulations in all speed cases. Highly impulsive acoustic pressure signals are identified in all simulation cases, and the magnitude of mid-frequency sound pressure level (SPL) increases significantly with increasing flight speed and lift offset. The strength of mid-frequency SPL, on the other hand, is reduced significantly as the rotor-to-rotor separation distance increases at 100 kt. However, the higher speed cases do not show a significant reduction in mid-frequency SPL with increasing separation distance.