Abstract:
A robust CFD/vortex particle coupling numerical method for simulating the three-dimensional flow field of a helicopter rotor is established in the present study. Taking the Caradonna-Tung rotor as the research model, the vortex ring state appeared in the vertical descent flight is simulated with the strategy of gradually increasing the vertical descent rate at a fixed collective pitch. The simulation successfully reveals the highly turbulent flow field around the rotor in the vortex ring state. Numerical simulation results also show that the rotor enters the deep vortex ring state when the vertical descent rate approaches the hovering induced velocity, where the rotor thrust and the lift force at each blade section decrease rapidly. Based on the numerical simulation data, the aerodynamic noise characteristics of the rotor are analyzed. With the increase of the vertical descent rate, the frequency spectra of the loading noise change dramatically, and the amplitude of noise with intermediate frequencies more than 10 times higher than the fundamental frequency increases. Based on this finding, a new method of installing microphones on the helicopter fuselage is proposed, which can provide a timely warning of the rotor encountering the vortex ring state by a real-time analysis of the rotor noise spectrum components.