Helicopter rotor noise prediction using a convected FW-H equation in the frequency domain

In this paper, high-speed impulsive noise radiated from helicopter rotors in hover and forward flight is computed by using a convected FW-H equation in the frequency domain with a rotating permeable data surface. Compared to the time-domain formulations, the frequency-domain formula avoids retarded time computation and flow data interpolation with respect to the retarded time. In addition, the complicated emission surface construction is obviated, while a supersonic rotating data surface is chosen to account for quadrupole sources outside the sonic cylinder. The selection of a rotating data surface close to the rotor blades improves noise source resolution and thus ensures noise prediction accuracy at a low computational cost, compared to a stationary data surface approach. Besides, the use of a rotating data surface for hovering helicopter rotors enables noise source identification by a steady flow simulation. Application of the methodology to hovering and forward-flight helicopter rotors has shown good agreement with experimental data, thus indicating that the approach is an accurate and efficient computational tool for noise prediction of helicopter rotors operating at high tip Mach number. Since the formula is derived in a moving medium frame, the effect of forward-flight speed on the radiated noise can be explicitly taken into account.