Passive Design for Isolated Blade-Vortex Interaction Noise Reduction

The noise from otherwise non-lifting rotors is examined for isolated blade-vortex interactions generated by a seperate vortex generator. An Euler/ Navier-Stokes solver is used in conjunction with the acoustic analogy. The prediction tool is validated against experiment for a simple rectangular blade undergoing a parallel interaction at 180 degrees of azimuth. The agreement with experimental data is excellent and the insensitivity of the solution to various input paramters is demonstrated. Various planforms are examined to determine the role of planform in isolated blade-vortex interaction noise reduction. All of the planform shapes are chosen to have identical nominal thrust, weighted area, and nominally zero pitching moments. Sweep is seen to play a powerful role in the forward flight thickness noise as well as the out-of-plane blade-vortex interaction noise. Favorable phasing effects can result in reduced focusing of the waves, and hence lower peak sound pressure levels. The results are also demonstrated to be applicable to oblique interactions. The unsteady loading noise is found to be relatively insensitive to the spanwise distribution of chord, allowing for other criteria to determine the use of increased chord or tapered sections.