Computational Comparisons between FW-H and Direct Acoustic Predictions

Acoustic calculations are made for a three blade single and dual rotors in hover using conventional acoustic analogy and directly from the CFD solver. The acoustic results directly from overset CFD solver with high order finite-difference schemes were able to produce remarkably similar results to the acoustic analogy solver out to ten radii. In the near field, the results directly from the solver were substantially higher than the acoustic analogy calculations. The off body grid density was increased to improve the resolution at up to ten radii away from the rotor. The computed results showed no numerical dissipation as long as there were at least four grid points per acoustic wavelength. Minimal dissipation was seen with at least two grid points with large numerical dissipation for less than one grid point per wavelength. The dual rotor results showed very similar patterns of acoustic constructive and destructive interference. The regions of interference showed some difference between the methods. Overall, acoustic computations directly from the CFD solver are shown to be comparable to conventional acoustic analogy computations while also having the ability to directly account for complex geometry. Grid density does need to be increased for the direct acoustic calculation but the overall increase can be limited to only the direction of interest. High frequency results at higher multiples of the blade passage frequency require greater grid density limiting direct calculations to the lower blade frequencies.