DISCRETIZATION ERRORS INHERENT IN FINITE-DIFFERENCE SOLUTION OF PROPELLER NOISE PROBLEMS

Global errors arising from the use of a finite difference approximation to solve propeller noise problems are investigated. It is found that the calculated waveforms of the finite difference solutions are subjected to severe distortion due to dispersive effects. This is so even when the Fourier amplitudes of the different blade-passage harmonics of the acoustic disturbance of the propeller are adequately predicted. In addition, it is found that finite difference solutions can produce spurious acoustic radiation. The spurious radiation is a form of aliasing error. The relative intensity of the spurious acoustic radiation can become significant at high subsonic flight Mach number and at high subsonic blade-tip rotational Mach number. Also, the relative intensity is higher for the higher order blade-passage harmonics.