AN EFFICIENT AND ROBUST METHOD FOR COMPUTING QUADRUPOLE NOISE

A new far-field approximation to the Ffowcs Williams-Hawkings quadrupo le source, which is valid for an in-plane observer and utilizes preint egration in the direction normal to the rotor disk, is presented. This approximation is new and unique in that no further approximation of t he quadrupole source strength is made and integrands with r(-2) and r( -3) dependence are retained, The far-field approximation leads to quad rupole integrals of the same form as thickness and loading noise, Quad rupole noise prediction is carried out in two parts: a preprocessing s tage in which the previously computed flaw field is integrated in the direction normal to the rotor disk, and a noise computation stage in w hich quadrupole surface integrals are evaluated for a particular obser ver position. The predicted noise agrees well with both experimental d ata and Euler predictions for a hovering rotor up to a hover Mach numb er M-H = 0.9, even though only subsonic quadrupole sources are utilize d in the present implementation, The method is robust and requires com puter resources comparable to thickness and loading noise prediction.