Boundary layer terms in high speed quadrupole noise

The paper summarizes some ideas about the meaning of different terms which arise when the quadrupole source distribution of the Ffowcs Williams-Hawkings equation is decomposed upon double generalized differentiation of the Lighthill stress tensor. Along with a surviving volume contribution, surface terms appear, corresponding to surfaces where some relevant flow quantities exhibit discontinuity. Considering a propeller blade as the only discontinuity surface in the flow field, the relative importance of different portions of the total quadrupole is analysed. The magnitude of any term depends on the flow model selected to describe the aerodynamic field around the blade, while the overall effect of the quadrupole source is to be preserved. So, it is crucial to correctly identify the flow quantities involved in the acoustic calculation, in order to reproduce noise prediction when different representations are employed to describe the flow field.