The role of viscosity in aerodynamic sound generation

The interpretation of unsteady surface shear stresses as dipole sound sources is discussed. Provided omega nu c2 <<1, it is legitimate to calculate the radiated pressure using viscous surface dipoles in combination with an inviscid-medium Green function. This is demonstrated via comparisons of acoustic-analogy predictions with exact solutions, for two plane-boundary problems. A Green function that is tailored to the linear relation between pressure and normal velocity on the boundary, as used by Powell [J. Acoust. Soc. Am. Vol. 32, 1960, pp. 982-990], highlights the role of viscous stresses in aeroacoustic calculations