The sound field of a rotating dipole in a plug flow.

An analytical far field solution for a rotating point dipole source in a plug flow is derived. The shear layer of the jet is modelled as an infinitely thin cylindrical vortex sheet and the far field integral is calculated by the stationary phase method. Four numerical tests are performed to validate the derived solution as well as to assess the effects of sound refraction from the shear layer. First, the calculated results using the derived formulations are compared with the known solution for a rotating dipole in a uniform flow to validate the present model in this fundamental test case. After that, the effects of sound refraction for different rotating dipole sources in the plug flow are assessed. Then the refraction effects on different frequency components of the signal at the observer position, as well as the effects of the motion of the source and of the type of source are considered. Finally, the effect of different sound speeds and densities outside and inside the plug flow is investigated. The solution obtained may be of particular interest for propeller and rotor noise measurements in open jet anechoic wind tunnels.

Comment on "On Dowell's simplification for acoustic cavity-structure interaction and consistent alternatives [J. Acoust. Soc. Am. 127, 22-32 (2010)]".

This Letter describes a correction to the equations used in Ginsberg [J. Acoust. Soc. Am. 127, 22-32 (2010)] to include the rigid-body (Helmholtz) cavity mode in the modal series solution when using Dowell's method. While the correction is easy to implement, it significantly affects the results and the conclusions when using the modal series solution. Specifically, the correct formulation of Dowell's method now predicts results that agree very well with the exact solution even to the lowest frequencies.