Foxwell, John Henry
Structural vibrations in acoustic fields.
University of Southampton, Institute of Sound and Vibration Research,
Theoretical studies are described on the response of certain structures to acoustic waves. The most general problem treated is the response of a long cylindrical shell periodically stiffened both longitudinally and circumferentially when excited by obliquely incident plane sound waves. Numerical calculations are given for a structure having similar characteristics to an aircraft fuselage. Calculations of the shell vibration and internal sound field are given.
A simplified mathematical model, related to the phasing of aircraft propellers, is used to show the effect on the internal sound field due to a phase difference between opposite running plane waves incident on the shell.
A procedure is described for calculating the acoustic loading on a rectangular flat plate set in a rigid wall and vibrating in sinusoidal modes. Results are given of the acoustic radiation damping.
Wide frequency band space correlation measurements of the pressure field close to a two-inch diameter cold air jet are described. The results are shown to be consistent with the hypothesis that the pressure field close to the jet is convected at a speed related to the local speed in the jet. Using the convection hypothesis a generalised non-dimensional form of the space correlation is derived and both model and full scale measurements are shown to conform.
The convection model predicts the form of the narrow frequency band space-correlation function. Tliis function is required for the calculation of structural response to jet noise. An approximate form of this function is utilised in a calculation of the response of a flat panel close to an air jet.
Space correlation measureinents in the region within five jet diameters of the orifice of a full scale jet engine exhaust show the acoustic nature of the radiated noise showing the pressure field propagating at the speed of sound as opposed to the near pressure field of the jet turbulence propagating downstream at about one half of the local jet speed.
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