The dispersion properties of the structural and acoustic waves in the coupled plate-fluid system are presented. The thin elastic plate considered as a simply supported one in the first case and as a part of an unlimited wall of a cannel with flowing fluid in the second case. The linear potential flow theory for inviscid fluid is used. The frequency and modal characteristics are analysed in the range of parameters where the coupling effects between vibration of plate and fluid are either weak or dominant.

The properties of simply supported steel plate interacting with air in cannel for some dimensionless parameters of system was studied by using ANSYS finite element code. As a result of this calculation some dispersion characteristics and fluid pressure distribution in the cannel were obtained.

The frequencies and modal properties of this system as a result of numerical solution (by using ANSYS modelling) are compared with some experimental results.

It is shown that in the fluid-elastic system the strongest acoustic-structural coupling exists if the resonances of acoustic and mechanical systems are sufficiently close. In this case the coupled fluid-mechanical system has two different natural frequencies that are neither pure structural properties nor acoustical. Consequently the fluid-plate arrangement can not be studied separately for the structural and acoustical resonances. Even a light medium can significantly change the spectrum of natural frequencies of a structure. Some expressions for calculation of critical flow velocities are derived.

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