structural acoustic modelling : Structural modes vs acoustic modes

[Marekcom]

Dear all,
Could anyone provide a clear explanation of the difference between acoustic mode, structural mode and coupled modes when performing vibro-acoustic modelling of a mechanical system ?
Once getting the natural frequencies and participation factors of a free vibration system using a structural acoustic coupling, how can I identify structural modes from acoustic ones ?
Thanks for any help you can provide.
Cheers.
Marek

Structural stress Engineer

 

[dmapguru]

In MSC Nastran, vibro-acoustics is called fluid-structure analysis. The fluid may be a gas or a liquid but in both cases the linear small displacement theory either allows the modes of the structure to be computed separately from the modes of the fluid or they can be computed as coupled modes.

In the uncoupled solution method, the modes of the fluid and structure are computed separately. The modes of the structure are computed as if the fluid was absent (called in vacuo). The modes of the fluid are computed as if the structure was absent; the free faces of the fluid cavity are assumed to have rigid behaviour (perfectly reflecting). In this case, it is easy to identify structure modes and fluid modes as they are in separate modal bases where the eigenvalues and eigenvectors are output separately. Any coupling occurs when the modal bases are used to reduce the physical problem to a modal one, where the physical coupling between fluid and structure is reduced to modal coupling using the 2 modal bases.

In the coupled solution method, the modes of the fluid and structure are computed together. Each provides a boundary condition for the other, and the presence of each changes the behaviour of the other.

If your fluid is a gas, like air, then the uncoupled method is usually the chosen one. This is because the presence of an air cavity (or cavities) does not significantly change the behaviour of the structure, and the presence of the structure does not significantly change the behaviour of the fluid. It is computationally less expensive and provides accurate results when the modal bases are used for dynamic response under the condition that enough modes are used. There are cases when the fluid is a gas, but the uncoupled method is not sufficiently accurate. I remember a case of a parabolic reflector destined for a geostationary satellite where the testing of the flimsy reflector did not correlate well with the MSC Nastran model because the presence of the air (principally its mass in this case) was not taken into account; when it was, the model behaved closer to the expected behaviour.

If your fluid is a liquid, like water or gasoline, the uncoupled assumption usually yields poor results because the presence of one affects the behaviour of the other. Computing coupled modes means there is only one modal basis, that of the coupled modes of the fluid-structure. It is computationally more expensive than the uncoupled solution method. You can of course simply look at the eigenvectors on only the structural degrees of freedom or only the fluid degrees of freedom, but for each eigenvalue there is only 1 eigenvector that yields the simultaneous displacement field in the structure and the pressure field in the fluid at the same cyclic frequency.

If you want to determine which modes are contributing to some dynamic response quantity, like a sound pressure level, then (response) participation factors may be used (see the PFMODE output request for example), but this requires a modal frequency response analysis and the definition of abstract collections of DOF called PANELs.

DG