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[Solution]

Contained fluid elements must be rectangular in shape and are in lower-order form only (due to how the mass is handled). They are displacement-based elements, so they result in symmetric matrices that can be solved by any equation solver. Acoustic fluid elements can be lower- or higher-order elements in ANSYS 13.0 (FLUID30 is 8-node brick, FLUID220 is 20-node brick, FLUID221 is 10-node tetrahedra). Acoustic elements are pressure-based elements, meaning that they result in unsymmetric matrices, so modal analyses require use of MODOPT,UNSYM or MODOPT,DAMP. Hydrostatic fluid elements were originally intended to provide pressure loading on a structure for an enclosed gas/liquid in a large-deflection analysis where the change in the volume of the trapped gas/liquid would result in an increase in pressure exerted on the structure. They can also be used in dynamic analyses. The treatment of mass is controlled by KEYOPT(5), where KEYOPT(5)>0 results in the mass of the liquid being lumped to the surface of the structure. Attached are three examples showing modal analyses using the 3 approaches. Because of the fact that the mass of the fluid is distributed on the surface for HSFLD242, the results are more approximate than the other two methods. For modal analysis, acoustic or contained fluid (or hydrostatic fluid) elements can be used.

Attachments:
1. 2019736.zip