I am interested in calculating the mode shapes and frequencies for a structure surrounded by a fluid (air or water) to get the reduced frequency from the fluid loading. When I perform an eigenfrequency study I get mode shapes and frequencies for both the structure, the surrounding fluid volume, and the coupled modes. Since I am not inerested in the modes of the surrounding fluid, only the added mass on the structure, I tried to add boundary elements to the surrounding fluid volume thinking that that would eliminate the modes of the fluid volume but nothing changed. I also tried adding the boundary elements on the surface of my structure to no effect. It looks like boundary elements are diabled in the eigenfrequency solver.
Are boundary elements always excluded in eigenfrequency analysis or am I doing something wrong?
Is there a way to calculate fluid loaded mode frequencies for a structure without also modeling and meshing a surrounding fluid volume and getting those associated modes?
Thanks, Phil