newbie FEBio question - contained fluid

[staed2023]

Greetings,

I am a relative newbie to FEBIO, I am more typically an ANSYS user. Sometimes I need to model a hollow object under pressure from an internal, contained fluid. In ANSYS I would use a specific element for that. It would still be a structural/solid simulation, not fluid or FSI.

Could anyone tell me what is the appropriate way of modeling such a thing on FEBIO?

Thanks for any help that can be provided.

[jthiesen]

Hi Staed.

If you are working with a biphasic (poroelastic) model, this pressure can be prescribed as an essential boundary condition over the pore pressure field. However, if your object is a single phase model, this can be achieved by specifying an external surface pressure over the inner surface of the hollow object, which acts as an external traction.

Best regards,
José.

[ateshian]

Hi,

Alternatively, if you want to use only a structural analysis where an enclosed domain is fluid-like, you can use the "udg-hex" element type with an uncoupled elastic material (e.g, "Mooney-Rivlin") where you set the elastic constants to be negligibly small and pick the bulk modulus to match that of the fluid. I have attached a sample problem ( TestMRasFluid.feb ) that illustrates how this can be done. Within FEBioStudio, you can set this element type by expanding Objects->Parts within the model tree, then select the part that needs to be modeled using udg-hex, Under Properties->Solid Formulation select "udg-hex".

Best,

Gerard

[staed2023]

Thank you both for the information.

Follow-up question: for udg-hex, do the underlying elements need to be of any specific shape? when I try in my mesh (where they are hex-4) I get the following error in the SolidDomain section: "unrecognized tag". In the TestMRasFluid example, the elements are hex-8 and this error does not occur.

Thanks once again.

[ateshian]

For tet4 elements use "ut4" for the solid formulation. It should have the same effect as udg-hex does for hex elements.

Best,

Gerard

[maas]

Another approach to consider is to use a volume preserving constraint. In that case, you can keep the "fluid" part hollow, and apply this constraint to the surrounding surface. The constraint will then apply a pressure that aims to preserve the volume. Take a look at the test suite cases vc01 and vc02 for examples (see here).

Best,

Steve