Here are some tips and tricks for modeling virtual fluid mass in OptiStruct to simulate the effect of fluid mass in dynamic analysis. I will as provide information on setting up the MFLUID card, as well as share an example and some best practices.
Modeling a Fluid-filled structure in OptiStruct with MFLUID
- The fluid mass can have a significant influence on the normal modes of a structure
- The fluid mass effect can be modeled with the MFLUID card for cases of non-pressurized fluids with OptiStruct
- Fluids are assumed to be incompressible (no change in volume) and inviscid (no viscosity)
- Sloshing or gravity effects for the fluid body are not considered
- The structural modes are below the incompressible fluid modes
- The virtual fluid mass is considered by sets of wetted element surfaces that meet the fluid
- The technique can be used to model virtual fluid mass externally and internally (floating structures, such as ship hulls, as well as fuel tanks and pipes)
Model Setup – MFLUID
- Define a coordinate system to reference the surface level of the fluid
- The Z-axis of the coordinate system should be normal to the direction of the free surface
- Define wetted element faces which meet the fluid by creating element face sets
- For shell elements, the damp side of the element is assumed to be on the side of the element normal.
For defining the model with an existing dynamic analysis:
- Create an MFLUID card image in a load collector
- Select the coordinate system with CID and input ZFS to represent the fluid level from the origin of the coordinate system: the free surface in distance on the Z-axis
- Define density of the fluid in contact with the wetted element sets with RHO
- Select the wetted element faces which meet the fluid by surface sets with WSURF1 or WSURF2
- Use WSURF2 only if both sides of the shell element set are touching the fluid
- Planes of symmetry for damp elements can be optionally be defined with PLANE1 and PLANE2
- Reference MFLUID in dynamic subcase setup section
Fluid Modes Calculation with PARAM,VMOPT and EIGRL/EIGRA
PARAM, VMOPT determines how the fluid modes are calculated
PARAM, VMOPT, 1 uses the virtual fluid mass in the mass matrix for the analysis.
PARAM, VMOPT, 2 calculates the dry modes of the structure first, and then approximates the wet modes based on the dry mode results
- This approach is recommended for models of any significant size, in OptiStruct, greater than 9000 elements on the wetted element surface.
- The trade-off is a slight loss in accuracy for what can be a much greater reduction in computation time.
For the choice of solver, LANZCOS vs AMSES, general recommendations apply
- Looking at the results, the natural frequencies are shifted downward as the effect of added virtual fluid mass is considered, and mode shapes can be significantly different as well
Free-free modes on a polypropylene tank with and without MFLUID
- The influence of virtual fluid mass has significant effect on natural frequency and model shapes
- With PARAM, VMOPT, 2, wet and dry modes are output to the .out and .h3d files
- A .wetel file is generated which can be imported to HM to review wetted element sets
Free-free modes with WSURF1, WSURF2, and NSM
- WSURF2 considers the element is wetted on both sides of the surfaces, so the virtual mass effect results in a lower frequency
- Note: modeling virtual fluid mass with MFLUID is accepted as a more accurate method than NSM based on comparison with test data [1]
Modeling Best Practices
MFLUID Solution Types
- PARAM, VMOPT, 2 is supported for modal dynamic subcases
- PARAM, VMOPT, 1 is supported for direct and modal dynamic subcases
- It is recommended to use PARAM, VMOPT, 2 where possible to save computational time
- Request 2-4X the number of dry modes for the number of accurate wet modes desired
Fluid Volumes
- MFLUID is not designed to model fully enclosed volumes of wetted grids (this can lead to singularities and solver errors)
- Removing a single element from an enclosed volume can remove the singularity
- A local coordinate system should be defined with ZFS indicating the proper fluid level on the MFLUID card
- Multiple fluid volumes can be defined with MULTI_MFLUID in HyperMesh
Solids vs Surfaces
- Solids – create sets of element exterior faces using SURF, ELFACE
- Shells – review element normals
