Modeling Virtual Fluid Mass in OptiStruct with MFLUID

Robert Hoglund
Robert Hoglund
Altair Employee
edited December 2021 in Altair HyperWorks

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

image

  • 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

image

  • 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

image

  • Reference MFLUID in dynamic subcase setup section

image


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

image


Free-free modes on a polypropylene tank with and without MFLUID


image

  • 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


image

  • 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

image

Tagged:

Categories