# MFLUID

Bulk Data Entry Used to define properties of a virtual fluid mass. A virtual fluid mass mimics the mass effect of an incompressible inviscid fluid in contact with a structure.

## Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
MFLUID SID CID ZFS RHO WSURF1 WSURF2 PLANE1 PLANE2
RMAX

## Example

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
MFLUID 25   32.0 42.0 45

## Definitions

Field Contents SI Unit Example
SID Unique set of identification numbers.

No default (Integer > 0)

CID Coordinate system identification number in which its z-axis is assumed to be normal to the free surface.

Default = 0 (Integer > 0, or blank)

ZFS Location of the free surface on the z-axis of the CID coordinate system.

Default = 1030 (Real)

RHO Fluid density.

No default (Real ≥ 0.0)

WSURF1 Set identification number of a SURF or SET Bulk Data Entry. This list identifies shell elements that are damp on only one side of the element or an exterior face of solid elements (see SURF,ELFACE).

Default = 0 (Integer ≥ 0, or blank)

WSURF2 Set identification number of a SURF or SET Bulk Data Entry. This list identifies shell elements that are damp on both sides of the element.

Default = 0 (Integer ≥ 0, or blank)

PLANE1 Symmetry type on x-z plane of fluid coordinate system (CID).
N (Default)
No symmetry
S
Symmetric
A
Antisymmetric

(Character)

PLANE2 Symmetry type on y-z plane of fluid coordinate system (CID).
N (Default)
No symmetry
S
Symmetric
A
Antisymmetric

(Character)

RMAX Interaction between two elements is ignored, if the distance between them is greater than RMAX.

Default = 1010 (Real > 0.0)

1. An MFLUID entry must be selected by the MFLUID command in the Subcase Information section.
2. More than one MFLUID entry may be specified to define multiple fluid volumes.
3. Fully enclosed wet surface without any open surface is currently not supported.
4. Either WSURF1 or WSURF2, or both must be specified and non-zero. In other words, WSURF1 and WSURF2 cannot be both blank or zero.
5. An element with all its vertices located on or above the free surface will be ignored in the fluid volume's mass calculation. An element vertex is considered to be located on the free surface, if the vertex is located within a distance that is $0.01\sqrt{2.0*\mathit{Element}\text{'}s \mathit{Area}}$ from the free surface.
6. By default for elements referenced by WSURF1, the damp side of the element is assumed to be on the same side as the element's normal. If this condition is not true for a given element, then the standard SURF entry format with ELFACE (not the alternative SET format) must be used to redefine the normal into the fluid by setting NORMAL=1 on SURF entry.
7. ELIST is an alternative to SET and SURF, but ELIST is intended only to provide compatibility with Nastran decks. It is recommended to use SURF/SET instead of ELIST. Additionally,
• The SURF and SET entries may not be combined with ELIST entries to define damp elements.
• ELIST entries are internally converted to SET entries. If ELIST is referenced on a minimum of one MFLUID entry, then all MFLUID entries in a model must reference ELIST entries. In such cases, if an MFLUID entry references a particular Set Identification Number (<SID>) to define WSURF1/WSURF2 fields without a corresponding ELIST,<SID> in the model, then OptiStruct will not search for a SET/SURF,<SID>.
8. Planes of symmetry may be defined with PLANE1 and PLANE2.
• "S" means zero displacement normal to the plane.
• "A" means zero pressure normal to the plane.
• Consistent structural boundary conditions should be applied to grids located on the planes of symmetry.
• If PLANE1 is "S" or "A", then all damp elements must not cross the X-Z plane and be on the same side of the X-Z plane as all other damp elements.
• If PLANE2 is "S" or "A", then all damp elements must not cross the Y-Z plane and be on the same side of the Y-Z plane as all other damp elements.
9. Dry mode output in the .out and .h3d files is available only when PARAM, VMOPT is specified.