PARAM, VMOPT

Bulk Data Entry Allows you to prevent the inclusion of the Virtual Mass Matrix in the Regular Mass Matrix for Modal Dynamic Subcases. In such cases, the virtual mass is added after the eigen solution and modes are modified based on the virtual mass matrix.

Parameter Values Description
VMOPT <Integer = 0, 1, 2>
=0 (Default) or 1
The virtual mass of the fluid is included in the mass matrix for all calculations. This can increase the calculation time because of the dense mass matrix on the damp surface of the structure.
=2
The eigenvalue solution is performed without adding the mass contribution from MFLUID, so that dry modes are calculated. A second eigenvalue analysis is performed to get the damp modes, which are used in modal dynamic solution. This solution is much faster with only a slight loss of accuracy.

Comments

  1. Dry mode output in the .out and .h3d files is available only when PARAM, VMOPT, 2 is specified.
  2. The mass from MFLUID in the case of PARAM,VMOPT,1 is included in the Grid Point Weight Generator output in the .out file, only if PARAM,VMMASS,YES is also specified.
  3. In order to produce the accurate wet modes results with VMOPT,2, it is recommended to request 2 to 4 times (or even higher, depending on the density of fluids) the number of dry modes than the desired number of wet modes. If the density of fluid is larger, the number of dry modes required could be larger accordingly in order to maintain the accuracy of wet modes that are based on the dry modes.