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SOL103 Normal Modes Analysis Using MFLUID

SOL103 Normal Modes Analysis Using MFLUID

SOL103 Normal Modes Analysis Using MFLUID

(OP)
I'm attempting to perform a normal modes analysis on a cylindrical aluminum tank filled with water. I am interested in determining the mode shapes and modal mass participation of the coupled structural/fluid response. I am attempting to model the water using a Fluid Region (fluid), but when I run the analysis I get a fatal error. My model is a simple cylindrical tank modeled with plane elements, that runs fine with the fluid region disabled. To apply the fluid region, I select the top, bottom, and 2 half-cylinder faces of the tank, uncheck the box for "Z Free Surface" because the analysis I am running is for a full tank, and specify the water mass density. Is there something in this process that I am missing which is causing the fatal error? Any help would be much appreciated; I haven't been able to find much information or tutorials regarding using Fluid Regions. Here is the fatal error that I get in the error log:

** SYSTEM FATAL MESSAGE 3034 (LNNHERR)
INTERNAL FAILURE IN THE LANCZOS PROCEDURE:
M-ORTHOGONAL QR PROCEDURE FAILED TO CONVERGE. PROBABLE CAUSE:
MASS MATRIX IS INDEFINITE (MODES) OR STIFFNESS MATRIX IS INDEFINITE (BUCKLING).
*** SYTEM FATAL MESSAGE 7340 (LNNHERR)
WARNING REPORTED BY SUBROUTINE LNNDRVD (IER= 728)
USER INFORMATION: GRAM-SCHMIDT DID NOT CONVERGE
*** SYSTEM FATAL MESSAGE 7340 (LNNRIGL)
TRIDIAGONAL QL DID NOT CONVERGE
THIS RUN IS TERMINATED. PARAM, BAILOUR, -1 MAY BE USED TO CONTINUE THE RUN

RE: SOL103 Normal Modes Analysis Using MFLUID

What version of Femap are you using? In Femap Help, see the section in Connections on Fluid Regions for descriptions of the inputs and setup.
I just executed the problem you described in v11.1.1 with no errors.

RE: SOL103 Normal Modes Analysis Using MFLUID

(OP)
I am running v11.0.0. I took a closer look at the help topic for MFLUID, and this part confused me:

"There is a Check box which enables you to turn the Z Free Surface OFF completely, so no value is written out to Nastran for the ZFS field. When the Z Free Surface is completely OFF and the XY Plane and YZ Plane fields are NOT set to Antisymmetry, Nastran will treat the MFLUID card as a special form of “external fluid”."

My takeaway from that is that turning the Z Free Surface off is not the proper way to model an internal fluid. I am able to get my analysis to run when I specify a Z Free Surface at the top of the tank, but I didn't think that was the correct method in my case since the tank is full and there are no free surfaces. For my case with a cylindrical tank, should I select just the bottom surface and the cylindrical sides, or should I select the top surface as well?

Thanks for your help!

RE: SOL103 Normal Modes Analysis Using MFLUID

Can you post your model?

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