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#12834 - 08/24/07 07:33 AM Mass model for dynamic input
A. Oertel Offline
Member

Registered: 02/26/01
Posts: 7
Loc: Merseburg, Germany
I need some help for dynamic input.

Since version 5.00 I can select between lumped and consistent mass modell.
For all my rigids and mass point I define lumped masses in the dynamic input screen.

I haven't found any hint, if the lumped mass input works in consisten mass model just as well in the lumped mass model.

What is the recommendation for building uniform mass models (forces/rigids/lumped masses) especially for load cases mit static/dynamic load combinations?

Thanks for help.


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A. Oertel

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#12837 - 08/24/07 07:59 AM Re: Mass model for dynamic input [Re: A. Oertel]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 6226
Loc: Houston, Texas, USA
The "lumped mass" versus "consistent mass" option controls how the "mass matrix" is formulated. (Read the "help text" on this item for details.) I'd use the "consistent mass" option - I believe it is more accurate.

I really don't like using the "lumped mass" input dialog. Typically anytime you see a value here, it represents something that should have been modeled differently upfront in the (static) piping model. CAESAR II will automatically account for the mass of the piping system and components that have been modeled. So the mass of the rigid elements will be accounted for - you don't have to do this in the Dynamic Input.

What we often see is that after building the static model, someone discovers a relatively heavy valve actuator exists. They put this into the static model as a concentrated force. After moving on to Dynamics, they realize forces are not masses, and use the Lumped Mass dialog to again define actuator. A better solution would have been to use dummy rigids to position the actuator, then another dummy rigid for the actuator in the static model. Then the mass of the actuator would have been automatically accounted for in Dynamics.

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Regards,
Richard Ay - Consultant

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