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12-19-2013 08:00 AM

9 REPLIES

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12-28-2013 02:19 PM

Dear Lionel,

Non-Structural Mass is a reasonable option, but as you are dealing with dynamic analysis then the position of center gravity of mass is important, then the use of one spider of RBE3 elements is also correct. Create both models and compare solution results.

An interesting option exist with NX NASTRAN to consider the fluid-structure interaction: **virtual fluid mass **(MFLUID Method). I think the MFLUID method would be much more reliable than the RBE3 method.

A virtual fluid volume produces a mass matrix which represents the fluid coupled to a boundary consisting of structural elements and other effects, such as free surfaces, planes of symmetry, and infinite fluids. The incompressible fluid produces a mass matrix defined with full coupling between accelerations and pressures on the flexible structural interfaces.

Although free surfaces are allowed, no gravity effects are included directly. Because the fluid is represented by a coupled mass matrix attached directly to the structural points, * this capability is allowed in all dynamic solution sequences*. This capability may be used to model a wide variety of fluid-structure interaction problems.

A free surface is defined as an x-y plane in any local rectangular coordinate system. The user simply specifies a value of z as the upper limit of the fluid volume. Different fluid volumes may have different levels and orientation. It is not required that the surface coincide with the element properties since partially wetted elements are allowed.

The virtual mass fluid option may be used in all NX Nastran dynamics solutions, including the following special approaches:

- Superelements (Residual Only)
*Nonlinear Analysis (SOL 129)*- Optimization (does not create sensitivity matrices)

The virtual mass method (MFLUID) is well suited for the following problem types:

- An infinite fluid surrounding part of a structure.
- A fluid with a free surface contained within a flexible structure.
- Multiple fluid volumes with combinations of the options above such as a half of a ship floating in water with several internal tanks, each with free surfaces and symmetric boundaries.

Note that compressibility and surface gravity effects are neglected. It is assumed that the important frequency range for the structural modes is above the gravity sloshing frequencies and below the compressible acoustic frequencies. It is further assumed that the density within a volume is constant and no viscous (rotational flow) or aerodynamic (high velocity) effects are present. In other words, a ship traveling at a high rate through a thick oil patch would require some extra modeling effort.

The MFLUID method is supported since years in FEMAP, but I do not see direct support of this feature in the GUI of NX Advanced Simulation V9.0, so this could be an important limitation.

In FEMAP the * Fluid Region *command is very similar to the

When a Fluid Region is present in your model, FEMAP provides an Output Request (Nastran only) called “**Fluid Pressure**” which will return an elemental “fluid-structure pressure” along with any other requested results.

Best regards,

Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director

IBERISA • 48004 BILBAO (SPAIN)

WEB: http://www.iberisa.com

Blog Femap-NX Nastran: http://iberisa.wordpress.com/

IBERISA • 48004 BILBAO (SPAIN)

WEB: http://www.iberisa.com

Blog Femap-NX Nastran: http://iberisa.wordpress.com/

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01-13-2014 07:24 AM

Thank you for your reply Blas,

It's a pity that the MFLUID method is not available in the NX GUI. I have started experminting with options that are available and so far an option that is giving ok results is to simply use a solid mesh representing the water. I think this might be the best way to do it for tanks that are completely full. I have given the water isotropic material properties. A youngs modulus as low as possible without heavily distorting elements (0.01 MPa) and a high Poisson's Ratio (0.49). I am still to experiment with the influence of the structural damping coefficient in dynamic runs. It would be nice to hear is someone else has experimented with this approach before, what isotropic properties worked out best and what their conclusions are.

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03-10-2014 07:57 AM

I have found out that there is a NX Open App to Define MFLUID from the NX GUI;

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03-10-2014 10:29 AM

where did you find this NX Open App? Is there somewhere a database/folder in the install directory of NX or did you find it online?

I would be interested if there are more helpfull apps available.

Thanks,

Georg

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03-10-2014 12:01 PM

See the Simulation SIG "NX Open Routines" library on the PLM World web site:

http://www.plmworld.org/p/do/si/topic=405

Note that you have to be a PLM World "Citizen" **and **a member of the Simulation SIG for the above link to work.

Regards,

Jim

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04-02-2014 12:51 AM

Ganesh

Regards,

Ganesh

Simcenter 3D Community Manager

Simulation and Test Solutions

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04-02-2014 09:14 AM

Please read the note in my original post:

"Note that you have to be a PLM World "Citizen" **and **a member of the Simulation SIG for the above link to work"

Using the PLM World member search tool, I don't see you listed as a Citizen, let alone a member of the SIG.

Regards,

Jim

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04-04-2014 02:07 PM

Dear Lionel & Georg,

I am checking with PLM world if the NX Open app that Jim referenced is something I can provide here. I'm hoping it is, if so it and other tidbits will be available on the community site

Stay tuned!

Regards,

Ganesh

Simcenter 3D Community Manager

Simulation and Test Solutions

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11-05-2015 07:22 AM

Is there any more information about this NX open app? I would really like to use this app, however i'm not a member of PLMworld

Br,

Bart

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