# Simulation on NX with the solver NX NASTRAN

Discussion in 'Stability' started by Thibaudinergy, Nov 18, 2010.

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### ThibaudinergyNew Member

Good morning everybody.
I have some requests and quesions.
In fact, I would like to represente the hydraustatic pressure on the boat due to a virtual mass in order to launch a modal analysis. I have some pdf by SIEMENS to understand what I have to do, and what is the correct writting. BUT, I don't know how can I write to introduce in the case control the virtual mass (=MFLUID).
For example, imagine a tank which contains a liquid (like gazoil), how can I represente in my simulation the hydrostatic pressure caused by the liquid on the tank's shell ?
For information, I use the solver NX NASTRAN 6.1 and the software NX 6.0.
I use as solution type : SEMODES 103-RESPONSE SIMULATION, and I would like to find out the modes of a system such as for example liquid within a tank. I know there are three sucbases. The first one is for the static part, the second one is for the stress stiffening part, and the last one is for the dynamic part.
There will be a statsub = 2 at the end of the case control to introduce the stiffness of the system calculated in the second part.
Could you give me some help please ? Or may be we could call us.
Best regards.
Tibo

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This is better represented by a varying pressure load. So at the top, the pressure is zero, at the base, it is the hydrostatic head, and just vary it linearly.

For modal analysis this is by far the “easiest” way.

If, however, you wish to have ‘elements’ that represent fluid, to be part of the main mesh of the FE, for ascertain the amount of dampening the fluid has, then this is seriously seriously complex. If you are unable to perform a simplified hydroelastic analysis, then I suggest you don’t use this method, as you won’t have any idea if the results are valid. The hand calculations are not a 5min job too.

Thus less is more, when using FEA.

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### ThibaudinergyNew Member

Thank you Ad Hoc for your response. How could I put the MFLUID in the case control, and the main bulk ? Thank you !

I am looking for someone who could help me for modal analysis with the software NX and the sovler NX NASTRAN. Indeed, I would like to represent the hydrostatic pressure exerted by a fluid on a tank's shell. As a result, would someone know how could I do ?

For informations, it's better to consider the liquid as a virtual mass. Therefore, the virtual mass carrys on the tank's shell a hydrostatic pressure represented by MFLUID. But i don't know how to write this in the INPUT FILE (.DAT). Particularly in the case control together with in the main bulk and the DECK NASTRAN. I use the solution type SEMODES 103 RESPONSE SIMULATION, and the hydrostatic pressure takes part in the static subcase. At this moment, I just did a real pressure hydrostatic, id est it's represented by PLOADD4 like a force, but for modal analysis, it's not exact if I launch a calcul with this force. Indeed, we are not interested in the effects of hydrostatic pressure, but we are interested in frequential responses of the system.I need your help, if someone has some ideas or contacts, tell me.

Regards!

NB : when i will have the MFLUID in the case control, Where I have to put MFLUID DEFINITION in the main bulk ? And HOW ?

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I am not familiar with NX and Nastran. However, as I noted above, if you wish to include the fluid, as part of the model for modal analysis, this is seriously complex and there is no accepted method of procedure nor verification.

May I suggest you read the paper “Modal analysis of a fast patrol boat made of composite material” by Santos, Temarel and Guedes Soares and also the paper
“The dynamic behaviour of a monohull in oblique waves using two and three-dimensional fluid-structure interaction model”, by Price, Salas Inzunza and Temeral.

Once you read these, you quickly grasp the fact what you are asking is not a 5min job and very complex and extremely difficult to validate.

Prof Price and Prof Temeral, whom are both serious geniuses in this field, and if they find it extremely difficult to do and still cannot achieve decent convergence and correlation with real world experiments, then no one can!

Thus, you need to really think about what it is you are trying to achieve, and how best to do this simply. Just because your program can dance, make tea and whistle the national anthem all at the same time, doesn’t mean it can what you want I to do, and with realistic answers.

Modal analysis with fluid-structure interactions of 3D models is extremely difficult to prove, and even more so to calculate by hand!

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### ThibaudinergyNew Member

Ad Hox, Thank you very much for your magic help. I know it's very complex and extremely difficult to validate my proposition of simulation model. Therefore, what do you think if I consider hydrostatic pressure in the main case ( static subcase), pre-stress in the stressstiffnening subcase, and gravity in the dynamic subcase ?
Imagine a tank held by a acoustic and vibration decoupling system. The attachment system has some pre-stress, etc. A liquid is inside the tank. My model is correct ? The stress are to keep attached the tank with the decoupling system ?
If you can answer me, I will be able to make the analogy between this model, and a boat on the sea !
Regards

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### JosephTSenior Member

You might want to run this by the NX CAE support team. I use NX myself and the CAE team should be able to provide an answer. They have a ship design module as well so I presume they will be able to find an answer for you one way or another. One of my colleagues suggested:

"The MFLUID entry must be selected with the Case Control command MFLUID = SID"

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Thibaudinergy

Rather than analogies, just state what it is you are trying to do. What the problem is and what answers you are seeking. Stating what it is you are trying to do, may provide clues to a more simplified method of analysis, or whether what you are proposing to do, is impossible or not.

Failing that, as JT suggests above, contact their CAE Dept.

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### khabiranJunior Member

As I know, WAMIT and Abaquse powerfull for solve hydroelastic analysis. of course this method use for larg floating vessels ;ships, platforms, floating city and airports... .
but there are some limits and aspects for solve these study, completly related to solve method ... 3d or 2d , wet or dry , time domain or frequency domain and etc.
some software such Nastran are very limited for solving may be sloshing only. as you know solving must be verifying by real measurement or model testing. that there is no sample for Nastran. or other. of course Auwa related to APDL Ansys could solve time and frequency domain.
for the more mail me... .

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### daiquiriEngineering and Design

This is a hell of a problem, I am not sure even folks at NX will be able to help you.
If I understand you well, you'll basically have to consider fluid dynamics coupled with structural response - if you want to carve out anything serious out of it. A hydrostatic pressure is just a part of the forces involved, and how do you account for dynamic pressures due to sloshing, fluid displacement, viscous forces, free-surface waves etc?
If your tanks' volume and free surface are very small compared to ship size, and if tanks are filled up to the top so that they can be approximated by concentrated masses of fluid with no free-surface effects, than I guess Nastran could cope with it (but you still have to validate results against known data, or else it's just a high-tech guesswork). Otherwise, I honestly think it's over the top. And besides, you'll need a huge computing power.
If you need to work out the vibrations and acoustic problems, then even just modelling the coupled hydrodynamic and elastic behaviour of the fluid volume and tank walls alone will be a feat, thinking about extending it to the whole ship is plain scary.

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### khabiranJunior Member

it's be confusing if you use mutual situation. hydroelasticity is more than a FSI problem. every response of structure effects on fluid flow and then different pressure disturbation.
For free surface effect (may be sloshing) situation is simpler. however I think it is better to denote your problems. sloshing, wave pressure, slamming or ...

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