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How to model an infinite fluid volume, to avoid wave reflexion

Hi,

I'm trying to simulate the interraction between a box and waves in the sea. Thus I'm using the material LAW37 (BIPHAS) in order to model the interface between water and air which are modeled in ALE.

The problem is that I don't know how to model the boundary conditions on the transverse faces in order to avoid wave reflexions on the wall (to simplify the computation, all DoFs are fixed on the longitudinal faces).

I can't use MAT/BOUND (LAW11) because it doesn't work properly with the LAW37.

So i'm trying to model these boundary conditions using a material LAW37 to create an 'air outlet' and a 'water outlet' with the same initial values as air except for :

-the liquid bulk modulus C_Liquid = 0.0

- gamma = 0.0

The computation runs well at the beginning and i get a pressure gradient in the fluid volume, so it's okay. But then an error appears :

' STOP : NEGATIVE RHO=-0.1322538772563E-09 FOR 3D-ELEMENT ID=3696
ERROR TERMINATION '

In the results file I can see that there is a problem with a really big velocity vector on top of the volume in the air part. I think this is linked, but I don't know how to avoid this.

Moreover, to model the interface between the box and the water, I use INTER/TYPE18 but I don't know how to set the right value for the StfVal.

Using the formula written in the Hyperworks Help i.e. StfVal=(rho * v² * Sel)/GAP , the box is floating in the air, and not in the water.

Do you have any suggestion?

Thank you very much in advance,

Hereby the .rad file

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Altair Forum User

Hi Fergon,

It seems the element in the air outlet component is getting deformed badly. Please double check the contact definitions, material properties, and units to make sure no mistakes were made in the model.

For Stfval, the density of the densest fluid in contact with the structural part must be considered for Stfval calculation. I'll try to get some information on Type 18 interface.

Please go through a case study where a similar parameters are used: http://www.altairatc.com/(S(rfs4o2whprtbiaaazxof4unk))/europe/presentations/Session12/Session12_Eurocopter_Majamaeki.pdf

Also can you try with Type 1 interface, which is used to describe the fluid-structure interaction.

Altair Forum User

Hi Georges P Johnson,

Thank you very much for your answer.

I've double-checked the contact definition, the material properties and unites, and everything seems well set.

Moreover I've tried to use the similar parameters as in the case study (initial gas pressure at 1e-20), the results show that the pressure gradient in the water doesn't appear anymore.

I've also tried to use INTER/TYPE1, but the problem is that the nodes on the faces have already several boundary conditions because of the outlet conditions, so the computation can't run.

Dhould you have any other suggestion, please let me know.

Thanks again