How to simulate the effects of water 2

[bisav]

Hi everybody,

I have a doubt in PATRAN.
How to simulate the effects of water weight effectively.

My model is simple cylindrical tank, in fact a huge tank. Tank is completely filled with water. Now I want to run it for the modal analysis(Normal Modes). How do I give the weight of the water effectively to the container. If I create a mass element at the centre and attach it with all the nodes of the tank using RBE2 connection, the system becomes very stiff. And if I give non structural mass (value is calculated by total mass divided by the total surface area) in the element property of the tank wall, I think that either is not the correct way to simulate water. Because each wall will be carrying huge loads which I feel is not the correct way to do it.

Can anybody help me.?

Thanks
Bibin

 

[Naffoussi]

Hi,

Hydrostatic pressure (P)?

P = Rho * g * h

Rho = fluid density , g = acceleration, h = fluid depth

Indeed the use of RBE2 is wrong and it will make the structure very stiff and cause numerical issues...

Here the variable is the water depth. With Femap it is easy to create such a pressure function of depth (so the element centroid, or node position) using data surface...Also I know that with Patran it is feasible too by creating spatial PCL function or tabular input. look at the attached pdf which explains how to create spatial and temporal variation of loads.


Hope this helps

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

Hai compsite curves,
Greetings for the day.

Thank you for the reply.
I have already applied this variable pressure by using spatial fields PCL function. That is for the stress analysis. Now that I want to do the MODAL analysis. This pressures won't be taken into account for modal analysis. But for modal analysis, I want to simulate the mass of the water effectively. I want to give the mass to the model in a correct manner. For this reason only in the modal analysis I removed the pressure function and created a mass element with the mass of the water at the center and attached it with tank using RBE2, but that is not the correct way, it makes the structure very stiff.

Bibin

 

[Naffoussi]

If you have a parametric base plate mesh then you could extrude the plate elements in order to obtain SOLID elements (i.e HEXA).
Then you have to deal with MAT10 and PFLUID (field n° 8 in the PSOLID nastran card) to represents the water properties and assign this properties to the solid elements.
You will need to define some information like the mass density, speed of sound to calculate the bulk modulus...

Regards,

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

Hello compositecurves,

But how can we give the connection between fluid element(hex elements and the side wall) ?
Node connection or any other connection. Node connection seems to be difficult since it is a big structure, so there will be so many nodes. And if I didn't connect it with the side walls will it be the correct way to approach to the problem???

Regards
Bibin.

 

[Naffoussi]

by merging nodes on hexa and side wall...

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

Inorder to give node merging nodes on the the wall should have its node positions similar to the hex elements right?

 

[bisav]

I checked the options, may be I can achieve this by adjusting my mesh.


Thanks Composite curves.

 

[Naffoussi]

Exactly !!

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

Hello
Good Morning

Node merging was not a problem. But while running it shows error. It seems like fluid elements can't be attached to structural elements.

The following is the error showing in F06,

USER FATAL MESSAGE 5419 (MODGM2)
STRUCTURAL ELEMENT QUAD4, ID = 1, REFERENCES FLUID GRID POINT ID = 1


And below is the error definition I found out,

*** USER FATAL MESSAGE 5419 (MODGM2) STRUCTURAL ELEMENT %1 ID =%2 IS ATTACHED TO A FLUID GRID POINT ID =%3



So is it not possible to attach a fluid elements with structural elements? If it is so how will we give the connection??

Regards
Bisav

 

[Naffoussi]

MPC (RBE2) node to node ?

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

MPC (RBE2) will be very very difficult, thousands of nodes are there.

 

[Naffoussi]

Could not create them in one click ? like using lists or groups ?

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

Is that possible at all.............!!! REBE2 can have only one independent node. How can we create the entire connection like that. If we are creating a centre node and attaching all the other nodes its okay. But I feel that is not the correct way. Guide me If I am wrong.

 

[Naffoussi]

The Idea was to create one RBE2 for each two coincident nodes (one node on CHEXA and one on CQUAD). Then each RBE2 will have one master node and one slave node...

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[bisav]

That is okay, but creating such one to one RBE2 connections for thousands of nodess seems little difficult....!!

 

[Naffoussi]

it should be feasible in an easiest way !!

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[nlgyro]

coupling for FSI simulations in NASTRAN is achieved by the ACMODL bulk data entry. The user supplies a search tolerance which Nastran uses to find a fluid grids to be associated to a structure grids. At the end of the job, Nastran echoes the list of the fluid grids mapped to structure grids in the f06 file.

 

[bisav]

how do I do that??

 

[nlgyro]

> how do I do that??

It is at this point that you need to pick up the patran users guide and read to find out how ACMODL is supported thro' patran.