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-   -   Tank sloshing in ansys cfx (http://www.cfd-online.com/Forums/cfx/83356-tank-sloshing-ansys-cfx.html)

enghamed December 24, 2010 06:03

Tank sloshing in ansys cfx
 
Hi,
I'm using 12.1 version and probably could move to 13.
My goal is to model water sloshing in a storage tank under siesmic forces. I believe I should use VOF. Any suggestion on how to model this will be appreciated. Should I do this by changing the gravity vector or moving wall B.C. or moving mesh?

Regrds;
Hamed

Haga December 27, 2010 12:39

the wall need not be movable. If the tank has a symmetrical geometry, a plane of symmetry also has to be incorporated in the model, in short 1/4th or 1/2 of the tank need to be modeled. the mesh should be moving mesh at the plane of symmetry. for 3D slosh model, the moving mesh should be given from symmetry plane towards the wall but not essentially up to the wall.

ghorrocks December 28, 2010 05:09

This question has been asked many times before. Do a search to find previous posts.

But in short the easiest way is to move the gravity vector, but it only approximates the true motion so you have to assess whether the approximation is good enough. Moving mesh is the most correct way, but is much more computationally expensive.

And sorry Haga, I have no idea what you are talking about. What have symmetry planes got to do with it?

Haga December 28, 2010 20:19

@ghorroks symmetry plane only for viewing the 2D slosh pattern. nothing else. am now trying different tank geometries. all are symmetric geometrically

enghamed January 4, 2011 19:16

Dear Ghorroks,

Thanks for your reply.
Can you please help me on the moving mesh? I have a circular tank of 10m diameter and 6 meter height. Where should I specify the mesh motion? Should it be specified displacement with any particular equation?
As Haga said can I model half of it and put symmetry BC or have to model all of it?

For the gravity change, which you said is approximation, where can I apply the equation for the gravity change?

I really appreciate your help.
Regards;
Hamed

enghamed January 4, 2011 19:47

Hi again,
Sorry but I have two more questions. My tank is completely closed (no inlet and out let) so can I use moving wall for simulating the sloshing or as you said the best way is mesh motion?
My second question: How can I know if I should use laminar or turbulent model? Any idea which turbulence model is better for this?

Regards;

singer1812 January 5, 2011 14:38

No, make the entire tank a subdomain and move the subdomain to simulate the tank movement.

enghamed January 6, 2011 02:50

Thanks singer1812,

Can you please tell me how can I move the subdomain? Will this involve mesh motion?

Regards;
Hamed

singer1812 January 6, 2011 10:01

It is moving mesh, but you are moving the entire mesh (domain). Just create the subdomain, with your input function for mesh (tank) motion on the subdomain.

enghamed January 10, 2011 01:54

Dear Singer1812,

Thank you very much for your help.

So if I have a tank which is fixed to ground which goes under a base movement of for example X=Asinwt, I should model the entire tank, put wall B.C on faces and create a subdoamin and put the mentioned equation for the sub domain as the mesh displacement input? Am i getting it right?

Many thanks,

singer1812 January 10, 2011 10:31

That is the general idea. The mesh movement will work fine. If it is a high frequency movement, I am not sure how well or easy the simulation will run.

Tantelise January 26, 2011 11:44

I usually just apply a general momentum source (on subdomain) with the known force definition. No mesh motion requred with this approach and no limitation on moving frequency.

"S=rho*a", where "a" denotes accerlation, =du/dt.

Care should be taken with the sign of "a", it's opposite to the tank moving direction.

ghorrocks January 26, 2011 19:22

Yes, this is a good approach, but note the application of an acceleration as a general momentum source is an approximation and may not be accurate for some types of flow. Need to assess whether it is suitable on a case-by-case basis.

Haga January 31, 2011 21:15

@enghamed
 
check for any available experimental data on this

vipul July 30, 2011 05:48

Quote:

Originally Posted by enghamed (Post 288374)
Hi,
I'm using 12.1 version and probably could move to 13.
My goal is to model water sloshing in a storage tank under siesmic forces. I believe I should use VOF. Any suggestion on how to model this will be appreciated. Should I do this by changing the gravity vector or moving wall B.C. or moving mesh?

Regrds;
Hamed


Hello, i am also solving similar problem. circular tank under seismic load i have acc. vs frequency(example frequency converted in time period (5 sec. i have acc. value for that time period now i have to divide time period in 4 intervals of 1.25 each and simulteniously changing the direction of acceleration to simulate motion similar to to and fro motion) values i am loking for pressure on all walls due to seismic effect.I also want suggestion to solve it with dynamic mesh or with playing with acceleration values in operation conditons.

vipul July 30, 2011 05:50

Hello, i am also solving soloshing problem. circular tank under seismic load i have acc. vs frequency chart(floor spectrum)(example frequency converted in time period (5 sec. i have acc. value for that time period now i have to divide time period in 4 intervals of 1.25 each and simulteniously changing the direction of acceleration to simulate motion similar to to and fro motion) values i am loking for pressure on all walls due to seismic effect.I also want suggestion to solve it with dynamic mesh or with playing with acceleration values in operation conditons.

vipul July 30, 2011 05:50

Quote:

Originally Posted by Tantelise (Post 292289)
I usually just apply a general momentum source (on subdomain) with the known force definition. No mesh motion requred with this approach and no limitation on moving frequency.

"S=rho*a", where "a" denotes accerlation, =du/dt.

Care should be taken with the sign of "a", it's opposite to the tank moving direction.

Hello, i am also solving similar problem. circular tank under seismic load i have acc. vs frequency(example frequency converted in time period (5 sec. i have acc. value for that time period now i have to divide time period in 4 intervals of 1.25 each and simulteniously changing the direction of acceleration to simulate motion similar to to and fro motion) values i am loking for pressure on all walls due to seismic effect.I also want suggestion to solve it with dynamic mesh or with playing with acceleration values in operation conditons.

ghorrocks July 30, 2011 08:07

I guess posting the same post three times is a quick way of getting your post count up.

To answer your question - you can solve it either by moving mesh or moving the gravity vector. Moving the gravity vector is simpler but does not account for all inertial effects - whether this is important in your case is up to you to decide.

vipul August 1, 2011 01:29

Quote:

Originally Posted by ghorrocks (Post 318140)
I guess posting the same post three times is a quick way of getting your post count up.

To answer your question - you can solve it either by moving mesh or moving the gravity vector. Moving the gravity vector is simpler but does not account for all inertial effects - whether this is important in your case is up to you to decide.

Thanks i am solving my problem in fluent for moving/dynamic mesh i have to write UDF for applying motion.do you have UDF for giving shm motion to the tank.can you give your email id?

ghorrocks August 1, 2011 07:48

This is th CFX forum, you won't get much Fluent help here. Try the fluent forum. And I do not give out my email to people on the forum. If you have a question or comment you can post it on the forum.


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