Free surface- Sloshing

hossein_sml · February 1, 2016, 05:07

Hello,

I am trying to simulate free surface in moving (rotating) cube ( water -sloshing)

unfortunately convergence is terrible. and the free surface's behavior is not logical.

As shown in below image, at start of movement a hollow is creating in surface.

After some steps it acts properly,

but it again become completely wrong.

I really need help!
What is wrong?
I have checked all parameters.

ghorrocks · February 1, 2016, 06:42

First of all - make sure your convergence is tight enough, your mesh is high quality and your time step size is small enough. You won't get anything sensible unless you do this.

The default free surface settings in CFX are not always optimal. In fact in same cases it can generate large spurious flows adjacent to the free surface which generates artefacts like this.

I recommend you look at all the free surface modelling parameters and adjust them all and see what helps and what does not. Doe this on a small mesh for a small number of time steps - enough to see the problems you are seeing starting up. This means you will have a few dozen simulations to do, so script them up to run together. You will find there are a few parameters which affect the model you are doing and you can normally find some settings which work better than the default values.

I have done this a few times for free surface modelling work I did and it improved the model a great deal. But it is different for each model so you really need to find out what works for you.

hossein_sml · February 24, 2016, 10:55

Dear Glenn
I have checked mesh sensitivity. and mesh is good.

I have set convergence tight enough, but it is not converging.

RMS: 1e-8
Max Coeff Loop : 50

Also time step is small enough

I have checked all parameters. I realy do not know what i should do!

ghorrocks · February 24, 2016, 18:23

Your result is typical of numerical instability, so something is making it go unstable. You have missed something important.

Quote:

I have checked all parameters

Really? Which parameters have you checked? In my experience the free surface parameters can make a big difference and are likely to be what is causing this problem so I suspect you have missed something important.

hossein_sml · March 5, 2016, 13:03

I should say that at first i tried to model free surface in a cube which was rotating on its corner. The results were OK.
But when I used that setting in cube which is rotating along center of its base side, it is not converging.
What is problem?

hossein_sml · March 5, 2016, 13:04

which setting you need to know to help me?

ghorrocks · March 6, 2016, 18:27

I have already said what is likely to be causing your problems in my experience.

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February 1, 2016, 06:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	First of all - make sure your convergence is tight enough, your mesh is high quality and your time step size is small enough. You won't get anything sensible unless you do this. The default free surface settings in CFX are not always optimal. In fact in same cases it can generate large spurious flows adjacent to the free surface which generates artefacts like this. I recommend you look at all the free surface modelling parameters and adjust them all and see what helps and what does not. Doe this on a small mesh for a small number of time steps - enough to see the problems you are seeing starting up. This means you will have a few dozen simulations to do, so script them up to run together. You will find there are a few parameters which affect the model you are doing and you can normally find some settings which work better than the default values. I have done this a few times for free surface modelling work I did and it improved the model a great deal. But it is different for each model so you really need to find out what works for you.

February 24, 2016, 10:55		#3
hossein_sml New Member hossein Join Date: May 2012 Posts: 28 Rep Power: 14	Dear Glenn I have checked mesh sensitivity. and mesh is good. I have set convergence tight enough, but it is not converging. RMS: 1e-8 Max Coeff Loop : 50 Also time step is small enough I have checked all parameters. I realy do not know what i should do!

March 5, 2016, 13:03		#5
hossein_sml New Member hossein Join Date: May 2012 Posts: 28 Rep Power: 14	I should say that at first i tried to model free surface in a cube which was rotating on its corner. The results were OK. But when I used that setting in cube which is rotating along center of its base side, it is not converging. What is problem?

March 5, 2016, 13:04		#6
hossein_sml New Member hossein Join Date: May 2012 Posts: 28 Rep Power: 14	which setting you need to know to help me?

March 6, 2016, 18:27		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	I have already said what is likely to be causing your problems in my experience.