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-   -   VOF mass loss (http://www.cfd-online.com/Forums/fluent/109806-vof-mass-loss.html)

Smaras November 27, 2012 05:31

VOF mass loss
 
Again one more time as i havent got any repsonse

Hello all,

I am doing VOF simulation with 2ddp. Now using two phase air and water. Taking water as first phase and air as second. With small inlet with 96 m/s air impingement over water surface. Lance to water bath distance is 154 mm with 100 mm depth of water bath and having 100 mm radius of cylinder. Outlet is at top defined as pressure outlet. Now the problem is that when i am doing calculation it's diverging but the results are totally disappointing as the penetration depth of the air jet should have reached 14 mm while it simulation its stuck at 7.5 and further there is a mass loss of around 0.1 Kg out of 3 Kg.
Have used coarser mesh and its showing better result. Now according to the theory the finer mesh should have given better result but here it's 180 degrees. :(

And further i am using URF as
pressure 0.2
momentum 0.3
Tur. dissipation rate 0.8
Tur. viscosity 0.8
Step size 0.0001

Non iterative with Factional Scheme
Volume Fraction = geo reconstruct
Turb. KE = quick

Would be thankful if some would give suggestions.

Regards,
Smaras

http://imageshack.us/a/img23/6368/kepfinal.th.jpg

http://imageshack.us/a/img23/6368/kepfinal.th.jpg

RodriguezFatz November 27, 2012 07:37

Quote:

Originally Posted by Smaras (Post 394346)
Now the problem is that when i am doing calculation it's diverging but the results are totally disappointing...

You should not look at results before you fix the numerics.
Get a converging simulation!

Smaras November 27, 2012 07:50

Quote:

Originally Posted by RodriguezFatz (Post 394369)
You should not look at results before you fix the numerics.
Get a converging simulation!

Thanks Rodriguez but i have a converging solution that's the problem. And the numerical calculation and also some research paper i am consulting also giving the same depth of penetration which is not confirmed with fluent.

Therefore i am doing some mistake. I need help in this regard.

Regards,
Smaras

RodriguezFatz November 27, 2012 07:55

Do you say that in your originial post you meant "converging" instead of "diverging"?

Smaras November 27, 2012 07:57

Quote:

Originally Posted by RodriguezFatz (Post 394375)
Do you say that in your originial post you meant "converging" instead of "diverging"?

Mistake,

I meant to say diverging from the numerical result and the results from the research paper

Thanks for correction.

Regards

RodriguezFatz November 27, 2012 08:01

Alright, now I get you.

Can you post some pics of your setup and mesh? And also the residuals...
What I don't understand: How can your simulation converge, when you have a mass loss?

Smaras November 27, 2012 09:14

Quote:

Originally Posted by RodriguezFatz (Post 394378)
Alright, now I get you.

Can you post some pics of your setup and mesh? And also the residuals...
What I don't understand: How can your simulation converge, when you have a mass loss?

That i cannot understand as well.

Mass-Loss:

http://imageshack.us/a/img441/5479/kepsmass0t.th.jpg

http://imageshack.us/a/img40/9773/kepsmasst.th.jpg


Mesh:

http://imageshack.us/a/img233/3048/kepsmesh0t.th.jpg

http://imageshack.us/a/img832/5681/kepsmesht.th.jpg

Penetration Depth:

http://imageshack.us/a/img402/547/ke...ondepth.th.jpg

Residuals:

http://imageshack.us/a/img502/961/kepsresiduals.th.jpg

Phases:

http://imageshack.us/a/img22/7638/kepsphaset.th.jpg


If anything else is left please i can post that as well. Vielen Dank Rodriguez

Regards,

Smaras

RodriguezFatz November 27, 2012 09:39

Just to be sure, I am getting your setup right:

1) The box you are simulating is a cylinder, the axis is on the left of your pictures?
2) On the bottom you got the water and a needle blasts air vertically on your water surface?
3) The needle is on the top left of your domain, where I can see the little gap in the green grid?
4) "Penetration depth" is the (average) difference of the height of the water surface at your symmetry axis, compared to the steady state without the needle fan?
5) Air from the needle escapes through the pressure outlet at the top?
6) On the right and on the bottom are walls?

Smaras November 27, 2012 09:59

Quote:

Originally Posted by RodriguezFatz (Post 394402)
Just to be sure, I am getting your setup right:

1) The box you are simulating is a cylinder, the axis is on the left of your pictures?
2) On the bottom you got the water and a needle blasts air vertically on your water surface?
3) The needle is on the top left of your domain, where I can see the little gap in the green grid?
4) "Penetration depth" is the (average) difference of the height of the water surface at your symmetry axis, compared to the steady state without the needle fan?
5) Air from the needle escapes through the pressure outlet at the top?
6) On the right and on the bottom are walls?

http://imageshack.us/a/img90/8106/unbenanntgj.th.png

I hope this image will clear the queries. While for the 4th
Penetration depth is the depth difference between the stable condition and the impingement of air onto water surface. the distance calculated is from the top i.e. Outlet.

And all dimension are in mm.

Regards.

RodriguezFatz November 27, 2012 10:04

Correct me if I am wrong, but as I understand it, the needle (lower left in your last picture) shouldn't be an axis, but a wall boundary. There can be just one cylinder axis.

Smaras November 27, 2012 10:09

Quote:

Originally Posted by RodriguezFatz (Post 394410)
Correct me if I am wrong, but as I understand it, the needle (lower left in your last picture) shouldn't be an axis, but a wall boundary. There can be just one cylinder axis.

Ohh it was by mistake typing error. The top left is nozzle wall. While the lower left having length 254 is axis.

RodriguezFatz November 27, 2012 11:33

Ok, is your simulation time dependent? The residuals screenshot looks a bit strange... it looks like your first 2000 iterations have a residual in continuity equation of about 1.0

Smaras November 27, 2012 11:47

Quote:

Originally Posted by RodriguezFatz (Post 394448)
Ok, is your simulation time dependent? The residuals screenshot looks a bit strange... it looks like your first 2000 iterations have a residual in continuity equation of about 1.0

Yup it is time dependent since i am using Fractional Scheme with NITA.

RodriguezFatz November 27, 2012 11:59

But look at your residuals. Something ugly happens during the first 2000 iterations.

You can also do this: Under "Monitors" add a surface monitor and integrate the water density over your domain. Plot it each timestep to a window. I guess when your residuals start to converge (after about 2000 iterations), your integrals won't change any more over time, thus the total mass will be conserved.

Smaras November 28, 2012 04:33

Quote:

Originally Posted by RodriguezFatz (Post 394456)
But look at your residuals. Something ugly happens during the first 2000 iterations.

You can also do this: Under "Monitors" add a surface monitor and integrate the water density over your domain. Plot it each timestep to a window. I guess when your residuals start to converge (after about 2000 iterations), your integrals won't change any more over time, thus the total mass will be conserved.

Rodriguez i decrease the time step size to 1e-3 from 1e-5.

RodriguezFatz November 28, 2012 04:35

Quote:

Originally Posted by Smaras (Post 394576)
Rodriguez i decrease the time step size to 1e-3 from 1e-5.

You could also try to keep the current stepsize and get convergence here! Maybe increase the number of iterations of each time step or change the solution algorithm.

Smaras November 28, 2012 04:41

Quote:

Originally Posted by RodriguezFatz (Post 394577)
You could also try to keep the current stepsize and get convergence here! Maybe increase the number of iterations of each time step or change the solution algorithm.

By currewnt step size you mean 1e-3 or 1e-5

RodriguezFatz November 28, 2012 04:43

I mean the old one (1e-3). I guess in your post above you meant "i decrease the time step size from 1e-3 to 1e-5" and not vice versa...

Smaras November 28, 2012 04:45

Quote:

Originally Posted by RodriguezFatz (Post 394583)
I mean the old one (1e-3). I guess in your post above you meant "i decrease the time step size from 1e-3 to 1e-5" and not vice versa...

other way i.e. from 1e-5 to 1e-3 that why its such a drastic change

RodriguezFatz November 28, 2012 05:04

I am confused... you can not decrease to 1e-3 from 1e-5. This is an increase.


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