[Smaras]

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

[RodriguezFatz]

Originally Posted by Smaras

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]

Originally Posted by RodriguezFatz

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]

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

[Smaras]

Originally Posted by RodriguezFatz

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]

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]

Originally Posted by RodriguezFatz

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:






Mesh:





Penetration Depth:



Residuals:



Phases:




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

Regards,

Smaras

[RodriguezFatz]

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]

Originally Posted by RodriguezFatz

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?

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]

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]

Originally Posted by RodriguezFatz

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]

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]

Originally Posted by RodriguezFatz

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]

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]

Originally Posted by RodriguezFatz

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]

Originally Posted by Smaras

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]

Originally Posted by RodriguezFatz

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]

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]

Originally Posted by RodriguezFatz

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]

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