Learning Forum

[SV]

Greetings

I am trying to model injecting air into water to see the formation of bubble. The figure shows that bubble is not axisymmetric in 2D and 3D modeling. Wall adhesion is taken into the consideration by adding contact angle. Please let me know if you know the problem.

[Rob]

Please can you repost the image with node values off? 

[SV]

This is a better picture. Height is 20 mm and 120 nodes. Base is 6 mm*6mm and 36 nodes*36 nodes.

[SV]

is there anyway to make a sharp interface between gas and liquid

[Rob]

And with node values off? 

[SV]

Please let me know what you mean and how to do that?

[Rob]

[SV]

Please look at following picture. is that what you want to see

[Rob]

I was hoping for the contour as before. The droplet is probably shifted sideways by a combination of mesh and initial condition. 

[SV]

 

Do you have any suggestion how to fix the problem. I do not have this problem when I inject air from following nozzle

 

[Rob]

Why do you think it's different? 

[SV]

In case a, the bubble is axisymmetric and in case b, I do not have axisymmetric bubble for 2D and 3D modeling. In case b, triple line can move freely and I do not have axisymmetric bubble.

[Rob]

Right, and looking at the above, why do you think it's behaving as it is? It's a sensible result, but one that's not quite as expected. Read up on the pitchfork bifurcation phenomena. 

[SV]

 

I can see the issue here. When I was checking the existing clips, I see some people can model it correct. I tried to model axisymmetric (half of bubble) but I get error. I have injected air into water for case b experimentally and I have got axisymmetric bubble experimentally. I was wondering if you can guide me in a right direction. is there anything I can do to fix the issue. 

 

[Rob]

Axi-symmetric will work fine, assuming you remember to scale the flow and follow the rules for axi-symmetric mesh/flow.

Asymmetry (or lack of) means you balanced a load of forces and had a perfect set up. In the experiments a slight nick in the entry port, off axis chamfer, spot of grease/fingerprint or fractionally higher flow could result in differences in the resulting bubble stream. In CFD the geometry should be good, but the mesh may be fractionally different, then there's multigrid etc. All tiny effects that can give a noticable effect. 

Consider a balance scale, if perfectly balanced and add a 0.001g mass to one side. Tiny change, but we now have a huge difference in the result. 

[SV]

When I try to model 2D and Axi-symmetric, I get following error

Error: floating point exception

Error Object: #f

I tried to reduce the time step size and mesh size to avoid the issue but it was not working

[Rob]

Did you rotate the mesh, ie did you check the axis rules for 2d axi-symmetric?

[SV]

The only change is in boundary conditions. Please look at boundary conditions and see the Axis.

is there anyway to make a sharp interface between gas and liquid

 

 

[Rob]

Please read the documentation for 2d axi-symmetric first. 

[SV]

Greetings

 

I have modeled different Axi-symmetric (2D) before like fluid flow in a pipe. I double checked. I do not see any issue. Please send me a link if you want me to read or double check certain area.

Thank you

 

[Rob]

You have gravity in the y direction, but the axis must be x=0. Hence checking. 

[SV]

Please check the geometry. 

 

[Rob]

That'll explain the problem.  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_GridQuality.html 

[SV]

Seems to me the geometry should be symmetric around x-axis. I changed the axis, and it is running now. We normally like to see gravity acts in y-axis. Thank you. 
Do you know how to make a sharp interface. Please look at the picture, the interface between liquid and air is not very sharp.

 

[Rob]

Check the mesh - it's why I still want to see that plot with node values off. To tighten up the free surface you'll need good convergence and a well refined mesh.