[Xilaf]

Hi,

I simulate the air bubble rising in water column. I work with small (d=1mm) bubble. Can you tell me if the VOF model is acceptable in my case? I attached the the .jpg file with the situation at the beggining of the rising.

Thank you

[CFDtoy]

For single bubble simulations, kindly go ahead and use VOF type..it will provide you with surface deformation of the bubble and other local transients along the interface.

For cluster of bubbles rising ..use eulerian type and try increasing the bubble diameter if you want the bubbles to move up faster

/CFDtoy

Originally Posted by Xilaf

Hi,

I simulate the air bubble rising in water column. I work with small (d=1mm) bubble. Can you tell me if the VOF model is acceptable in my case? I attached the the .jpg file with the situation at the beggining of the rising.

Thank you

[Xilaf]

I need simulate the rise velocity of small bubbles (0.5-2 mm).

These are the results from VOF:
A small bubble (d = 1 mm) rises in the simulation cca 30 mm/s. It is too slow. It should rise 200 mm/s, how can I see from literature (R.Clift at al,1978) and from experiment with camera.
A bubble d = 2 mm rises cca 200 mm/s. It should rise 300 mm/s.

In the VOF I can't set the drag coeficient. Is it possible to set it in another model?
You say Eulerian. And what about Mixture model?

I haven't worked with these models yet.

Thank you

[CFDtoy]

I understand your concern. The problem with lift of bubbles in, atleast vof case, is due to the density difference. Make sure you input the density values properly. Also, try and run different density values to see if the bubble rises faster.

It is true that you cannot set drag coefficient for VOF..the flow is homogeneous - ie..the liquid and gas share the same velocity field.

In the eulerian - two velocity fields for each phase applicable - using drag coefficient (or bubble diameter) effectively you can increase / decrease the bubble rise velocity. problem with this is that ..the interface of liquid-gas interface may be little smeared.

Now, i suggest the following, increase the grid density (if it is 2D then great..) twice and run eulerian multi-fluid approach - play with the drag and make sure you get the rate of bubble rise as you want.

btw, i did want to make sure you switched on gravity right?? Check how much flow you have in the system...is there a lot of flow (is it thermal driven flow ?? - if so try increasing the boussinessq coefficient - to enhance flow)..even with VOF this should work well if there is sufficient flow.

Hope this helps.

Regards,

CFDtoy

Originally Posted by Xilaf

I need simulate the rise velocity of small bubbles (0.5-2 mm).

These are the results from VOF:
A small bubble (d = 1 mm) rises in the simulation cca 30 mm/s. It is too slow. It should rise 200 mm/s, how can I see from literature (R.Clift at al,1978) and from experiment with camera.
A bubble d = 2 mm rises cca 200 mm/s. It should rise 300 mm/s.

In the VOF I can't set the drag coeficient. Is it possible to set it in another model?
You say Eulerian. And what about Mixture model?

I haven't worked with these models yet.

Thank you

[jomin1986]

can you teach me how to obtain the velocity of the bubble in FLUENT software?

[amasoudi]

Hi,
Does anybody know how can I monitor rising velocity of a single bubble in a 2D column?

[Zaktatir]

Look after the centroid of the isosurface equal to void*void and then track the velocity of the bubble.

[amasoudi]

Hi
Thank you for your answer but unfortunately I did not understand what you mean

[Zaktatir]

Look after the centroid of the isosurface equal to void*void and then track the velocity of the bubble.

[gaurav4rt]

Originally Posted by jomin1986

can you teach me how to obtain the velocity of the bubble in FLUENT software?

In the left panel in ansys workbench fluent there will be MONITOR column. Click it and turn on the x y and z velocity fields and the continuity field and after doing the calculation click on countour button and in that select velocity as the criteria and click compute.It will show you the maximum and minimum velocity. Then click display...