[sega]

Originally Posted by Xilaf

Yes. I've just run the calculation in axisymmetric space. The bubble rises 2x faster, than in planar space, but still slower, than the reality expects.

How much slower?

[thecfduser]

Why dont u do a 3D simulation and get a good result?????
even at small sizes, the trajectory of ure bubble is not necesserly vertical (unless it gets near a wall) I am not sure of the ranges of Re with in u get this behavior, but t is due to the instability of the vortex ring.
The values of velocity that u are comparing with: are u sure that those experiences have been done in a very clean water???? in normal water, the behavior of bubbles is different. By exemple for stokes regime, the drag will be 24/Re, while it is 16/Re in a very clean water....

[sega]

Can't blame him. Do you know which computational power he can access?
I'm pretty sure a 3D simulation with acceptable interface resolution will take days ...

Lebaigue et. al. (http://test.interface.free.fr/Case01.pdf) have accomplished good results in 2D and Xilaf is simulation even smaller bubbles (and lower Reynolds) ...

@Xilaf:
What you should check is the approximation of the error the terminal velocity will have inside your domain: Equation (1.4) in the above paper.
If this is in the range your bubble rises slower you may want to increase your computational domain.

[almostafa67]

hi dear all...
i was running fluent solving my 2D case after 35 time step an error message showed up (Divergence detected in AMG solver: x-momentum) i decreased under relaxation factor for momentum from 0.7 to 0.1 but again this message...
i will be thankful if u share any similar experience

[pranab_jha]

Originally Posted by almostafa67

hi dear all...
i was running fluent solving my 2D case after 35 time step an error message showed up (Divergence detected in AMG solver: x-momentum) i decreased under relaxation factor for momentum from 0.7 to 0.1 but again this message...
i will be thankful if u share any similar experience

You can try decreasing your time step. That should do it.

@All
I am also running simulations of similar nature. I would like to know how to find out the terminal velocity of the bubble in fluent. I got qualitative results of two bubbles rising in a tank (walls have free shear so that they do not affect the bubble). I want to know the Reynold's number. For that I need the terminal velocity.

[almostafa67]

Originally Posted by pranab_jha

You can try decreasing your time step. That should do it.

@All
I am also running simulations of similar nature. I would like to know how to find out the terminal velocity of the bubble in fluent. I got qualitative results of two bubbles rising in a tank (walls have free shear so that they do not affect the bubble). I want to know the Reynold's number. For that I need the terminal velocity.

dear pranab thank u for ur response
i,ve tried all of this,i changed time step from 0.0001 to 1e-7 but no luck
and about ur question:in my case rotational velocity is constant but i think u can read velocity from display>xy plot>velocity>...
hope it helps

[pranab_jha]

Originally Posted by almostafa67

dear pranab thank u for ur response
i,ve tried all of this,i changed time step from 0.0001 to 1e-7 but no luck
and about ur question:in my case rotational velocity is constant but i think u can read velocity from display>xy plot>velocity>...
hope it helps

Do you mean I create a line which passes through the center of the bubble and through to the top of the domain. Then plot my velocity on the line and see when it goes constant? I am trying this. Let me know if you think differently.

Also, for your problem, I read somewhere (cannot find the reference right now) that if your cell volume is less than 1e-6 cubic meter, then you could try with 3ddp version of fluent instead of 3d. Give it a try and let me know. It worked for me in one case. But have not required it again.

[almostafa67]

Originally Posted by pranab_jha

Do you mean I create a line which passes through the center of the bubble and through to the top of the domain. Then plot my velocity on the line and see when it goes constant? I am trying this. Let me know if you think differently.

Also, for your problem, I read somewhere (cannot find the reference right now) that if your cell volume is less than 1e-6 cubic meter, then you could try with 3ddp version of fluent instead of 3d. Give it a try and let me know. It worked for me in one case. But have not required it again.

yeah,it's a good suggestion u can try this but i dont know how much realible it is!
while ago i tried to open my 2D case with 3D fluent but i faced an error;i'll try that,tnx for ur suggestion

[Leen_1]

Originally Posted by Xilaf

The infinite cylinder doesn't mean, that the water column is infinity, but that the circle midhip doesn't represent the bubble but the infinite cylinder orthogonal on the 2D geometry.

I set the values of bubble (diameter) and both phases (density, viskosity). and the interaction between phases well, I think. (0.073 mN/m)

0.073mn/m is small,it should be set 0.073 n/m,best regard