How much slower?

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....

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.

Divergence detected in AMG solver: x-momentum
 

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:o

Terminal vel
 

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.

x-momentum
 

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:o
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.

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:)

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