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-   -   bubbleFoam problems unstable and strange behaiviour (http://www.cfd-online.com/Forums/openfoam-solving/105679-bubblefoam-problems-unstable-strange-behaiviour.html)

matt.mech.eng August 6, 2012 09:31

bubbleFoam problems unstable and strange behaiviour
 
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Hi all,

I am running a simulation on an aeration tank and have been running into a few problems..

To try and troubleshoot an issue I have been having I have run a solution without having any gas enter the domain from the inlet. If you see the attached image which shows a plot of alpha and a vector plot of the continuous phase overlayed.

I keep getting similar results no matter what I try changing, it appears the continuous phase, b starts to move up the wall!!! This happens for both turbulent and laminar cases, if alpha is initialized (ie no interface between the 2 fluids alpha is initially zero all over the domain), and regardless of whether or not there is a gas inflow at the inlet boundary.

I am stumped and can not figure out what is going wrong- maybe someone here can?

Kind Regrads

Matt

Mat_fr August 30, 2012 09:42

Hello Matt,

What did you put as a boudary condition at the inlet for the continuous phase ?
And, I'm not sure if I get it : you said that if the tank is only filled with water, and without gas injection, the liquid is still moving ?

Best,

Mat

matt.mech.eng August 30, 2012 19:44

Hi Mat,

Thanks for your reply- I assume you are referring to the velocity of the continuous phase at the inlet? If so then this was set to fixed value zero. Yes that is correct- no inlet velocity for either phase and no internal fields initialized for either velocity.

This seems to only occur when the outlet is larger than the inlet.. You could set up a small case using the bubblefoam tutorial case and change blockmeshdict so that the top of the domain is larger (vee shaped column)- if you run a laminar case (so that the turbulence doesn't give you stability issues) with no inlet velocity specified and have alpha set to zero at the inlet you will observe the same problem I am having!

Kind regards,


Matt

Mat_fr September 5, 2012 07:41

Indeed, I tried this modified case (starting from bubbleFoam tuto), and I observed a low recirculation near the walls.
I have no idea why, sorry.

Good luck

Mat


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