[ROY4]

Hello,
I'm modeling a bubble column using CFX and for boundary conditions in outlet I set degassing and for inlet I have a wall that source points are defined on it to inject air in to the domain. I solved it and I found the mass conservation correct. and the results were some how near to the experimental data. But now I have two really serious problems that I feel they might make my solution completely wrong.
--------------------------------------------------------------------
1) For modeling, I did not set initial conditions and volume fractions. Instead I used the steady state solution(which in there no volume fractions was set either) as my initial guess. Can this make my solution wrong?
--------------------------------------------------------------------
2) I want to get data about the pressure of the gas in outlet but the pressure that it gives to me is about 600 pa which is not correct, the air is entering the domain at 1 atm. the degassing boundary gives 600 pa and it can not be specified for every fluid separately...that is not true because then the flow does not exit the domain. I thought maybe Im getting the degassing boundary meaning wrong. In CFX-Pre I didn't specify any thing for the pressure. Now I feel that this makes my solution wrong. please if some one know any thing about the pressure definition in CFX, answer my question that: should I define some expression for pressure before defining my run to get the true results?
or maybe my settings are all wrong from the first place?

I appreciate your helps in advance

thank you

[ghorrocks]

1) If the initial condition is important then yes, it could be significant. It sounds like what you did was equivalent to having a water tank with no bubbles then turning the air on. If this is a reasonable approximation of the actual situation then your initial condition sounds fine.

2) The variable pressure is relative to the reference pressure. And for buoyant simulations there is an additional factor because the pressure is relative to a hydrostatic pressure obtained from the buoyancy reference density and location. Read the documentation about the pressure variable for more details.

[ROY4]

Originally Posted by ghorrocks

1) If the initial condition is important then yes, it could be significant. It sounds like what you did was equivalent to having a water tank with no bubbles then turning the air on. If this is a reasonable approximation of the actual situation then your initial condition sounds fine.

2) The variable pressure is relative to the reference pressure. And for buoyant simulations there is an additional factor because the pressure is relative to a hydrostatic pressure obtained from the buoyancy reference density and location. Read the documentation about the pressure variable for more details.

Dear ghorrocks
thanks a lot for answering my question.
1)for your first answer I should say no. there is air injection via source points in steady state too. I mean that I solved the whole problem in steady state condition and did not specify a volume fraction in any place of the settings. the only thing I did was not to activate transient mode...I let it go in steady state for about 20000 steps and then I use it as an initial condition for transient flow,
now do you think this is wrong to model the unsteady two phase problem in steady state mode and then use its information as the initial guess for the transient solution?
2) can you please explain what do you mean by documentation about pressure variable? because I checked CFX tutorial and CFX guide for more explanation of pressure but it didn't give enough information. and there is not pressure contour in literature related to my case
3) by the way can you please tell me what is the difference between total pressure and pressure in CFD post? Is pressure static pressure?
because from both of them i get this result that the pressure increase with increasing height...it is leveled but it does not make sense to my knowledge

[ghorrocks]

1) This approach is good, if this is representative of what you are modelling (starting a transient run from a steady state run).

2) See CFX theory manual, section 1.3, Buoyancy. Eqn 1-128 is what I am talking about.

3) Total pressure = Static Gauge Pressure + Dynamic pressure; Pressure = Static gauge pressure.

[ROY4]

Originally Posted by ghorrocks

1) This approach is good, if this is representative of what you are modelling (starting a transient run from a steady state run).

2) See CFX theory manual, section 1.3, Buoyancy. Eqn 1-128 is what I am talking about.

3) Total pressure = Static Gauge Pressure + Dynamic pressure; Pressure = Static gauge pressure.

Dear ghorrocks
All the best for you..Thanks a lot for your help and fast answering
But may I ask you do you think that the pressure in a bubble column must increase with height or decrease?
I ask it again to see if you have any sense about it(like any personal idea)?because as I said before I don't have.
In my knowledge as the height increases the pressure decrease...but I'm experiencing vise versa in here..pressure increase with height increasing. How is that possible?can buoyancy and turbulent flow effect this some how?