[raagh77]

Hello all,

Anyone please tell me how "degassing boundary condition" (which is used in CFX) is implemented in OpenFOAM ?

Regards
Raghavendra

[hjasak]

Can you provide some information on this boundary condition:
- what is it used for
- how is it implemented

There may be some papers or information in publicly available sources as the name itself does not tell me what to do.

Hrv

[raagh77]

Hi..Hrvoje Jasak
my task is to simulate air particle being injected (at the center) to the water as continuous medium. (Becker case)

(Domain being simple 2D rectangular region sides and bottom (except at the center) being walls. Velocity inlet at the center and pressure outlet at the top)


Boundary conditions

1) alpha 0 gradients at the wall
2) inlet uniform 1
3) 3/4 th water (continous phase) filled so alpha being eual to 0 and remaining 1/4 air(discrete phase) and alpha being 1
4) alpha being 1 at the "top" where pressure outlet boundary conditions are set

But I was suggested to implement degassing boundary condition at the top so that only gas (air) can pass through it but not the liquid (continuous phase - water )

I am just wondering how this condition can be implemented in OpenFOAM..

Regards
Raghavendra

[holger_marschall]

Hi,

Degassing Boundary Condition is only available when one fluid is "Continuous" and the other "Dispersed Fluid". If the second fluid is
"Dispersed Solid", it is not available.

Degassing Boundary Conditions are used to model a free surface from which dispersed bubbles are permitted to escape, but the liquid phase
is not. They are useful for modelling flow in bubble columns. When Degassing Boundary Condition is selected as the Flow Specification of an outlet, the continuous phase sees this boundary as a free-slip wall and does not leave the domain. The dispersed phase sees this boundary as an outlet. However, the outlet pressure is not specified. Instead, a pressure distribution is computed on this fixedposition boundary, and can be interpreted as representing the weight of the surface height variations in the real flow.

Since the Degassing Boundary Condition does not specify a pressure value, a fixed pressure reference point will be set automatically for a domain with an incompressible flow with degassing boundaries, but no pressure boundaries.

from Physical Models Fluid Properties and Boundary Conditions in CFX-5

But in your case: why don't you simulate the free-surface in the bubble column as the free-surface it is The degassing b.c. in CFX is just a model for a quiscient free-surface being spatial fixed at the boundary. Just fill the bubble column to the level it was filled in the Becker experiment (compare to Henrik Rusche's PhD thesis).

Anyway I think you can "build" your boundary condition from the existing b.c.'s in OpenFOAM.

best regards
Holger

[raagh77]

Hello Holger Marschall

Thanks for your reply..
Yes, as you said am using the existing boundary conditions which is in twoPhaseEulerFoam tutorials.

But with slight modifications.
1.alpha 3/4 domain height 0 and top 1/4 is 1.
2. and K and epsilon being 1e-08 and 0.1 in the internal fields.
3. Small timestep 1e-04 (also with even less value!! )

but after few iterations
simulation crashes
1.sometimes with high courant number
2.sometimes with high continuity error and high cournat number.

Do I need to do some modifications for the solver and recompile it again
(also I tried with bubbleFoam Solver but without success )

awaiting for your reply..

[holger_marschall]

Hi Raghavendra,

what do you mean by "sometimes"? What else did you change? Can you post the error message?

However, it would be much more interesting to code that b.c. ... Do you have any informations on how the outlet pressure is computed? Are there some papers/reports?

best regards
Holger

[raagh77]

"sometimes" I meant was changing timestep to smaller values..but didn't work

This is the error message i get at the last iterations

#0 Foam::error::printStack(Foam:stream&) in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#1 Foam::sigFpe::sigFpeHandler(int) in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#2 Uninterpreted: [0xb7fda400]
#3 Foam::GAMGSolver::scalingFactor(Foam::Field<double >&, Foam::Field<double> const&, Foam::Field<double> const&, Foam::Field<double> const&) const in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#4 Foam::GAMGSolver::scalingFactor(Foam::Field<double >&, Foam::lduMatrix const&, Foam::Field<double>&, Foam::FieldField<foam::field,> const&, Foam::UPtrList<foam::lduinterfacefield> const&, Foam::Field<double> const&, unsigned char) const in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#5 Foam::GAMGSolver::Vcycle(Foam::PtrList<foam::lduma trix::smoother> const&, Foam::Field<double>&, Foam::Field<double> const&, Foam::Field<double>&, Foam::Field<double>&, Foam::Field<double>&, Foam::PtrList<foam::field<double> >&, Foam::PtrList<foam::field<double> >&, unsigned char) const in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#6 Foam::GAMGSolver::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libOpenFOAM.so"
#7 Foam::fvMatrix<double>::solve(Foam::Istream&) in "/home/ragh/OpenFOAM/OpenFOAM-1.5/lib/linuxGccDPOpt/libfiniteVolume.so"
#8 main in "/home/ragh/OpenFOAM/OpenFOAM-1.5/applications/bin/linuxGccDPOpt/twoPhaseEulerFo am"
#9 __libc_start_main in "/lib/tls/i686/cmov/libc.so.6"
#10 Foam::regIOobject::readIfModified() in "/home/ragh/OpenFOAM/OpenFOAM-1.5/applications/bin/linuxGccDPOpt/twoPhaseEulerFo am"
Floating point exception


at this timestep
Courant Number mean: 0.00349919 max: 67.8872


can I mail you the case setup files?
It is just 8mb..

Regards
Raghavendra

[raagh77]

it is just 1.6mb ..

[holger_marschall]

Sure. Please finde my e-mail in my profile!

regards
Holger

[enoch]

To. Raghavendra

Have you ever implemented this BC?

Thanks.

[hester]

Hello Raghavendra,

I am also curious about how to implement the degassing boundary condition. Have you ever figured out how to calculate the outlet pressure?

Regards,
hester

[BlnPhoenix]

Hey, i have the same trouble as described by raagh77 in this thread. Has anybody succesfully implemented a degassing boundary in OpenFoam?

I'm curious what i should try esp. in U.air, U.water, alpha.air and p files?! I tried stuff like inletOutlet but i get the exact error message as raagh77. Can anybody help me with this please?

[ancolli]

Originally Posted by BlnPhoenix

Hey, i have the same trouble as described by raagh77 in this thread. Has anybody succesfully implemented a degassing boundary in OpenFoam?

I'm curious what i should try esp. in U.air, U.water, alpha.air and p files?! I tried stuff like inletOutlet but i get the exact error message as raagh77. Can anybody help me with this please?

Degassing Boundary Condition