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Blown Up Courant Number: twoPhaseEulerFoam

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Old   December 18, 2017, 22:25
Unhappy Blown Up Courant Number: twoPhaseEulerFoam
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Hi all,
I have recently encountered a strange problem that troubles me a lot. I am running a two-phase case the involves particles and air. The geometry is simply a 2D slice of a cylindrical body.
The simulation is done with variable time step with Courant Number smaller than 1, so the problem is discovered that deltaT suddenly drops to an extremely low level after simulating for a while, which means the Courant Number actually blows up at that position.
To simplify this problem, I forced the volume fraction of particles to be 0, so what is simulated is actually a single phase flow.
The problem happens at around 1.5s of simulation, and the velocity field is attached: before and after the blowing up of Courant Number, nearly all quantities behave quite well, including k, epsilon, velocity, etc. The only observable irregularity is observed with the pressure field. Pictures of the velocity field and the pressure field are attached.
I appreciate it a lot if you could be helping me, since it has really troubling me for quite a long time.
Thanks a lot guys!
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File Type: png U_air.png (42.4 KB, 22 views)
File Type: png pressure.png (26.8 KB, 20 views)
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Old   December 19, 2017, 00:19
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In most such situations the b.c. are not correct. I would check them first.
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Old   December 19, 2017, 00:31
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Quote:
Originally Posted by piu58 View Post
In most such situations the b.c. are not correct. I would check them first.
Hi piu58,
Thanks a lot for your reply!
However, I have checked my boundary conditions and changed them several times as well. It seems that they are not that problematic.
Since I have observed in my openfoam that the cells are affected from the outlet, I hereby provide my b.c. for the outlet:
Code:
velocity: zeroGradient
pressure: fixed value (uniform, atmospheric pressure)
p_rgh: prghPressure
k: inletOutlet
epsilon: inletOutlet
Is there any problem in the B.C. here?
(To be honest, for the boundary conditions, I mostly followed the fluidised bed in the tutorial cases)
Another important thing is that I have actually run the same case before, using the icoFoam solver,which gives quite good results.
Thanks again!
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Old   December 19, 2017, 09:03
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You need a pressure reference point somewhere. I don't know whether atmospheric pressure gives it.
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Old   December 19, 2017, 20:26
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Quote:
Originally Posted by piu58 View Post
You need a pressure reference point somewhere. I don't know whether atmospheric pressure gives it.
Actually what I am doing is simply setting the atmospheric pressure as internal field as an initial condition. I am not quite aware whether this gives so-called reference point
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Old   December 21, 2017, 02:25
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Hey guys
Update of this problem: I have consulted my professor, and he told me to focus on p_rgh instead of p alone. But the problem is that I could not find the suggested boundary conditions for p_rgh. Is there anything that we can refer to? Thanks guyz
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Old   December 22, 2017, 02:36
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Are you simulating a pipe flow? You mesh is same with mine.

I would suggest you switch off the turbulence to check if the problem comes from turbulence. The small value of deltaT implies your velocity's too large, which probably comes from a wrong prediction of turbulence fields.

Just a guess.
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Old   December 22, 2017, 02:43
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Quote:
Originally Posted by sharonyue View Post
Are you simulating a pipe flow? You mesh is same with mine.

I would suggest you switch off the turbulence to check if the problem comes from turbulence. The small value of deltaT implies your velocity's too large, which probably comes from a wrong prediction of turbulence fields.

Just a guess.
Hi Dongyue
Thanks a lot for your suggestion. Actually, just now I turned off the adjustable runtime, and make it fixed to a low enough but still acceptable fixed deltaT, and let it run from the beginning. Strangely, this seems to be working for the simplified single-phase case. I am now running it for two-phase, and I think it would be OK.
BTW thanks a lot for your website and your papers: it really helps me a lot to get more familiar with CFD and OpenFOAM.
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