buoyantSimpleFoam : problem with the value of viscosity

laurentD · June 29, 2016, 11:44

Hello everybody,

i am currently trying to use the buoyantSimpleFoam solver and i am facing a problem. When i take the real value of the viscosity of the air (2.e-05), the solver doesn't want to run, speaking about a printstack error.

But when i use the viscosity of the water (0.001), it runs...
Can anybody explain it ?

Thanks a lot.

Laurent

Coris · June 30, 2016, 02:50

Do you only change the value of the viscosity?

Can you post the full error?

jherb · June 30, 2016, 05:44

You do not give enough information to really help you, but just one guess: Due to the higher viscosity of water, some problems are "smeared" out.

Have you tried to reduce the under-relaxation factors?

laurentD · June 30, 2016, 09:02

I have tried to reduce the under-relaxation. I see an effect but the problem is still present.

Currently i am decreasing the value of viscosity slowly.
I explain :
visc. = 0.001 until convergence.
Then using the resulting fields, visc. = 5e-04 until convergence... etc.
Doing it i succeed to reach the good viscosity.

I have to add the following information :
the studied flow is very turbulent.

I think the transition between the sleeping-state and the turbulent one is too strong.

TobM · July 1, 2016, 03:42

Edit:

Now I read your last post correctly, sry. You can try to work with velocity ramping BCs, where the velocity is build up slowly.
Also, check if your turbulence model is fit for the turbulence you have in your domain.
If you have effects like vortex shedding, you have to be careful if you can still use a steady state solver, maybe it is better to change to buoyantPimpleFoam.

June 29, 2016, 11:44	buoyantSimpleFoam : problem with the value of viscosity	#1
laurentD Senior Member Laurent DASTUGUE Join Date: May 2014 Location: Chartres, France Posts: 122 Rep Power: 11	Hello everybody, i am currently trying to use the buoyantSimpleFoam solver and i am facing a problem. When i take the real value of the viscosity of the air (2.e-05), the solver doesn't want to run, speaking about a printstack error. But when i use the viscosity of the water (0.001), it runs... Can anybody explain it ? Thanks a lot. Laurent Last edited by laurentD; June 30, 2016 at 02:44.

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June 30, 2016, 02:50		#2
Coris New Member Joris C. Join Date: Jan 2013 Posts: 29 Rep Power: 13	Do you only change the value of the viscosity? Can you post the full error?

June 30, 2016, 05:44		#3
jherb Senior Member Joachim Herb Join Date: Sep 2010 Posts: 650 Rep Power: 21	You do not give enough information to really help you, but just one guess: Due to the higher viscosity of water, some problems are "smeared" out. Have you tried to reduce the under-relaxation factors?

June 30, 2016, 09:02		#4
laurentD Senior Member Laurent DASTUGUE Join Date: May 2014 Location: Chartres, France Posts: 122 Rep Power: 11	I have tried to reduce the under-relaxation. I see an effect but the problem is still present. Currently i am decreasing the value of viscosity slowly. I explain : visc. = 0.001 until convergence. Then using the resulting fields, visc. = 5e-04 until convergence... etc. Doing it i succeed to reach the good viscosity. I have to add the following information : the studied flow is very turbulent. I think the transition between the sleeping-state and the turbulent one is too strong.

July 1, 2016, 03:42		#5
TobM Member Join Date: Sep 2014 Location: Germany Posts: 88 Rep Power: 11	Edit: Now I read your last post correctly, sry. You can try to work with velocity ramping BCs, where the velocity is build up slowly. Also, check if your turbulence model is fit for the turbulence you have in your domain. If you have effects like vortex shedding, you have to be careful if you can still use a steady state solver, maybe it is better to change to buoyantPimpleFoam.