# Equalizing Pressure between Two Tanks

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November 7, 2017, 08:27
Equalizing Pressure between Two Tanks
#1
Member

CWL
Join Date: Nov 2015
Posts: 55
Rep Power: 7
I was asked about a supposedly easy problem but Fluent seems to give wrong results. I hope someone can shed some insights for me.

Two interconnected tanks connected with a small pipes (as shown) with no inlet/outlet. The initial pressure and temperature of the two tanks are different.
The black lines are the walls with no-slip conditions.

Wanted to simulate the transient process of mixing. It is obvious that the final velocity should be about 0, pressure and temperature be the averaged value of the initial ones. However, when simulation started, within one or two time-steps, the pressure, temperature and velocity all becomes zeroes. I somehow suspect one cannot put wall all around the boundary - somewhere must have an inlet/outlet condition for the simulation to work. Can anyone shed some light on the problem?

Thanks.
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April 7, 2020, 22:02
#2
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navid toussi
Join Date: Nov 2015
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Quote:
 Originally Posted by cwl6750084 I was asked about a supposedly easy problem but Fluent seems to give wrong results. I hope someone can shed some insights for me. Two interconnected tanks connected with a small pipes (as shown) with no inlet/outlet. The initial pressure and temperature of the two tanks are different. The black lines are the walls with no-slip conditions. Wanted to simulate the transient process of mixing. It is obvious that the final velocity should be about 0, pressure and temperature be the averaged value of the initial ones. However, when simulation started, within one or two time-steps, the pressure, temperature and velocity all becomes zeroes. I somehow suspect one cannot put wall all around the boundary - somewhere must have an inlet/outlet condition for the simulation to work. Can anyone shed some light on the problem? Thanks.
Hi,
I was wondering if you have managed to solve your problem at the end?

 April 8, 2020, 05:34 Case Setup #3 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,948 Blog Entries: 1 Rep Power: 31 Wall boundary condition is not the problem. That is alright. However, you need to look at it from the perspective of reality. Why would these two fluids mix? And which model is being used. If both are same fluids but at different temperature, then mixing would depend upon whether the natural convection is enabled or not. If not, then there won't be any fluid convection and thermal energy will get distributed purely because of thermal distribution. If these are two different fluids, then they should either be gases or miscible liquids, such as, water and alcohol. This would require species transport model to be enabled. Gravity can also play a role if density ratio is substantially far from unity. If the fluids are immiscible, then it would require multiphase model and gravity is must. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

April 13, 2020, 19:19
#4
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navid toussi
Join Date: Nov 2015
Posts: 17
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Quote:
 Originally Posted by vinerm Wall boundary condition is not the problem. That is alright. However, you need to look at it from the perspective of reality. Why would these two fluids mix? And which model is being used. If both are same fluids but at different temperature, then mixing would depend upon whether the natural convection is enabled or not. If not, then there won't be any fluid convection and thermal energy will get distributed purely because of thermal distribution. If these are two different fluids, then they should either be gases or miscible liquids, such as, water and alcohol. This would require species transport model to be enabled. Gravity can also play a role if density ratio is substantially far from unity. If the fluids are immiscible, then it would require multiphase model and gravity is must.

Hi,
what if the fluid is only air with different pressures in each tank, in one of them 10 bar and the other on 1 bar. The Temperature at the beginning is room Temperature. There is no heat transfer through the walls.
I have such a case and the system is closed, there is no inlet/outlet boundary conditions except walls. How do you suggest me to set such a case?

 April 14, 2020, 04:39 Air #5 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,948 Blog Entries: 1 Rep Power: 31 If the fluid is gas and at different pressure values, then it must be modeled using ideal gas as density. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

April 14, 2020, 07:35
#6
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navid toussi
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Quote:
 Originally Posted by vinerm If the fluid is gas and at different pressure values, then it must be modeled using ideal gas as density.
which solver of openfoam do you suggest me to use? the speed in the connection pipe have the possibility to reach near the speen of sound!! (Ma=1)
I am using rhoPimpleFoam with k and epsilon turbulent model, but simulation after some iterations blows up.

 April 14, 2020, 07:39 OpenFOAM! #7 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,948 Blog Entries: 1 Rep Power: 31 I am afraid that you are in the wrong Forum; this is Fluent Forum. You should pose the question in OpenFOAM forum. rhoPimpleFoam should work as long as rho is driven by ideal gas law. will work alright. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.