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Equalizing Pressure between Two Tanks

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Old   November 7, 2017, 08:27
Default Equalizing Pressure between Two Tanks
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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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Old   April 7, 2020, 22:02
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Originally Posted by cwl6750084 View Post
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?
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Old   April 8, 2020, 05:34
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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.
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Old   April 13, 2020, 19:19
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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?
Thank you in advance
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Old   April 14, 2020, 04:39
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If the fluid is gas and at different pressure values, then it must be modeled using ideal gas as density.
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Old   April 14, 2020, 07:35
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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.
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Old   April 14, 2020, 07:39
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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. k-\varepsilon will work alright.
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