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Yousef

No flow BC condition

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I am doing reactive transport simulation in 2D rectangle area. I would like to know how I can set up a flow BC condition? I want to set a no flow BC condition. any help is really appreciated.

Yousef

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By default, boundaries in TOUGH2 and "no-flow". There are several tricks for making them fixed with respect to pressure, temperature, or both.

To make a boundary have both constant pressure and temperature, you should select the cells in the boundary and make them "fixed state". This bumps up the volume of the cells by a very large amount, which means the any flow or heat transfer in our of the very large cells do not affect the conditions within the cells.

The PetraSim User's Guide has a good discussion of boundary conditions starting on page 61.

To make a boundary "fixed temperature", but not fixed pressure, then you should create a new material type with a permeability of zero. This keeps fluid from flowing in and out of the cells. You should them make the cells "fixed state", with an initial Temperature equal to the desired fixed temperature.

If you want a fixed pressure boundary that acts as a barrier to heat flow, then make the thermal conductivity of the cells equal to 0. This will allow fluid to flow in and out of the cells, but not heat.

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To make a boundary have both constant pressure and temperature, you should select the cells in the boundary and make them "fixed state". This bumps up the volume of the cells by a very large amount, which means the any flow or heat transfer in our of the very large cells do not affect the conditions within the cells.

Alison,

that confuses me. I'm having a hard time understanding the difference between "high volume" boundary conditions and "fixed state"/"inactive elements". According to Tough2 manual these are too different things if I understand that correctly. Primary thermodynamic variables of "high volume" cells are still part of the unknowns set - although their value will not change significantly due to their large volume as you mention, while with "fixed state" cells they are treated as knowns, thus reducing calculation time.

Are there any conditions to be considered when to use either of them? Any advantages of one over the other - beside less computational time?

Thanks,

Julia

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This really depends on the simulator you are using. You are correct that the TOUGH2 simulator includes an option for treating fixed state cells "inactive" cells (denoted with a cell volume of 0 or less). However, TOUGHREACT and TMVOC don't include this options, so you are then forced to bump up the volume of the cells to force a fixed state. If you are using PetraSim, then the program takes the TOUGH2 approach for TOUGH2 models, and the large cell volume approach for TOUGHREACT and TMVOC models.

I think that in most cases, if you are using TOUGH2, the inactive cell approach is the most appropriate. This is what TOUGH2 impliments for Fixed State cells.

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I see. As I'm only working with TOUGH2 I did not know about the TOUGHREACT/TMVOC implementation of fixed state. Thanks for the clarification.

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