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Species Transport in Saturated Porous Media -> why is porosity considered constant in the weak forms???
Posted May 8, 2011, 10:44 a.m. EDT 5 Replies
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Hi all,
I'm modeling species transport in saturated porous media. My species transport equation includes:
- Time-dependent
- Convection
- Diffusion
- No adsorption
- No dispersion
If you look at the weak forms for a Mobile Fluid-Immobile Solid, you find (after having removed useless terms equal to 0):
-chpm.theta*ct*test(c)-chpm.De_c*cx*test(cx)-chpm.u*cx*test(c)
There should be a time derivative with the porosity in it: -c*chpm.thetat*test(c)
i.e. the second term in equation (3-107) in the Chemical Reaction Engineering Module User's Guide,
but it's not there! Anobody knows why?
I guess the assumption is that the porosity is considered constant, but it's not mentionned anywhere.
My porosity evolves with time, so it's not a valid assumption in my case...
I'm modeling species transport in saturated porous media. My species transport equation includes:
- Time-dependent
- Convection
- Diffusion
- No adsorption
- No dispersion
If you look at the weak forms for a Mobile Fluid-Immobile Solid, you find (after having removed useless terms equal to 0):
-chpm.theta*ct*test(c)-chpm.De_c*cx*test(cx)-chpm.u*cx*test(c)
There should be a time derivative with the porosity in it: -c*chpm.thetat*test(c)
i.e. the second term in equation (3-107) in the Chemical Reaction Engineering Module User's Guide,
but it's not there! Anobody knows why?
I guess the assumption is that the porosity is considered constant, but it's not mentionned anywhere.
My porosity evolves with time, so it's not a valid assumption in my case...
5 Replies Last Post May 10, 2011, 1:52 a.m. EDT
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Posted:
1 decade ago
Hi
normally time derivative terms are only added if you are in transient analysis (and it's clearly identified by COMSOL), so sometimes, when you have several combined solver steps, you need to load the initial values of the transient analysis to get the formulas updated (or after a first full solver case).
its the same for the time "t", that gets defined only from the moment you run a transient analysis. If you start with a stationary case and solve that only you might have formulas that imply "t" and this i flagged "undefined" => workaround, define a PARAMETER t=0[s] and COMSOL finds out of it. Conclusions: never fully trust a formula before you have run the solver ;)
--
Good luck
Ivar
normally time derivative terms are only added if you are in transient analysis (and it's clearly identified by COMSOL), so sometimes, when you have several combined solver steps, you need to load the initial values of the transient analysis to get the formulas updated (or after a first full solver case).
its the same for the time "t", that gets defined only from the moment you run a transient analysis. If you start with a stationary case and solve that only you might have formulas that imply "t" and this i flagged "undefined" => workaround, define a PARAMETER t=0[s] and COMSOL finds out of it. Conclusions: never fully trust a formula before you have run the solver ;)
--
Good luck
Ivar
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Posted:
1 decade ago
Hi Ivar,
I'm running a transient analysis since the beginning, over and over. No transient term has appeared in the weak forms after simulation.
The transient term for the concentration (c) is present in the equation from the start: -chpm.theta*ct*test(c).
What I'm saying is that there should be a time derivative of the porosity in order to respect the equation in the User's Guide (see eq. 3-107 in the attached pdf). This derivative would have this form if we respect the default comsol nomenclature: -c*chpm.thetat*test(c)
with
c: concentration
chpm.theta: porosity
But this term is never there! As if the porosity was ALWAYS considered constant...which is wrong.
Is it clear now? Sorry if my explanations are not clear, english is not my native language.
I'm running a transient analysis since the beginning, over and over. No transient term has appeared in the weak forms after simulation.
The transient term for the concentration (c) is present in the equation from the start: -chpm.theta*ct*test(c).
What I'm saying is that there should be a time derivative of the porosity in order to respect the equation in the User's Guide (see eq. 3-107 in the attached pdf). This derivative would have this form if we respect the default comsol nomenclature: -c*chpm.thetat*test(c)
with
c: concentration
chpm.theta: porosity
But this term is never there! As if the porosity was ALWAYS considered constant...which is wrong.
Is it clear now? Sorry if my explanations are not clear, english is not my native language.
Attachments:
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Posted:
1 decade ago
Hi
To make it easier to compare, pls specify CM version and dimension (2-3D?)
From what you say the issue was not in the solver not being updated then (what I thought and mentioned in the previous answer).
If you are in v4.0 there might be a missing term indeed (as that was an early version after a major software change), but that should be corrected in the newer one if so. But I have problems to find back your formula in my patched 4.1 ;) So it's difficult for me to say much more
Hei, just one thing still: I see a "ct" = dc/dt in Comsol notation, there in the first term of your original mail:
-chpm.theta*ct*test(c)
----------------^^^
--
Good luck
Ivar
To make it easier to compare, pls specify CM version and dimension (2-3D?)
From what you say the issue was not in the solver not being updated then (what I thought and mentioned in the previous answer).
If you are in v4.0 there might be a missing term indeed (as that was an early version after a major software change), but that should be corrected in the newer one if so. But I have problems to find back your formula in my patched 4.1 ;) So it's difficult for me to say much more
Hei, just one thing still: I see a "ct" = dc/dt in Comsol notation, there in the first term of your original mail:
-chpm.theta*ct*test(c)
----------------^^^
--
Good luck
Ivar
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Posted:
1 decade ago
Hi,
There should be TWO time derivatives: one for the concentration (c) and one for the porosity (chpm.theta).
Please look at eq.3-107 in the attached pdf of my second post.
I work in 1D.
Have a look at the weak espressions in the attached jpg here, and compare with eq. 3-107: the second time derivative is not there, as if porosity was considered constant.
No matter the value of the porosity (I tried many things: time dependent polynomial, etc.), the second time derivative never appears.
I'm running 4.1.0.185.
There should be TWO time derivatives: one for the concentration (c) and one for the porosity (chpm.theta).
Please look at eq.3-107 in the attached pdf of my second post.
I work in 1D.
Have a look at the weak espressions in the attached jpg here, and compare with eq. 3-107: the second time derivative is not there, as if porosity was considered constant.
No matter the value of the porosity (I tried many things: time dependent polynomial, etc.), the second time derivative never appears.
I'm running 4.1.0.185.
Attachments:
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Posted:
1 decade ago
Hi
you should get this clarified by "Support", as you are using v4.1 with the latest patch
--
Good luck
Ivar
you should get this clarified by "Support", as you are using v4.1 with the latest patch
--
Good luck
Ivar