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Ill Conditioned Preconditioner

Posted 20 feb 2013, 02:00 GMT-5 Fluid & Heat, Heat Transfer & Phase Change, Mesh Version 4.2 3 Replies

Gary Yu
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Hi everyone,

I haver been trying to untangle this ill conditioned error with different meshings, which I think is the cause of the error.

The temperature distribution in the soil (from 3D plot group - Isosurface) seem to be reasonable, but with that error, I am not sure if the results are correct or not.

I have attached the file.

Please take a look and suggest what I should do to correct this.

Thank you in advance.
3 Replies Last Post 21 feb 2013, 02:01 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 20 feb 2013, 16:35 GMT-5
Hi

first of all as you do not really need the details of the fluid flow, you would be much better off with the new "pipe flow" module, that does not solve the flow in such detail.
Your issues most probably comes from the fluid meshing set-up, you have no boundary layer, and the mesh is too coarse, then you should add the initial conditions with a parabolic flow velocity all over and a Poiseille type initial pressure drop (check the formula on Wiki) anyhow solving the flud flow in detail is very RAM and CPU consuming

You could even try only the SPF flow alone to get your model to converge, then transfer the settings to your NITF model and add the thermal part

--
Good luck
Ivar
Hi first of all as you do not really need the details of the fluid flow, you would be much better off with the new "pipe flow" module, that does not solve the flow in such detail. Your issues most probably comes from the fluid meshing set-up, you have no boundary layer, and the mesh is too coarse, then you should add the initial conditions with a parabolic flow velocity all over and a Poiseille type initial pressure drop (check the formula on Wiki) anyhow solving the flud flow in detail is very RAM and CPU consuming You could even try only the SPF flow alone to get your model to converge, then transfer the settings to your NITF model and add the thermal part -- Good luck Ivar
Gary Yu
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Posted: 1 decade ago 20 feb 2013, 20:18 GMT-5
Thanks Ivar, for your comments and suggestions.

One thing I would like to point out is I actually tried to use a very fine mesh along the pipes, but that came up with 'very ill conditioned'. I was trying to attach to reflect this situation, but the file size was too big (just over 5MB) for this forum to allow me to attach. Therefore I made the meshing coarser just for me to be able to attach the file.

So do you think the model has already given the correct results (if i just want the temperature distributions in the soil)?

I will try and digest that pipe flow module business along with the other suggestions that you just gave me.

Please feel free to advise further.

Kind regards,

Gary Yu



Hi

first of all as you do not really need the details of the fluid flow, you would be much better off with the new "pipe flow" module, that does not solve the flow in such detail.
Your issues most probably comes from the fluid meshing set-up, you have no boundary layer, and the mesh is too coarse, then you should add the initial conditions with a parabolic flow velocity all over and a Poiseille type initial pressure drop (check the formula on Wiki) anyhow solving the flud flow in detail is very RAM and CPU consuming

You could even try only the SPF flow alone to get your model to converge, then transfer the settings to your NITF model and add the thermal part

--
Good luck
Ivar


Thanks Ivar, for your comments and suggestions. One thing I would like to point out is I actually tried to use a very fine mesh along the pipes, but that came up with 'very ill conditioned'. I was trying to attach to reflect this situation, but the file size was too big (just over 5MB) for this forum to allow me to attach. Therefore I made the meshing coarser just for me to be able to attach the file. So do you think the model has already given the correct results (if i just want the temperature distributions in the soil)? I will try and digest that pipe flow module business along with the other suggestions that you just gave me. Please feel free to advise further. Kind regards, Gary Yu [QUOTE] Hi first of all as you do not really need the details of the fluid flow, you would be much better off with the new "pipe flow" module, that does not solve the flow in such detail. Your issues most probably comes from the fluid meshing set-up, you have no boundary layer, and the mesh is too coarse, then you should add the initial conditions with a parabolic flow velocity all over and a Poiseille type initial pressure drop (check the formula on Wiki) anyhow solving the flud flow in detail is very RAM and CPU consuming You could even try only the SPF flow alone to get your model to converge, then transfer the settings to your NITF model and add the thermal part -- Good luck Ivar [/QUOTE]
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 21 feb 2013, 02:01 GMT-5
Hi

use a boundary mesh, or better just use the default COMSOL physical induced mesh, then add a parabolic flow for the initial conditions, and add a pressure drop, take the initial value from a simple Poiseuille case.

I agree your model looks quite logical, but that is only a qualitative statement, for quantitative values you must really validate it correctly with some hand calculations (I often use Maple to check my results analytically, for some simple cases)

--
Good luck
Ivar
Hi use a boundary mesh, or better just use the default COMSOL physical induced mesh, then add a parabolic flow for the initial conditions, and add a pressure drop, take the initial value from a simple Poiseuille case. I agree your model looks quite logical, but that is only a qualitative statement, for quantitative values you must really validate it correctly with some hand calculations (I often use Maple to check my results analytically, for some simple cases) -- Good luck Ivar

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