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Compressible Navier-Stokes with Non-newtonian viscosity in PDE module

Posted Mar 13, 2014, 6:24 a.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD), Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3 4 Replies

Daniel Restrepo
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Hello,

I am new to COMSOL and I've been trying to model some incompressible Navier-Stokes system with variable viscosity, a Carreau viscosity to be specific. I have used the CFD module to model this with the single phase laminar flow which also has Carreau fluid properties, and now I'd like to use the PDE module, but I have no idea how to input the equations.
I have also seen there are several ways to do it: Coefficient, General and Weak forms. I have my Navier-Stokes equations in cylindrical coordinates (better for a pipe flow) and in differential format, so my question is what would be the best way to do it and how?

Thanks in advance,
Daniel
4 Replies Last Post Jun 18, 2014, 4:43 p.m. EDT
Alex M.
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Posted: 1 decade ago Mar 13, 2014, 1:55 p.m. EDT
Hi,

Turns out you don't need the PDE module!! Just type ' 1+(100-1)*(1+(1*spf.sr)^2)^-0.25 ' in for mu under the fluid properties node. I would suggest using parameters for mu_inf, mu_0, lambda, and N but it is up to you.

See attached file, I would suggest doing a power law model first and compare to the analytical solution to make sure everything is working as expected.

Good luck!

Edit: I should add spf.sr is the variable name COMSOL uses for the shear rate in most fluid flow applications. If you are doing Fluid Structure Interaction you have to use fsi.sr (I think). you can always check variable names using equation view.


-- Alex
Hi, Turns out you don't need the PDE module!! Just type ' 1+(100-1)*(1+(1*spf.sr)^2)^-0.25 ' in for mu under the fluid properties node. I would suggest using parameters for mu_inf, mu_0, lambda, and N but it is up to you. See attached file, I would suggest doing a power law model first and compare to the analytical solution to make sure everything is working as expected. Good luck! Edit: I should add spf.sr is the variable name COMSOL uses for the shear rate in most fluid flow applications. If you are doing Fluid Structure Interaction you have to use fsi.sr (I think). you can always check variable names using equation view. -- Alex
Daniel Restrepo
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Posted: 1 decade ago Mar 18, 2014, 5:07 a.m. EDT
Hello Alex,

Thanks for answering! I appreciate your suggestion and also I think it's helpful to know what you wrote on the second part of your post about the shear rate "spf.sr", it might be useful for me later.

Thanks a lot,
Daniel
Hello Alex, Thanks for answering! I appreciate your suggestion and also I think it's helpful to know what you wrote on the second part of your post about the shear rate "spf.sr", it might be useful for me later. Thanks a lot, Daniel
Brian Berg
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Posted: 1 decade ago Jun 17, 2014, 10:03 a.m. EDT

Hi,

Turns out you don't need the PDE module!! Just type ' 1+(100-1)*(1+(1*spf.sr)^2)^-0.25 ' in for mu under the fluid properties node. I would suggest using parameters for mu_inf, mu_0, lambda, and N but it is up to you.

See attached file, I would suggest doing a power law model first and compare to the analytical solution to make sure everything is working as expected.

Good luck!

Edit: I should add spf.sr is the variable name COMSOL uses for the shear rate in most fluid flow applications. If you are doing Fluid Structure Interaction you have to use fsi.sr (I think). you can always check variable names using equation view.


-- Alex


Could you explain a little bit more about spf.sr and fsi.sr? I'm trying to make a model using a carreau fluid but I don't have the package so I'm inputting it manually. For some reason spf.sr isn't in my equation view (I'm using version 4.3)

[QUOTE] Hi, Turns out you don't need the PDE module!! Just type ' 1+(100-1)*(1+(1*spf.sr)^2)^-0.25 ' in for mu under the fluid properties node. I would suggest using parameters for mu_inf, mu_0, lambda, and N but it is up to you. See attached file, I would suggest doing a power law model first and compare to the analytical solution to make sure everything is working as expected. Good luck! Edit: I should add spf.sr is the variable name COMSOL uses for the shear rate in most fluid flow applications. If you are doing Fluid Structure Interaction you have to use fsi.sr (I think). you can always check variable names using equation view. -- Alex [/QUOTE] Could you explain a little bit more about spf.sr and fsi.sr? I'm trying to make a model using a carreau fluid but I don't have the package so I'm inputting it manually. For some reason spf.sr isn't in my equation view (I'm using version 4.3)
Alex M.
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Posted: 1 decade ago Jun 18, 2014, 4:43 p.m. EDT
spf.sr and fsi.sr are the variable names for shear rate in two different modules. I'm not sure which module you're using or what the shear rate is called. I would recommend solving any problem in the module of interest. After you have a solution plot the shear rate. When you plot the shear rate, the variable name is entered into the box "Expression" on your plot (line graph for example). That expression is the variable name for the shear rate, it's what you need as the independent variable in your carreau model.

Good Luck -- Alex
spf.sr and fsi.sr are the variable names for shear rate in two different modules. I'm not sure which module you're using or what the shear rate is called. I would recommend solving any problem in the module of interest. After you have a solution plot the shear rate. When you plot the shear rate, the variable name is entered into the box "Expression" on your plot (line graph for example). That expression is the variable name for the shear rate, it's what you need as the independent variable in your carreau model. Good Luck -- Alex

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