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dimensionless NS equations

Posted Jan 20, 2011, 11:02 a.m. EST Interfacing, Chemical Reaction Engineering Version 4.0a 3 Replies

Lu Han
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Hi,

I am trying to model phase separation occuring in a biodiesel reactor. I am couping NS equations with Cahn-Hilliard equations in PDE format. I ran into some convergence problems with dimensional variables so I thought to move to using dimensionless equations instead. Using some literature cited scaling, the non-dimension NS equations were as follows: Re*rho*du/dt = -del(P) + eta*del(v)^2 + rho1*cB/Ca/Ch*rho*(mu1*del(c1))

How do I input the Reynolds number in the Comsol format of the NS equations? Do I have to change the format? Please help.

Thank you

Sincerely,
Lu
3 Replies Last Post Jan 26, 2011, 3:21 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 21, 2011, 1:44 a.m. EST
Hi

do not forget that you have the scaling parameters in the solver so this means for me that you can keep the units and use scalings to normalize when needed. Comsol is rather good to propose itself reasonable scaling factors, but you might need to fine tweak. There is also often an option to ask for verbose log that gives more info (not that much, I'm missing a level "2" to better understand the underlaying).

I have only observed during some of the COMSOL courses how the "teachers" played with these parameters, there are also a few of the model library items that require manual scaling

--
Good luck
Ivar
Hi do not forget that you have the scaling parameters in the solver so this means for me that you can keep the units and use scalings to normalize when needed. Comsol is rather good to propose itself reasonable scaling factors, but you might need to fine tweak. There is also often an option to ask for verbose log that gives more info (not that much, I'm missing a level "2" to better understand the underlaying). I have only observed during some of the COMSOL courses how the "teachers" played with these parameters, there are also a few of the model library items that require manual scaling -- Good luck Ivar
Lu Han
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Posted: 1 decade ago Jan 26, 2011, 2:30 p.m. EST
Thank you for your input. You discussed that there are scaling parameters already built in the solver for normalization. Can you point them out where they are documented and if they can be changed?
For example, pressure should be normalized by: (pressure)*(length) / (velocity) / (density)
and viscosity should be normalized by: viscosity * (interfacial length)^2 / (gradient density parameter)/ (bulk concentration)

Thank you,
Lu
Thank you for your input. You discussed that there are scaling parameters already built in the solver for normalization. Can you point them out where they are documented and if they can be changed? For example, pressure should be normalized by: (pressure)*(length) / (velocity) / (density) and viscosity should be normalized by: viscosity * (interfacial length)^2 / (gradient density parameter)/ (bulk concentration) Thank you, Lu
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 26, 2011, 3:21 p.m. EST
Hi

in some solver settings they can be adapted, these are numerical scaling parameters, and I believe without "units", but I haven't really though about the unit aspect (to be verified with COMSOL support).

These scaling parameters are described briefly in the solver section. And then, somewhere in some of the 4.1 solver windows you have a "verbose" option for the log file that gives you some more info, including scaling values

--
Good luck
Ivar
Hi in some solver settings they can be adapted, these are numerical scaling parameters, and I believe without "units", but I haven't really though about the unit aspect (to be verified with COMSOL support). These scaling parameters are described briefly in the solver section. And then, somewhere in some of the 4.1 solver windows you have a "verbose" option for the log file that gives you some more info, including scaling values -- Good luck Ivar

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