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Multiphase modelling of dense suspension help

Posted 05.08.2015, 06:47 GMT-4 Computational Fluid Dynamics (CFD), Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.2, Version 4.3 2 Replies

Nikhil Nambiar
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I was going through the Multiphase model of dense suspensions tutorial and have a doubt in some of the steps. For the uniform concentration, there is no term "eps" in the expression for gamma. But in Case 2 there is a term eps added to it.

What does this eps stand for? I assumed it was the turbulent dissipation rate, but we are working in the laminar flow regime.

Also, in the equations given in the model there is a term mdc/rhod and mdc/rhoc about which there is no mention in the tutorial pdf file. Is that taken as 0.

I would also like to add small alterations to the equations in this model to suit the system im working on. Solving PDEs from scratch seems a daunting task to me. All ive got to know about modifying equations is that I should go to the equations view. Is it the week function section that I have to modify. I am not able to decipher the syntax of the expressions. Does anyone have links to materials I could go through to understand the syntax of week functions or rather anything on modifying equations in equation view?

I would be very grateful for any help.
2 Replies Last Post 06.08.2015, 08:51 GMT-4
Niklas Rom COMSOL Employee

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Posted: 9 years ago 06.08.2015, 07:33 GMT-4
Hi,
"eps" is a reserved name in COMSOL that produces a number close to zero. As explained in the application documentation it is used here to avoid division by zero in during the iterations. We will add a clarifying text about the meaning of eps to the application doc for the next release.

"md" is the mass transfer rate between phases. It is only active if you add a Mass Transfer feature to the Mixture Model interface. You can read more about this the help section "Theory for the Mixture Model
Interfaces".

If you hit F1 while in COMSOL, the help panel appears. In the search field, type "weak form" and you will be directed to several sections in the user guide that will guide you. Please note that COMSOL cannot provide any support on applications with modified formulations in the equation view. Also, as a word of warning, the fluid flow interfaces have rather involved stabilization schemes that will probably be corrupted of you manually modify the equation view formulations. You run a big risk of getting unstable models or even wrong results.
Hi, "eps" is a reserved name in COMSOL that produces a number close to zero. As explained in the application documentation it is used here to avoid division by zero in during the iterations. We will add a clarifying text about the meaning of eps to the application doc for the next release. "md" is the mass transfer rate between phases. It is only active if you add a Mass Transfer feature to the Mixture Model interface. You can read more about this the help section "Theory for the Mixture Model Interfaces". If you hit F1 while in COMSOL, the help panel appears. In the search field, type "weak form" and you will be directed to several sections in the user guide that will guide you. Please note that COMSOL cannot provide any support on applications with modified formulations in the equation view. Also, as a word of warning, the fluid flow interfaces have rather involved stabilization schemes that will probably be corrupted of you manually modify the equation view formulations. You run a big risk of getting unstable models or even wrong results.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 9 years ago 06.08.2015, 08:51 GMT-4
Hi

the "standard for "eps" in binary floating point numbers is defined here:

en.wikipedia.org/wiki/Machine_epsilon

or here

www.mathworks.com/matlabcentral/newsreader/view_thread/339346
stackoverflow.com/questions/6970705/why-cant-i-get-a-p-value-smaller-than-2-2e-16
--

Good luck
Ivar
Hi the "standard for "eps" in binary floating point numbers is defined here: https://en.wikipedia.org/wiki/Machine_epsilon or here http://www.mathworks.com/matlabcentral/newsreader/view_thread/339346 http://stackoverflow.com/questions/6970705/why-cant-i-get-a-p-value-smaller-than-2-2e-16 -- Good luck Ivar

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