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matlab/comsol unit problem

Posted Apr 9, 2010, 6:07 a.m. EDT Fluid & Heat, Interfacing, Heat Transfer & Phase Change, Parameters, Variables, & Functions Version 4.4 4 Replies

Priya Pathak
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My problem is

Diffusion eq + Navier–Stokes eq
I am solving diffusion equation which has unit (mm) in Matlab and Navier–Stokes eq in Comsol (SI unit).

I could not decide how to get out of unit problem.

what unit I should take in Comsol?

If I have structure of 35 mm length 5 mm width geometry.

4 Replies Last Post Dec 17, 2014, 6:27 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 9, 2010, 11:00 a.m. EDT
Hi

I always use SI units, its easier to handle, othervise you must always check that you have the right scaling factors everywhere, and you almost always miss a couple ;)

Ivar
Hi I always use SI units, its easier to handle, othervise you must always check that you have the right scaling factors everywhere, and you almost always miss a couple ;) Ivar
Guo Shusen
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Posted: 1 decade ago Aug 4, 2014, 5:53 a.m. EDT
thanks Ivar！
I have a question in transport of the diluted species，when I set the diffusion of the species to 1.77e-9[m^2/s], it is very slow to convergence,and there is a error says"the following feature has encounted a problem:
undefined value found
-Detail:NaN or Inf found when solving linear system using SOR
-Feature:stationary solver1(sol1/s1)"
I think it is because i set the diffusion coefficient to small， so i change the unit of my pde，I change all coefficients' unit to 1（you can see it in picture），but in comsol the geometry and physics still have the unit such as [m] and so on, my question is whether it will influence the solution of the pde，will i get the solution i want by inverse transformation.
thanks a lot!
thanks Ivar！ I have a question in transport of the diluted species，when I set the diffusion of the species to 1.77e-9[m^2/s], it is very slow to convergence,and there is a error says"the following feature has encounted a problem: undefined value found -Detail:NaN or Inf found when solving linear system using SOR -Feature:stationary solver1(sol1/s1)" I think it is because i set the diffusion coefficient to small， so i change the unit of my pde，I change all coefficients' unit to 1（you can see it in picture），but in comsol the geometry and physics still have the unit such as [m] and so on, my question is whether it will influence the solution of the pde，will i get the solution i want by inverse transformation. thanks a lot!
Aaron Ayal
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Posted: 10 years ago Dec 17, 2014, 5:00 a.m. EST
I have a similar problem.

I am trying to implement a MATLAB script to explore a set parameter space in a fluid problem.
The issue is that I am working with dimensionless equations and numbers.
I know how to implement this with the COMSOL desktop, but I've no idea how to set the unit system to dimensionless in the model created with MATLAB. I also seem to be unable to find it in the manuals provided for Livelink implementation.

EDIT.
The problem has been resolved.
I have a similar problem. I am trying to implement a MATLAB script to explore a set parameter space in a fluid problem. The issue is that I am working with dimensionless equations and numbers. I know how to implement this with the COMSOL desktop, but I've no idea how to set the unit system to dimensionless in the model created with MATLAB. I also seem to be unable to find it in the manuals provided for Livelink implementation. EDIT. The problem has been resolved.
Lasse Murtomäki
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Posted: 10 years ago Dec 17, 2014, 6:27 a.m. EST
In the uppermost level, in the end of the screen, select Unit System to None.
In the uppermost level, in the end of the screen, select Unit System to None.

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