# Discussion Forum

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## matlab/comsol unit problem

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 17.12.2014, 06:27 GMT-5

Posted: 10 years ago
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

Posted: 5 years ago
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!

Posted: 5 years ago
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.