# Discussion Forum

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Hi, I am using Comsol 5.3a. At the moment using Darcy as well as transport of dilute species modulus by coupling these two equations . I am doing parametric study from small range of diffusion coefficient from 10^-9 to 10^-15, such that investigating concentration of solute across my geometry. However, each time I decrease Diffusion to 10^-14 to 10^-15 range, I get negative concentration next to narrower part of channel. I even used small meshes with distribution of fixed numebr of 100, still getting negative concentration in some parts of domain. Is there anyone who khows how I should solve this problem?

Many thanks, Azin

1 Reply Last Post 14 août 2019 à 07:51 UTC−4

Posted: 6 months ago
Updated: 6 months ago

Hi Azin, if you simulate a transport of diluted species you have to note the Cell-Peclet number. The Peclet number (https://en.wikipedia.org/wiki/Péclet_number) is the ratio of advective and diffusive transport. You can calculate the cell Peclet number as follows: Pec=h*spf.U/tds.Dav_c.

From the from a strong mathematical point of view the numerical method accurately solves the PDE with Cell-Peclet-Numbers < 1 ... 2. COMSOL Multiphysics® is very robust and it gives you satisfying results in many cases with Cell-Peclet-Numbers >> 2.

If you decrease the diffusion coefficient in such a way you mentioned above you dramatically increase the Cell Peclet Number, that means the ratio of advective and diffusive transport. I recommend you to check the Cell Peclet number.

Best regards Michael Rembe

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Michael Rembe
Rembe Consulting PartG mbB
www.rembe-consulting.de

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