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Negative concentration and strange profiles in microfluidic? help!

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Hi everyone!

I am modelling a microfluidic device. There's an initial concentration of reagent in the chamber and I apply a flow with nearly 0 concentration at one side. (I put the file at the bottom of this message)

The thing is, I obtain slightly negative concentrations (when plotted in linear) and when I plot the concentration on a logaithmic scale (log(c/c0)) the profile goes crazy!

I've been looking around forums and already tried :

- A ramp function for the flow of the nearly 0 concentration to avoid sharp transitions.
- To slow down the fluid flow
- To add inconsistant stabilization
- Some tricks like a new variable C defined as "if(c<0,eps,c)" and to plot C instead of C

But I can not manage to suppress this effect!

If somebody have an other idea or want to have a quick look I would be so GRATEFUL!


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