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Negative concentration and strange profiles in microfluidic? help!
Posted Nov 14, 2014, 10:22 a.m. EST Fluid & Heat, Chemical Reaction Engineering Version 4.2a 1 Reply
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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!
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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1 Reply Last Post Nov 17, 2014, 5:23 a.m. EST
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Posted:
1 decade ago
Dear William,
the problem in your model areises from the fact that your initial and boundary condition at the inlet are inconsistent. In other words, it is unphysical to have c0=3e-4 in the whole domain as initial, but at the same time cin=1e-12 on the boundary. This constitutes an infinitely sharp onset.
Make sure that you ramp your inlet concentration in a finite time from c0 top the desired value (e.g. using a step function). Then make sure that first time step is significantly smaller than that ramp time.
This is what the recommendation
"- A ramp function for the flow of the nearly 0 concentration to avoid sharp transitions."
actually means but I dont see it in your model.
This should solve the problem - if not, feel free to contact us in support www.ch.comsol.com/support
Best regards,
Sven
the problem in your model areises from the fact that your initial and boundary condition at the inlet are inconsistent. In other words, it is unphysical to have c0=3e-4 in the whole domain as initial, but at the same time cin=1e-12 on the boundary. This constitutes an infinitely sharp onset.
Make sure that you ramp your inlet concentration in a finite time from c0 top the desired value (e.g. using a step function). Then make sure that first time step is significantly smaller than that ramp time.
This is what the recommendation
"- A ramp function for the flow of the nearly 0 concentration to avoid sharp transitions."
actually means but I dont see it in your model.
This should solve the problem - if not, feel free to contact us in support www.ch.comsol.com/support
Best regards,
Sven