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Re: How to solve the problem of negative concentration

You means that it is not a good idea to solve this problem by the method of logarithmic transformation? Except for choosing finer meshes and the method of lograrithmic transformation, it seems that...

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Re: How to solve the problem of negative concentration

Thank you. Various stabilization technique? Which parameters are related to this technique? I know that if the the (relative/absolute) tolerance is larger, it is more easy to get convergence. The...

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Re: How to solve the problem of negative concentration

Hello. I return to work. i had tried your suggestion. But it still did not pass the problem of negative concentration. It showed that 'Attempt to evaluate real logarithm of negative number. - Function:...

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Re: How to solve the problem of negative concentration

Hi, Here is one trick to solve negative concentrations. I made variable "C" which is defined by "if(c<0,eps,c)" under Definitions => Variables. Instead of concentration "c" I used variable "C"....

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Re: How to solve the problem of negative concentration

Thank you! I just returned to work yesterday from a vacation. I will try to do it like that and tell you the results. I try to use the boundary layer mesh. The appropriate velocity field could be...

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Re: How to solve the problem of negative concentration

Thank you. Your method seems available. But if another condition that the concentration value exceeds the expectable largest value also occurs, how to set the variable C? That is, if c<0, c=eps; if...

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Re: How to solve the problem of negative concentration

Hi, Try this: if(c<0,eps,if(c>1,1,c)). Best regards Tero

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Re: How to solve the problem of negative concentration

Dear Tero, It is so good. You are good on using comsol physics. I should learn from experienced experts on this field like you! Thank you so much! Hi, Try this: if(c<0,eps,if(c>1,1,c)). Best...

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Re: How to solve the problem of negative concentration

Hello Tero, I think about this issue again. I use the method that you mentioned, all positive values of concentration could be obtained. However, maybe it just related to the range of the values that...

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Re: How to solve the problem of negative concentration

Hi, I not sure did I understand you correctly...? This is just modeling trick how to get over problems during solving. Like Ivar said other things (mesh etc) are still main solution to this problems...

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Re: How to solve the problem of negative concentration

Hello, good morning. For my model, the concentration should be between 0-1 mol/l. However, some values are over 1, the largest values are about 1.03 (3%of the expectable largest value), and some values...

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Re: How to solve the problem of negative concentration

Hi often in time series models you have steep gradients at t=0 i.e. concentration at a boundary, nothing inside the domain. Then I have noticed that using a "boundary mesh" on the c=1 (or T=cnst)...

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Re: How to solve the problem of negative concentration

Hello Ivar, Thank you so much. My model is stationary model with laminar flow and transport of diluted specie models. To my model I used the boundary layer mesh for the laminar flow, and got physical...

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Re: How to solve the problem of negative concentration

Hi indeed my remark was more for time dependent cases with high gradients due to initialcondition steps. The next on the list, in my view, is the numerical stabilisation parameters, but I'm not use to...

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Re: How to solve the problem of negative concentration

Hi, Try this: if(c<0,eps,if(c>1,1,c)). Best regards Tero Thank you so much, Tero. I appreciate your helps.

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Re: How to solve the problem of negative concentration

if(c<0,eps,if(c>1,1,c)) Dear Tero, First i would like to say thank you because ,i am doing my MS project "modeling of electrosorption" using comsol version 4.3 at Hindustan Unilever...

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Re: How to solve the problem of negative concentration

Hi, Eps is coming from Comsol and it is about 2.2e-16. I have used it instead of zero. If c is between 0 and 1 it is keeping original value of c. For example if: c is -1 => return value is 2.2e-16...

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Re: How to solve the problem of negative concentration

Hi, Here is one trick to solve negative concentrations. I made variable "C" which is defined by "if(c<0,eps,c)" under Definitions => Variables. Instead of concentration "c" I used variable "C"....

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Re: How to solve the problem of negative concentration

Hi, Diffucult to say without seeing your model... have you check your model mesh? I have never tested this method with unsteady case. Best regards Tero

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Re: How to solve the problem of negative concentration

Hi Suresh, Thanks for the analysis. Is there any way to solve with logarithmic concentration(or any alternative approach) for time dependent drift-diffusion equation. Physically why we can't use your...

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