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: log
'
How can I use comsol physcis well? So far I do not get the right results. It seems that they is very few parameters that can be controlled. Just mesh and solvers can be controlled............ Maybe I am wrong.....
I also attach my simulation as following:
Hi,
For stationary you have (ignore my tensorial mistakes),
- u . grad(c) = - grad . [D . grad(c)]
where c is the primary variable defined in comsol.
In comsol model builder, define a variable
c1=log(c)
Therefore,
c = exp(c1) and
grad(c) = exp(c1)*grad(c1)
So, your transformed equation is:
u . exp(c1)*grad(c1) = grad (D*exp(c1)*grad(c1))
Now go to "transport in dilute species" and click on convection and diffusion equation.
In the velocity field, u, feed
velocity in x = exp(c1)*ux
velocity in y = exp(c1)*uy
ux and uy are the velocities you get from your flow solution.
Now, go to the diffusion field, and feed:
D*exp(c1)
where D is the effective diff coeff.
Finally, if you are solving with initial conc = 0, then put a very small number say 10^-20 so that the variable c1 does not have a problem (log(0)).
This is roughly the derivation, but I have never tried this. If there is a mistake let me know.
You could switch on adaptive meshing with a reasonable initial meshing as a starting point.
Suresh
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: log
'
How can I use comsol physcis well? So far I do not get the right results. It seems that they is very few parameters that can be controlled. Just mesh and solvers can be controlled............ Maybe I am wrong.....
I also attach my simulation as following:
Hi,
For stationary you have (ignore my tensorial mistakes),
- u . grad(c) = - grad . [D . grad(c)]
where c is the primary variable defined in comsol.
In comsol model builder, define a variable
c1=log(c)
Therefore,
c = exp(c1) and
grad(c) = exp(c1)*grad(c1)
So, your transformed equation is:
u . exp(c1)*grad(c1) = grad (D*exp(c1)*grad(c1))
Now go to "transport in dilute species" and click on convection and diffusion equation.
In the velocity field, u, feed
velocity in x = exp(c1)*ux
velocity in y = exp(c1)*uy
ux and uy are the velocities you get from your flow solution.
Now, go to the diffusion field, and feed:
D*exp(c1)
where D is the effective diff coeff.
Finally, if you are solving with initial conc = 0, then put a very small number say 10^-20 so that the variable c1 does not have a problem (log(0)).
This is roughly the derivation, but I have never tried this. If there is a mistake let me know.
You could switch on adaptive meshing with a reasonable initial meshing as a starting point.
Suresh