[swak4Foam] dynamic no-flux BC for electrokinetics, can groovyBC do this?

OmarEGB · October 26, 2017, 20:03

Hi,
I have a solver for a Poisson-Nernst-Planck system, that is, the Poisson equation gives me an electric field $-\nabla \phi$ and that electric field pushes the ions according the the Nernst-Planck equation, together with diffusion . The ion flux would be something like this

$J=-D(\nabla c + z_i c \nabla \phi )$

now, as a boundary condition, I need this flux to be zero in one of my boundaries. Obviously, making the condition $\nabla c = 0$ isn't enough in this case, since $\nabla \phi$ has a constant value (that comes from the boundary conditions to the Poisson equation), and additionally, the simulation starts from a non-uniform

field, so making that zero isn't an option either. I would need to specify a condition for the gradient in the form

$\nabla c = -z c \nabla \phi$

where the

in the right hand side would be the previous concentration value at the boundary (from the previous timestep) and $\nabla \phi$ is known (since this happens to be a boundary condition for the Poisson equation).

I was looking at my options and apparently It looks like could use groovyBC, or otherwise I would need to write a library specifically for that boundary condition.
I have no experience whatsoever with swak4foam or writing custom boundary conditions so I would like to take the easiest path.
Could anyone enlighten me on whether this can be done with groovyBC? is there any other alternative?

Snapy · February 8, 2018, 12:03

Hi OmarEGB,

did you manage to add that type of boundary condition? I would like to add a similar BC.

I would appreciate any hint

gschaider · February 8, 2018, 22:22

Quote:

Originally Posted by Snapy

Hi OmarEGB,

did you manage to add that type of boundary condition? I would like to add a similar BC.

I would appreciate any hint

Code:

snGrad(phi)

gives you the gradient in the normal direction of the patch

Snapy · February 9, 2018, 06:00

Thanks for the fast response and hint

So I could use snGrad() in refCast to set the BC for a specific patch:

Am I correct?

Quote:

fixedGradientFVPatchScalarField& c =

refCast<fixedGradientFvPatchScalarField>

(c.boundaryFieldRef()["patchID"]);

c.gradient() = phi.boundaryField()["patchID"].snGrad();

October 26, 2017, 20:03	dynamic no-flux BC for electrokinetics, can groovyBC do this?	#1
OmarEGB New Member Omar E. Godinez Join Date: May 2016 Posts: 8 Rep Power: 10	Hi, I have a solver for a Poisson-Nernst-Planck system, that is, the Poisson equation gives me an electric field $-\nabla \phi$ and that electric field pushes the ions according the the Nernst-Planck equation, together with diffusion . The ion flux would be something like this $J=-D(\nabla c + z_i c \nabla \phi )$ now, as a boundary condition, I need this flux to be zero in one of my boundaries. Obviously, making the condition $\nabla c = 0$ isn't enough in this case, since $\nabla \phi$ has a constant value (that comes from the boundary conditions to the Poisson equation), and additionally, the simulation starts from a non-uniform field, so making that zero isn't an option either. I would need to specify a condition for the gradient in the form $\nabla c = -z c \nabla \phi$ where the in the right hand side would be the previous concentration value at the boundary (from the previous timestep) and $\nabla \phi$ is known (since this happens to be a boundary condition for the Poisson equation). I was looking at my options and apparently It looks like could use groovyBC, or otherwise I would need to write a library specifically for that boundary condition. I have no experience whatsoever with swak4foam or writing custom boundary conditions so I would like to take the easiest path. Could anyone enlighten me on whether this can be done with groovyBC? is there any other alternative?

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February 8, 2018, 12:03		#2
Snapy New Member Chris Join Date: Jun 2016 Posts: 2 Rep Power: 0	Hi OmarEGB, did you manage to add that type of boundary condition? I would like to add a similar BC. I would appreciate any hint