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Poisson Nernst Planck and Navier Stokes equation

Posted 11.04.2011, 01:42 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.3b 6 Replies

mitchel
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Hi everyone,

I am trying to simulate the electro-osmotic flow in a nano-channel using the Poisson-Nernst-Planck and Navier-Stokes equations coupling them from the Electrostatics,Transport of dilute Species and Laminar flow modules respectively using stationary solver in Comsol 4.0a. The nano-channel has surface charge on the walls and is filled with an electrolyte solution,and an external electric field is applied to bring about the flow.

After applying the boundary conditions,on computing I am getting the error message "No convergence even when using minimum damping factor. Returned solution has not converged"...

Now the boundary conditions for the Navier Stokes have normal flow,no pressure at the inlet and outlet. When I am increasing the pressure to some positive value,my solution converges,but the conditions for my simulation have to be zero pressure conditions. The boundary where i am having surface charge has been provided with finer mesh and the no of mesh elements seem to be enough.

So it would be very helpful,if somebody can point out what is the reason for getting such an error message and how can I go about getting a converged solution.

Regards,
Mitchel
6 Replies Last Post 03.01.2014, 02:47 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 11.04.2011, 03:07 GMT-4
Hi

I have noticed that NS solving often converges easier if you set up as initial condition with some first flow ands pressure drop. Even a few Pa is often enough ;)

And using the latest COMSOL version helps alot ;)

--
Good luck
Ivar
Hi I have noticed that NS solving often converges easier if you set up as initial condition with some first flow ands pressure drop. Even a few Pa is often enough ;) And using the latest COMSOL version helps alot ;) -- Good luck Ivar
mitchel
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Posted: 1 decade ago 11.04.2011, 08:17 GMT-4
Thanx Ivar...

I too think that putting a few Pa is gonna help,just wanted to know any other way to debug the simulation to get a convergent solution.

Mitchel
Thanx Ivar... I too think that putting a few Pa is gonna help,just wanted to know any other way to debug the simulation to get a convergent solution. Mitchel
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 11.04.2011, 08:54 GMT-4
Hi again

when working with simple tubing I usually set up a head loss/pressure drop from Poiseuille law, + a parabolic initial and win mostly a lot of solver time, but with complex flows or diffuse/porous media flow, I'm not sure what to use at best ;)

--
Good luck
Ivar
Hi again when working with simple tubing I usually set up a head loss/pressure drop from Poiseuille law, + a parabolic initial and win mostly a lot of solver time, but with complex flows or diffuse/porous media flow, I'm not sure what to use at best ;) -- Good luck Ivar
Gaspard Pardon
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Posted: 1 decade ago 29.02.2012, 09:33 GMT-5
Hi,

I am trying to simulate about the same thing.
I stumble with setting appropriate boundary condition at the ends of the channel to get convergence.

I set 0.5 V and 0 V at each end.
Inflow, 0 pressure and outflow zero pressure.
An fix the concentration at the bulk value for the specie. But, this seems to be the problem. I have tried hard to work around it by having more realistic boundary conditions, but I still get convergence problem, or non realistic results.

Can you help?

Thanks
Hi, I am trying to simulate about the same thing. I stumble with setting appropriate boundary condition at the ends of the channel to get convergence. I set 0.5 V and 0 V at each end. Inflow, 0 pressure and outflow zero pressure. An fix the concentration at the bulk value for the specie. But, this seems to be the problem. I have tried hard to work around it by having more realistic boundary conditions, but I still get convergence problem, or non realistic results. Can you help? Thanks
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 29.02.2012, 11:17 GMT-5
Hi

I would propose to use a v input and a p=0 output then you set a fixed point for both dependent solutions and I believe the uniqness of your PDE solving will be iproved
when you mix also V not to forget that ALL dependent variables need enough BCs to give only one solution

--
Good luck
Ivar
Hi I would propose to use a v input and a p=0 output then you set a fixed point for both dependent solutions and I believe the uniqness of your PDE solving will be iproved when you mix also V not to forget that ALL dependent variables need enough BCs to give only one solution -- Good luck Ivar
Fang Hao
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Posted: 1 decade ago 03.01.2014, 02:47 GMT-5
Hi

I want to ask one question about CFD module.
I'm working on estimating the energy loss of my reactor.
The modle I built up in COMSOL is a pipe reactor with one inlet and one outlet above.
The physics I added is turbulent flow,kappa-epsilon.
And I set mass flow(0.5t/h) as inlet boundary condition,0Pa as outlet boundary condition.
After setting,meshing and studying,I make surface intergration of the velocity magnitude on inlet and outlet surface in order to count the energy loss rate.
But the data says that the intergration of Vout is larger than Vin.
How to explain that phenomenon?
Is there any good way to estimatem the head loss in COMSOL 4.3b?
I will be grateful for your answer,thank you!
Hi I want to ask one question about CFD module. I'm working on estimating the energy loss of my reactor. The modle I built up in COMSOL is a pipe reactor with one inlet and one outlet above. The physics I added is turbulent flow,kappa-epsilon. And I set mass flow(0.5t/h) as inlet boundary condition,0Pa as outlet boundary condition. After setting,meshing and studying,I make surface intergration of the velocity magnitude on inlet and outlet surface in order to count the energy loss rate. But the data says that the intergration of Vout is larger than Vin. How to explain that phenomenon? Is there any good way to estimatem the head loss in COMSOL 4.3b? I will be grateful for your answer,thank you!

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