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-   -   Can we handle steep gradients with blok command? (http://www.cfd-online.com/Forums/phoenics/52073-can-we-handle-steep-gradients-blok-command.html)

 Kali Sanjay June 23, 2006 13:30

Can we handle steep gradients with blok command?

Dear Friends!

I am solving a porous medium problem and activated CONPOR in some region. So my AEAST changed in some region as per conpor. However the fluid and concentrations in the porous medium are flowing into adjuscent non-porous medium which is also part of computational domain. When I activated conpor, the mass balance is distrubed severly. The fluid is same in the porous region as well outside (single phase). Looks like problem at the interface and mass flux is inconsistant with mass in domain increasing with time. However the initial concentration gradients are too steep near the interface which must be causing this mass imbalance. Can someone help what to do when we activate conpor command? Do we have to do some blok corrections at the interface? If so how to? Thanks in advance and looking forward to Mike for help!

Sanjay

 Mike June 26, 2006 04:27

Re: Can we handle steep gradients with blok comman

I advise solving for the superficial velocity in the porous medium because this eliminates the velocity discontinuity at the interface of the porous medium with the external fluid. This in turn means that the momentum source term associated with the flow resistance through the porous media must be based on the superficial velocity rather than the velocity at the minimum flow area.

 Kali Sanjay June 26, 2006 15:31

Re: Can we handle steep gradients with blok comman

Dear Mike!

Thank you for your reply. Actually in my system, the flux is due to migration. I am solving for concentrations and am giving flux at faces by upwind scheme which is -u*c*dp/dx, where u is mobility, c is concentration and dp/dx is potential drop which has units moles/area/time. At the interface, this flux is not balancing ie. flux from porous medium to free area is less, but concentration in the cell next to porous medium is increasing and disturbing mass balance. Please help me!

Thank you in advance,

Sanjay

 Mike July 4, 2006 03:15

Re: Can we handle steep gradients with blok comman

Do you solve a momentum and overall continuity equation? Or are you solving for concentration only?

 Kali Sanjay July 4, 2006 04:35

Re: Can we handle steep gradients with blok comman

Dear Mike, I am solving for Concentration only and 'Convection' term in terms is off. I have diffusion active and source term for migration, I am upwinding. Sanjay

 Mike July 4, 2006 08:36

Re: Can we handle steep gradients with blok comman

How do you determine the potential field?

 Kali Sanjay July 4, 2006 12:40

Re: Can we handle steep gradients with blok comman

Dear Mike, I am solving Laplace equation d/dx(dv/dx)=0, where v is potential. Sanjay

 Mike July 5, 2006 04:14

Re: Can we handle steep gradients with blok comman

Kali,Thank you. I now understand that you are solving two equations, one for the concentration, and one for some potential. You have not given a complete description of the physcial process you are trying to simulate, but I suspect your potential equation may be derived from Darcy's Law and the continuity equation, thereby eliminating the overall continuity and momentum conservation equations in favour of a Laplace-type equation for the potential. Darcy's law is usually formulated in terms of the superficial velocity, in which event it is inconsistent to define area porosities within the porous media, as the correct flow resistance will be represented by the permeability/hydraulic conductivity. I think what I am saying is that it must be possible to formulate your model entirely in terms of the superficial velocity, which will obviate the need to define area porosities within the porous media.

 Kali Sanjay November 4, 2006 12:19

Re: Can we handle steep gradients with blok comman

Dear Mike, I am sorry I was disconnected with my modeling work due to some very personal preoccupation and couldn't respond to your reply....however I am back and yet to solve the problem. My system is sort of a tube in which, part of it is filled with sand. I am solving transient problem with diffusion and migration. Migration term I am giving as flux at each wall(u*c*dp/dx, where u is mobility of ion, c, concentration and dp/dx the potential gradient).Migration actually is d(u*c*dp/dx)/dx, which I am specifying as u*c*dp/dx at east and west walls and using upwind method. For getting potential (dp/dx), I am solving laplace equation d(K dp/dx)=0, where K is conductivity. There is no convection at present, which I want to activate later once this program runs.

I activated CONPOR as CONPOR(CAREA,0.4,CELL,#2,#2,1,1,1,1), the second region is the one filled with sand.

This shows my AEAST is same in porous region and non-porous region. Even though my AEAST is same in all the regions, I presume PHOENICS takes volume of cells in porous region actually is DXU2D * AEAST * 0.4

Is it correct?

I tried to use BLOK correction for all variables.

However the mass in the system is increasing. especially in the non-porous region. Can you please suggest what wrong I am doing.

Thank you in advance.....

Sanjay

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