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July 13, 2004, 08:43 
UDS_FLUX

#1 
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Dear Everybody, There have been some doubts about UDS_FLUX, here are two more: i) has anybody written udf for electric charge continuity equation? It is similar to the mass continuity, but in place of the fluid velocity we have the charge velocity, and in place of the density we charge density. If yes, I would have some practical questions
ii) What are exactly the NV_VEC(), NV_D() and NV_S() functions and how do they work? I saw these functions being referred in the UDF manual, but they were not explained. 

July 14, 2004, 10:02 
Re: UDS_FLUX

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I dont know about the NV_VEC() that you have mentionedbut have used
NV_V(viscous_force,= ,wallshear); this copies one vector to the other. Look under vector utilities and you might find out more. Ajay 

July 14, 2004, 14:48 
Re: UDS_FLUX

#3 
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Hi, I'm trying to write an UDF for charge conservation, but I consider i (current density (A/m2)= =conductivity*gradient of potential), instead of charge density, but I think that we have the same problem. By reading previous messagges we have to define an UDS, with diffusivity=conductivity.
Ari 

July 15, 2004, 04:33 
Re: UDS_FLUX

#4 
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Hi Ari, Thanks for your answer. It is good to know that somebody has been working on a similar problem, so that we could help each other. In principle current density can be written as:
J=conductivity*gradient of potential+ charge density * particle velocity I am modelling high velocity (>mach 1) flow where the electric charge is attached to dpm particles and the carrier stream is air. Because of the high velocities and nonconductiing media I am considering electron drift alone to be negligible, but perhaps I am wrong. I have some papers where this was suggested (although not very recent ones), and I have some others where the second term in the equation above was neglected. In these, fluid velocities were considerably lower. What about your case? Are you considering the current density as a UDMI or a UDS? What are the boundary condition for your J? I have already implemented the Poisson equation it works fine. Regrads, Szabolcs 

July 15, 2004, 11:33 
Re: UDS_FLUX

#5 
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Hi Szabolcs,
I'm modeling charge conservation in a porous media (=electrolyte in a fuel cell) and so, in my case, the second term is negligible in electroneutrality's condition. Perhaps your problem is more complicated than mine...btw what are you modeling??? 

July 16, 2004, 04:36 
Re: UDS_FLUX

#6 
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Hi, I am modelling an electrogasdynamic power converter (converts fluid kinetic energy to electric power without moving parts of equipment). I had (have) to add to FLUENT two extra equations. Poisson's, that is:
div^2 (el. potential)= charge dens/permitivity where the charge density is calculated from the dispersed phase concentration. The other equation is the electric charge continuity, that I have more problem with. However, I think that since the eletric charge is attached to the particles, once particle continuity exists, electric charge continuity should exist too.? Anyway, I would like to calculate current density in order to estimate electric power output. Regards, Szabolcs 

July 20, 2004, 11:29 
Re: UDS_FLUX

#7 
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Hi Szabolcs!!!
Sorry for my late reply, but I cannot give a reply to your model's question. I'm still confused about UDS for potential "phi", for example how could I link boundary conditions for "phi" (or for current density i)for two adjacent subdomain??? In a previous post there is a sign to UDMI that is not very clear for me. Thanks in advance for your responses!!! Ari 

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