# Boundary

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 April 19, 2013, 14:51 Boundary #1 New Member   Aria Join Date: Nov 2012 Posts: 5 Rep Power: 6 Hi everybody, I am writing a solver for concentration diffusion. Somehow I need to set a boundary condition for concentration so that instead of a fixedValue for a wall, the value at the wall could have a new value after each time step and the value is the average of for example concentration (concentration has already been solved in solver). what type of boundary type should i use and how? thanks

 April 19, 2013, 15:51 #2 Senior Member   Lieven Join Date: Dec 2011 Location: Leuven, Belgium Posts: 297 Rep Power: 15 Hi Aria, Is it the scalar transport (convection-diffusion) equation that you are solving? If so, I would advise you to pick a zeroGradient BC. This prevents any diffusion over the wall boundary from occurring. Cheers, L

 April 20, 2013, 10:03 #3 New Member   Aria Join Date: Nov 2012 Posts: 5 Rep Power: 6 Hi, Thanks for your fast reply, Yes, thats what i solve, but the particel at surface has to be renewed after each five second with a concentration equal to average one. Thats why i need a flexible BC.

 April 21, 2013, 03:13 #4 Senior Member   Lieven Join Date: Dec 2011 Location: Leuven, Belgium Posts: 297 Rep Power: 15 Hi Aria, The boundary condition you want to impose is not really clear to me. Do you have a mathematical expression for it to clearify it? E.g. the five seconds, is this equal to the preset time step? Cheers, L

 April 22, 2013, 08:16 #5 Senior Member   Olivier Join Date: Jun 2009 Location: France, grenoble Posts: 265 Rep Power: 11 hello, Take a look at groovyBC: you can use a mixed BC (zeroGradient/value), and switch with the expression value you want (like every 5 s), and calculate the average concentration you need. NB: And like Lieven say, take care when you use fixedValue, since you have no control over the diffusive flux. regards, olivier

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