U, phi

ariaballack · March 15, 2013, 04:44

hi everybody,
I am solving concentration using diffusion equation and for the convection term I need the velocity (U) . but after 2 seconds, the velocity remains constant (steady) and solving it anymore is just wasting time. Does anyone know how can I read phi after 2 second and continue without solving U anymore?
thanks

Bernhard · March 15, 2013, 05:29

I don't know how important your start-up phase is (I assume it isn't important), but you can

*write a solver that first calculates the velocity field, and afterwards calculate the diffusion/convection.
* update the present solver with an if( t>2 ) around the solution step for U
* use a solver for flow first, then switch to a seperate solver for convection/diffusion.

JimKnopf · March 21, 2013, 15:32

A not so elegant but very fast way would be, to write a if statement around the #include "uEqn.H" or the code where you solve the momentum equation. You should also stopp solving the pressure correction.

This approch increases the computational work by one if statement every iteration, wouldn't be very flexible and only valid if U and p are constant after some time. But it is done within a minute.

Greets
Jim

March 15, 2013, 04:44	U, phi	#1
ariaballack New Member Aria Join Date: Nov 2012 Posts: 5 Rep Power: 13	hi everybody, I am solving concentration using diffusion equation and for the convection term I need the velocity (U) . but after 2 seconds, the velocity remains constant (steady) and solving it anymore is just wasting time. Does anyone know how can I read phi after 2 second and continue without solving U anymore? thanks

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March 15, 2013, 05:29		#2
Bernhard Senior Member Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21	I don't know how important your start-up phase is (I assume it isn't important), but you can write a solver that first calculates the velocity field, and afterwards calculate the diffusion/convection. update the present solver with an if( t>2 ) around the solution step for U * use a solver for flow first, then switch to a seperate solver for convection/diffusion.

March 21, 2013, 15:32		#3
JimKnopf Member Jim Knopf Join Date: Dec 2010 Posts: 60 Rep Power: 15	A not so elegant but very fast way would be, to write a if statement around the #include "uEqn.H" or the code where you solve the momentum equation. You should also stopp solving the pressure correction. This approch increases the computational work by one if statement every iteration, wouldn't be very flexible and only valid if U and p are constant after some time. But it is done within a minute. Greets Jim