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-   -   False Transient UnderRelaxation (http://www.cfd-online.com/Forums/cfx/22681-false-transient-underrelaxation.html)

Ihab Sraj June 8, 2006 16:17

False Transient UnderRelaxation
 
I have a question regarding the underRelaxation technique that CFX uses with the help of the false trasient time step.

Is it added to the mometum equation only or the continuity equation. Because also CFX asks for the time to be used with the contiuity equation...

If yes how should it be implemeted for an incomressible case where the transiet term is totally removed. Thanks in advance . . . . .

Robin June 9, 2006 09:10

Re: False Transient UnderRelaxation
 
The transient term is not removed from the equations. At steady state, the difference between the old and new transient term goes to zero (since the solution isn't changing) thus it's influence on the equations goes away.

Ihab Sraj June 9, 2006 12:52

Re: False Transient UnderRelaxation
 
Even for the case of incompressible fluid flow where the density is constant , and therefore the density transient term in the continuity equation can't be transformed into pressure to add stability??????????

Robin June 9, 2006 13:27

Re: False Transient UnderRelaxation
 
Ihab,

CFX is not a density based code.

-Robin

Ihab Sraj June 9, 2006 13:37

Re: False Transient UnderRelaxation
 
Dear Robin, I know that CFX is a pressure based code, this is why i asked that the transietn term appearing in the contiuity equation cannot be used as underRelaxaing parameter for the pressure equation because it is zero . . . . . . . . .. .. .!!!!!!!!!!!! for incompressible flow.....

AM i right????

Robin June 11, 2006 12:42

Re: False Transient UnderRelaxation
 
I guess I never thought of that.

You have to keep in mind that the continuity equation is not solved on it's own, but coupled with the momentum equations, where the transient term is not zero.

-Robin


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