# False Transient UnderRelaxation

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 June 8, 2006, 16:17 False Transient UnderRelaxation #1 Ihab Sraj Guest   Posts: n/a 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 . . . . .

 June 9, 2006, 09:10 Re: False Transient UnderRelaxation #2 Robin Guest   Posts: n/a 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.

 June 9, 2006, 12:52 Re: False Transient UnderRelaxation #3 Ihab Sraj Guest   Posts: n/a 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??????????

 June 9, 2006, 13:27 Re: False Transient UnderRelaxation #4 Robin Guest   Posts: n/a Ihab, CFX is not a density based code. -Robin

 June 9, 2006, 13:37 Re: False Transient UnderRelaxation #5 Ihab Sraj Guest   Posts: n/a 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????

 June 11, 2006, 12:42 Re: False Transient UnderRelaxation #6 Robin Guest   Posts: n/a 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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