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-   -   separate time step for mass fraction? (http://www.cfd-online.com/Forums/cfx/24390-separate-time-step-mass-fraction.html)

joe August 10, 2007 06:40

separate time step for mass fraction?
 
hello, i simulate a problem with cfx 11 including a mass fraction. since that mass fraction moves with a very low settling velocity (downwards slip velocity)- in comparison to the flow of the constraint, i would like to solves the equation of the mass fraction with a bigger time step than the "global time step size". Does anyoune kmow if this is possible with a transient simulation? thanks! joe

Glenn Horrocks August 12, 2007 18:30

Re: separate time step for mass fraction?
 
Hi,

Why would you want to do this in a transient simulation? You won't get a true time history then. You should be able to use quite a large timestep if the bulk flow is pretty much steady state.

An alternative if the velocity and mass equations do not change but the mass fraction evolves then you can turn the velocity and mass equations off and only solve the mass fraction. This will speed the simulation up hugely if this assumption is valid.

Glenn Horrocks


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