URF vs Time Step Size
 

Hi,
What is better to achieve the convergence?
Lowering URF or reducing Time step size?
Thanks

In general, i would only touch URF if the solution is diverging.

Thanks for replying
But don't you think reducing time step has a bigger effect on solution than URF?
and what is the lowest value of URF you can use?
Regards

To be honest i can not give you a good answer to when to prefer a lower URF instead of a lower time step, since i never faced such a situation. But my guess is, if lowering the time step again and again doesn't not help at all, i would look into lowering the URF for a given time step size. There is then probably a optimum (calculation time wise) of time step and URF when you have a difficult problem that is not easy to stabilize.
Theoretically speaking the lowest URF would be something very small but > 0.

Thank you for your reply

Isn't URF somehow related to the Courant number as well. If that is the case then URF will effect courant number, likewise, time-step also effects courant number. Hence, stability of the problem would depend on URF and time-step size!!

I'm not aware of any such relation. Can you give a source for this?

Regards

In Fluent theory guide (V19)>21.4.4 Steady-state iterative algorithm>21.4.4.2 Under-relaxation of equation.

The CFL is a solution parameter in the pressure-based coupled algorithm and can be written in terms of :

(1-)/=1/CFL

Under-relaxation factor=

I don't understand this. I know the CFL definition only for time dependent simulations, here it is used for steady state applications. Also URF = 1 is commonly used in transient simulations for certain equations. The given formula here however yields CFL = 0 for = 1.

So maybe this is an alternative usage/definition of CFL, which in turn i don't see a use for in multiphase flows since these are pretty much always transient..

But thanks for making aware of this!

The definition I told you is for implicit relaxation of equations.