[Destry]

Hi all...

i'm just running a case of fluid flow around a pipe in FLUENT..
but i got some problem in iterating...
i'm a new user
so i need some explanation about limitation of time step size
because i have run my case but still not finish yet..

i make 0.17 for time step
600 for number of time step
30 for max iter for time step

based on tutorial http://flowlab.fluent.com/exercise/pdfs/cylinder.pdf

thx before


Destry

[dobiarz]

Hi
I had got very similar problem, this what I've found in Flunet Documentation and some presentation from Fluent User Services Center:
"

- Max Iterations per Time Step: When FLUENT solves the time-dependent
equations using the implicit formulation, iteration is necessary at each
time step. This parameter sets a maximum for the number of iterations
per time step. If the convergence criteria are met before this
number of iterations is performed, the solution will advance to the
next time step.
- Time Step Size: The time step size is the magnitude of t. Since the
FLUENT formulation is fully implicit, there is no stability criterion

that needs to be met in determining t. However, to model transient

phenomena properly, it is necessary to set t at least one order of
magnitude smaller than the smallest time constant in the system being
modeled. A good way to judge the choice of t is to observe the
number of iterations FLUENT needs to converge at each time step.
The ideal number of iterations per time step is 10-20. If FLUENT
needs substantially more, the time step is too large. If FLUENT needs
only a few iterations per time step, t may be increased. Frequently
a time-dependent problem has a very fast \startup" transient that
decays rapidly. It is thus often wise to choose a conservatively small
t for the rst 5-10 time steps. t may then be gradually increased
as the calculation proceeds.
For time-periodic calculations, you should choose the time step based
on the time scale of the periodicity. For a rotor/stator model, for
example, you might want 20 time steps between each blade passing.

For vortex shedding, you might want 20 steps per period.

"
Solving SMM Problems
Choose appropriate Time Step Size and Max Iterations Per Time step to ensure good convergence with each time step.
Time Step Size should be no larger than the time it takes for a moving cell to advance past a stationary point:
t <= s/(omega*R)
s = average cell size
omega*R= translational speed
Good luck
Regards

[Destry]

thx for information
i really appreciate that...

Regards

Destry