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July 18, 2010, 11:26 
Problem in iterating

#1 
New Member
Destry Marulitua Siagian
Join Date: Jul 2010
Posts: 20
Rep Power: 7 
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 

July 19, 2010, 08:36 

#2 
New Member
Join Date: Jul 2010
Posts: 6
Rep Power: 7 
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 timedependent that needs to be met in determining t. However, to model transientequations 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 phenomena properly, it is necessary to set t at least one order of For vortex shedding, you might want 20 steps per period.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 1020. 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 timedependent problem has a very fast \startup" transient that decays rapidly. It is thus often wise to choose a conservatively small t for the rst 510 time steps. t may then be gradually increased as the calculation proceeds. For timeperiodic 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. " 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 

July 20, 2010, 09:48 

#3 
New Member
Destry Marulitua Siagian
Join Date: Jul 2010
Posts: 20
Rep Power: 7 
thx for information
i really appreciate that... Regards Destry 

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