Hi,
It is our general practice while computing unsteady case of CFD codes(Fluent), we gradually increase the step size. For example, Initially we adopt step size 0.0005 and after 10 second of simulation we increase the step size to 0.001. After getting stability, step size changes to 0.005 when simulation was at 40 second.
I notice that, whenever we change the step size during the simulation the graphical pictures( Animation ) change; i.e, some time expand and some time contract its flow profile.
My question is , " What is this Phenomenon ? Does Time step size effect the accuracy?"
Please guaid !!
Dear Muhammad,
If the cells are too small compared with the time step, numerical smearing may occur leading to instability and in some cases divergence. To avoid this, a match between fluid velocity, cell size and time step is needed to ensure convergence. By introducing the Courant number (CFL) this problem can be assessed quantitatively:
CFL=(U ∆t)/∆x)
Where U is the fluid velocity, Δt is the time step and Δx is the characteristic cell length.
see more in (http://www.witpress.com/elibrary/cmem-volumes/4/2/1147)
Best wishes.
Hi
In addition to Zahid's comment, i should say YES. Time step size can affects on the accuracy. Specially in situations that in which speed of changes is high.
What is your physical problem? What properties are you referring to? If your problem is unsteady, it might be just how the physics behave. If it is a steady case, I'll say it is not converge as yet.
Dear Zahid I. Al-Hashimy ,
Thank you for your suggestions. My simulation is Pressure based. This CFL option is just open when density based simulation ON . Isn't It ?? I tried many time to find CFL option. It never appears when pressure based option On. ...
So, how should I tune this CFL in my simulation ??
I addition to the answer by Mr. Zahid you may read following CFD forums -
http://scicomp.stackexchange.com/questions/16063/fluent-time-step-size-transient-implicit-solver
http://scicomp.stackexchange.com/questions/7702/importance-of-the-time-step-value-for-the-accuracy-of-a-transient-cfd-simulation
All the best for your successful simulations.
Dear Tyagi Parul,
Thank you very much for sharing this informative link.. I got my answer ! Stack Exchange website is much informative...
Regards
Yes, whenever time step is not enough small, it can affect the consistency. I saw that in cranck-nicolson method.
In a time-dependent simulation, there is always an error term due to the discretization.
Depending on the scheme you are using, this term scales with the time-step size somehow (e.g., linearly).
Thus, YES reducing the time step reduces this error and thus affects the solution you get.
Practically, there is a limit below which one does not get a meaningful improvement since other sources of errors (like round-off) become dominant.
Thank you everyone to join this question and give me better suggestions..
Regards
Furqan
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