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Result dependance on Iterations per Time Step, URANS 2D cylinder.

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Old   April 25, 2018, 14:45
Question Result dependance on Iterations per Time Step, URANS 2D cylinder.
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Leonardo
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Hello,

I am performing a mesh independence of a 2d Cylinder, kw SST model, Reynolds related to cylinder= 1.4e5 and Y+=0.9. And have this problem when increase iterations per time step, from 20 ( suggested by fluent) to 50, 70 or 100 my results change dramatically.
iterations per ts.png

My CourantFL is 0.3, and time step = 0.0001.

Solution methods are: PISO, Pressure PRESTO!, momentum, turbulent kinetic energy and specific dissipation rate are 2d Order upwind. And Transient formulation Second Order Implicit.


The problem I think is the continuity residuals... they stay very high.... and solution doesnt converge in every timestep because of it.

I say high because some papers consider converged when reach 1e-6... and I am getting 1e-4 with 100 iterations!

residuals.png

What would you do? Thank you for reading.
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Old   April 26, 2018, 01:53
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All right. So I found my mistake.

Was a big mistake: my Courant number was 300, not 0.3. And because of that, my time step was way too big.

So to keep a CFL=1 my time step has to be 3e-07. And that is too small... will take weeks to calculate.

Now I am considering CFL = 30, time step = 1e-05. Need 8 iterations to converge and everything seems good.

Any comentary would be great. Thanks.
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Old   April 26, 2018, 02:01
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When you used PISO, did you set all your underrelaxation factors to 1?

Don't do PISO with too many iterations (it defeats the point of PISO) and don't use PISO with large time-steps (it's unstable unless you lower the urf's, but this again defeats the point of PISO). For large time-steps & when you want to take many iterations anyway, use SIMPLE or the COUPLED solver.
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Old   April 26, 2018, 02:12
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Hello LuckyTran, I am keeping under relaxation Factors as default... I saw some people change pressure under relaxation from 0.3 to 0.7. But I am afraid of it since I will use CFL = 30 (I dont know if you could read my last reply, I found my mistake calculating the CFL).

I only knew PISO was good for unsteady simulations, thank you for the advice about the number of iterations. Right now I solved my problem and have about 8-10 iterations per time step.
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Old   April 26, 2018, 02:50
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8-10 iterations per time-step is in general not bad, but I think you have some have room for improvement. The way you are using PISO is SIMPLE-like approach and that's why I suggest to use SIMPLE.

PISO is SIMPLE with an extra correction step at the end. That's why it's better to go with SIMPLE if you plan to do many iterations. You don't need to waste these extra correctors. Remember, SIMPLE and PISO are cousins! One iteration of PISO is almost like 2 iterations of SIMPLE, but in PISO you don't update the outer solution until you do the 2nd corrector.

It's preferable to go with the fewest number of iterations (1 or 2, 3 for highly complex cases), with largest urf's, and largest time-step that PISO can still converge with (this is the Courant number < 1 regime) or switch to SIMPLE, use urf's and also largest time-step without diverging.

Last edited by LuckyTran; May 15, 2018 at 23:07.
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Old   April 26, 2018, 03:59
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Thanks! I actually do want to have larger time-steps, or the simulations will never end! (have only 8 cores). So, I will try the following: Higher Time step=1e-4, SIMPLE, pressure urf 0.7. But courant number would be about 300. I am trying this with a coarse mesh and hope it doesnt diverge. By now I am getting about 7-8 iterations per time step with this configuration . Will try with finer meshes and post a reply.

Btw, do you know any literature reference or paper recommended about this?
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Old   April 26, 2018, 09:46
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If using SIMPLE, I would have kept the 0.3 and 0.7 urf's for pressure and momentum. Or I would change it to 0.5, 0.5.

With a finer mesh, your time-step would have to be even smaller and overall simulation time goes up even more (due to needing more time-steps and having a bigger mesh). Either buckle down for a long simulation or give up. This is why it's even more important to play with your case and learn how it responds when your first set it up.

You can find SIMPLE/PISO algorithm even on wikipedia these days. "Anyone can become a scientist if they know what to search for."

PISO Usage and URF's in Fluent Manual
PISO explanation in Fluent Manual
Merged PISO/SIMPLE in OpenFOAM

Last edited by LuckyTran; April 26, 2018 at 14:06.
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Old   May 15, 2018, 21:14
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Well, I finally could get some coherent results with 6 grids, from coarse to "fine".
Smallest Courant I could handle is 30. I actually kept SIMPLE method and 0.3 and 0.7 urf for P. and M. (higher values for P. urf gave me problems with finer meshes).

By now I am trying to find a method so I could get stabilized results faster. So I start with a steady simulation, then set high time steps (Courant = 3000), and making it smaller until get stabilized values with a small time step (Courant=30). And I am getting about 15 iterations per time step... and I think this is ok. Results very close to similar studies.

Thank you for your advice LuckyTran.
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