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January 2, 2017, 07:42 
solver algorithm for LES

#1 
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raunak jung pandey
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Hello I am using Fluent 15.0 to carry out my LES simulations.
Which solver algorithm is appropriate for fast and accuracte results ? ITA/NITA/FSM/PISO/SIMPLEC My simulations run normally and converge when using SIMPLE/SIMPLEC and Coupled but takes long time My simulation shows follwoing error when using PISO and NITA Pressure Correction = 0.3 Moment Correction = 0.7 " divergence detected in AMG solver : pressure correction" 

January 2, 2017, 09:38 

#2 
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Filippo Maria Denaro
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Try noniterative fractional step setting second order time and space accuracy


January 4, 2017, 08:29 

#3 
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raunak jung pandey
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[QUOTE=FMDenaro;631794]Try noniterative fractional step setting second order time and space accuracy[/QUOTE
Thank you for your answer. I am still facing convergence problem while using PISO and NITA. THis doesnt exist when I am using SIMPLE/SIMPLEC or Coupled and second order Implicit transeint formulation. WHat maybe the reasons for this ? 

January 4, 2017, 08:32 

#4 
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Filippo Maria Denaro
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are you using the dynamic SGS model? Instability can happen for the eccessive value of the time step.
Could you plot the total kinetic energy versus the time? 

January 4, 2017, 09:34 

#5  
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raunak jung pandey
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Quote:
I am using LES WALE and Smagorinsky model. My time step size is 0.00001 sec giving courant no under 1. 

January 4, 2017, 11:24 

#6 
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Filippo Maria Denaro
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why you can not? have you tried to reduce the CFL number of one order?


January 11, 2017, 06:59 

#7 
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If you use PISO in combination with NITA, you should employ 23 neighbor corrections. Besides, a highquality mesh is desirable. NITA is naturally less stable than ITA since the coupling among equations is actually not enforced  there are only inner iterations, no outer iterations. Which implies that it is not applicable to all situations. The equation "faster computational method = the same stability & accuracy as the slower computational method" simply does not work
@FMDenaro  I am not aware of any simple method how to plot the turbulence kinetic energy in LES. Since it is defined via velocity fluctuations, one actually has to get there through instantaneous velocities and averaged velocities. To get averaged velocities means to save the instantaneous velocities at many time steps and to compute the mean and fluctuations after the calculation has finished (or have data from a previous calculation). Correct me if there is an easier way to get k. 

January 11, 2017, 09:35 

#8  
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Filippo Maria Denaro
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Quote:
Just to be clear: I wrote total kinetic energy that is available from a LES solution. If the flow is incompressible then the solution v_bar allows to compute k_LES =v_bar . v_bar. Of course, that is not equivalent to k_bar. However, k_LES can be used to assess a statistical steady state or an energy pileup leading to numerical instability. what do you mean for: NITA is naturally less stable than ITA since the coupling among equations is actually not enforced  there are only inner iterations, no outer iterations. Which implies that it is not applicable to all situations. 

January 12, 2017, 05:14 

#9 
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That is a pretty loose statement. I should have rather said that NITA is not suitable for every application. Obviously, strong coupling between equations would have adverse effects on convergence. I can imagine that to achieve a good convergence in multiphase flow modeling may be challenging using NITA. And just to quote the Fluent guide: the NITA solver is not recommended for highly viscous fluid flow.


January 12, 2017, 05:30 

#10 
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Filippo Maria Denaro
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Despite of any high order accurate timeintegration, the fractional step method has an intrinsic error due to the timesplitting. I don't know much about multiphase flows but I do not understand the warning about viscous flows...


January 12, 2017, 07:17 

#11  
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Arjun
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Quote:
I created a fractional solver where I could include this correction and could demostrate that when including this nonlinear correction I could achieve results like iterative scheme for the test cases similar to what Milovan used (ie vortex shedding from cylinders). By switching this term off got results same as PISO. So yes, what you say has real effect and could be demostrated. This is why I created hybrid solver for multiphase flows where courant determines times step and using iterative scheme is expensive while NITA is unstable. I have been using this in FVUS for VOF calculations to cut the calculation cost by at least half. 

January 12, 2017, 08:31 

#12  
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Filippo Maria Denaro
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Quote:
What type of correction have you implemented? Some years ago I deeply analysed the errors appearing in the fractional step method and the sources were highlighted in the splitting but also in the bc.s. Thus, some remedies were proposed to get full high order accuracy https://www.researchgate.net/publica...ary_conditions 

January 12, 2017, 09:00 

#13  
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Arjun
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Quote:
What I did was that I stored the momentum matrix and once the velocity correction was available I corrected flux and added another correction to source to momentum eqn. Then solved again (rather than setting up whole thing). I wrote down outline in a ppt, I try to find it and attach here. (it was long time ago so don't remember the steps exactly). Give me a bit of time to find that ppt. 

January 12, 2017, 09:11 

#14  
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Arjun
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Okay found the pdf.
slide 10 first chart is from Prof. Milovan's previous test. Second and third from my tests with on and off of this correction. This outlines the algo that I tried. Quote:


January 12, 2017, 09:48 

#15 
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Filippo Maria Denaro
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No specific corrections in the BC.s? Have you checked the convergence slope on some analytical test?


January 12, 2017, 10:19 

#16  
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Arjun
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Quote:
Anyway did not work much on it but on a provided test case from industry it beat PISO of starCD by factor of 2 or so. (105 seconds vs 265 seconds of PISO in that test case). All in all I feel it is worth exploring but I have no time. May be in future I try it in FVUS, as for now there is lot to do there so no time. 

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