[raunakjung]

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"

[FMDenaro]

Try non-iterative fractional step setting second order time and space accuracy

[raunakjung]

[QUOTE=FMDenaro;631794]Try non-iterative 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 ?

[FMDenaro]

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?

[raunakjung]

Originally Posted by FMDenaro

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?

No I couldnt plot total kinetic energy vs time.
I am using LES WALE and Smagorinsky model.
My time step size is 0.00001 sec giving courant no under 1.

[FMDenaro]

why you can not? have you tried to reduce the CFL number of one order?

[Zbynek]

If you use PISO in combination with NITA, you should employ 2-3 neighbor corrections. Besides, a high-quality 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.

[FMDenaro]

Originally Posted by Zbynek

If you use PISO in combination with NITA, you should employ 2-3 neighbor corrections. Besides, a high-quality 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.

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 pile-up 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.

[Zbynek]

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.

[FMDenaro]

Despite of any high order accurate time-integration, the fractional step method has an intrinsic error due to the time-splitting. I don't know much about multiphase flows but I do not understand the warning about viscous flows...

[arjun]

Originally Posted by FMDenaro

Despite of any high order accurate time-integration, the fractional step method has an intrinsic error due to the time-splitting. I don't know much about multiphase flows but I do not understand the warning about viscous flows...

Once to compare these solvers Prof. Peric (Milovan Peric) did some tests and demostrated that even with time step as small as 5 times of iterative method PISO could not produce the results same as iterative method.

I created a fractional solver where I could include this correction and could demostrate that when including this non-linear 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.

[FMDenaro]

Originally Posted by arjun

Once to compare these solvers Prof. Peric (Milovan Peric) did some tests and demostrated that even with time step as small as 5 times of iterative method PISO could not produce the results same as iterative method.

I created a fractional solver where I could include this correction and could demostrate that when including this non-linear 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.

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

[arjun]

Originally Posted by FMDenaro

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

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.

[arjun]

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.

Originally Posted by FMDenaro

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

[FMDenaro]

No specific corrections in the BC.s? Have you checked the convergence slope on some analytical test?

[arjun]

Originally Posted by FMDenaro

No specific corrections in the BC.s? Have you checked the convergence slope on some analytical test?

For BCs I chose to keep it in two forms Dirichlet or correction = 0 on fixed value types (inlet etc) or Neumann or 0 grad for correction. I took care while updating source (I think) but I feel it is worth investigating. (i don't remember details well).

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.