
[Sponsors] 
February 24, 2013, 10:31 
LES with Simple Algorithm for steady situations

#1 
Senior Member
Bobi
Join Date: Oct 2012
Location: Chicago, Illinois
Posts: 402
Rep Power: 8 
Dear Foamers
Hi I have a question about LES. Can we have a LES solution with Simple Algorithm rather than Pimple. I mean if we use a steady algorithm like Simple for a steady problem like a free jet, Is it possible or even right? Regards Bobi 

March 18, 2013, 15:12 

#2 
Senior Member
ATM
Join Date: May 2009
Location: United States
Posts: 104
Rep Power: 10 
No. LES is for solving turbulence problems, which are inherently Unsteady. Therefore you should not use a steady solver for these cases. You have to run the unsteady case for a certain time period, and then average the instantaneous solutions to obtain the turbulent flow statistics.


March 19, 2013, 02:22 

#3 
Senior Member
Bobi
Join Date: Oct 2012
Location: Chicago, Illinois
Posts: 402
Rep Power: 8 
Dear ATM
Hi with your response I conclude that LES is not applicable with Simple algorithm and always I should employ the Pimple one. Nevertheless, RAS models can use Simple algorithm although they characterize turbulent flows. They average the solutions during marching to find the favre solution. My intension was that can we use the same strategy for LES? I mean instead of having real time marching in Pimple algorithms, employing unreal marching in Simple solvers and then averaging the instantaneous solutions to find the ultimate solution. As we can do in RAS model. Regards Bobi 

March 19, 2013, 02:35 

#4 
Senior Member
ATM
Join Date: May 2009
Location: United States
Posts: 104
Rep Power: 10 
Bobi,
You are right about RAS., and LES runs on a modified SIMPLE method. However, in LES you are resolving much smaller length scales than you do in RAS. Hence time stepping is very important as it should be very small. Since you resolve very small scales , their associated time scales are very small too. So, to obtain good statistics, you have to have a sufficiently small timestep. A courant no. of <0.5 is usually recommended. Therefore, even pimpleFoam would not be a good choice since its a "Large timestep transient solver for incompressible flow using the PIMPLE (merged PISOSIMPLE) algorithm" (OF website) My best bet would be in dedicated transient solvers like pisoFoam, channelFoam etc. for LES.A Steady State solver is unphysical for LES. Last edited by atmcfd; March 19, 2013 at 10:53. Reason: edit 

March 19, 2013, 03:26 

#5 
Senior Member
Bobi
Join Date: Oct 2012
Location: Chicago, Illinois
Posts: 402
Rep Power: 8 
Dear ATM
Hi Thnks for ur suggestions buddy. Regards Bobi 

March 21, 2013, 16:46 

#6 
Member
Jim Knopf
Join Date: Dec 2010
Posts: 60
Rep Power: 8 
This is not a question of piso, simple or pimple, which names the way the pressure coupling is done, but more a question of spartial and time resolution.
Afaik you can use SIMPLE als pressure coupling for solving Equations with a time derivative. But if you are using LES  which means Large Eddy Simulation  you have to bear in mind that you have to resolve these large eddies. Afaik they are commonly the eddy with a length scale above the isotropic ones (but I'm not sure if this is a correct formulation). Also you have to bear in mind that a lenght scale is coupled with a time scale via the velocity. The CFLcriterium gives the time step size if you know length and velocity scales, is has to be 1 for the smalest eddies. In RANS or RAS you think of turbulence as of a viscosity, whicht provides you with the energie dissipation you get in LES via the breakup of eddies. If you use a to coarse mesh or a to large time step size, it does not mean you get divergence or no result. It does only mean you get a not correct result, what ever correct is. LES is not a easy to use tool. Greets Jim 

March 22, 2013, 03:11 

#7 
Senior Member
Bobi
Join Date: Oct 2012
Location: Chicago, Illinois
Posts: 402
Rep Power: 8 
Dear jim
Hi Thnks for your suggestions. My question is not about time steps or grid sizes, cause as u know there are some limitations for LES. As Pope has mentioned in his book, For instance LES should resolve about 80% of the turbulent kinetic energy. I was wondering that if we had a steady turbulence condition, can we use SIMPLE algorithm for coupling between pressure and velocity in conjunction with LES turbulence model. Regards Bobi 

March 22, 2013, 12:58 

#8 
Member
Jim Knopf
Join Date: Dec 2010
Posts: 60
Rep Power: 8 
Hi Bobi,
but steady or unsteady is not a question of pressure coupling, it's a question of the Equations you're solving. What I mean is that you can't infer from SIMPLE that it's steady, without time resolution. You are right that most (all?) of the esi/OF delivered solvers with SIMPLE algorithm are steady solvers but there are enough unsteady solvers with SIMPLE pressure coupling. And definetly you are right with saying that LES and steady does not work or say does not bring meaningful results. Regards, Jim 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
SIMPLE algorithm confusion  lost.identity  Main CFD Forum  1  October 7, 2010 11:48 
LES basic and simple questions.  dshawul  Main CFD Forum  0  October 4, 2010 10:56 
What is advantage of SIMPLE algorithm?  GeonHong  Main CFD Forum  1  May 18, 2010 07:46 
SIMPLE algorithm for variable density flow  mauz  Main CFD Forum  1  May 15, 2009 15:13 
SIMPLE algorithm  Jonathan Castro  Main CFD Forum  3  December 10, 1999 05:59 