
[Sponsors] 
September 16, 2013, 16:31 
Simulations with large time steps (high CFL)

#1 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Hi everyone!
I have been trying to run simple 2D RANS simulations over an incompressible airfoil, from 0deg to 15deg. I thing I need some help! Everything works just fine when I use a steady solver. However, things do not work quite so well in timeaccurate mode. They work, but the CFL constraint forces me to use a very low time step. Basically, it takes 32,000 time steps for a particle to go from leading edge to trailing edge. Slightly excessive I'd say. If I use 5 outer correctors in pimpleFoam, the CFL remains more or less steady, around 80 (the value that I want). Still, I am afraid that sooner or later, it will keep on increasing and eventually explode... Is there any way to run such simulations with larger timesteps, while maintaining spatial and time accuracy? Some colleagues use OverFlow and Fun3D and they have no problem with that. Thank you very much for your help! Joachim 

September 17, 2013, 13:52 

#2 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
no one?
I can't believe there is no way to run a 2D airfoil in time accurate mode with OpenFOAM... 

September 17, 2013, 18:33 

#3 
Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 8,659
Blog Entries: 34
Rep Power: 87 
Greetings Joachim,
Have you checked OpenFOAM's tutorial "incompressible/pimpleDyMFoam/wingMotion"? As for the "trick" to not having high CFL is to have a good mesh that doesn't have very small cells in high speed zones. You can use the Co utility for generating the Courant Number field for your case, so that you can find where the CFL values are higher. Another thing is to check the mesh statistics given by checkMesh, namely the minimal cell volume. Best regards, Bruno
__________________


September 17, 2013, 19:54 

#4 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Thanks wyldckat,
However, I do not have much choice in this case, for 2 reasons: 1. The airfoil has a blunt trailing edge. I need really fine cells to properly capture the flow there. 2. A colleague and I are running the same case on both OpenFOAM and Overflow. We are therefore using the same mesh. > I cannot change it. It is pretty frustrating! In Overflow and Fun3D, they don't have that issue with the CFL. 

September 18, 2013, 03:20 

#5 
Senior Member
Anton Kidess
Join Date: May 2009
Location: Delft, Netherlands
Posts: 921
Rep Power: 17 
Find out what makes Overflow and Fun3D faster. What algorithms do they implement? Are all other settings the same? It wouldn't surprise me if those codes implement algorithms that outperform OpenFOAM in specific cases, and if so you can either stick to using those codes, or if you really need OpenFOAM and the improved performance, implement the algorithms into OpenFOAM.
__________________
*On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer. *Help define the OpenFOAM stackexchange Q&A site: http://area51.stackexchange.com/prop...oamtechnology 

September 18, 2013, 11:18 

#6 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Hey!
I would like to check the solver implementation in Overflow and Fun3D, but I can't! I am an international student, and therefore I am not allowed to see the code...that's why I am using OpenFOAM. 

September 18, 2013, 11:30 

#7 
Senior Member
Anton Kidess
Join Date: May 2009
Location: Delft, Netherlands
Posts: 921
Rep Power: 17 
Even without access to the code you should be able to get everything you need from theory guides or white papers or such.
__________________
*On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer. *Help define the OpenFOAM stackexchange Q&A site: http://area51.stackexchange.com/prop...oamtechnology 

September 18, 2013, 11:36 

#8 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 8 
You should be able to find the properties of the algorithm: SIMPLE or PISO, how many correctors, localtime stepping tricks, ... ?


September 19, 2013, 21:00 

#9 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Actually, I am pretty confused right now. Is the PISO algorithm implemented in OpenFOAM really a PISO algorithm? The solver is supposed to be coupled and not segregated. However, it clearly seems to solve first the momentum equations over the entire domain, and then apply the pressure equation.
If you google "Pressure Velocity Coupling  Hochschule Luzern", you'll find a powerpoint presentation (OF workshop 2010) where two people apparently implemented a "real" PISO algorithm. They were able to run simulations with Courant numbers above 500 without any problem. That is pretty strange. That was three years ago, but the algorithm is still not included in the official version of OF. I am going to contact them directly to see if there was anything wrong with their solver. Else, it seems that a vector coupled solver was added in the release OF 2.2.0 (http://www.openfoam.org/version2.2.0/matrixsolvers.php). However, I tried to run the cavity case given as an example, while refining the mesh (> increasing the CFL), and the simulation still diverged for CFL > 1. 

September 20, 2013, 08:35 

#10 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 8 
Be careful when you see presentations like this...
Most of the time, people make those beautiful tries on OF 1.6 ext which is not an official OpenFoam release ! The "extend" project is independent from OpenFoam (2.2 now). The extend version implement whatever they want and even sometimes code that is under copyrights (see the problem OF Ext vs Ansys few months ago). They can do it because it is an opensource project without any "leader" to blame (or to judge). What you saw in the power point is really attractive but it requires lots of code that was not natively in OF. It requires extra work, extra maintenance and extra files. Moreover, as far as I remember, the way OF works doesn't allow a straight forward coupled algorithm. To be clear: PISO, SIMPLE and COUPLED are 3 different algorithms. Sometimes you can also see SIMPLEC which is an "consistent" version of SIMPLE. Basically, SIMPLE solve the equation one by one and use a corrector. PISO is an "improvement" of SIMPLE since it adds a second corrector stage that helps coupling pressure and velocity. COUPLED is another algorithm where pressure and velocity are solved at the same time. PISO and SIMPLE solves two systems (pressure and velocity) while COUPLED solve a unique system. The biggest problem is that COUPLED uses much more memory and it can be in some cases as slow as PISO if you have a very turbulent flow. 

September 20, 2013, 09:07 

#11 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Oh thank you, I did not know that.
But basically, it means that right now, OpenFOAM does not have any coupled solver and therefore cannot run simulations with arbitrarily large time steps? 

September 20, 2013, 09:13 

#12 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 8 
Yes, that's right
And be careful, some "researchers" would call you crazy if you use a coupled solver with low speed. Indeed, the coupled algorithm was originally designed for high mach problems (Mach > 0.4). It turns out that the algorithm is actually quite efficient for other purposes (low turbulent flow) but people often want to stick to the original use. 

September 20, 2013, 09:18 

#13 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
But then, is it actually possible to run subsonic external aerodynamics simulations in OpenFOAM?
I mean, if you have a mesh with roughly y+ = 1 and a freestream velocity corresponding to M = 0.2 for example. You will immediately get very high CFL, if you use time steps that would be short enough to capture all the unsteadyness of the flow. Still, the calculation will blow up... Is there any way around this? Is the compressible solver more stable than its incompressible counterpart? Thanks! 

September 20, 2013, 09:36 

#14 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 8 
Haha, that is a main problem of CFD...
First, you can improve your mesh. On airfoils, you can have area of high velocity but low pressure gradient. Therefore you don't need small cells in this area (wide rectangles should be good, aspect ratio of 40). You can also try to reduce as much as possible the number of cell in your domain. Second, you can try to play with the number of corrector loops. Third, you can use some implicit algorithms that allow larger time steps. But be very careful ! An implicit solver with large dt that works (no crash) doesn't mean that your solution is right... In some cases, the solver doesn't crash but still give you inaccurate solution. Fourth, you can use a steady state solution to initialize your problem. Fifth, no matter what, you cannot use time steps larger that the frequency of your flow. You can refer to the "Strouhal" number to get an idea of the vortex frequency and then you apply the Nyquist–Shannon sampling theorem. 

September 20, 2013, 09:49 

#15 
Senior Member
Joachim
Join Date: Mar 2012
Location: Atlanta
Posts: 140
Rep Power: 6 
Actually, this is my big problem! If I want to have a CFL of 1, the time steps are so small that it takes 32,000 iterations for a particle to go from leading edge to trailing edge. I really need this accuracy in time, but it seems that I don't really have the choice.
I'll get rid of my blunt trailing edge and coarsen the grid then! Thank you for the recommendations. 

September 20, 2013, 09:53 

#16 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 8 
What is your number of cell ? What is you current time step ?
If you want to get accuracy, you don't really have choice. Any drastic improvement will be made at a cost... 

October 1, 2014, 01:28 

#17  
Senior Member
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,118
Rep Power: 16 
Quote:
If this is true, why can Fluent handle arbitrarily large time steps with their "transient simple", which is pretty much the same as pimpleFoam? This is a segregated solver as well. I can run really large time steps (and thus co numbers) in Fluent and with the very same mesh I need to have like 1020 times smaller time steps in pimpleFoam.
__________________
The skeleton ran out of shampoo in the shower. 

October 1, 2014, 03:39 

#18 
Senior Member
Anton Kidess
Join Date: May 2009
Location: Delft, Netherlands
Posts: 921
Rep Power: 17 
Transient SIMPLE is not like Pimple (Pimple is still basically PISO). The tradeoff between SIMPLE and PISO is that you need to apply underrelaxation (yet another tuneable parameter!) and a lot more iterations per time step for convergence using SIMPLE to get the large time steps.
__________________
*On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer. *Help define the OpenFOAM stackexchange Q&A site: http://area51.stackexchange.com/prop...oamtechnology 

October 1, 2014, 03:55 

#19 
Senior Member
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,118
Rep Power: 16 
I don't really get what you say, sorry. I run Fluent "transient SIMPLE" with 15 Iterations per time step and it is stable. pimpleFoam with 15 it/dt isn't. Both use the same underrelaxation (pressure 0.3, momentum 0.7 and turbulence 0.8) and I tryed to set Openfoam numerics as "Fluentish" as possible.
1) I know PISO isn't SIMPLE, but the two solvers (Fluent and OpenFoam) in my case both use inner iterations with underrelaxation each time step to account for coupling of pressure and velocity. 2) How does the argument hold, that the reason is the segregated solver if both programms use them?
__________________
The skeleton ran out of shampoo in the shower. 

October 1, 2014, 05:16 

#20 
Senior Member
Anton Kidess
Join Date: May 2009
Location: Delft, Netherlands
Posts: 921
Rep Power: 17 
Just because you apply underrelaxation and more iterations does not make your PISO loop (which pimple still is) SIMPLE. You are still comparing two different algorithms which both have their pros and cons.
I couldn't understand the point you were trying to make in 2).
__________________
*On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer. *Help define the OpenFOAM stackexchange Q&A site: http://area51.stackexchange.com/prop...oamtechnology 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Superlinear speedup in OpenFOAM 13  msrinath80  OpenFOAM Running, Solving & CFD  18  March 3, 2015 06:36 
How to write k and epsilon before the abnormal end  xiuying  OpenFOAM Running, Solving & CFD  8  August 27, 2013 15:33 
dynamic Mesh is faster than MRF????  sharonyue  OpenFOAM Running, Solving & CFD  14  August 26, 2013 07:47 
A simple problem about Adaptive Time steps  sakurabogoda  CFX  9  December 12, 2012 06:22 
Could anybody help me see this error and give help  liugx212  OpenFOAM Running, Solving & CFD  3  January 4, 2006 19:07 