# How to choose the right courant number?

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 19, 2013, 16:36 #21 Senior Member   zaynah K. Join Date: Jun 2012 Location: Mauritius Posts: 138 Rep Power: 7 Dear Mads, Thanks to you analyssi, I have solved all other queries concerning the courant numbers.. I have one thing which is quite bothering me and i hope you can help me out of this tight spot. I have a case where i am simulating wind flow over a sphere ( 2D), its incompressible, turbulent and stready. I used pisoFoam and i got the same results which i obtained with Fluent for same mesh. Now i am using simpleFOam, and solution converged. But in paraview i get completely nonsense results.. Your conclusion would be more than welcome dear Mads.. Thanking you in advance if you can help me out from this Tight spot.. Best Regards Zaynah.

 February 11, 2013, 16:41 #22 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,210 Rep Power: 19 hi how is it possible to add "adjustableRunTime" into sonicFoam?

February 11, 2013, 17:25
#23
Senior Member

Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 612
Rep Power: 22
Quote:
 Originally Posted by immortality hi how is it possible to add "adjustableRunTime" into sonicFoam?
See http://www.cfd-online.com/Forums/ope...tml#post407241
__________________
Dan

 May 30, 2013, 16:23 #24 New Member   Arthur Piquet Join Date: Mar 2013 Posts: 16 Rep Power: 6 Hi, I'm trying to resolve supersonic compressible flow with OpenFOAM and I have some questions about the Courant Number calculation. I fix my Courant number in the ControlDict file, and the time step try to adapt himself. I look up on the C/H file of the courant number calcultation and I found that the formulation used looks like: CFL = 0.5 * (sumFace of phi/rho) / VolumeCell ~ 0.5 * [ u * dt / dx + v * dt / dy + w * dt / dz ] Thus, I don't understand why they put 0.5 coefficient, but I maybe misreaded the code... I have also another question. For compressible flow where shock occurs, the CFL needs to be (for resolving acoustic and shock) CFL_comp = dt * (U + a) / dt (a: speed of sound) Does anyone know if it's the correct formulation for shocked flow?? or the normal form is better? If I summarize all the CFL calculation (OpenFOAM + compressible/shock), the equation becomes : CFL_comp = CFL_wanted (ex=0.5) / ( 2 * ( 1 + max(Mach number) ) ) (2 is for compensating the 0.5 coefficient of OpenFOAM and ( 1 + max(Mach number) ) is for (U + a)) right?? thx immortality likes this.

 October 3, 2013, 08:28 Courant number for non-newtonian fluids #25 Senior Member   Albrecht vBoetticher Join Date: Aug 2010 Location: Zürich, Swizerland Posts: 214 Rep Power: 10 Dear modellers, I have nice results for an adapted InterMixingFoam solver that works with a pressure dependent viscosity in one phase. I use maxCo 0.25 because that is what I got recommended for Large Eddy Simulations. Here I have no turbulence, but when I change maxCo, say to 0.4, the viscosity of the fluid increases! Has anyone experience or ideas of explanation for this behavior with pressure dependent viscosities?

 October 14, 2014, 20:07 #26 New Member   Joana Join Date: Oct 2014 Posts: 1 Rep Power: 0 Hi, Mads! I'm quite new here and I'm trying to understand Delf 3D so I can work in a model of a river to see the water level changing with this new construction. I am sorry for my ignorance, but I can't understand how the velocity can affect Courant number. So far I've just added the grid and bathymetry. Can you please explain me? Thanks a lot! Joana

 July 21, 2016, 04:19 #27 New Member   Alaska1964 Join Date: May 2015 Posts: 16 Rep Power: 4 HiMads and everyone, I am trying to save computational costs by increasing the time step unti I get some CFL# of 0.5. When my timestep is 10^-6 my mean Courant number (CFL#) is 0.0032. How can I define the most appropriate Courant number? We know that CFL# should be less than 1, but how shall I know the lower limit of Courant number? How shall I know if 0.4 is better of 0.1 of 0.0032 ? Is there another criteria to restrict our choices on time-step to reduce the calculation cost? Thanks in advance for help and ur time,

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Luiz Eduardo Bittencourt Sampaio (Sampaio) OpenFOAM Mesh Utilities 42 January 8, 2017 13:55 maka OpenFOAM Pre-Processing 6 August 12, 2010 09:01 andre OpenFOAM 5 June 23, 2008 10:37 liugx212 OpenFOAM Running, Solving & CFD 3 January 4, 2006 19:07 michele OpenFOAM Other Meshers: ICEM, Star, Ansys, Pointwise, GridPro, Ansa, ... 2 July 15, 2005 04:15

All times are GMT -4. The time now is 10:30.