RMS Courant Number vs MAX Courant Number

zoozoozoo · June 15, 2010, 02:32

Hi all,

sorry for confronting you guys with such a stale topic.

I thought when everybody is talking about the Courant Number, such as CFL has to be less than 1 etc. ...
it is addressed to the max Courant number.
Now I've read the following sentence in Ansys CFX, 12.1 Help:
"For accuracy, the average Courant (or CFL) number should be in the range of 0.5-1. "
What does it mean? Do I have to consider the RMS Courant number.

Help will be appraciated

Thanks in advance.
zzz

f_sizer · August 19, 2010, 10:38

This is related also to mesh size.

Since the courant number is calculated using dx (distance between cell centre or interface depending on solver) what this means is that in regions where the flow is fast you need smaller cells to achieve the courant number value of between 0.5 and 1.

Danial Q · June 11, 2012, 02:24

IN my transient simulation of homogeneous multiphase problem involving free surface also, i have RMS courant Number = 0 , Max Courant nUmber = 0

, is it OK??? If not what could be the problem??

Thanks

Ford Prefect · June 12, 2012, 14:44

Accuracy and stability. If you use an explicit scheme and violate the CFL condition then stability should suffer. However the Courant number should be as close to unity as possible for accuracy. When it comes to accuracy then I guess that you could use an average value for the entire mesh.

June 15, 2010, 02:32	RMS Courant Number vs MAX Courant Number	#1
zoozoozoo New Member MG Join Date: Aug 2009 Posts: 7 Rep Power: 17	Hi all, sorry for confronting you guys with such a stale topic. I thought when everybody is talking about the Courant Number, such as CFL has to be less than 1 etc. ... it is addressed to the max Courant number. Now I've read the following sentence in Ansys CFX, 12.1 Help: "For accuracy, the average Courant (or CFL) number should be in the range of 0.5-1. " What does it mean? Do I have to consider the RMS Courant number. Help will be appraciated Thanks in advance. zzz

June 12, 2012, 14:44		#4
Ford Prefect Senior Member Join Date: Mar 2009 Posts: 157 Rep Power: 17	Accuracy and stability. If you use an explicit scheme and violate the CFL condition then stability should suffer. However the Courant number should be as close to unity as possible for accuracy. When it comes to accuracy then I guess that you could use an average value for the entire mesh. __________________ "Trying is the first step to failure." - Homer Simpson

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
[mesh manipulation] Mesh Refinement	Luiz Eduardo Bittencourt Sampaio (Sampaio)	OpenFOAM Meshing & Mesh Conversion	42	January 8, 2017 13:55
DecomposePar unequal number of shared faces	maka	OpenFOAM Pre-Processing	6	August 12, 2010 10:01
On the damBreak4phaseFine cases	paean	OpenFOAM Running, Solving & CFD	0	November 14, 2008 22:14
IcoFoam parallel woes	msrinath80	OpenFOAM Running, Solving & CFD	9	July 22, 2007 03:58
Could anybody help me see this error and give help	liugx212	OpenFOAM Running, Solving & CFD	3	January 4, 2006 19:07

August 19, 2010, 10:38		#2
f_sizer New Member Freddie Sizer Join Date: Apr 2009 Posts: 19 Rep Power: 17	This is related also to mesh size. Since the courant number is calculated using dx (distance between cell centre or interface depending on solver) what this means is that in regions where the flow is fast you need smaller cells to achieve the courant number value of between 0.5 and 1.

June 11, 2012, 02:24	Hi	#3
Danial Q Senior Member Danial Join Date: Nov 2011 Posts: 179 Rep Power: 15	IN my transient simulation of homogeneous multiphase problem involving free surface also, i have RMS courant Number = 0 , Max Courant nUmber = 0 , is it OK??? If not what could be the problem?? Thanks