# RMS Courant Number vs MAX Courant Number

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 June 15, 2010, 01:32 RMS Courant Number vs MAX Courant Number #1 New Member   MG Join Date: Aug 2009 Posts: 7 Rep Power: 16 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

 August 19, 2010, 09:38 #2 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, 01:24 Hi #3 Senior Member   Danial Join Date: Nov 2011 Posts: 179 Rep Power: 14 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

 June 12, 2012, 13:44 #4 Senior Member     Join Date: Mar 2009 Posts: 151 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