Calculation of the CFL-number

Felix_Sp · December 16, 2021, 06:06

Hello everyone,

I was just wondering how the calculation of the CFL-number works in REEF3D.
It is stated in https://doi.org/10.1016/j.compfluid.2016.09.01 that the CFL-number is calculated with the contribution of advection, diffusion and the maximum of the source terms:

$\Delta t \leq 2\left(\left(\frac{\left | u \right |_{max}}{dx}+D\right)+\sqrt{\left(\frac{\left | u \right |_{max}}{dx}+D\right)^2+\frac{\left | S \right |_{max}}{dx}}\right)^{-1}$

As far as I understand is that if I now apply D 20 2 and T 11 1 (and do not have sediment or heat transport) the equation simplifies to:

$\Delta t \leq \frac{dx}{\left | u \right |_{max}}$

I then use N 47 as a scaling factor, let's say $\alpha$ , for the calculated time step, e.g., 0.3, which leads ultimately to:

$\Delta t_{applied} = \Delta t * \alpha = \Delta t * 0.3$

Is this the way the actual time step is calculated? Or did I misunderstand something? Thanks for your help in advance!

kamath · December 16, 2021, 07:35

That is correct

Felix_Sp · December 16, 2021, 09:57

Thanks for the quick reply! You guys are always really fast!

kamath · December 17, 2021, 05:07

Thank you Felix

Hope you continue enjoying working with REEF3D

December 16, 2021, 06:06	Calculation of the CFL-number	#1
Felix_Sp Member Felix S. Join Date: Feb 2021 Location: Germany, Braunschweig Posts: 88 Rep Power: 6	Hello everyone, I was just wondering how the calculation of the CFL-number works in REEF3D. It is stated in https://doi.org/10.1016/j.compfluid.2016.09.01 that the CFL-number is calculated with the contribution of advection, diffusion and the maximum of the source terms: $\Delta t \leq 2\left(\left(\frac{\left \| u \right \|_{max}}{dx}+D\right)+\sqrt{\left(\frac{\left \| u \right \|_{max}}{dx}+D\right)^2+\frac{\left \| S \right \|_{max}}{dx}}\right)^{-1}$ As far as I understand is that if I now apply D 20 2 and T 11 1 (and do not have sediment or heat transport) the equation simplifies to: $\Delta t \leq \frac{dx}{\left \| u \right \|_{max}}$ I then use N 47 as a scaling factor, let's say $\alpha$ , for the calculated time step, e.g., 0.3, which leads ultimately to: $\Delta t_{applied} = \Delta t * \alpha = \Delta t * 0.3$ Is this the way the actual time step is calculated? Or did I misunderstand something? Thanks for your help in advance!

December 16, 2021, 07:35		#2
kamath Senior Member Arun Kamath Join Date: Nov 2014 Location: Trondheim, Norway Posts: 265 Rep Power: 14	That is correct __________________ Arun X years with REEF3D

December 17, 2021, 05:07		#4
kamath Senior Member Arun Kamath Join Date: Nov 2014 Location: Trondheim, Norway Posts: 265 Rep Power: 14	Thank you Felix Hope you continue enjoying working with REEF3D __________________ Arun X years with REEF3D

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December 16, 2021, 09:57		#3
Felix_Sp Member Felix S. Join Date: Feb 2021 Location: Germany, Braunschweig Posts: 88 Rep Power: 6	Thanks for the quick reply! You guys are always really fast!