# Cfl

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 June 19, 2017, 09:00 Cfl #1 New Member   antony morgan Join Date: Jun 2017 Posts: 18 Rep Power: 2 hello friends. i'm trying to model a 3D dynamic airfoil. i'm using LES model. and SIMPLEC in pressure velocity coupling but i don't know what is the proper courant number in this case (LES & SIMPLEC). because of mesh study i need to know what is the proper CFL number. setup: U_infinity=75.5 m/s viscous model =LES pressure velocity coupling =SIMPLEC chord lenght =0.2 m

June 19, 2017, 09:55
#2
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,751
Rep Power: 41
Quote:
 Originally Posted by nhhamidsa hello friends. i'm trying to model a 3D dynamic airfoil. i'm using LES model. and SIMPLEC in pressure velocity coupling but i don't know what is the proper courant number in this case (LES & SIMPLEC). because of mesh study i need to know what is the proper CFL number. setup: U_infinity=75.5 m/s viscous model =LES pressure velocity coupling =SIMPLEC chord lenght =0.2 m

I do not understand your question ... the CFL is a numerical parameter for the numerical stability constraint. It depends on the mesh sizes (3d), time step and velocity components. It is combined with the diffusive parameter.
The general procedure is to fix the mesh sizes and determine the time step that allows for the numerical stability of the used scheme. It is important for explicit scheme.

 June 19, 2017, 16:17 #3 New Member   antony morgan Join Date: Jun 2017 Posts: 18 Rep Power: 2 hello again. my question is that what is the desired CFL number in 3D modeling of dynamic airfoil when we use SIMPLEC for velocity pressure coupling and LES for viscous modeling

June 19, 2017, 16:26
#4
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,751
Rep Power: 41
Quote:
 Originally Posted by nhhamidsa hello again. my question is that what is the desired CFL number in 3D modeling of dynamic airfoil when we use SIMPLEC for velocity pressure coupling.

It is not a "desired" value but it is constrained by the numerical stability. It depends on the type of discretization. A classical constraint is cfl<=1, valid for some explicit schemes.

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