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Old   January 11, 2007, 04:48
Default CFL numer
  #1
vivek
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HiFriends I am a new user and learning FLUENT..I like to know what is CFL number and its importance. What is the criteria to define time step size(s), number of time step and number of iteration per time step. I am studying atomization please send me appropriate information thanks in advance vivek
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Old   January 12, 2007, 11:12
Default Re: CFL numer
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Mubassh
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Dear Vivek,

The Boundary information waves should not cross morethan one grid space per unit time. If you consider some big timestep, then Boundary information will propagate or move many grid cells in a timestep and finally you cant get correct flux entering or leaving through each cell face.

The information wave speed is based on fluid velocity and the velocity of sound max u+c ... so your timestep mustbe less than (delx or cell size)/u+c. If you select some big timesize, then discretize that into number of subiterations.

Hope it will help you.
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Old   January 16, 2007, 15:39
Default Re: CFL numer
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Jonas Bredberg
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Mubassh is correct if you are solving with coupled/explicit solver. If you on the other hand are using the implict solver you will be able to use CFL-number (=U*dt/dx) higher than 1. As stated by Fluent, solving the flow with new NITA scheme (6.2) you may under fortunately circumstances be able reach CFL-numbers of 100!
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