Time step for the implicit case

rizwan4081rizwan4081 Member

Hi. I am performing an analysis where I analyze the ventilation inside a tunnel with a body emitting heat inside. I came across the CFL condition for determining the time step for running the calculation. But I read its applicable to only explicit cases. How can I determine the minimum time step for my case study?

 

 

Comments

  • KremellaKremella Admin
    edited May 12

    Hello,

    Yes, you are correct. 

    Ideally, CFL number gives you a criterion for min time-step required for solution stability in the case of an explicit solver. For fully implicit solvers, the time-step size can be generally higher than the predictions from the CFL number. However, it is important to understand that in Fluent it is required to obtain a converged solution every single time-step. Please use the time-step obtained from the CFL number as your guide and change the time-step to see that you are converging your solution every single time-step. The idea is to optimize the time-step size and number of iterations to get a good and faster-converged solution.

    Also, under Velocity, you should be able to plot the Convective Courant Number as a contour plot. This will give you a good insight into the CFL number based on your current time-step.

    Finally, please note that your time-step size is lower than the physical time-scales for your problem. If the time-scales required to resolve the diffusion of heat (for example) is 1e-2 s, then your time-step size must be lower than this number. Otherwise, there is a good chance you will miss the physics you are trying to capture.

    Hope this helps.

    Best,

    Karthik

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