Fluids

Fluids

Density based solver implicit formulation default courant number?

    • PTW99
      Subscriber
      Hi, I have a question when i use transient density based solver with implicit formulation in LES it appears to have a default courant number of 5. From what i know the courant number should be < 1 in order to make sure that the flow doesn't travel more than 1 cell per time step The question is what does this courant number in this scenario means? And also if we use fixed time stepping method how do we monitor cell convective courant number in density based solver ? Thank you in advance
    • Nikhil Narale
      Ansys Employee
      Hello,

      The criteria for the courant number to be less than 1 stands strong for the explicit formulation. The solution will be unstable for large time step size, which exceeds the CFL criteria. This is because, explicit formulation uses information from the previous time step to find the solution of the present time step and hence it is important to use smaller time step size (courant number).
      On the other hand, the implicit solvers don't have instability issue. It is stable even at larger courant number (larger time step size). Though the solver is stable, this will not necessarily give accurate solution. For accuracy, the time step size must be smaller than the inherent unsteadiness of the actual problem. The implicit formulation majorly uses the information from neighboring cell of the current time step to find the solution.
      As mentioned in the documentation, you can start with lower courant number at the start of the solution and can increase it as the solution progresses.
      For more information, please refer to this documentation: 32.4.1. Changing the Courant Number (ansys.com)
      If you are not able to access the link, refer this discussion:Using Help with links ÔÇö Ansys Learning Forum

      Nikhil
    • PTW99
      Subscriber
      Hi
      Thank you so much for answering my question so quick. I use an implicit solver and my model has a minimum edge length 3.81e-2 and has maximum velocity 10.67 m/s. So the fixed time step for this should be less than (1*3.81e-2)/10.67 in order to make sure that courant number in all of my domain will be less than 1 and achieve accurate solution am i doing it right?
    • Nikhil Narale
      Ansys Employee
      Please be informed that the 'Delta x' in the courant number formula corresponds to the length between cell elements (cell size) and not the geometrical length of the model. Rest, what you are saying is right.

      Nikhil
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