[hitzhwan]

How to set the flow courant number in the Coupled model of Fluent? The default is 200, do I need to set it as 0.5?

[LuckyTran]

It depends whether you are steady or transient and if transient, how large is your time-step. In most cases, 200 is fine for steady cases. 100 used to be an old default and you can revert back to that if you are having trouble converging. You rarely want to decrease it to 0.5, that would be, a huge waste of time. For transient cases, values up to 1e7 are often used.


Honestly if you have to ask, I recommend you avoid using the solver and stick to tried and true methods.

[hitzhwan]

Originally Posted by LuckyTran

It depends whether you are steady or transient and if transient, how large is your time-step. In most cases, 200 is fine for steady cases. 100 used to be an old default and you can revert back to that if you are having trouble converging. You rarely want to decrease it to 0.5, that would be, a huge waste of time. For transient cases, values up to 1e7 are often used.


Honestly if you have to ask, I recommend you avoid using the solver and stick to tried and true methods.

Thank you so much, but in the PISO , the CFL is need to control under 0.5 to assure the robustness of the calculation, if you set it to 1e7 for the transient cases , would it be easily divergence?

In addition, I use the PISO model, but it divergence at first , I do not know the reason, so I changed to Coupled model, it does not divergence at first, but it sometimes divergence, if I lower the CFL, it will not happen, but it takes a lot time, how I can I solve this problem?

[LuckyTran]

Firstly, CFL number is a misnomer. I recommend you to start calling it Courant number like it should be called. But for the sake of discussion I'll refer to it as the CFL-Courant number just to illustrate the point.

Let's take the example of PISO. You have URF's in PISO and a time-step size. The CFL-Courant number is determined by the time-step size. The URF for PISO is 1.

With the coupled flow solver you have a Flow Courant number setting and a time-step size. The CFL-Courant number is determined by again the time-step size exactly as in PISO regardless of what you set the Flow Courant number setting to be. Turning up and down the Flow Courant number setting controls the convergence rate of your iterative loop the same way as a URF. A URF=1 is exactly identical to a Flow Courant number of infinity. You can make PISO or the Coupled flow solver more stable by lowering these settings, you can make them converge faster by increasing them.

In transient problems, you have two strategies: large time-steps or small time-steps. With small time-steps with low CFL-Courant number, you crank up the URF for PISO and Flow Courant number setting to the max because your time-step is small enough to be stable and accurate. For large time-steps, you are more unstable due to CFL condition so you lower the URF and Flow Courant number setting to be more stable. However, lowering URF and Flow Courant number means your problem is not converged, so you must increase the number of sweeps taken by either solver.

[hitzhwan]

Originally Posted by LuckyTran

Firstly, CFL number is a misnomer. I recommend you to start calling it Courant number like it should be called. But for the sake of discussion I'll refer to it as the CFL-Courant number just to illustrate the point.

Let's take the example of PISO. You have URF's in PISO and a time-step size. The CFL-Courant number is determined by the time-step size. The URF for PISO is 1.

With the coupled flow solver you have a Flow Courant number setting and a time-step size. The CFL-Courant number is determined by again the time-step size exactly as in PISO regardless of what you set the Flow Courant number setting to be. Turning up and down the Flow Courant number setting controls the convergence rate of your iterative loop the same way as a URF. A URF=1 is exactly identical to a Flow Courant number of infinity. You can make PISO or the Coupled flow solver more stable by lowering these settings, you can make them converge faster by increasing them.

In transient problems, you have two strategies: large time-steps or small time-steps. With small time-steps with low CFL-Courant number, you crank up the URF for PISO and Flow Courant number setting to the max because your time-step is small enough to be stable and accurate. For large time-steps, you are more unstable due to CFL condition so you lower the URF and Flow Courant number setting to be more stable. However, lowering URF and Flow Courant number means your problem is not converged, so you must increase the number of sweeps taken by either solver.

Thank you so much for your answer. I have some questions for you:
1.What is the URF?
2. Is the flow Courant number not useful, because the CFL-Courant number only depends on the time-steps in transient problem, so I think setting the flow Courant number is not valid， is that right?