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• # CFL Number for steady flows

Posted by on February 23, 2024 at 9:13 am

Dear Sir,

ANSYS Fluent uses CFL number for steady flows also so how is it interpreted theoretically? The default value is 200. And CFL is only used for density-based solvers. Why is it not used for pressure-based solvers? Also, recently ANSYS has said that both solvers can be used for high-speed compressible flows (Although density-based solvers may result in better accuracy). So what are the stability criteria for a pressure-based solver for transient flows?

Regards,

2 Members · 2 Replies
• 2 Replies
• ### Sujith FTL

Member
February 26, 2024 at 9:27 am

Hello,

In ANSYS Fluent, the CFL number is used for steady-state simulations to control the stability of iterative solvers by adjusting the convergence behavior. It ensures that the solver progresses stably through each iteration. The CFL number is more relevant for density-based solvers, commonly used for compressible flows, because they use explicit time-stepping schemes. Pressure-based solvers, often used for incompressible flows, rely on implicit methods where the time step size is determined internally by the solver. While pressure-based solvers don’t have a direct CFL-based stability criterion, they ensure stability in transient flows through careful selection of solver settings, discretization schemes, and numerical algorithms to prevent numerical instabilities while accurately capturing transient flow behavior.

Regards,

Sujith

• ### Sidharth Kamble

Member
February 26, 2024 at 8:39 pm

Thanks Sujith for the explanations. I was helpful. Do you have any theory guide for CFL numbers for steady flows? For example, for transient flow, we can define CFL as [Velocity. (delta t)/ (delta x)] so similarly can we quantify CFL number for steady flow

Regards,