I am working on gas-liquid slug flow in vertical pipe, and changing pipe diameter is one of the parameters that i aim to investigate, so when changing diameter i need to update the time step size. My question is what are the main considerations when calculating time step size for transient vof method in Ansys Fluent?
Calculate a time step such that the flow should not cross one cell. Refer CFL number.
Adjust the time step size such that the Courant number remains lower, say 2.0. However, FLUENT accept maximum 250.0 Courant number but it is better to keep its value in the lower range.
If you don't know appropriate value for the time step size or you are not able to determine time step size value in your cases then you can adopt variable time step size option available in FLUENT. In this approach, FLUENT will automatically considers a time step size (within the time step size range you specify in this case) such that the Courant number remains low (by default 2.0).
In addition, the range of accepted Courant number depends whether you use the segregated solver (Courant number near 1) or coupled solver (Courant number up to 200 or 400).
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