Adaptive time stepping

MFiX
1

Hello everyone,

I am using adaptive time stepping with a constant ratio and would like to clarify the role of dt_fac. The documentation states that dt_fac must be ≤ 1.0. Does this mean that dt_fac only controls time step reduction (dt_fac < 1.0 decreasing dt ), while dt_fac = 1.0 keeps dt constant? If so, what parameter in MFiX allows the time step to grow adaptively? Given the following settings, can we expect a faster simulation?

time = 0.0
tstop = 5.0
dt = 1.0000e-03
dt_min = 1.0000e-07
dt_max = 1.0000e-02
dt_fac = 1.2 (if not allowed, what should be used for adaptive growth?)

2

DT_FAC is used both to decrease and decrease the time step. We multiply by DT_FAC to decrease the time step while we divide by DT_FAC to increase the time step.

1 Like
3

Thank you very much, Jeff.
By default, is DT_FAC automatically adjusted to either multiply or divide during the simulation, or is it explicitly set to only one operation?

Additionally, if it is set for only one operation by default, could you please guide me on how to configure the simulation so that DT_FAC can automatically adjust in both directions to optimize the time step and improve simulation speed?

4

If you set DT_FAC to a value less than one, it will adjust the time step both up and down, based on the simulation convergence. There is typically not need to adjust it, DT_FAC=0.9 is the recommended value.

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5

Thank you very much, Jeff.