
As mentioned in Section 25.17.1, it is possible to have the size of the time step change as the calculation proceeds, rather than specifying a fixed size for the entire calculation. This section provides a brief description of the algorithm that FLUENT uses to compute the time step size, as well as an explanation of each of the parameters that you can set to control the adaptive time stepping.

Adaptive time stepping is available only with the pressurebased and densitybased implicit formulations; it cannot be used with the densitybased explicit formulation. In addition, it cannot be used with the discrete phase model, secondorder time integration, EulerEuler multiphase models (Section
23.2.1), or userdefined scalars (Section
9.3).

The Adaptive Time Stepping Algorithm
The automatic determination of the time step size is based on the estimation of the truncation error associated with the time integration scheme. If the truncation error is smaller than a specified tolerance, the size of the time step is increased; if the truncation error is greater, the time step size is decreased.
An estimation of the truncation error can be obtained by using a predictorcorrector type of algorithm [ 129] in association with the time integration scheme. At each time step, a predicted solution can be obtained using a computationally inexpensive explicit method (forward Euler for the firstorder unsteady formulation, AdamsBashford for the secondorder unsteady formulation). This predicted solution is used as an initial condition for the time step, and the correction is computed using the nonlinear iterations associated with the implicit (pressurebased or densitybased) formulation. The norm of the difference between the predicted and corrected solutions is used as a measure of the truncation error. By comparing the truncation error with the desired level of accuracy (i.e., the truncation error tolerance), FLUENT is able to adjust the time step size by increasing it or decreasing it.
Specifying Parameters for Adaptive Time Stepping
The parameters that control the adaptive time stepping appear in the Iterate panel, as described in Section 25.17.1.
These parameters are as follows:
It is a good idea to perform a few fixedsize time steps before switching to the adaptive time stepping. Sometimes spurious discretization errors can be associated with an impulsive start in time. These errors are dissipated during the first few time steps, but they can adversely affect the adaptive time stepping and result in extremely small time steps at the beginning of the calculation.

When the solution tends to exhibit incomplete convergence, rather than increasing the time step size or keeping the same time step size in the next step,
FLUENT reduces the time step size by at least half for the next time step (making sure that the time step size does not go below the specified minimum time step size.

Specifying a UserDefined Time Stepping Method
If you want to use your own adaptive time stepping method, instead of the method described above, you can create a userdefined function for your method and select it in the UserDefined Time Step dropdown list. The other inputs under Adaptive Time Stepping will not be used when you select a userdefined function.
See the separate UDF Manual for details about creating and using userdefined functions.