I am interested to know how does Fluent decide the time step size when I am using adaptive time stepping. It depends on the mesh size and the convergence (residues) of the last time step. But, exactly which formula is used by the Fluent?
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 (based on Minimum/Maximum Step Change Factor); if the truncation error is greater, the time step size is decreased. 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.
If the time step change factor (f) is computed as the ratio between the specified truncation error tolerance and the computed truncation error, the size of time step (dtn) is computed as follows:
If 1< f < fmax, dtn is increased to meet the desired tolerance;
If 1 < fmax < f, dtn is increased, but its maximum possible value is fmax*dt(n-1);
If fmin < f< 1, dtn is unchanged;
If f < fmin < 1, dtn is decreased.
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