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25.13 Setting Multi-Stage Time-Stepping Parameters

The most common parameter you will change to control the multi-stage time-stepping scheme is the Courant number. Instructions for modifying the Courant number are presented in Section  25.10.1. The Multi-Stage Parameters panel is accessible when using the density-based explicit formulation.



Changing the Multi-Stage Scheme


It is possible to make several changes to the multi-stage time-stepping scheme itself. You can change the number of stages and set a new multi-stage coefficient for each stage. You can also control whether or not dissipation and viscous stresses are updated at each stage. These changes are made in the Multi-Stage Parameters panel (Figure  25.13.1).

Solve $\rightarrow$ Controls $\rightarrow$ Multi-Stage...

Figure 25.13.1: The Multi-Stage Parameters Panel
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You should not attempt to make changes to FLUENT's multi-stage scheme unless you are very familiar with multi-stage schemes and are interested in trying a different scheme found in the literature.

Changing the Coefficients and Number of Stages

By default, the FLUENT multi-stage scheme uses 3 stages for steady-state solutions with coefficients of 0.2075, 0.5915, and 1.0, and 4 stages for unsteady solutions with coefficients of 0.25, 0.3333, 0.5, and 1.0. You can decrease or increase the number of stages using the arrow buttons for Number of Stages in the Multi-Stage Parameters panel. (If you want to increase the number of stages beyond five, you will need to use the text-interface command solve/set/multi-stage.) For each stage, you can modify the Coefficient. Coefficients must be greater than 0 and less than 1. The final stage should always have a coefficient of 1.

Controlling Updates to Dissipation and Viscous Stresses

For each stage, you can indicate whether or not artificial dissipation and viscous stresses are evaluated. If a Dissipation box is selected for a particular stage, artificial dissipation will be updated on that stage. If not selected, artificial dissipation will remain "frozen'' at the value of the previous stage. If a Viscous box is selected for a particular stage, viscous stresses will be updated on that stage. If not selected, viscous stresses will remain "frozen'' at the value of the previous stage. Viscous stresses should always be computed on the first stage, and successive evaluations will increase the "robustness'' of the solution process, but will also increase the expense (i.e., increase the CPU time per iteration). For steady problems, the final solution is independent of the stages on which viscous stresses are updated.

Resetting the Multi-Stage Parameters

If you change the multi-stage parameters, but you then want to return to the default scheme set by FLUENT, you can click the Default button in the Multi-Stage Parameters panel. FLUENT will change the values to the defaults and the Default button will become the Reset button. To get your values back again, you can click the Reset button.


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