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32.4.21 Solve/Iterate...

The Solve/Iterate... menu item opens the Iterate panel.



Iterate Panel


The Iterate panel is used to start the solver iterations. See Sections  25.16 and 25.17 for details about the items below.

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Controls

Time   contains time step parameters. These items will appear only for unsteady flow calculations that use one of the implicit unsteady formulations. See Section  25.17.1 for details.

Time Step Size   sets the magnitude of the (physical) time step $\Delta t$.

Number of Time Steps   sets the number of time steps to be performed.

Time Stepping Method   contains options for how the time step is determined.

Fixed   selects a fixed time step, equal to the specified Time Step Size.

Adaptive   selects a time step that is initially equal to the specified Time Step Size, but gets modified by FLUENT as the calculation proceeds. See Section  25.17.2 for details.

Variable   enables variable time stepping method. The inputs are same as for the adaptive time stepping method, with the exception of specifying a global Courant number.

Options   contains options related to unsteady calculations.

Data Sampling for Time Statistics   enables the sampling of data during an unsteady calculation. See Sections  25.17.1 and 25.17.4 for details.

Sampling Interval   allows you to specify the frequency of the Data Sampling for Time Statistics.

Iteration   contains iteration parameters.

Number of Iterations   (for steady flow calculations) sets the number of iterations to be performed. (For unsteady calculations using the explicit unsteady formulation, this will specify the number of time steps, since each iteration will be a time step.)

Max Iterations per Time Step   (for unsteady flow calculations using an implicit unsteady formulation) sets the maximum number of iterations to be performed per time step. If the convergence criteria are met before this number of iterations is performed, the solution will advance to the next time step. See Section  25.17.1 for details.

Reporting Interval   sets the number of iterations that will pass before convergence monitors will be printed and plotted. The default is 1 (i.e., reports will be updated after each iteration).

UDF Profile Update Interval   sets the number of iterations that will pass before user-defined functions for boundary profiles, properties, etc. will be updated.

Number of FV Sub-Iterations   specifies the number of finite-volume iterations to be performed per global iteration. This item is available only with the composition PDF transport model for steady-state flows.

Number of PDF Sub-Iterations   specifies the number of PDF transport particle steps to be performed per global iteration. This item is available only with the composition PDF transport model for steady-state flows.

Iterations in Time Average   specifies the number of iterations over which to average the statistical fluctuations. This item is available only with the composition PDF transport model for steady-state flows.

Time Average Increment   specifies the increment by which to increase the Iterations in Time Average. Once you have obtained a steady-state solution, it is recommended to use a value of 0.2 for this parameter in order to gradually decrease the residuals over further iterations. See Section  18.3.7 for details about solution parameters for the composition PDF transport model.

Adaptive Time Step Parameters   contains parameters related to adaptive time stepping. This section of the panel will appear when Adaptive is selected as the Time Stepping Method. See Section  25.17.2 for information about using adaptive time stepping.

Truncation Error Tolerance   specifies the threshold value to which the computed truncation error is compared. Increasing this value will lead to an increase in the size of the time step and a reduction in the accuracy of the solution. Decreasing it will lead to a reduction in the size of the time step and an increase in the solution accuracy, although the calculation will require more computational time. For most cases, the default value of 0.01 is acceptable.

Ending Time   specifies an ending time for the calculation. Since the ending time cannot be determined by multiplying the number of time steps by a fixed time step size, you need to specify it explicitly.

Minimum/Maximum Time Step Size   specify the upper and lower limits for the size of the time step. If the time step becomes very small, the computational expense may be too high; if the time step becomes very large, the solution accuracy may not be acceptable to you. You can set the limits that are appropriate for your simulation.

Minimum/Maximum Step Change Factor   limit the degree to which the time step size can change at each time step. Limiting the change results in a smoother calculation of the time step size, especially when high-frequency noise is present in the solution. 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 $\Delta t_n$ is computed as follows:

  • If $1 < f < f_{\rm max}$, $\Delta t_n$ is increased to meet the required tolerance.

  • If $1 < f_{\rm max} < f$, $\Delta t_n$ is increased, but its maximum possible value is $f_{\rm max}\Delta t_{n-1}$.

  • If $f_{\rm min} < f < 1$, $\Delta t_n$ is unchanged.

  • If $f < f_{\rm min} < 1$, $\Delta t_n$ is decreased.

Number of Fixed Time Steps   specifies the number of fixed-size time steps that should be performed before the size of the time step starts to change. The size of the fixed time step is the value specified for Time Step Size under Time.

User-Defined Time Step   contains a drop-down list of available user-defined functions.

Variable Time Step Parameters   contains parameters related to variable time stepping. This section of panel will appear when Variable is selected as the Time Stepping Method. Variable is selectable when the VOF multiphase model is enabled. See Section  25.17.3 for details.

Global Courant Number   allows you to specify Courant number. The default value for the Global Courant number is 2.

Iterate   starts the iterations. While the calculation is in progress, a Working dialog box will appear. Clicking the Cancel button or typing <Control-C> in the console window will interrupt the calculation (as soon as it is safe to stop).

Apply   applies the current settings in the panel.


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