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25.18.5 Monitoring Volume Integrals

At the end of each solver iteration or time step, the volume or the sum, volume integral, volume average, mass integral, or mass average of a field variable or function can be monitored in one or more cell zones. You can print and plot these convergence data, and also save them in an external file. The external file is written in the FLUENT XY plot file format described in Section  28.8.5. The report types available are the same as those in the Volume Integrals panel, as described in Section  29.6.

Monitoring volume integrals can be used to check for both iteration convergence and grid independence. For example, you can monitor the average value of a certain variable in a particular cell zone. When this value stops changing, you can stop iterating. You can then adapt the grid and reconverge the solution. The solution can be considered grid-independent when the average value in the cell zone stops changing between adaptions.

Overview of Defining Volume Monitors

You can use the Volume Monitors panel (Figure  25.18.7) to create volume monitors and indicate whether and when each one's history is to be printed, plotted, or saved. The Define Volume Monitor panel (Figure  25.18.8), opened from the Volume Monitors panel, allows you to define what each monitor tracks (i.e., the volume or the sum, integral, or average of a field variable or function in one or more cell zones).

Defining Volume Monitors

You will begin the volume monitor definition procedure in the Volume Monitors panel (Figure  25.18.7).

Solve $\rightarrow$ Monitors $\rightarrow$ Volume...

Figure 25.18.7: The Volume Monitors Panel

The procedure is as follows:

1.   Increase the Volume Monitors value to the number of volume monitors you wish to specify. As this value is increased, additional monitor entries in the panel will become editable. For each monitor, you will perform the following steps.

2.   Enter a name for the monitor under the Name heading, and use the Plot, Print, and Write check buttons to indicate the report(s) you want (plot, printout, or file), as described below.

3.   Indicate the frequency at which you want to plot, print, or write the volume monitor by entering a number under Every. A default value of 1 will allow you to monitor at every Iteration or Time Step.

4.   Indicate whether you want to update the monitor every Iteration or every Time Step by selecting the appropriate item in the drop-down list below When. Time Step is a valid choice only if you are calculating unsteady flow. If you specify every Iteration, and the Reporting Interval in the Iterate panel is greater than 1, the monitor will be updated at every reporting interval instead of at each iteration (e.g., for a reporting interval of 2, the monitor will be updated after every other iteration. If the reporting interval is 2 and monitor frequency is at Every 3 Iterations, then the monitoring will be done at multiples of six, which is the least common multiple of the two numbers). If you specify every Time Step, the reporting interval will have no effect; the monitor will always be updated after the specified number of time steps.

5.   Click the Define... button to open the Define Volume Monitor panel (Figure  25.18.8). Since this is a modal panel, the solver will not allow you to do anything else until you perform steps 6-11, below.

Figure 25.18.8: The Define Volume Monitor Panel

6.   In the Define Volume Monitor panel, choose the integration method for the volume monitor by selecting Volume, Sum, Max, Min, Volume Integral, Volume-Average, Mass Integral, or Mass-Average in the Report Type drop-down list. These methods are described in Section  29.6.

7.   In the Cell Zones list, choose the cell zone(s) on which you wish to integrate.

8.   Specify the variable or function to be integrated in the Field Variable drop-down list. First select the desired category in the upper drop-down list. You can then select one of the related quantities in the lower list. (See Chapter  30 for an explanation of the variables in the list.)

9.   If you are plotting the data or writing them to a file, specify the parameter to be used as the $x$-axis value (the $y$-axis value corresponds to the monitored data). In the X Axis drop-down list, select Iteration, Time Step, or Flow Time as the $x$-axis function against which monitored data will be plotted or written. Time Step and Flow Time are valid choices only if you are calculating unsteady flow. If you choose Time Step, the $x$ axis of the plot will indicate the time step, and if you choose Flow Time, it will indicate the elapsed time.

10.   If you are plotting the monitored data, specify the ID of the graphics window in which the plot will be drawn in the Plot Window field. When FLUENT is iterating, the active graphics window is temporarily set to this window to update the plot, and then returned to its previous value. Thus, each volume-monitor plot can be maintained in a separate window that does not interfere with other graphical postprocessing.

11.   If you are writing the monitored data to a file, specify the File Name.

12.   Remember to click OK in the Volume Monitors panel after you finish defining all volume monitors.

Printing, Plotting, and Saving Volume Integration Histories

There are three methods available for reporting the selected volume integration. To print the volume integration in the text (console) window after each iteration, turn on the Print option in the Volume Monitors panel. To plot the integrated values in the graphics window indicated in Plot Window (in the Define Volume Monitor panel), turn on the Plot option in the Volume Monitors panel. If you want to save the values to a file, turn on the Write option in the Volume Monitors panel and specify the File Name in the Define Volume Monitor panel. You can enable any combination of these options simultaneously.


If you choose not to save the volume integration data in a file, this information will be lost when you exit the current FLUENT session.

Plot Parameters

You can modify the attributes of the plot axes and curves used for each volume-monitor plot. Click the Axes... or Curves... button in the Define Volume Monitor panel for the appropriate monitor to open the Axes panel or Curves panel for that volume-monitor plot. See Sections  28.8.8 and 28.8.9 for details.

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