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25.18.4 Monitoring Surface Integrals

At the end of each solver iteration or time step, the average, mass average, integral, flow rate, or other integral report of a field variable or function can be monitored on a surface. 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 Surface Integrals panel, as described in Section  29.5.

Monitoring surface 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 on a surface. 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 on the surface stops changing between adaptions.

Overview of Defining Surface Monitors

You can use the Surface Monitors panel (Figure  25.18.5) to create surface monitors and indicate whether and when each one's history is to be printed, plotted, or saved. The Define Surface Monitor panel (Figure  25.18.6), opened from the Surface Monitors panel, allows you to define what each monitor tracks (i.e., the average, integral, flow rate, mass average, or other integral report of a field variable or function on one or more surfaces).

Defining Surface Monitors

You will begin the surface monitor definition procedure in the Surface Monitors panel (Figure  25.18.5).

Solve $\rightarrow$ Monitors $\rightarrow$ Surface...

Figure 25.18.5: The Surface Monitors Panel

The procedure is as follows:

1.   Increase the Surface Monitors value to the number of surface 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 surface 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 Surface Monitor panel (Figure  25.18.6). Since this is a modal panel, the solver will not allow you to do anything else until you perform steps 5-10, below.

Figure 25.18.6: The Define Surface Monitor Panel

6.   In the Define Surface Monitor panel, choose the integration method for the surface monitor by selecting Integral, Standard Deviation, Flow Rate, Mass Flow Rate, Volume Flow Rate, Area-Weighted Average, Mass-Weighted Average, Sum, Facet Average, Facet Minimum, Facet Maximum, Vertex Average, Vertex Minimum, or Vertex Maximum in the Report Type drop-down list. These methods are described in Section  29.5.

7.   In the Surfaces list, choose the surface or surfaces on which you wish to integrate.

8.   Specify the variable or function to be integrated in the Report of 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 surface-monitor plot can be maintained in a separate window that does not interfere with other graphical postprocessing.

In order to have multiple monitors display in a single graphics window, you can set the Plot Window ID to correspond to the same ID for different monitors. This is useful when you have multiple monitor displays on the screen, you can set all monitors to the same display. For example, for three different monitors, you can set the Plot Window ID to 0 for each of the different monitors in order to display all three monitors in a single window. The name of the monitors ( monitor-1.out, etc.) will be different, but only the Plot Window ID will remain the same. So that each monitor has data that is stored in a different file, but the data is displayed in the same window.


Note that surface and volume monitors cannot be displayed on the same window.


If multiple monitors are plotted in the same window, make sure you set an axes range that can be applied to all the monitors. This axes range will be the same for all the monitors in the shared plot window. Otherwise, the most recently defined monitor, sharing the same window as the other monitors, will determine the axes range. If the default option of Auto Range is enabled for all the monitors sharing the same plot window, then the default value for Minimum will be the minimum value of all the monitors, and the default Maximum will be the maximum value of all the monitors.

Modifying plot attributes can be achieved by clicking the Curves and the Axes button. See Sections  28.8.8 and 28.8.9 for details about plot options.

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

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

Printing, Plotting, and Saving Surface Integration Histories

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


If you choose not to save the surface 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 surface-monitor plot. Click the Axes... or Curves... button in the Define Surface Monitor panel for the appropriate monitor to open the Axes panel or Curves panel for that surface-monitor plot. See Sections  28.8.8 and 28.8.9 for details.

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