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25.18.3 Monitoring Forces and Moments

At the end of each solver iteration, the lift, drag, and/or moment coefficient can be computed and stored to create a convergence history. You can print and plot this convergence data, and also save it to an external file. The external file is written in the FLUENT XY plot file format described in Section  28.8.5. Monitoring forces can be useful when you are calculating external aerodynamics, for example, and you are especially interested in the forces. Sometimes the forces converge before the residuals have dropped three orders of magnitude, so you can save time by stopping the calculation earlier than you would if you were monitoring only residuals. (You should be sure to check the mass flow rate and heat transfer rate as well, using the Flux Reports panel described in Section  29.2, to ensure that the mass and energy are being suitably conserved.)


The force and moment coefficients use the reference values described in Section  29.10. Specifically, the force coefficients use the reference area, density, and velocity, and the moment coefficients use the reference area, density, velocity and length.


Only the processed force coefficient data is saved. If you decide to change any of the parameters controlling the force monitoring, such as the reference values, force vector, moment center, moment axis, or wall zones, you may see a discontinuity in the data: the previous data is not updated. Usually, if you have made changes you will want to delete the previous force coefficient data before continuing to iterate.

Monitoring Forces in Unsteady Flow Calculations

If you are calculating unsteady flow, the specified force reports will be updated after each time step, rather than after each iteration. All other features of the force monitor and the related setup procedures are unchanged.

Overview of Using the Force Monitors Panel

You can use the Force Monitors panel (Figure  25.18.4) to print, plot, and save the convergence history of the drag, lift, and moment coefficients on specified wall zones.

Solve $\rightarrow$ Monitors $\rightarrow$ Force...

Figure 25.18.4: The Force Monitors Panel

In this panel you will indicate the types of reports that you want (printouts, plots, or files), and specify which coefficients and wall zones are of interest. Additional information will be entered for each coefficient that is monitored. You can also modify the plot parameters.


Remember to click Apply after making the desired modifications to the setup for each coefficient report.

Specifying the Force Coefficient Report

For each coefficient that you choose to monitor, you will set all appropriate parameters in the Force Monitors panel and click Apply. You can monitor one, two, or all three of the coefficients (drag, lift, and moment vector component) while iterating. When you select the desired coefficient, the current (or default) panel settings are shown for that coefficient. Clicking the Apply button will save the current panel settings for the selected coefficient.

The procedure for specifying a force coefficient report is listed below:

1.   Indicate the type of report you want (printout, plot, or file), as described below.

2.   If you want to monitor the force or moment on individual walls in a single printout, plot, or file, turn on the Per Zone option. See below for details.

3.   Choose the coefficient of interest by selecting Drag, Lift, or Moment in the Coefficient drop-down list.

4.   In the Wall Zones list, select the wall zone(s) on which the selected coefficient is to be computed. If you are monitoring more than one coefficient, the selected wall zones are often the same for each coefficient. If you want, however, you can have each coefficient computed on a different set of zones.

5.   Depending on the coefficient selected, do one of the following:

  • If you are monitoring the drag or lift coefficient, enter the X, Y, and Z components of the Force Vector along which the forces will be computed. The Force Vector heading will appear only if you have selected Drag or Lift in the Coefficient drop-down list. By default, drag is computed in the $x$ direction and lift in the $y$ direction.

  • If you are monitoring the moment coefficient, enter the Cartesian coordinates ( X, Y, and Z) of the Moment Center about which moments will be computed. The Moment Center heading will appear only if you have selected Moment in the Coefficient drop-down list. The default moment center is (0,0,0). You will also need to specify which component of the moment vector you wish to monitor. Presently, you can monitor only one component of the moment vector at a time. Select X-Axis, Y-Axis, or Z-Axis in the About drop-down list. (This list is active only if you have selected Moment in the Coefficient drop-down list.) For two dimensional flows, only the moment vector about the $z$-coordinate axis exists.

6.   Click Apply and repeat the process for additional coefficients, if desired.

Printing, Plotting, and Saving Force Coefficient Histories

There are three methods available for reporting the selected force coefficients. To print the coefficient value(s) in the text (console) window after each iteration, turn on the Print option under Options in the Force Monitors panel. To plot the coefficient in the graphics window indicated in Plot Window, turn on the Plot option. If you want to save the values to a file, turn on the Write option and specify the File Name. You can enable any combination of these options simultaneously.


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

If you wish to display a plot of the current force-coefficient history, simply click the Plot button.

Plot Parameters

If you choose to plot the force coefficients (either interactively during the solution or using the Plot button after calculations are complete), there are several display parameters you can modify.

In the Plot Window field, you can specify the ID of the graphics window in which the plot for each force coefficient will be drawn. 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, the force-coefficient plots can be maintained in separate windows that do not interfere with other graphical postprocessing.

You can also modify the attributes of the plot axes and the coefficient curves. The same attributes apply to all force-monitor plots. Click the Axes... or Curves... button to open the Axes panel or Curves panel. See Sections  28.8.8 and 28.8.9 for details.

Monitoring Forces and Moments on Individual Walls

By default, FLUENT will compute and monitor the total force or moment for all of the selected walls combined together. If you have selected multiple walls and you want to monitor the force or moment on each wall separately, you can turn on the Per Zone option in the Force Monitors panel. The specified force vector or moment axis will apply to all selected walls.

If the monitor results are printed to the console (using the Print option), the force or moment on each wall will be printed in a separate column. If the results are plotted (using the Plot option), a separate curve for each wall will be drawn in the specified plot window. If the results are written to a file (using the Write option), the file will be in a tab-separated column format based on the XY plot file format described in Section  28.8.5.

Discarding the Force-Monitoring Data

Should you decide that the data gathered by the force monitor are not useful (e.g., if you are restarting the calculation or you changed one of the reference values), you can discard the accumulated data by clicking on the Clear button. The Clear button will delete all monitoring data for the coefficient selected in the Coefficient drop-down list, including the associated history file (with the name in the File Name field). When you use the Clear button, you will need to confirm the data discard in a Question dialog box. Only the force-monitoring data is removed using this operation; the solution data is not affected.

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Up: 25.18 Monitoring Solution Convergence
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