OpenFOAM v11 User Guide

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7.4 Sampling and monitoring data

There are a set of general post-processing functions for sampling data across the domain for graphs and visualisation. Several functions also provide data in a single ﬁle, in the form of time versus values, that can be plotted onto graphs. This time-value data can be monitored during a simulation with the foamMonitor script.

7.4.1 Probing data

The functions for probing data are boundaryProbes, internalProbes and probes as listed in section 7.3.12 . All functions work on the basis that the user provides some point locations and a list of ﬁelds, and the function writes out values of the ﬁelds are those locations. The diﬀerences between the functions are as follows.

probes identiﬁes the nearest cells to the probe locations and writes out the cell values; data is written into a single ﬁle in time-value format, suitable for plotting a graph.
boundaryProbes and internalProbes interpolate ﬁeld data to the probe locations, with the locations being snapped onto boundaries for boundaryProbes; data sets are written to separate ﬁles at scheduled write times (like ﬁelds). data.

Generally probes is more suitable for monitoring values at smaller numbers of locations, whereas the other functions are typically for sampling at large numbers of locations.

As an example, the user could use the pitzDailySteady case set up in section 2.1 . The probes function is best conﬁgured by copying the ﬁle to the local system directory using foamGet.

foamGet probes

The user can modify the probeLocations in the probes ﬁle as follows.

12
13#includeEtc "caseDicts/postProcessing/probes/probes.cfg"
14
15fields (p U);
16probeLocations
17(
18 (0.01 0 0)
19);
20
21// ************************************************************************* //

The conﬁguration is completed by adding the #includeFunc directive to functions in the controlDict ﬁle.

functions
{
#includeFunc probes
... other function objects here ...
}

When the simulation runs, time-value data is written into p and U ﬁles in postProcessing/probes/0.

7.4.2 Sampling for graphs

The graphUniform function samples data for graph plotting. To use it, the graphUniform ﬁle can be copied into the system directory to be conﬁgured. We will conﬁgure it here using the pitzDaily case as before. The ﬁle is simply copied using foamGet.

foamGet graphUniform

The start and end points of the line, along which data is sampled, should be edited; the entries below provide a vertical line across the full height of the geometry 0.01 m beyond the back step.

14
15start           (0.01 -0.025 0);
16end             (0.01  0.025 0);
17nPoints         100;
18
19fields          (U p);
20
21axis            distance; // The independent variable of the graph. Can be "x",
22                          // "y", "z", "xyz" (all coordinates written out), or
23                          // "distance" (from the start point).
24
25#includeEtc "caseDicts/postProcessing/graphs/graphUniform.cfg"
26
27// ************************************************************************* //

The conﬁguration is completed by adding the #includeFunc directive to functions in the controlDict ﬁle.

functions
{
#includeFunc graphUniform
... other function objects here ...
}

The simulation can be then re-run or the user could run the post-processing with the following command.

foamPostProcess -solver incompressibleFluid

Either way, distance-value data is written into ﬁles in time directories within postProcessing/graphUniform. The user can quickly display the data for $\text{[math]}$ -component of velocity, $\text{[math]}$ in the last time e.g. 285, by running gnuplot and plotting values.

    gnuplot
    gnuplot> set style data linespoints
    gnuplot> plot "postProcessing/graphUniform/285/line_U.xy" u 2:1

This produces the graph shown in Figure 7.5. This graph corresponds to the velocity inlet with a uniform proﬁle, rather than a boundary layer proﬁle.

$\relax \special {t4ht=$

Figure 7.5: Graph of $\text{[math]}$ at $\text{[math]}$ = 0.01, uniform sampling

The formatting of the graph is speciﬁed in conﬁguration ﬁles in $FOAM_ETC/caseDicts/postProcessing/graphs. The graphUniform.cfg ﬁle in that directory includes the conﬁguration as follows.

8
9#includeEtc "caseDicts/postProcessing/graphs/graph.cfg"
10
11sets
12(
13    line
14    {
15        type            lineUniform;
16        axis            $axis;
17        start           $start;
18        end             $end;
19        nPoints         $nPoints;
20    }
21);
22
23// ************************************************************************* //

It shows that the sampling type is lineUniform, meaning the sampling uses a uniform distribution of points along a line. The other parameters are included by macro expansion from the main ﬁle and specify the line start and end, the number of points and the distance parameter speciﬁed on the horizontal axis of the graph.

An alternative graph function object, graphCell, samples the data at locations nearest to the cell centres. The user can copy that function object ﬁle and conﬁgure it as shown below.

13
14start   (0.01 -0.025 0);
15end     (0.01  0.025 0);
16fields  (U p);
17
18axis    distance; // The independent variable of the graph. Can be "x",
19                  // "y", "z", "xyz" (all coordinates written out), or
20                  // "distance" (from the start point).
21
22#includeEtc "caseDicts/postProcessing/graphs/graphCell.cfg"
23
24// ************************************************************************* //

Running the post-processing produces the graph in Figure 7.6 .

$\relax \special {t4ht=$

Figure 7.6: Graph of $\text{[math]}$ at $\text{[math]}$ = 0.01, mid-point sampling

7.4.3 Sampling for visualisation

There are several surfaces and streamlines functions, listed in Section 7.3.16 , that can be used to generate ﬁles for visualisation. The use of streamlinesLine is already conﬁgured in the pitzDailySteady case.

To generate a cutting plane, the cutPlaneSurface function can be conﬁgured by copying the cutPlaneSurface ﬁle to the system directory using foamGet.

foamGet cutPlaneSurface

The ﬁle is conﬁgured by setting the origin and normal of the plane and the ﬁeld data to be sampled. We can edit the ﬁle to produce a cutting plane along the pitzDaily geometry, normal to the $\text{[math]}$ -direction.

16
17fields (p U);
18
19interpolate true; // If false, write cell data to the surface triangles.
20 // If true, write interpolated data at the surface points.
21
22#includeEtc "caseDicts/postProcessing/surface/cutPlaneSurface.cfg"
23
24// ************************************************************************* //

The function can be included as normal by adding the #includeFunc directive to functions in the controlDict ﬁle. Alternatively, the user could test running the function using the solver post-processing by the following command.

foamPostProcess -solver incompressibleFluid -func cutPlaneSurface

This produces VTK format ﬁles of the cutting plane with pressure and velocity data in time directories in the postProcessing/cutPlaneSurface directory. The user can display the cutting plane by opening ParaView (type paraview), then doing File->Open and selecting one of the ﬁles, e.g. postProcessing/cutPlaneSurface/285/U_zNormal.vtk as shown in Figure 7.7 .

$\relax \special {t4ht=$

Figure 7.7: Cutting plane with velocity

7.4.4 Live monitoring of data

Functions like probes produce a single ﬁle of time-value data, suitable for graph plotting. When the function is executed during a simulation, the user may wish to monitor the data live on screen. The foamMonitor script enables this; to discover its functionality, the user run it with the -help option. The help option includes an example of monitoring residuals that we can demonstrate in this section.

Firstly, include the residuals function in the controlDict ﬁle.

    functions
    {
    #includeFunc  residuals
    ... other function objects here ...
    }

The default ﬁelds whose residuals are captured are p and U. Should the user wish to conﬁgure other ﬁelds, they should make copy the residuals ﬁle in their system and edit the fields entry accordingly. All functions ﬁles are within the $FOAM_ETC/caseDicts directory. The residuals ﬁle can be located using foamInfo:

foamInfo residuals

It can then be copied into the system directory conveniently using foamGet:

foamGet residuals

The user can then run the case in the background.

foamRun > log &

The user should then run foamMonitor using the -l option for a log scale $\text{[math]}$ -axis on the residuals ﬁle as follows. If the command is executed before the simulation is complete, they can see the graph being updated live.

foamMonitor -l postProcessing/residuals/0/residuals.dat &

It produces the graph of residuals for pressure and velocity in Figure 7.8 .

$\relax \special {t4ht=$

Figure 7.8: Live plot of residuals with foamMonitor

OpenFOAM v11 User Guide - 7.4 Sampling and monitoring data