[dcan]

Hello,

I am trying to learn the post-processing capabilities of OpenFoam. I have a simple pipe flow case. I want to plot velocity variation across a cross-section. For this purpose, I can use sampling utility or define probes and get the wanted data. But what is the difference between them? Is there any critical consideration I should take into account before going into the solution?

Thanks

[HPE]

In OpenFOAM's finite-volume framework, in a given mesh, field information (e.g. U) is stored at the cell centre.

When this information is to be sampled by using "probes", cell(s) encapsulating/containing the "probe" coordinate is found - and "probe" returns the field value of the cell (stored at its centre) no matter where the probe coordinate resides inside the cell or how close/far it is to the cell centre. No effects from neighbouring cells.

When this information is to be sampled by using "sample" utility, the information at the "sample" coordinate is found by interpolation of the field value to the "sample" coordinate from cells surrounding the "sample" coordinate based on the chosen "sample" interpolation scheme.

In summary, "probe" picks the value from the cell in which the "probe" coordinate resides without any interpolation, and "sample" calculates the value from cells around the "sample" coordinate by an interpolation.

"Probe" is expected to be cheaper to compute than "sample" due to the lack of interpolation computations.

Hope this helps.

[Steben]

Originally Posted by HPE

In OpenFOAM's finite-volume framework, in a given mesh, field information (e.g. U) is stored at the cell centre.

When this information is to be sampled by using "probes", cell(s) encapsulating/containing the "probe" coordinate is found - and "probe" returns the field value of the cell (stored at its centre) no matter where the probe coordinate resides inside the cell or how close/far it is to the cell centre. No effects from neighbouring cells.

When this information is to be sampled by using "sample" utility, the information at the "sample" coordinate is found by interpolation of the field value to the "sample" coordinate from cells surrounding the "sample" coordinate based on the chosen "sample" interpolation scheme.

In summary, "probe" picks the value from the cell in which the "probe" coordinate resides without any interpolation, and "sample" calculates the value from cells around the "sample" coordinate by an interpolation.

"Probe" is expected to be cheaper to compute than "sample" due to the lack of interpolation computations.

Hope this helps.

thanks,I have encountered this problem before. It is not clear from the source program whether the probes extract grid center or node interpolation result. Thanks again!

[ARG]

This is from OpenFOAM V11 user guide regarding the probe function differences:

https://doc.cfd.direct/openfoam/user...phs-monitoring

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.

Please note that these functions work with both foamRun and foamPostProcess.