solidPointDisplacement

Overview

solidPointDisplacement is a function object (library libmultiRegionFOs) that monitors the displacement of one point of a solid region. At start-up it picks the mesh point closest to a user-given location. At every execution it writes the time, the three components and the magnitude of the point displacement field pointD at that point to a .dat file. pointD is provided by the solidStVK region type. The typical use is the tip displacement of an elastic structure in FSI benchmarks such as Hron-Turek FSI3.

Method

1. Region. The region keyword names the solid mesh. If it is omitted, solid is used when a mesh of that name exists, otherwise region0.

2. Point selection (constructor, once). The vector point is read. On each processor the mesh point with the smallest distance $|\mathbf{x}_i - \mathbf{x}_\text{probe}|$ is found by a brute-force search over all points of the reference (undeformed) mesh. Only the processor(s) holding the global minimum distance keep their point. If several processors tie (e.g. a point on a processor boundary), the one with the highest processor number is kept. Each processor that keeps a point prints its distance to the given location. If no processor ends up with a point, the run aborts.

   Note: this search always uses the mesh solid if it exists, otherwise region0, regardless of the region keyword (see Known Limitations).

3. Sampling (execute(), once per time step by default). In mesh region, the pointVectorField pointD is looked up. Its value at the chosen point is taken without interpolation. All other processors contribute zero, and the values are summed over processors. The master writes one line

   $$ t \quad D_x \quad D_y \quad D_z \quad |\mathbf{D}| $$

   If pointD does not exist in that mesh, a message pointVectorField pointD not found is printed and nothing is written for that step.

write() does nothing and read() ignores the dictionary, so changes to the entry during the run (runTimeModifiable) have no effect.

Usage

system/controlDict entry (function object):

functions
{
    displacement
    {
        type            solidPointDisplacement;
        libs            (multiRegionFOs);
        region          solid;
        point           (0.6 0.2 0.0075);
    }
}

The standard function object controls that OpenFOAM handles outside the object itself (e.g. executeControl, executeInterval, timeStart, timeEnd) can be added. The object itself reads no other keywords.

Input / Output

Read

Written (master processor only):

<case>/postProcessing/<startTime>/solidPointDisplacement_<name>.dat

<name> is the function object entry name (e.g. displacement) and <startTime> is the case start time. In parallel runs the file is still written to the undecomposed case directory (processor*/../postProcessing). The file starts with the header # Time Dx Dy Dz magD, followed by one space-separated line per execution. The file is opened anew at start-up, so a restart from the same start time overwrites the earlier file.

Known Limitations

• region is only partly honoured. The closest-point search always uses mesh solid (or region0 if there is no solid mesh). The sampling uses region. In practice the solid region must be called solid. With another name the search falls back to region0, which does not exist in a multi-region case, and the run aborts.
• Nearest mesh point, no interpolation. The monitored location is the closest existing mesh point, not the exact point. The distance is printed at start-up, so check it in the log.
• Requires pointD. Only region types that register a pointVectorField called pointD (currently solidStVK) can be monitored. Other fields cannot be selected.
• Reference configuration. The point is selected on the solid mesh points at start-up. This is correct for the total-Lagrangian solidStVK, whose mesh never moves.
• Brute-force search. The point search is $O(N_\text{points})$ per processor. This is negligible since it runs only once.
• Distributed cases. With distributed data (no shared case directory), writing to processor*/../postProcessing can fail.
• Header comment. The class description in the header refers to “temperature” and least-squares vol-to-point interpolation. Both are wrong: the object reads pointD directly.

Tutorials