6.5 Sampling data

The sampleDict dictionary can be generated by copying an example sampleDict from the sample source code directory at $FOAM_UTILITIES/postProcessing/sampling/sample. The plateHole tutorial case in the $FOAM_TUTORIALS/solidDisplacementFoam directory also contains an example for 1D line sampling:

---

Keyword Options Description interpolationScheme cell cellPoint cellPointFace Cell-centre value assumed constant over cell Linear weighted interpolation using cell values Mixed linear weighted / cell-face interpolation setFormat raw gnuplot xmgr jplot Raw ASCII data in columns Data in gnuplot format Data in Grace/xmgr format Data in jPlot format surfaceFormat null foamFile dx vtk raw stl Suppresses output points, faces, values file DX scalar or vector format VTK ASCII format values for use with e.g.gnuplotsplot ASCII STL; just surface, no values fields List of fields to be sampled, e.g. for velocity U: U Writes all components of U.component(0) Writes component 0, i.e. U.component(1) Writes component 1, i.e. mag(U) Writes magnitude, i.e. sets List of 1D sets subdictionaries -- see 6.4 surfaces List of 2D surfaces subdictionaries -- see 6.5 and 6.6 Table 6.3: keyword entries for sampleDict.

---

The dictionary contains the following entries:

interpolationScheme

the scheme of data interpolation;

sets

the locations within the domain that the fields are line-sampled (1D).

surfaces

the locations within the domain that the fields are surface-sampled (2D).

setFormat

the format of line data output;

surfaceFormat

the format of surface data output;

fields

the fields to be sampled;

The interpolationScheme includes cellPoint and cellPointFace options in which each polyhedral cell is decomposed into tetrahedra and the sample values are interpolated from values at the tetrahedra vertices. With cellPoint, the tetrahedra vertices include the polyhedron cell centre and 3 face vertices. The vertex coincident with the cell centre inherits the cell centre field value and the other vertices take values interpolated from cell centres. With cellPointFace, one of the tetrahedra vertices is also coincident with a face centre, which inherits field values by conventional interpolation schemes using values at the centres of cells that the face intersects.

The setFormat entry for line sampling includes a raw data format and formats for gnuplot, Grace/xmgr and jPlot graph drawing packages. The data are written into a sets directory within the case directory. The directory is split into a set of time directories and the data files are contained therein. Each data file is given a name containing the field name, the sample set name, and an extension relating to the output format, including .xy for raw data, .agr for Grace/xmgr and .dat for jPlot. The gnuplot format has the data in raw form with an additional commands file, with .gplt extension, for generating the graph. Note that any existing sets directory is deleted when sample is run.

The surfaceFormat entry for surface sampling includes a raw data format and formats for gnuplot, Grace/xmgr and jPlot graph drawing packages. The data are written into a surfaces directory within the case directory. The directory is split into time directories and files are written much as with line sampling.

The fields list contains the fields that the user wishes to sample. The sample utility can parse the following restricted set of functions to enable the user to manipulate vector and tensor fields, e.g. for U:

U.component()

writes the th component of the vector/tensor, ;

mag(U)

writes the magnitude of the vector/tensor.

The sets list contains sub-dictionaries of locations where the data is to be sampled. The sub-dictionary is named according to the name of the set and contains a set of entries, also listed in 6.4, that describes the locations where the data is to be sampled. For example, a uniform sampling provides a uniform distribution of nPoints sample locations along a line specified by a start and end point. All sample sets are also given: a type; and, means of specifying the length ordinate on a graph by the axis keyword.

---

Required entries Sampling type Sample locations name axis start end nPoints points uniform Uniformly distributed points on a line face Intersection of specified line and cell faces midPoint Midpoint between line-face intersections midPointAndFace Combination of midPoint and face curve Specified points, tracked along a curve cloud Specified points Entries Description Options type Sampling type see list above axis Output of sample location x ordinate y ordinate z ordinate xyz coordinates distance distance from point 0 start Start point of sample line e.g.(0.0 0.0 0.0) end End point of sample line e.g.(0.0 2.0 0.0) nPoints Number of sampling points e.g.200 points List of sampling points Table 6.4: Entries within sets sub-dictionaries.

---

The surfaces list contains sub-dictionaries of locations where the data is to be sampled. The sub-dictionary is named according to the name of the surface and contains a set of entries beginning with the type: either a plane, defined by point and normal direction, with additional sub-dictionary entries a specified in 6.5; or, a patch, coinciding with an existing boundary patch, with additional sub-dictionary entries a specified in 6.6.