Postprocessing

BlobFlow$^{\rm TM}$ basis functions are seldom a desirable form of output for interpretation or analysis. Most graphics packages like to read data on some form of regular grid. The program egrid projects vortex basis functions onto a regular grid. As a tool, egrid is fairly unfriendly. It will look for a file called egrid.default which should have five numbers in it: (x₀,y₀), (x₁,y₁) and n, where the first coordinate pair specifies the lower left corner of the domain onto which you wish to project your data. The second coordinate is the upper right corner, and n is the desired number of mesh points. If it cannot find egrid.default, it will query the user for this information.

There are two switches for egrid. Ordinarily, egrid will keep the user informed of its progress row by row. However, if the -q switch is set, egrid will run silently. The -x switch generates the x-antisymmetric projection of the vorticity field corresponding to the XANTISYMM compiler directive. There used to be a separate program to perform the x-antisymmetric projection, but this has been deprecated.

Since users typically want to project an whole group of files onto a regular grid, there is a Python front end for egrid which presents all this information in one place, and once the user sets all the parameters, will repeatedly run egrid.

Another useful, but somewhat unfriendly tool, is vtx2vel. It reads .vtx files and computes the induced velocity field on the regular grid defined by the egrid.default file. The -x flag will impose x-antisymmetry for computations using the XANTISYMM compiler directive.

Note that one can easily hack any of these code to recover the vorticity or velocity field wherever one wants by rewriting the top level subroutines.