GfsFunction

From Gerris

Functions can be used in most objects which require a numerical parameter. A function can be a constant or a piece of C code taking coordinates (x,y,z), time t or any of the domain variables as arguments and returning a floating-point value.

The syntax in parameter files is as follows:

-1.78e-3

or a C function

{
  double a = sin (x + y);
  double b = cos (x - z);
  double c = sin (M_PI*t);
  return a + b + c;
}

or a C expression

40.*(P - 1.)

or a GTS file

myfunction.gts

The GTS file must be a planar (preferably Delaunay) triangulation of a set of points. The value of the function at a given (x,y) coordinate is then calculated by computing the z-coordinate of the intersection of a vertical line passing through the point at (x,y,0) with the triangulation defined by the GTS file.

Gradients of variables can be computed using the dx(), dy() and dz() functions. For example, the z-component of the vorticity would be computed as:

(dx("V") - dy("U"))

More details on C functions

Comments should use the C syntax; i.e. opening /* and closing */ not necessarily on the same line rather than the usual parameter file syntax of a line beginning with a #. This is to allow the use of C preprocessor directives in C functions in GfsFunctions.

Example:

• Coalescence of a pair of Gaussian vortices (Gerris logo), in the InitVorticity command.