V = vortex returns a 128x128 vector field containing at the center a
Burger's vortex (Gaussian vorticity distribution) of radius 8 mm and
vorticity 1 s^-1. V is a standard PIVMat structure, which can be
displayed using showf.
V = vortex(N,R,W) returns a N*N vector field containing a vortex of
radius R (in mm) and vorticity W (in s^-1).
V = vortex(...,MODE) specifies the vorticity distribution shape:
'burgers' (by default) or 'rankine' (top-hat vorticity). For the
'burgers' mode, a divergence may also be specified by V =
vortex(...,'burgers',D), with D (in s^-1) of order of W (D=W by