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Comparison between analytic and computed vorticity, where:
r = sqrt( (x - x0)^2 + (y - y0)^2 + (z - z0)^2)
vx = y/r
vy = x/r
vz = 0

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VRTX MIN DIFF: 0.0 MAX DIFF: 0.000460

VRTY MIN DIFF: 0.0 MAX DIFF: 0.000460

VRTZ MIN DIFF: 0.0 MAX DIFF: 0.000395