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This animation of pictures of a vortex (diameter 0.1m) separated with a 0.25s time step illustrates two things:
  1. The 3 first pictures from t=0.005s to t=0.05s show a fast growing vortex has the instability is building up. At the same time the peek velocity is climbing too (up to 1,39 m/s at t=0.05s).
  2. Then the vortex is not gaining amplitude anymore. As the vortex is spiraling the velocity vectors form an almost steady circular pattern causing the "cats eyes". The peek velocity is falling towards 1 m/s.
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