Vortices with a maximum Reynolds number of 5000 were randomly distributed throughout a square 2D domain and allowed to evolve numerically. Grey contours show the resulting fluid displacement and the red and blue lines show the Lagrangian Coherent Structures, as indicated by the peaks in the Hessian of the Lyapunov exponent.
The merger and coalescence of dissipating vortices —

Vortices with a maximum Reynolds number of 5000 were randomly distributed throughout a square 2D domain and allowed to evolve numerically. Grey contours show the resulting fluid displacement and the red and blue lines show the Lagrangian Coherent Structures, as indicated by the peaks in the Hessian of the Lyapunov exponent.

https://fluids.ac.uk/files/09_Photo_180331.1522825660.png