Nonlinear excursions of particles in ideal 2D flows

A number of problems related to particle trajectories in ideal 2D flows are discussed. Both regular particle paths, corresponding to integrable dynamics, and irregular or chaotic paths may arise. Examples of both types are shown. Sometimes, in the same flow, certain particles will follow regular paths while others follow irregular paths. Even in the chaotic region the amount of regularity or irregularity of a path depends on initial conditions and system parameters. The notion of a transported fluid region or "atmosphere" is mentioned. Various conclusions, ideas and queries are formulated based on the examples given. The intimate mix of regular and chaotic trajectories complicates a purely Lagrangian approach to fluid flow problems.