| Complex or intelligent entities from birds and fish, to balloon and glider pilots, fishermen chasing their catch, and even oceanographers on campaign often modify their behaviour as a result of the opportunities and constraints imposed by their fluid environment, driven by the need to achieve an objective in some optimal way; for instance, shark feeding has been found to involve Levy flights. Such behavioural strategies of entities within a fluid are particularly interesting when they give rise to a collective phenomenon produced by mutual interactions between multiple entities; either interactions via some visual, auditory, or chemical mechanism, or else via the fluid flow itself. For example, aerobic bacteria in overpopulated regions around a limited oxygen source develop swimming patterns that generate bioconvection: mixing fluid currents that transport oxygen to themselves. Similarly, biological cilia on the surface of an organism can organize their movements to develop a collective flow, as do bees flapping their wings to cool their hive, and further instances of such interactions are seen in the V-flight of flocks of birds, porpoises riding the bow wave of a ship, and the peloton of racing cyclists, all of which are of mutual benefit to those involved; on the other hand an antagonistic example of interacting entities is a racing yacht taking the wind out of another boat's sails. Such entity-induced flow is a conceptual opposite to the advection of passive scalars: there flow affects the entities, while here the entities affect the flow. This conceptual blend of chaotic advection and agent-based dynamics has not been systematically explored and provides some interesting perspectives for future research. |
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