Particle capture in a model chaotic flow with a moving capture unit

To investigate the capture of passive scalars in geophysical flows, we computationally study capture in the simpler, but still chaotic, time-dependent 2d double-gyre flow. For a range of model parameters, the double-gyre flow consists of a rapidly mixing chaotic region interspersed with non-mixing islands in which particle trajectories are regular. Here we consider a moving capture unit (plant) rather than a fixed one, because the former can potentially capture more material. We restrict plant motion to be along a straight line connecting the centers of the flow’s two gyres. Three prescriptions of plant motion are studied for flow conditions ranging from non-chaotic to fully chaotic: motion based on maximum capture for non-chaotic flow but applied to chaotic flow; a method based on Lagrangian coherent structures (LCS) in which the plant follows a maximum in the finite-time Lyapunov exponent field; and a biologically-inspired approach that tracks concentration gradients. The capture efficiency depends on flow conditions. The maximum capture approach based on non-chaotic flow is effective when the flow is less chaotic, while the LCS method is better for fully chaotic flow. The biologically-inspired approach works well when plant motion is not restricted to a straight line.