Strategies for particle capture in a chaotic flow with a moving capture unit

Motivated by the challenge of capturing passive scalars such as particles, pollutants and greenhouse gases in chaotic geophysical flows, we study capture computationally in the simpler, but still chaotic, time-dependent two-dimensional double-gyre flow. For a range of model parameters, the domain of the double-gyre flow consists of a chaotic region, characterized by rapid mixing, interspersed with non-mixing islands in which particle trajectories are regular. Past work considered the efficacy of stationary capture units, whereas here we consider moving capture units, which can potentially capture more material more rapidly. Four different capture strategies are considered (fixed direction, fixed point, fixed angle and chasing a moving point), all of which rely on periodic determinations of the local particle concentration within a certain distance to determine the motion of the capture unit. For all strategies, the capture results depend on a combination of the flow conditions and the capture unit velocity. The fixed direction strategy is most effective when the capture unit velocity is high and the flow is more chaotic, while the fixed angle strategy works well when the unit velocity is slow and the flow is mostly nonchaotic.