Optimal paths in time-varying flow fields

Dhanushka N. Kularatne

doi:10.17918/D8N658

The use of autonomous marine vehicles (AMV) have seen a significant growth in the last few decades. This growth has been driven not only by advances in vehicle technology, but also by a need from the scientific and industrial communities for increased autonomy in marine environments. The use of AMVs for scientific activities in aquatic environments has increased data availability, reliability and consistency while their use in commercial activities has made marine based systems safer and more reliable. AMVs have been used for scientific activities such as migration tracking, measurement of temperature and salinity profiles, and monitoring of harmful algae blooms. AMVs have also been used in mapping and characterizing ocean structures, monitoring submerged pipelines and power transmission cables, locating plane crash and ship wreckage, and assessing oil spills. They have helped us to better understand oceanic processes ranging from biological phenomena to climate change. However, these platforms are generally small and resource constrained. While this helps with maneuverability and allows for unobtrusive monitoring of marine phenomena, it also means that the AMV missions have limited durations. To increase the utility of these vehicles and further increase their adoption in marine applications, their mission durations have to be made longer. To this end, developers of AMVs are actively looking at ways of optimizing on-board resource usage, from using better senors and computers to more efficient hull designs. In this work, efficient navigation is explored as an alternate method to improve mission durations. The high inertia environments that AMVs operate in presents a unique opportunity for vehicles to exploit the surrounding flows for more efficient navigation. Trajectories of AMVs should try to use a 'go with the flow' strategy to reduce its energy consumption. To this end, the thesis considers the general problem of optimal path planning in timevarying flow fields for resource constrained marine vehicles. A graph based framework to obtain solutions to this problem is developed. The developed framework encompasses the inherently time varying nature of the problem and also considers uncertainties in flow velocity forecasts available for planning. The presented methods are validated in extensive simulations and in experiments. In addition, a method is developed to track a class of flow structures known as Lagrangian coherent structures that are beneficial to energy efficient navigation in the ocean. These structures can be used as energy efficient pathways in the ocean, when explicit flow velocity information is unavailable to plan optimal paths using the graph based methods.

Optimal paths in time-varying flow fields

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Optimal paths in time-varying flow fields

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