On the dynamics of amphibious vehicles in surf and shallow water

Amphibious craft are a unique class of vehicle – having the ability to both operate on land as well as in water – with broad applications in the littoral region of the ocean that include cargo connectors, search and rescue, and coastal monitoring amongst others. Interest in autonomous amphibious vehicles to perform these tasks has grown significantly in recent years due to the decrease in risk to human life in austere environments, persistent monitoring, and freeing up people to perform more critical tasks. Any amphibious vehicle operating in the naval context, at the very least, must pass through the surf zone, a region of the ocean that extends from the point of wave breaking to the shoreline (J. M. Smith, 2003; Svendsen, 1992). When waves break on a beach, they dissipate large amounts of energy as turbulence, driving currents both onshore and offshore (Michallet, Cienfuegos, Barthélemy, & Grasso, 2011; J. M. Smith, 2003; Svendsen, 1992). These circumstances present a clear challenge to vessel controllability and survivability (Arnold, Behara, Martin, et al., 2020; Behara, Arnold, Martin, et al., 2020).

Amphibious craft, like many other cross-country ranging ground vehicles, must contend with shallow-water environments both in the surf zone and during water fording. Despite the wide applicability of fording, the literature on the topic mirrors that of surf zone studies. There is a lack of systematic experimental studies on the hydrodynamics of ground vehicles operating in shallow-water.

Furthermore, recent advances in both experimental devices and system identification unlock the potential for rapid and accurate identification of model parameters that form the basis for the formulation of automatic control models for marine vehicles.

Presented in this thesis are a number of investigations on amphibious vehicles aimed at developing the experimental methods and for the development of dynamic models. Experiments cover surf zone transit of a free-running craft, where results were used to validate numerical codes as well as found the phasing of the first wave encounter and momentum entering the surf zone determined the subsequent wave encounters. A shallow-water experimental campaign was conducted to investigate the hydrodynamic forces on a ground vehicle during water fording. The results of this study were used to validate a machine learning model and to find a scaling relationship for the total resistance coefficient. Finally, a system identification methodology was developed that used phase-optimized orthogonal multisine excitations to determine the added mass and damping matrices of a submerged body. These model parameters can then be extended to contemporary vehicle-specific maneuvering models, where automatic control schemes may be used with full system knowledge to drive an uncrewed underwater vehicle autonomously.