Seakeeping behaviour of a surfaced underwater vehicle

The applications of underwater Vehicles (UV) range from military operations to the collection of scientific data or industrial surveys. These applications are typically performed at depth; hence the design of UVs is focused on the optimisation for submerged operations. Most applications yet require the UV to navigate on the surface to complete essential tasks like getting air, running engines, or communicating. At the surface, a UV is subject to waves, where its seakeeping performance becomes critical as it defines the ability of a UV to carry out its mission. However, most of the seakeeping and resistance knowledge applicable to conventional surface ships is not relevant to UVs due to the significant differences in their hull forms.
This thesis aims to investigate the seakeeping behaviour of a surfaced UV, by means of experimental work conducted in a towing tank and numerical simulations. A novel experimental rig was developed to provide enhanced testing capabilities in high sea states. The captured data was essential in illustrating the hull’s motion response to waves, as well as its resistance to the forward motion. The data was also used as validation for the numerical work.
A well-defined double-peak pitch behaviour was found experimentally. It was linked to the hull shape coupled with the heave peak response in waves. The double peak pitch response was the key difference to conventional surface ship motions and benefitted from a comprehensive set of test conditions to map out the phenomenon. Furthermore, it was found that the seakeeping response of the surfaced UV was linked with a regime of negative added resis?tance in waves at moderate Froude numbers. This negative added resistance as well as the double peak pitch behaviour were linked to some aft appendages’ effect. Experiments on a model without aft appendages helped to show their importance in the damping of the peak motion response and the amplification of the negative added resistance. The experiments also highlighted that the acceleration induced by the heave motions are a primary source of concern for operators and that current aft appendages fail to mitigate them effectively.
The numerical investigation proved that a panel code solver was not able to accurately predict the motion response of a hull in waves. Even when damping coefficients were added to account for the appendages’ effect, the viscosity of the wave run-up on the UV bow was critically missing. It was then shown that RANS-based Computational Fluid Dynamics can predict the resistance of the UV. The viscosity parameters in the CFD model are essential to account for the viscosity influence of the wave run-up on the bow.
Results show that the surfaced UV seakeeping and resistance is composed of very complex and highly non-linear hydrodynamic behaviour. Both the dynamic effects of the aft appendages and the viscosity effect on the UV bow are of importance, but some other flow behaviours are also at stake.
This thesis brings a new understanding of surfaced underwater vehicle performance through the clarification of a 3 degrees of freedom motion response coupling. It also highlights that the aft appendages should be the primary point of focus for designers to provide a surfaced hull shape with superior seakeeping abilities. Operators are also provided with applicable speed and wave height targets for optimum seakeeping and resistance operations.