Optimisation of a fleet of autonomous underwater vehicles to minimise energy dissipation

Abstract

The range of an AUV is dictated by its finite energy source and minimising the energy consumption is required to maximise its endurance. For an individual AUV, this may be achieved by obtaining the optimum hydrodynamic hull shape design. For a fleet of multiple AUVs, this may be targeted for both individuals and the entire fleet. The purpose of this work is, firstly, to develop a rational approach to find an optimal hull shape, secondly, to provide guidance for operators on suitable configuration for multiple AUVs' missions, finally, to investigate the influence of the propeller on the drag of twin self-propelled AUVs.

An AUV hull form has been optimised to obtain low resistance hull. Hydrodynamic optimisation of hull form has been carried out by employing five parametric geometry models with a streamlined constraint. Three Genetic Algorithm optimisation procedures are applied by three simple drag predictions which are based on the potential flow method. The results highlight the effectiveness of considering the proposed hull shape optimisation procedure for the early stage of AUV hull design. The influence on the drag of the fleet of multiple towed prolate spheroids is investigated with various configurations and spacings. A series of three-dimensional simulations are performed using a commercial RANS-CFD code ANSYS CFX 12.1 with the SST turbulence closure model at the length Reynolds Number of 3:2 x 106. The results show that the spacing between two hulls determines the drags. Seven zones based on the drag characteristic of twin towed models are classified. Both the multi vehicle vee and echelon configurations show limited influence against that of the entire fleet's energy budget. Then the investigation extended to determine the combined drag of a pair of propelled prolate spheroids at various longitudinal offsets and transverse separations.

The RANS-HO propeller models are selected to estimate the time averaged thrust and torque of the propeller. The results show that the self-propelled vehicles experience an additional drag which is dominated by the thrust distribution of the propeller rather than torque. The drag of the following AUV is increased due to the upstream propeller, defined as a propeller race deduction. The two sources of self-propelled drag increment are the viscous interaction and a direct result of proximity to the propeller race upstream. The result highlights the importance of both thrust deduction and propeller race deductions when calculating the propulsive power consumption. Based on this optimisation procedure and this numerical data, operators can design the optimal hull shape of an individual AUV including the determination of the optimal configurations in transverse separation and longitudinal offset based on energy considerations of fleets of multiple AUVs, it can be very effective at the early design stage.

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ORCID for P.A. Wilson: orcid.org/0000-0002-6939-682X

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