This paper proposes a bio-inspired hull shape (BHS) for AUV by studying and modeling the body shape of humpback whales. Among factors affecting hydrodynamic characteristics, this paper considers both the hull drag and displacement volume to optimize the BHS, which profitably improve the space utilization and voyage of the AUV. The optimization is conducted by a surrogate model using response surface methodology (RSM), during which the translational propagation Latin hypercube design (TPLHD) is adopted to obtain sampling points. In order to verify the optimization results of BHS, the drag computations for BHS and eight typical hull shapes of existing typical or bionic AUVs are performed for comparison under conditions of similar volume, wet surface area, body length and attachments. A scaled-down model of BHS with attachments is then designed, manufactured and carried out a towing tank test. The drag measured in the towing tank test is basically the same as the simulation result, with the average relative error of 3.68% at 4m/s. The result of the shape optimization performed with the RSM is effective. Furthermore, the proposed BHS is highly suitable for underwater vehicles with requirements for longer distance, higher speed or better sensor carrying capacity.

Supercavitation has been recently presented as an effective method for the drag reduction of underwater vehicles. However, maintaining the supercavitating state requires a lot of energy, making vehicles difficult to control. Therefore, it is necessary to design an underwater vehicle with low drag in the fully wetted state while being able to move at ultra-high speed in the supercavitating state. In this study, a detachable fairing design for underwater vehicles is proposed, which has the advantage of increasing the total voyage and avoiding the problem of difficult steering in the supercavitating state. On the other hand, the study of non-body-of-revolution (non-BOR) has become a prevalent area of interest in the shape design of underwater vehicles. The cavity generated by an elliptical disk-shaped cavitator is studied numerically. It is found that the cavity profile on the cross-section near the cavitator is approximately elliptical. The cavity length of an elliptical disk-shaped cavitator is almost the same as that of a disk-shaped cavitator when they have the same inflow area. Based on these two characteristics, the parameters of the internal elliptical disk-shaped cavitator are optimized, which provides a promising strategy for the issue of cavitators increasing drag in a fully wetted state.

Hybrid-driven Underwater Glider (HUG) is a new type of underwater vehicle which integrates the functions of an Autonomous Underwater Glider (AUG) and an Autonomous Unmanned Vehicle (AUV). Although HUG has the characteristics of long endurance distance, its maneuverability still has room to be improved. This work introduces a new movement form of the neck of the underwater creature into HUG and proposes a parallel mechanism to adjust the attitude angle and displacement of the HUG’s bow, which can improve the steering maneuverability. Firstly, the influence of bow movement and rotation on the hydrodynamic force and flow field of the whole machine is analyzed by using the Computational Fluid Dynamics (CFD) method. The degree of freedom, attitude control range and movement amount of the Movable Bow Mechanism (MBM) are obtained, and then the design of MBM is completed based on these constraints. Secondly, the kinematic and dynamic models of MBM are established based on the closed vector method and the Lagrange equation, respectively, which are fully verified by comparing the results of simulation in Matlab and Adams software, then a Radial Basis Function (RBF) neural network adaptive sliding mode controller is designed to improve the dynamic response effect of the output parameters of MBM. Finally, a prototype of MBM is manufactured and assembled. The kinematic, dynamics model and controller are verified by experiments, which provides a basis for applying MBM in HUGs.

It remains a great challenge for hybrid-driven underwater glider (HUG) to carry out fine-scale ocean observations by limitations of mobility and flexibility. Inspired by the neck motion of underwater creatures, such as penguins and dolphins, this article proposed an HUG with a movable bow (HUGMB), which was designed to have high maneuverability for oceanographic exploration. The detailed design of HUGMB was given, and its dynamic model was established by considering the correlation between hydrodynamic coefficients and the attitude parameters of the movable bow. Based on the dynamic model, the motion performance of HUGMB was simulated, mainly involving gliding performance, steering capability, spiraling motion, and depth-fixed performance. The minimum spiraling radius was reduced to 1.83 m with an angular velocity of 12.3°/s, the maximum pitch angle reached 64.7°, and the variation in depth can be reduced to nearly 0 m during depth-fixed navigation, which is almost impossible for traditional HUGs. Finally, a prototype of HUGMB was created, and a tank experiment was carried out to verify the simulation results. These proved that the proposed movable bow can effectively improve the maneuverability of HUGs. This study also provides guidance for improving the motion performance of other underwater vehicles.

The buoyancy-driven underwater glider serves as a highly efficient tool for achieving ocean-wide, long-term, and continuous fine-scale observations. However, the performance of underwater gliders in the gliding range and speed is significantly influenced by biological attachments. To investigate potential solutions for this issue, this study explores the drag reduction mechanism of the Petrel-L underwater glider's main body based on the biomimetic superhydrophobic surface (BSHS). The flow field surrounding the underwater glider is analyzed through particle image velocimetry, and mechanical measurements are obtained with force balance techniques. The drag reduction effect of the BSHS with/without biological attachments is accurately determined through force balance analysis. Additionally, the impact mechanism of the boundary layer on Petrel-L with/without biological attachments is investigated with a range of analytical techniques, including proper orthogonal decomposition, conditional phase averaging, finite-time Lyapunov exponent, and quadrant analysis. At an angle of attack (AOA) of 3°, the BSHS realizes a drag reduction of approximately 15.2% when there are no biological attachments. When biological attachments are introduced, the flow drag of the underwater glider increases significantly, but BSHS achieves a drag reduction of about 16.8%. The drag reduction ability of BSHS is mainly reflected in its reduction of the streamwise fluctuation velocity within the boundary layer and achieves relaminarization of the boundary layer under the influence of AOA. These findings suggest that the BSHS retains its remarkable drag reduction capability for the underwater glider, even in the presence of adverse AOA and biological attachments. The current study demonstrates the immense potential of BSHS in the realm of underwater vehicles and offers theoretical and empirical support for future investigations into hydrodynamic performance optimization and passive drag reduction technologies for underwater vehicles.

The vibration caused by the movement of internal actuating components within an acoustic underwater glider can interfere with onboard sensors. However, as a new vibration-damping material, phononic crystals can effectively reduce this impact. Using simulation and an underwater test, this work studied the vibration-damping mechanism of the phononic crystal suspension (PCS) designed by Tianjin University, China. The bandgaps and the modes of PCS were calculated first, which offered basic data for the following simulation. Then, the relationship between the modes and attenuation zones (AZs) were broadly considered to reveal the variation law of the AZs with the change in modes, both in the air and under water. Finally, an underwater test was carried out to verify the good vibration-damping effect of the PCS. The results show that the cutoff frequency of the AZs could be predicted by finding the relevant modes. The PCS showed a good vibration-damping effect from 170 Hz to 5000 Hz in the underwater test, with a maximum decrease of 6 dB at 2000 Hz. Finally, the damping of the PCS could suppress the overlap of modes that resulted from Bragg scattering. This work will also provide theoretical guidance for further study on the optimization of phononic crystal mechanisms for vibration damping.