# Analysis of a New Twin Hybrid Autonomous Underwater Vehicle
## 1 Introduction  
Underwater gliders (UG), autonomous underwater vehicles (AUV), and remotely operated vehicles (ROV) [1, 2, 3] have been developed for decades as major underwater monitoring platforms. Compared with ROV, AUV are untethered, pre-programmed operating platforms, which means they have a wider working envelope and longer mission cycles [4]. UG is a monitoring system driven by buoyancy, which makes UG possess the characteristics of low energy consumption and longer working time. In the literature, [5] studied a vertically aligned two-compartment autonomous underwater vehicle and performed motion simulations. The hybrid autonomous underwater vehicle (HAUV) combines the advantages of low energy consumption of UG and strong mobility of AUV, alternating the working modes of UG and AUV to complete a wide range of underwater data collection and subsea monitoring. In reference [6], a nonlinear kinematic and dynamic model of a hybrid autonomous underwater vehicle (HAUV) is derived for the two modes of locomotion, namely, propelled (AUV) and gliding (UG) modes. The HAUV adopts the UG operation mode in the process of underwater data acquisition, i.e., by adjusting the magnitude and direction of buoyancy to make it alternately operate underwater diving and surfacing. It can also adjust the buoyancy to make it hover horizontally and use thrustersto drive it to complete the acquisition of data such as seafloor topography. After finishing the sampling work in a certain water area, the AUV mode is used to change the course to the next collection region. The THAUV is a kind of HAUV with two buoyant bodies. The fixed wing plays a vital role in the work of the UG and the HAUV; the lift generated by the wing drives the UG and HAUV forward during the dive and surfacing states and CFD numerical calculations are used to study the hydrodynamic characteristics of torpedo-type underwater gliders. [7]. The acquisition of hydrodynamic parameters is crucial to the simulation prediction of the UG and the HAUV. Common methods include the experimental method, parameter identification method, and computational fluid dynamic (CFD) simulation. The hydrodynamic performance and parameters of the underwater glider were calculated using experimental methods as described in reference [8]. In reference [9], the authors described the identification of model parameters for SLOCUM using experimental flight test data. In particular, the focus is on data from a steady linear glide. CFD is widely used for the calculation of hydrodynamic parameters of underwater equipment because of its low cost and fast result acquisition. The application of CFD methods for modeling and hydrodynamic analysis of underwater gliders is described in detail in Reference [10].  
The operating modes of this THAUV can be divided into two types according to the operating mechanism, i.e., glide mode and AUV mode. In the glide mode, the net buoyancy and the position of the center of gravity are regulated by the displacement change of the piston, which drives the glider to move downward (gravity over buoyancy) or move upward (buoyancy over gravity) in the vertical plane. In AUV mode, the piston is adjusted to keep the glider in a neutral state, and the thruster is used to drive the glider to move in the horizontal plane for tasks such as target search and seafloor scanning, while the elevator is responsible for adjusting the attitude of the glider to keep it horizontal (the angle of attack is 0). In this paper, the gliding mode is taken as the research objective, and suitable airfoil selection is obtained through simulation of the gliding motion.