# Numerical Simulation of the Hydrodynamic Performance and Self-Propulsion of a UUV near the Seabed
## 1 Introduction  
UUV is a kind of underwater vehicle, which integrates technologies of underwater communication, automatic control, mode recognition, and artificial intelligence. It is capable of performing underwater tasks autonomously without any operator and is important equipment for exploring the ocean. The UUV has been widely used in marine environment exploration, undersea oil and gas pipeline and cable inspection, hydrological research, etc. The operability and rapidity of UUV are receiving more and more attention, which reflects the safe operations and navigation efficiency of the vehicles. They play an important role in traveling distance and duration of travel. The operability is mainly represented by the hydrodynamic characteristics of the vehicle, and the rapidity generally needs to consider interferences between hull and thruster. In recent years, a great number of studies have studied the hydrodynamic characteristics of UUV and the interaction between hull and thruster through experiments and CFD methods. The drag and self-propulsion analysis of SUBOFF with E1619 propeller was carried out using CFD method by [PERSON] et al. [1, 2]. The propellers of self-propelled submarines were modeled using the actuated disc and real propellers, respectively. The numerical results are compared with the available experimental data. The suitability of CFD methods for the prediction of self-propulsion performance of underwater vehicles is discussed. [PERSON] et al. [3] reported the results of self-propelled SUBOFF and compared with the towing test results. They find that the bimodal distribution of turbulent stresses in the towing case is different with the axial peak of turbulent kinetic energy in self-propulsion due to thebackpressure gradient induced by the stern section. [PERSON] et al. [4] studied the effect of mesh refinement on propeller performance and wake flow for E1619 propeller and SUBOFF by CFD method. The results showed the applicability of different turbulence models for propeller wake flow and blade tip vortex analysis, and analyzed the self-propulsion performance of SUBOFF. The near-surface and subsea conditions are two extreme conditions for UUV, which put the maneuverability of UUV to severe challenges. The UUV need to navigate near the surface in order to share, transmit information, obtain energy and update their position. In the near-subsea condition, the UUV detects submarine oil and gas pipelines and fiber optic cables. These two conditions need to consider the nonlinear effect of waves on the vehicles near the surface and the blocking effect of the seabed. Studies on the interaction between the hull and the propeller near the surface and fully submerged were carried out in a towed tank [5]. The experimental results show that the drag coefficient of UUV in the surface condition is larger than that in the submerged condition. Besides, the reliability of CFD method is verified by the experimental results. It provides a new method to study the interference between hull and propeller. [PERSON] et al. [6] used the two-phase flow calculation CFD method to study the performance of UUV under the wave-current coupling. It's found that the lift of the UUV is greatly influenced by waves, and the average drag increases with the increase of wave height and the decrease of submerged depth. [PERSON] [7] studied the hydrodynamic performance of the SUBOFF with an E1658 propeller near the water surface by experiments and CFD overlapping mesh method. The interaction between the vessel and free surface strongly influences the inflow and wake of the propeller, generating high local propulsion coefficients and blade loads near the propeller surface. Strong surface fluctuations at small axial depths lead to instability and collapse of propeller tip vortices in the near field. The performance of the UUV with a propeller near water surface was simulated by [PERSON] et al. [8] using the slip grid technique and the VOF method.