# Numerical Simulation of the Hydrodynamic Performance and Self-Propulsion of a UUV near the Seabed
## 1 Introduction
Results show that the effect of the free surface on the drag force is greater than that of the self-propulsion. Hydrodynamic characteristics and self-propulsion performance of self-propelled of UUV near the water surface have been carried out by a large number of researchers using CFD and experimental methods. However, there are few papers studying the near-subsea conditions to the authors' knowledge. [PERSON] et al. [9; 10; 11] proposed two subsea stationing strategies, hydraulically supported and mooring chain types, for UUV that require permanent fixed-point exploration. Two simulation models of the landing process of UUV were established. The results show that the lateral force of UUV with vertical thrust is 5-15% larger than that without vertical thrust. [PERSON] et al. [12] developed a mathematical model of critical instability considering the complex coupling between the glider and the seabed as well as the flow field in which the underwater glider resides stably. The results show that the center of buoyancy is 0.3 m from the seabed and the angle of attack of the glider has a strong influence on the stability of the system. However, the UUV analyzed in these papers are stationary without motion. [PERSON] et al. [13] conducted a model resistance test for UUV at different distances near the seabed and near the sea surface, and established a model test and analysis method to obtain the influence rules of UUV drag near the seabed and near the sea surface. The results demonstrate that considering only the effect of the free surface, it can be assumed that there is no wave resistance when the vessel travels at a depth of over 1/3 of its length. Taking into account only the influence of the seabed wall, the effect of the seabed is no longer considered when the submerged body travels at a distance of over 1/4 of its length from the seabed. When the influences of both free water surface and sea bed wall exist, to completely avoid the interference of restricted water, it is required that submerged hull dive depth exceeds 1/3 of the hull length, and the distance between the seabed and the axis of the submerged hull should not be less than 1/2 of the hull length. [PERSON] et al. [14] developed a numerical model of the near-subsea UUV maneuverability and investigated the effects of distance from subsea, \(Re\) and angle of attack on the hydrodynamic performance of the UUV. The results show that the drag coefficient of UUV increases with decreasing distance from the seabed and with increasing angle of attack. The absolute value of the lift coefficient increases with decreasing distance and with the increment of the angle of attack. The absolute value of the pitching moment coefficient increases as the distance decreases and the angle of attack increases. [PERSON] et al. [15] established a numerical model of a near-subsea UUV in order to obtain the effects of subsea boundary effects and complex terrain on the operational stability and maneuverability. The effects of subsea distance, \(Re\), angle of attack, and inclined terrain on the hydrodynamic performance of the UUV were analyzed. In summary, there is little experimental and CFD literature on UUV near the seabed. The studies focus on the hydrodynamic characteristics of UUV, while those of self-propelled UUV are still relatively few. Therefore, it is meaningful to investigate the hydrodynamic performance and self-propulsion performance of the UUV near the seabed through CFD method in this paper. Two models, bare UUV and self-propelled UUV near the seabed, were established and analyzed the effects of \(Hd\) and \(Re_{L}\) on the performance of vessel.