# Numerical Simulation of the Hydrodynamic Performance and Self-Propulsion of a UUV near the Seabed
## 4 Results and Discussion
### The Hydrodynamic Characteristics of Near-Seabed Bare UUIV
#### 4.1.2 Correlation between the Hydrodynamic Characteristics of Bare UUV and \(Re_{L}\)  
From Figures 6-8, some remarks can be concluded. The UUV drag coefficient decreases with the increase of \(Re_{L}\). The absolute of lift coefficient increases with the increase of \(Re_{L}\). When \(Hd\) is greater than 7, the relationship between lift coefficient and pitching moment coefficient and \(Re_{L}\) becomes weaker. From Figure 10, it can be found that the \(C_{p}\) near the nose of the UUV decreases with increasing \(Re_{L}\), while the absolute value of \(C_{p}\) near the tail increases with increasing \(Re_{L}\). As a result, the differential pressure drag of the UUV decreases with increasing \(Re_{L}\), while the frictional drag coefficient remains constant in this \(Re_{L}\) range. Finally, the drag coefficient of the UUV decreases with increasing \(Re_{L}\). The \(C_{p}\) values of UUV are less than 0 behind 3.5% \(L_{A}\), and the \(C_{p}\) values decrease with increasing \(Re_{L}\). This explains that the absolute values of UUV lift and pitching moment increase with increasing \(Re_{L}\). The variation rule of pitch moment coefficient with \(Re_{L}\) is not consistent with the lift coefficient because the increment of pitch moment with velocity is smaller than the second-squares increment of velocity. Hydrodynamic Characteristics of Self-Propelled UUV near the seabed and the Self-Propelled Performances  
.2.1 Hydrodynamic Characteristics of the Self-Propelled UUV in Relation to the \(Hd\) and \(Re_{L}\)  
The drag coefficient of the self-propelled UUV decreases continuously with the increase of \(Hd\) and finally tends to be stable, and the variation rate of the coefficient is the largest near the seabed. Figure 11 shows that the drag coefficient of self-propelled UUV near the seabed is greater than that away from seabed. When \(Hd\) is greater than 5, the drag coefficient tends to be stable and remains constant. The drag coefficient of the self-propelled UUV increased by 16.74-17.88% compared to that of the bare UUV. The absolute value of the lift coefficient of the self-propelled UUV decreases continuously with the increase of \(Hd\) and finally tends to be around 0.008, and the variation rate of the coefficient is the largest near the seabed. As can be seen from Figure 12, the absolute value of the lift coefficient of self-propelled UUV near the seabed is greater than that away from seabed. When \(Hd\) is greater than 7, the lift coefficient of UUV tends to be constant. It is consistent with the trend of the lift coefficient of bare UUV with \(Hd\). However, the lift coefficient of the self-propelled UUV increases by 69-130% compared to that of the bare UUV when the \(Hd\) less than 3. Similar to the bare UUV, the self-propelled UUV takes the buoyancy center of the UUV with a propeller as the origin of the pitching moment. The pitch moment coefficient of the self-propelled UUV decreases with the increase of \(Hd\) and finally tends to be around 0.005, and the variation rate of the coefficient is the largest near the seabed. As can be seen from Figure 13, the pitching moment coefficient near the seabed is larger than that away from the seabed, which causes the dive phenomenon near the seabed without angle of attack. The pitching moment curve of the self-propelled UUV varies with \(Hd\) and \(Re_{L}\) according to the same trend as that of the bare UUV. However, the pitching moment coefficient of the self-propelled UUV increased by 100-300% relative to that of the bare UUV when the \(Hd\) less than 3. Figure 11: Self-propelled UUV drag characteristics. Figure 12: Self\(-\)propelled UUV lift characteristics. The correlation of hydrodynamics coefficients of the self-propelled UUV and \(Re_{L}\) is consistent with that of the bare UUV. The drag coefficient decreases with the increase of \(Re_{L}\). The absolute of lift coefficient increases with the increase of \(Re_{L}\).