# 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
#### 4.1.1 Correlations between Hydrodynamic Characteristics of Bare UUV and \(Hd\)  
The drag coefficient of the bare 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. As shown in Figure 6, the drag coefficient of UUV is greater near the seabed. When \(Hd\) is greater than 5, the drag coefficient tends to be stable and remains constant. This blocking effect between the UUV and the seabed causes an increase in the differential pressure in the X-direction on the UUV surface. Figure 9 shows that the \(C_{p}\) value is greater than 0 until 3.5% \(L_{A}\), and decreases with increasing \(Hd\). When \(Hd\) is greater than 5, the \(C_{p}\) is almost constant. The value \(C_{p}\) near the nose of the UUV decreases continuously with the increase of \(Hd\), that is, the pressure near the nose along the X-direction decreases with the increase of \(Hd\). The pressure component in the X-direction of the parallel section of the UUV is 0, so it is ignored. The \(C_{p}\) value near 83.45% \(L_{A}\) is less than 0, and the absolute value of \(C_{p}\) decreases continuously with the increase of \(Hd\). That is, the pressure value there increases continuously with the increase of \(Hd\) increases. The drags of the bare UUV are mainly contributed from the sum of the pressure values of the nose and tail. The variation of the tail pressure value is one order of magnitude smaller than that of the nose pressure. When \(Hd\) is greater than 5, both are of the same order of magnitude. When \(Hd\) is less than 5, the drag of the UUV is mainly determined by the component of the pressure at the nose of the UUV along the X-direction. As \(Hd\) increases, the drag of UUV decreases. When \(Hd\) is greater than 5, the drag of UUV is almost constant. * Lifting Characteristics  
Absolute value of lift coefficient of bare UUV decreases continuously with the increase of \(Hd\) and finally tends to be zero, and the variation rate of the coefficient is the greatest near the seabed. As shown in Figure 7, the absolute value of lift coefficient of UUV near seabed is larger than that away from seabed, which is shown as the ground diving phenomenon near seabed without angle of attack. This phenomenon benefits from the asymmetric distribution of pressure on the upper and lower surfaces of the UUV, as shown in Figure 15. A further reason is due to the blocking effect between the UUV and the seabed. This  
Figure 10: \(C_{p}\) distribution of bare UUV at cross section a for different \(Re_{L}\). leads to an increase in the fluid velocity between them and consequently a decrease in the static pressure value at that position, which eventually leads to a lift of the UUV less than 0. Figure 9 shows that the pressure at the nose and parallel segments varies with \(Hd\). When \(X/L_{A}\) is greater than 3.5%, the surface pressure coefficient of the UUV is less than 0. Additionally, the absolute value of the pressure coefficient decreases continuously with the increase of \(Hd\). The absolute value of the parallel segment \(C_{p}\) decreases continuously with the increase of \(Hd\). When \(Hd\) is greater than 7, the absolute value of parallel segment \(C_{p}\) is almost unaffected by the seabed. Therefore, the absolute value of UUV lift keeps decreasing with the increase of \(Hd\), and when \(Hd\) is greater than 7, the UUV lift is almost constant. * Pitch Moment Characteristics  
The center of buoyancy of the bare UUV is treated as the origin of the pitching moment. The pitching moment coefficient of the bare UUV decreases with the increase of \(Hd\) and finally tends to be zeros. Additionally, the variation rate of the coefficient is the greatest near the seabed. As shown in Figure 8, the pitching moment coefficient of UUV near seabed is greater than that away from seabed. There is a tendency of the UUV to dive in the near-seabed case. The lift coefficients on the UUV surface are unevenly distributed along the X-direction.