# 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
#### 4.2.2 Effects of \(Hd\) and \(Re_{L}\) on UUV Self-Propelled Performance  
The relevant self-propulsion coefficients are derived from Equations (8)-(11). To analyze the effects of \(Hd\) and \(Re_{L}\) on the interactions between the UUV and the thruster, parameters such as thrust reduction, hull efficiency, effective wake fraction, and rotational efficiency are selected for analysis. The first two of these parameters are the effects of the thruster on the UUV, and the last two reflect the effect of the UUV on the thruster. * The Variation of Thrust Reduction
* \(t\) describes the effect of propeller at the wake of the hull on the UUV. Due to the rotation of the thruster at the tail of the UUV, velocity of flow near the tail increases and the static pressure here decreases relative to the static pressure of the bare UUV, resulting in an increase in the drag of the UUV. For the increased UUV drag, its convention is generally attributed to the reduced thrust of the propellor. As can be seen from Figure 14, \(t\) decreases firstly with the increasing of \(Hd\), then increases slowly and finally tends to stable. Near the seabed, the wall blocking effect of the seabed increases the pressure difference between the lead and tail of the UUV. Additionally, there is an abrupt change in the \(t\) curve for \(Hd\) equal to 2.5. This is because that although the trends of \(R_{0}\), \(R\) and \(T\) with \(Hd\) are consistent, the inconsistency of the rapidity of their changes leads to an abrupt change in \(t\) when \(Hd\) is 2.5. In addition, comparing the \(t\) values for different \(Hd\), it is found that the relative change of \(t\) with respect to that at \(Hd\) of 10 is less than 1%. It suggests that the effect of the seabed wall on \(t\) is relatively weak. It is difficult to capture the small changes by numerical methods. \(t\) increases with the increase of \(Re_{L}\). Figure 13: Self-propelled UUV Pitch moment characteristics. Figure 15 shows the pressure distribution of self-propelled UUV at cross-section (a) with \(Hd\) = 1.5 and oncoming flow velocities of 0.5, 1, 2, 4 m/s, where (a) is the pressure distribution of bare UUV and (b) is the pressure distribution of UUV with propeller. Comparing the two columns (a) and (b), it can be seen that the pressure at the tail end of the UUV decreases due to the suction effect of the propeller, which in turn causes an increase in the drag of the vehicle, corresponding to a reduction in the thrust of the UUV. Figure 14: Thrust reduction for self-propelled UUV. Figure 15: The pressure clouds at the bare UUV as shown columns (**a**) and self–propelled UUV as shown columns (**b**) at cross–section a. Figure 16 shows the pressure distribution of UUV at a cross section with incoming flow velocity of 0.5 m/s and _Hd_ of 1.5, 2, 2.5, 3, 5, 7, 9, 10, where (a) column is the pressure distribution of bare UUV and (b) column is the pressure distribution of UUV together with propeller. Comparing the two columns (a) and (b), it can be seen that the pressure at the trailing end of the UUV is reduced due to the suction of the propeller, which in turn causes an increase in the drag of the UUV. Figure 16: The pressure clouds at the bare UUV as shown columns (**a**) and self-propelled UUV as shown columns (**b**) at a cross-section. * Hull Efficiency \(\eta_{H}\) depicts the hull efficiency and represents the combined effect of thrust reduction and wake flow. As can be seen from Figure 17, this efficiency decreases with increasing \(Re_{L}\). As \(Hd\) increases, this efficiency gradually increases and remains stable when \(Hd\) is greater than 3. * Effective Wake Fraction of Self-Propelled UUV  
As shown in Figure 18, \(w\) increases slowly with the increasing of \(Hd\) and finally stabilizes, and \(w\) decreases with the increase of \(Re_{L}\). Figure 19 shows the distribution of the \(w\) at the UUV propeller disc with \(Hd\) of 1.5 and oncoming velocities from top to bottom of 0.5 m/s, 1 m/s, 2 m/s, and 4 m/s.