# Research on the Influence of Turbulent Flow Induced by Dunes on AUVs
## 3 Results and Discussion  
This study focuses on the instantaneous variation in hydrodynamic coefficients of an AUV within the turbulent flow induced by underwater dune terrain. Considering computational efficiency and the full development of the flow field, the simulation time is set to the duration required for the incoming flow to pass twice the length of the computational domain. The chosen incoming flow velocities are 0.5 m/s, 1.0 m/s, and 1.5 m/s. Notably, due to the more pronounced flow variations at the velocity of 1.5 m/s, the simulation time is extended to the duration required for the incoming flow to pass four times the length of the computational domain at this velocity. The computational results reveal that, during the initial phase when the incoming flow passes one length of the computational domain at the set velocity, the variation in the AUV's hydrodynamic coefficients remains relatively stable. In other words, the flow field stays in a relatively stable state without reaching full development. Thus, this study analyzes the variation in the AUV's hydrodynamic coefficients during the subsequent time intervals beyond this stage. When \(H/D=1.5\), the instantaneous variation in the hydrodynamic coefficients of the AUV at different incoming flow velocities is shown in Figure 7. It can be observed that the hydrodynamic coefficients of the AUV are influenced by the turbulent flow field and fluctuate within a certain range. As the flow velocity increases, the fluctuations become more pronounced. Regarding the total resistance coefficient, which is divided into the shape resistance coefficient and friction resistance coefficient, it becomes apparent that the fluctuation in the shape resistance coefficient is more intense. Moreover, the variation trend in the shape resistance coefficient aligns completely with that of the total resistance coefficient. This suggests that the changes in the AUV's total resistance are primarily driven by variations in the shape resistance, while the friction resistance coefficient shows relatively minor fluctuations and remains nearly stable under different flow velocities. The shape resistance and friction resistance are determined by integrating pressure and shear forces over the AUV's surface. At a flow velocity of 1.5 m/s, the pressure distribution on  
Figure 6: Comparison between the numerical results and the experimental results: (**a**) 0.13 m; (**b**) 0.48 m; (**c**) 0.82 m; and (**d**) 1.27 m.  
the upper and lower sides of the AUV's longitudinal section in a center plane at different instances is shown in Figure 8. It is evident that due to the presence of the seafloor, the pressure variations in the lower surface of the AUV, especially in the bow and stern sections, are more pronounced. Figure 9 presents the distribution of the skin friction coefficient on the AUV's surface at different instances. Similarly, significant variations in shear forces can be observed in the bow and stern sections of the AUV, with minor fluctuations in the midsection. Overall, among the various hydrodynamic coefficients, the lift coefficient presents the most pronounced fluctuations. Figure 7: Instantaneous variation in the hydrodynamic coefficients of the AUV at different incoming flow velocities; \(H/D=1.5\): (**a**) total resistance coefficient; (**b**) pressure resistance coefficient; (**c**) friction resistance coefficient; (**d**) lift coefficient; and (**e**) pitch moment coefficient. Figure 8: Pressure distribution on the upper and lower sides of the AUV’s longitudinal section in the center plane at different times; \(u=1.5\) m/s and \(H/D=1.5\): **(a)** upper side and **(b)** lower side.