# Research on the Influence of Turbulent Flow Induced by Dunes on AUVs
## 3 Results and Discussion
Within the range of \(H/D=1.5\) to \(3.5\), the hydrodynamic coefficient time-domain curves present larger amplitudes of high-frequency components. The vorticity distribution contours at different near-seafloor distances in the final time step are shown in Figure 13. Due to the shear layer generated by the seafloor terrain, the flow field within this region is more turbulent. Moving away from this region, the intensity of vortex motion decreases, leading to a reduction in the frequency of flow field variations, and the dominance of low-frequency components in the hydrodynamic coefficient time-domain curves becomes evident. Furthermore, it is noteworthy that the dominant frequency component in the pitch moment coefficient time-domain curves is higher than the other two coefficients. This indicates that the AUV is prone to experiencing pitching motion in this turbulent flow field. As the AUV moves away from this region, the frequency and intensity of pitching motion will significantly decrease. However, during near-seafloor processes, the intense pitching motion could potentially pose collision risks. Along the length of the AUV, the turbulent kinetic energy \(k\) is calculated at \(0.05\) m from the upper and lower surfaces of the AUV in the longitudinal section of the AUV in the center plane. The distribution of turbulent kinetic energy near the AUV is shown in Figures 14 and 15, where \(l_{x}\) is the longitudinal distance between the AUV surface and the AUV bow. It is evident that as the distance from the seafloor decreases, the turbulent kinetic energy near the AUV increases, which indicates that the fluid possesses greater energy and is more prone to forming vortex structures. Interestingly, it can be observed that when \(H/D=2.5\), the turbulent kinetic energy beneath the AUV is higher than when \(H/D=1.5\). However, such a phenomenon is not observed in the variation in turbulent kinetic energy above the AUV. Analyzing Figure 13 reveals that this is because, at smaller near-seafloor distances, the presence of the AUV hampers the development of vortex structures between the AUV and the seafloor. To depict the vortex characteristics within the flow field more distinctly, the \(Q\)-criterion method is used for identification. Figure 16 displays the vortex structures corresponding to \(Q=10\) in the flow field. For \(H/D=1.5\), the presence of the AUV  
Figure 6:0 Vorticity distribution contours at different near-seafloor distances at the final time step: \(u=1.5\) m/s: (**a**) \(H/D=1.5\); (**b**) \(H/D=2.5\); (**c**) \(H/D=3.5\); (**d**) \(H/D=4.5\); and (**e**) \(H/D=5.5\). limits the development of vortex structures beneath it, reducing their quantity. However, the AUV remains within the vortex structures, leading to continually changing forces exerted on the AUV. Moreover, the presence of the AUV affects the evolution of vortex structures. Notably, it can be observed that at smaller near-seafloor distances, the turbulent kinetic energy around the lower surface of the AUV gradually diminishes along the AUV's longitudinal direction. This phenomenon arises because the bow of the AUV is located within a region with stronger vortex structures, which may result in vibration in practical applications. Below is an analysis of the average characteristics of the AUV hydrodynamic changes. The average values of the AUV hydrodynamic coefficients under different incoming flow velocities and near-seafloor distances are listed in Figure 6:1 Higher flow velocities correspond to a reduction in the resistance coefficient value. As near-seafloor distance varies, there is no distinct pattern in the variation in the hydrodynamic coefficients. Overall, the total resistance coefficient presents an increasing trend with the increase in near-seafloor distance. This increase in the total resistance coefficient is primarily driven by the rise in the friction resistance coefficient.