# Research on the Influence of Turbulent Flow Induced by Dunes on AUVs
## 3 Results and Discussion
Figure 9: Skin friction coefficient distribution on the AUV’s surface at different times; \(u=1.5\) m/s and \(H/D=1.5\): **(a)** 15 s; **(b)** 20 s; **(c)** 25 s; **(d)** 30 s; and **(e)** 35 s.  
When \(u=1.5\) m/s, the variation in the hydrodynamic coefficients of the AUV at different distances from the seafloor is shown in Figure 10. It can be observed that the coefficients are influenced by the underwater dune terrain, and the influence on the AUV diminishes as the distance between the AUV and the seafloor increases. When \(H/D>5.5\), the fluctuations in the AUV's hydrodynamic coefficients become negligible. However, concerning the friction resistance coefficient, significant fluctuations only occur when the AUV is positioned relatively close to the seafloor. This indicates that the instantaneous influence of turbulence variations on the AUV's friction resistance is minimal. To analyze the characteristics of hydrodynamic coefficient variations in turbulent flow fields, a Fourier analysis is performed on the time-domain variation curves of the AUV's hydrodynamic coefficients for different incoming flow velocities. This analysis yields the amplitude-frequency variation curves of the hydrodynamic coefficients. The peaks are selected from these curves, and within a 2% bandwidth on either side of the peak frequency in the amplitude-frequency curve, a bandpass filter is applied to the original curves. This process results in the time-domain variation curves of hydrodynamic coefficients corresponding to the peak frequency. Figure 10: Cont. Figure 10: Instantaneous variation in the hydrodynamic coefficients of the AUV at different distances from the seafloor; \(u=1.5\) m/s: (**a**) total resistance coefficient; (**b**) pressure resistance coefficient; (**c**) friction resistance coefficient; (**d**) lift coefficient; and (**e**) pitch moment coefficient. When \(H/D=1.5\), the Fourier analysis and filtering results of the time-domain hydrodynamic coefficient curves for different incoming flow velocities are shown in Appendix A, Figures A1-A9. It can be observed that higher amplitude is associated with high-frequency components when the flow velocity is relatively high. As shown in Figures 11 and 12, within the turbulent region on the leeward side of the dune, a continuous process of vortex evolution and fragmentation occurs. When the flow velocity is high, the flow field possesses increased energy, leading to more intense vortex motion and promoting the fragmentation of smaller vortices. The dominance of high-frequency components in the flow field, along with the increased amplitudes of high-frequency phenomena, indicates the accelerated development of small vortices, enhancing the randomness of the flow field and resulting in pronounced variations in the AUV's hydrodynamic coefficients. As the flow velocity decreases, the dominant components in the curves shift toward lower frequencies, accompanied by a reduction in the amplitude of high-frequency components. This shift implies a weakening in the phenomenon of vortex evolution and fragmentation, with large-scale vortices constituting the main components of the flow field while evolving at a slower pace. Furthermore, it is evident that with decreasing flow velocity, the dominance of low-frequency components is most pronounced in the resistance coefficient's time-domain curve. However, for the lift coefficient and pitch moment coefficient time-domain curves, the frequency components dominating the behavior at low flow velocities are higher than those in the resistance coefficient curve. This indicates that the AUV's lift and pitch moment present higher sensitivity to variations in the turbulent flow field, with the potential for significant changes still present at lower flow velocities. When \(u=1.5\) m/s, the Fourier analysis and filtering results of the time-domain hydrodynamic coefficient curves for different near-seafloor distances are presented in Appendix A, Figures A10-A21.