Unmanned underwater vehicles (UUV) face maneuverability and rapidity challenges when they are applied for detecting and repairing submarine oil and gas pipelines, and fiber cables near the seabed. This research establishes numerical models of the bare UUV and self-propelled UUV near the seabed using the computational fluid dynamics (CFD) method. The effect of dimensionless distance Hd and $R{e}_L$ on the hydrodynamic performance of the vehicle and the interaction between the hull and the propeller is investigated. The range of Hd is 1.5D–10D, and the $R{e}_L$ is 9.97 × 10⁵~7.98 × 10⁶. Findings indicate that: (1) There is an obvious strong coupling between the hydrodynamic performance of the bare UUV and Hd. With the increase of Hd, the hydrodynamic performance such as $C_d$, the absolute value of $C_l$ and $m_y$ decreases continuously and finally tends to be stable. The absolute values of $C_d$ and $C_l$ increase with the increase of $R{e}_L$. The change trend of $m_y$ is opposite to that of $C_l$. (2) The variation trend of hydrodynamic performance of the self-propelled UUV with Hd is consistent with those of the bare UUV. Additionally, it increases to some extent, respectively, compared with the bare UUV. (3) The self-propelled characteristics such as $t$, ${\eta }_H$, $w$ and ${\eta }_i$ are weakly related to Hd. The $t$ and ${\eta }_i$ increase with the increasing of $R{e}_L$, while ${\eta }_H$ and $w$ decrease with the increasing of $R{e}_L$.

This study investigates the effectiveness of UUV formations during navigation to designated target areas. The research focuses on propeller-equipped UUVs and employs a computational fluid dynamics (CFD) methodology to analyze the hydrodynamic interactions among multiple UUV formations while en route to their targeted exploration areas. Utilizing the relative drag coefficients ($r_l \mathrm{a}\mathrm{n}\mathrm{d} r_f$) and static thrust ($R_{fleets}$) as analytical parameters, this paper defines the relative distances ($a$ and $b$) between UUVs within a formation and conducts a comparative analysis of the hydrodynamic performance between individual UUVs and formation configurations. The study establishes correlations between relative distances and the hydrodynamic performance of formations. The findings reveal the following: 1. For both the lead UUV and the following UUV within the formation, the $r_l \mathrm{a}\mathrm{n}\mathrm{d} r_f$ heatmaps exhibit two distinct regions: a thrust region and a drag region. Notably, these regions significantly overlap. The maximum $r_l$ is 31.23%, while the minimum $r_f$ is −20.9%, corresponding to relative distances of $a$ = 0.12 and $b$ = 1.5. Conversely, the minimum $r_l$ is −12.2%, while the maximum $r_f$ is 22.03%, with relative distances of $a$ = 1.1 and $b$ = 0.2; 2. An analysis of formation static thrust $R_{fleets}$ reveals that it can be up to 7% greater than the drag experienced by self-propelled UUVs when relative distances $a$ and $b$ are set to 1.1 and 1, respectively. This highlights the enhanced performance achievable through formation navigation. The results presented in this paper offer valuable theoretical insights into the optimal design of relative distances within UUV formations, contributing to the advancement of UUV formation navigation strategies.

Recently, inspired by the flippers of humpback whales, researchers have been widely studying leading-edge tubercles for use as passive flow control devices. In this research, we numerically investigated the effects of leading-edge tubercles on a three-dimensional flapping foil coupled with rolling and pitching motions. Appropriate spanwise flexibility is considered to mimic the real flapping motion of humpback whales, and the profile of the angle of attack was analyzed in a representative section under the effects of spanwise flexibility. The motion of flexible foils was decomposed into rigid motion and flexible deflection by using the sliding mesh and dynamic mesh methods, respectively. Then, the hydrodynamic performance of the flexible flapping foils was estimated by solving the unsteady Reynolds Averaged Navier–Stokes equations. The effects of the shape and kinematic parameters on thrust, power consumption, and propulsive efficiency were studied and the mechanism behind these effects was investigated. A maximum efficiency loss of 19.4% was observed for the sharpest tubercle shape. Although the hydrodynamic advantages of leading-edge tubercles were not observed in the present study, the tendency of flow separation over peaking sections was suppressed under low angles of attacks. The results suggest that leading-edge tubercles are more suitable for foils with steady or quasi-steady motions, such as propellers or turbines.