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Underwater Vehicles (UVs), including Autonomous and Remotely Operated Vehicles, are increasingly utilized in marine applications such as exploration, surveying, and defense. The hydrodynamic performance of UVs, particularly their resistance and lift characteristics near the free surface, plays a crucial role in their design and operational efficiency. This research employs Computational Fluid Dynamics (CFD) simulations to analyze the behavior of an underwater vehicle operating at various depths near the free surface. Two configurations of the DARPA SUBOFF model: the bare hull (AFF-1) and the fully appended configuration (AFF-8) were investigated. Simulations were conducted under different operating conditions, and numerical results were validated against experimental and existing numerical data to ensure accuracy and reliability. The interaction between the underwater vehicle and the free surface is analyzed to understand its effects on hydrodynamic performance. The findings demonstrate a significant impact of the free surface on resistance and lift, with the appendages in the AFF-8 configuration leading to more pronounced hydrodynamic effects, particularly at higher speeds where wave generation and interaction with the free surface are increased. These results highlight the effects of speed, depth, and vehicle configuration on hydrodynamic performance, providing valuable insights for the design and optimization of UVs. This study serves as a valuable foundation for further exploration of operational strategies and the development of UVs across diverse marine applications.

To investigate the hydrodynamic coefficients of the fully appended DARPA SUBOFF model under various motion modes, this study develops user-defined functions within a computational fluid dynamics (CFD) model to simulate sting-supported planar motion mechanism tests. A dimensionless yawing velocity (r′) is proposed to evaluate the influence of motion periods and amplitudes on the hydrodynamic coefficients. Taking advantage of high-resolution turbulence simulation, reduced model dependency and applicability to complex geometries, this study integrates large eddy simulation and the dynamic mesh method to simulate each motion mode. Discrepancies between simulation results and experimental data for some velocity-dependent force derivatives in oblique towing tests may be linked to the rotational centers of the planar motion mechanism (PMM) supports. Variations in PMM devices affect some acceleration-dependent moment derivatives in pure sway and heave tests. Finally, turning circle maneuvers were conducted to verify the effectiveness of the simulated hydrodynamic coefficients.

Submarines are required to have good performance, which is influenced by their type of hull, hull conditions, and operational conditions. This study compares the resistance between a Modified-U209 (U209) submarine and the DARPA Suboff. The former is an older hull geometry with both surface and submerged operation considered, whereas the latter represents a modern nuclear-powered submarine designed for submerged operations only. The two geometries were scaled to give the same usable volume, and all results were non-dimensionalized using this to ensure consistency. A Computational Fluid Dynamics (CFD) method was utilized to predict resistance by employing the Reynolds-averaged Navier–Stokes (RANS) equations. The results show that the total resistance coefficient for the U209 bare hull is approximately 6% higher than the Suboff bare hull. When a casing was added to the U209 geometry the increase in total resistance coefficient was approximately 8%. The addition of the sail resulted in an increase in total resistance coefficient ranging from approximately 4% (Suboff sail added to U209) to approximately 14% (U209 sail added to U209). An existing empirical prediction technique was used to predict the resistance, with the total resistance coefficient predicted being consistently about 5% lower than the values obtained using CFD.

The maneuverability of a submarine in the vertical plane is a key indicator of navigation safety. However, existing studies typically evaluate maneuvering performance based on hydrodynamic coefficients, often neglecting the flow-field evolution induced by different steering strategies. In this study, a high-fidelity numerical model for the vertical-plane motion of the DARPA SUBOFF submarine is established using the Reynolds-Averaged Navier–Stokes (RANS) method and validated against benchmark data. Unlike traditional analyses that employ a fixed rudder angle, this work systematically compares three steering strategies with continuously varying rudder angles—trapezoidal, step, and linear steering—examining their motion responses, hydrodynamic performance, and unsteady flow-field evolution. The results show that, although step steering produces the fastest response with the strongest transient characteristics, it also triggers pronounced flow separation and significant unsteady effects. Linear steering yields a smoother but the weakest motion response, with reduced rudder effectiveness and a noticeable lag effect. In contrast, trapezoidal steering maintains a stable flow field around the submarine, with uniformly concentrated vorticity distribution, ensuring smooth and safe motion and achieving a favorable balance between response speed and flow stability. The findings provide theoretical reference for research on submarine vertical-plane steering motion, rudder-angle control, and flow-field stability.

A thorough numerical introspection for assessing the particular issues of large flow separations around a submersible hull by using various turbulence models is described. The generic Defense Advanced Research Projects Agency (DARPA hereafter) Suboff hull is considered in the present study. Detailed descriptions of the mathematics behind the hybrid Shear Stress Transport (SST), Detached Eddy Simulation (DES) and the Improved Delayed Detached Eddy Simulation (IDDES) are given. The ISIS solver of the FineTM/Marine package is used to solve the flow problems. An adaptive mesh refinement is employed for resolving the flow inside the areas hosting significant flow gradients. Two sets of computations are analyzed: one refers to the straight-ahead course, whereas the other is focused on the static drift motions. Four angles of incident flow and three different incoming flow velocities are proposed for clarifying the details of the flow separation. Extensive grid convergence tests are performed for both working regimes and for all the meshes used in the present investigation. Extended verification and validation (V&V hereafter) of the numerical approach is performed through extensive comparisons with the experimental data. Global hydrodynamic performance of the hull as well as the local flow features are discussed in detail. The study is concluded by a series of final remarks aimed at providing useful information for further similar investigations.

Accurate simulation of the flow field around the hull is of great significance to predict hydrodynamic performances of a submarine. Judging by the resistance of the hull, researchers will be able to know whether the flow field was finely simulated. In this paper, LES (Large-Eddy Simulation) is used to predict the resistance and wake of a standard submarine model DARPA SUBOFF at a high Reynolds number of 2.65 × 10⁷. The results of the simulation were analyzed in detail and compared with the experimental data; they showed satisfactory agreement with the experimental data both quantitatively and qualitatively.

The safety of underwater operation depends on the accuracy of its speed logs which depends on the location of its probe and the calibration thoroughness. Thus, probes are placed in areas where the flow of water is smooth, continuous, without high velocity gradients, air bubbles, or vortical structures. In the present work, the flow around two different submarines is numerically described in deep-water and near-surface conditions to identify hull zones where probes could be installed. First, the numerical setup of a multiphase solver supplied with OpenFOAM v7 was verified and validated using the DARPA SUBOFF-5470 submarine at scaled model including the hull and sail configuration at H/D=5.4 and Fr=0.466. Later, the grid sensitivity of the resistance was assessed for the full-scale Type 209/1300 submarine at H/D=0.347 and Fr=0.194. Free-surface effect on resistance and flow characteristics was evaluated by comparing different operational conditions. Results shows that the bow and near free-surface regions should be avoided due to high flow velocity gradient, pressure fluctuations, and large turbulent vortical structures. Moreover, free-surface effect is stronger close to the bow nose. In conclusion, the probe could be installed in the acceleration region where the local flow velocity is 15% higher than the navigation speed at surface condition. A 4% correction factor should be applied to the probe readings to compensate free-surface effect.

Feng, D.; Wang, X.; Jiang, F., and Zhang, Z., 2015. Large eddy simulation of DARPA SUBOFF for Re = 2.65 × 107. Accurate simulation of the flow field around the hull is of great significance to predict hydrodynamic performances of a submarine. Judging by the resistance of the hull, researchers will be able to know whether the flow field was finely simulated. In this paper, LES (Large-Eddy Simulation) is used to predict the resistance and wake of a standard submarine model DARPA SUBOFF at a high Reynolds number of 2.65 × 107. The results of the simulation were analyzed in detail and compared with the experimental data; they showed satisfactory agreement with the experimental data both quantitatively and qualitatively.