SUBOFF Model Research Articles

Influence of Full and Symmetrical Domains on the Numerical Flow around a SUBOFF Submarine Model using OpenFOAM

In this research, we consider the influence of two kinds of domain on the numerical flow around a submarine model. A fully appended SUBOFF submarine model was used, and the structure and characteristics of the flow were investigated under a full domain and a symmetrical domain arrangement. The numerical simulation was carried out using the OpenFOAM software, and the flow was numerically modelled as single-phase and incompressible. The SST k-ω turbulence model was used in both domains, together with an insensitive Spalding wall function to represent the boundary layer near the wall. The results showed that simulations in both the full and symmetrical domains could accurately predict the total resistance. Compared to the symmetrical domain, the resistance value obtained with the full domain was more precise; the symmetrical domain under coarse grid conditions had an error value of 1.34%, whereas the full domain using the same grid size had an error value of 0.6%. Hence, the full domain was superior in terms of predicting the resistance with a coarse grid. Next, the pressure coefficient comparison at the leading edge of the rudder was calculated, where = 0.92, and the symmetric domain was found to have a value of 0.0747 whereas the full domain had a value of 0.236. Compared with the results from experiment (=0.302), the symmetric domain appears to give an underestimate for the pressure distribution at this position. In addition, the flow structures and properties in both domains differ, particularly in terms of the vortical structures generated by the sail and rudders. The simulation results for the full domain reveal that the flow around the SUBOFF model is asymmetric. The full domain was able to capture the flow structures in more detail than the symmetrical domain, and represented the velocity distribution at the propeller plane better. As a result, the full domain must be considered when carrying out propeller analysis and self-propulsion simulations.

Numerical Simulation on Free Motion Response of a Submarine Induced by Internal Solitary Wave

Abstract：The internal solitary waves (ISWs) in the ocean carry huge energy and pose a serious threat to the safety of underwater vehicle. In order to obtain the dynamic response of the submarine under the action of ISWs, the amplified SUBOFF model was placed in a large numerical water tank with a length of 5000m, a width of 200m and a depth of 500m. The Korteweg-de Vries (KdV) theory was adopted to simulate the generation of ISWs in the two-layer flow, and overset grid technology was used to ensure the grid quality during the submarine movement. The results show that before the ISWs peak reaches the position of the submarine, the submarine will move, which causes the submarine to sink, move laterally and pitch. The longitudinal velocity is obviously greater than the lateral velocity, and the submarine finally hits the water tank bottom. With the increase of the ISWs amplitude, the time needed for submarine to reach the bottom increases, and the pitching angle increases greatly. The ISWs amplitude has no effect on the motion trajectory of the submarine's center of gravity, and has little effect on the lateral and longitudinal velocity. With the decrease of submergence depth of the submarine, the time required for submarine to reach the bottom will also increase, and the motion trajectory will change, but the trend of change is basically the same. The submergence depth has little influence on the variation range of lateral velocity, longitudinal velocity and pitching angle.

The Effect of the Second-order Difference-frequency Wave Force on the Motions of an Underwater Vehicle Near the Surface

_ When an underwater vehicle performs tasks, such as power recharge, air exchange, deployment, and retrieval near the surface, the low-frequency suction force and pitch moment will interfere with its motion. This paper presents a study on the effect of second-order difference-frequency wave force on the motions of the DARPA SUBOFF model at various submergence depths and environmental conditions. Ignoring the second-order wave force component induced by the second-order difference-frequency force may underestimate the second-order effect in irregular waves in the time domain. Comparisons were made between Newman’s approximation, which sets the off-diagonal difference-frequency quadratic transfer function (QTF) value to the average over the corresponding diagonal values, and the full QTFs based on the Pinkster approximation in the frequency and time domains. The QTFs-based prediction is significantly larger values than Newman’s approximation. Time-domain simulations were conducted, to investigate the effect of the second-order difference-frequency wave loads on motion responses at different nondimensional submergence depths and various sea conditions. The results of the computation indicate that ignoring the second-order difference-frequency wave force may underestimate the motion and dynamic responses of underwater vehicles; in addition, the pitch and heave motion responses are dominated by the low-frequency response. Introduction When an underwater vehicle travels close to the water surface, it experiences an upward force and pitch moment, known as surface suction. This can significantly affect the underwater vehicle’s behavior when power recharge, air exchange, deployment, and retrieval are incorporated into the underwater vehicle simulations. Veillon et al. (1996) noted that for a 10,000-ton submarine at a depth of 50 m, the surface suction due to waves will require around 20–30 tons of compensation to stop the submarine from surfacing. In addition, Hirom (1974) indicated that in waves, the fluctuating forces on a submarine can be on the order of 1000 kN, with a steady component of approximately 10 kN. In calm water, suction occurs owing to a higher flow velocity and a lower pressure above an underwater vehicle when sailing near the surface, and the wave pattern generated by the underwater vehicle complicates the surface effect. The longitudinal position of the center of pressure is after the amidships, which induces the pitch moment. In the presence of waves, the first-order wave force, which is the oscillatory force at the wave frequency, acts on the underwater vehicle. In addition, waves and model motions induce low-frequency suction force and pitch moment. Using any internal compensation and control surfaces to offset the induced suction force and pitch moment requires a good and early understanding of the magnitude of these forces, for designing sufficient-capacity compensation tanks (Crossland 2013). Therefore, the surface suction and pitch moment effects are essential components of the hydrodynamic characteristics of underwater vehicles sailing near the water surface.

Lines optimisation of an underwater vehicle using SMOTE and adaptive minimise LCB based dynamic surrogate models

ABSTRACT The lines optimisation of an underwater vehicle based on a dynamic surrogate model is studied. Four performances including rapidity, manoeuverability, energy consumption and structure of the underwater vehicle are considered in the optimisation framework constructed by a generalised collaborative optimisation method. Expert knowledge based analytic hierarchy process is conducted to obtain the optimisation object that involves the four performances. Numerical simulation is performed to accurately analyze the rapidity, manoeuverability and structure performances of the underwater vehicle. To reduce the calculation burden, dynamic surrogate models are proposed to replace numerical simulation in the optimisation framework. To guarantee the optimisation efficiency and accuracy, a synthetic minority oversampling technique (SMOTE) and adaptive minimise lower confidence bound (LCB) are combined in constructing the dynamic surrogate models. The proposed optimisation strategy is applied to the SUBOFF model and compared with other dynamic surrogate models. Comparison results prove the advantages of the proposed dynamic surrogate model.

Numerical Simulation of the Maneuvering Motion Wake of an Underwater Vehicle in Stratified Fluid

When a vehicle moves underwater, disturbance is generated and a wake remains that destroys the original free surface and produces a new wake. In order to study the mechanism and characteristics of the wave-making wake generated by the maneuvering motion of an underwater vehicle in density-stratified fluid, the k-ε model and the VOF method that is based on the RANS equation were used in this paper to analyze the SUBOFF model in stratified fluid at different drift angles. Numerical simulation of the maneuvering motion was carried out under these angles, and the corresponding changes in flow field caused by this motion were analyzed. The results from the comparison and analysis of the surface wave wakes under different drift angles in stratified fluid show that with the increasing drift angle, the motion wake of the vehicle still exhibits obvious Kelvin wave system characteristics. However, there are significant changes in hydrodynamic performance. The asymmetry of the surrounding flow field will increase with the increase in the drift angle. The pressure of the underwater vehicle is inversely proportional to the velocity of the surrounding flow field, and the amplitude of the peak and trough of the free surface wave is linearly related to the change in the drift angle. The numerical simulation can serve as a reference for the non-acoustic detection of the motion heading of an underwater vehicle and the motion trajectory of anti-reconnaissance underwater vehicles under actual sea conditions.

Reducing underwater radiated noise of a SUBOFF model propelled by a pump-jet without tip clearance: Numerical simulation

A pump-jet without tip clearance is proposed to propel a scaled SUBOFF model. The hydrodynamic performance of the pump-jet behind the SUBOFF, modal characteristics and vibro-acoustic responses of the SUBOFF – pump-jet under unsteady excitations are reported and compared with those by the pump-jet with a 2 mm tip clearance. The hydrodynamics are obtained by CFD (Computational Fluid Dynamics) and vibro-acoustic responses are predicted by coupled FEM (Finite Element Method) and direct BEM (Boundary Element Method). The CFD model of the pump-jet without tip clearance is validated by experimental results in a cavitation tunnel through steady hydrodynamics. Fluctuation pressure, standard deviation of fluctuation pressure and spectra of the new pump-jet behind SUBOFF are demonstrated. It is shown that fluctuation pressure on a single blade and fluctuation force of the rotor are suppressed remarkably, as well the amplitudes at BPF (blade passing frequency) and its multiples is greatly attenuated for the monitoring points on the rotor and duct. Since a shroud is added for rotor blades and stiffness is presented between the shroud and the duct due to hydrodynamic lubrication, a new mode of coupled rotor and duct deformation is borne and modal frequency at longitudinal mode of shaft is increased due to change of the cantilever supporing state of rotor blades. New force transmission paths for the rotor via the duct to excite the hull, and that for the duct via the shafting to excite the hull are presented, which will transmit part of vibration energy when applying excitation forces on the rotor or duct. The sound radiation power level is greatly suppressed after eliminating the tip clearance. Especially at the characteristic structural modes due to combined effect of alternating the structural modes to be far away from the characteristic frequencies of excitation forces and suppressing fluctuation pressures on the rotor and duct.

Numerical investigation of flow structures around the DARPA SUBOFF model

A large eddy simulation (LES) of a fully appended SUBOFF model was performed by the Boundary Data Immersion Method (BDIM) with the Reynolds number of 1.2 × 107. Numerical simulation is carried out through Fortran-based code and the geometric structure used in this paper is an idealized submarine (DARPA SUBOFF, with appendages). The computational uniformly distributed orthogonal grids consist of approximately 108 million nodes and are used to capture all essential flow structures. The pressure coefficients and streamwise velocity distribution obtained by the large-eddy simulation (LES) are in good agreement with the experimental data. The objective of this paper is to evaluate the ability of BDIM to predict the flow over axisymmetric hull, to investigate the evolution of the junction and tip flows on the body of the SUBOFF. Three different types of large-scale vortex structures were found around the sail, including horseshoe vortex, hairpin vortex and necklace vortex. The different vortex identification methods, namely, ω criterion, Q criterion and Liutex method, are used to capture and analyze the formation mechanism of various vortex structures. Meanwhile, by extracting the rotation and shear relative parameter ε around the sail, it can be found that the rotation is dominant in the vortex distribution profile.

Numerical Study on Multiple-Blade-Rate Unsteady Propeller Forces for Underwater Vehicles

_ The unsteady propeller forces of an underwater vehicle were numerically simulated using computational fluid dynamics to investigate the effects of the axial location of the stern planes. A benchmark study was undertaken using a three-bladed propeller; experimental results of the nominal inflow wake profile were analyzed and the unsteady propeller forces were measured. The numerical method was applied to predict the unsteady propeller forces in the SUBOFF model’s wake by varying the axial locations of the stern planes. Several remarks were made on the primary harmonics of the hull’s wakes and blade-rate propeller forces. Introduction The hydroacoustic noise, which matches multiples of the number of propeller blades and its rotational speed, known as “blade-rate (BR) noise,” has been increasingly used to manage hydroacoustics for naval vessels. BR noise can be caused by alternating blade loads owing to fluctuations in the angle of attack of the blades because marine propellers are operated in the nonuniform wake of ships’ hulls. The unsteady blade load produces unsteady propeller forces that are transmitted via the propeller shaft and bearing, thus producing undesirable vibration and noise. Although the resultant BR noise is a common issue for marine vessels, in particular, submarines and other underwater vehicles deployed for undersea defense systems and oceanographic survey systems require strict specifications for the acoustic signature. Therefore, the unsteady propeller forces must be improved for reduced detectability, because the vehicles should be able to operate without being discovered while sonar detection technology continues to improve.

On surface flow induced by internal waves generated by a slender body moving at low internal Froude number in a sharply stratified fluid

The studies on internal waves(IWs) generated by a moving body in a stratified fluid generally focused on wave amplitude phase characteristics in underwater space. The surface flow induced by internal waves(IWs) generated by an underwater moving body in stratified fluid is generally weak, and is rarely studied. In the present paper, the surface flow induced by body-generated IWs of a slender body(SUBOFF model) moving at Fri = 0.547～2.189 and high Reynolds number (Re = 2.0 × 106～8.0 × 106) in a sharply stratified fluid with finite depth is investigated numerically and experimentally. For the theoretical calculation, the Euler equations and the free surface boundary conditions are linearized, and the source-sink distribution method is used to calculate the velocity field of body-generated IWs of SUBOFF model using the potential flow theory combined with the associated eigenvalue problem on vertical velocity. Meanwhile Particle Image Velocimetry (PIV) is used to measure the surface flow induced by body-generated IWs of SUBOFF model under the same conditions (object size, fluid stratification and movement parameters) at a large stratified fluid tank (25m × 3m × 1.5m) and validate the theoretical calculation method. The wave patterns, the maximum peak-to-peak velocity value of surface flow and its variation with Fri are analyzed.

Numerical investigation of flows around an axisymmetric body of revolution by using Reynolds-stress model based hybrid Reynolds-averaged Navier–Stokes/large eddy simulation

Flows around an axisymmetric body of revolution at a zero yaw angle were studied using a hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) approach, which employed a full Reynolds stress model (RSM) in the RANS branch with the aim of accounting for the Reynolds stress anisotropy, streamline curvature, and flow separations in the boundary layer. The SUBOFF model without appendages was applied to conduct the simulations, and the Reynolds number based on the free-stream velocity and the length of the body is ReL=1.2×106. The results, including time-averaged Cp, Cf, and velocity statistics, were compared with the experimental data and wall-resolved LES results available in the literature, and the overall agreement of the comparisons was satisfactory. To assess the performance of the RSM-based hybrid RANS/LES approach, we carried out shear-stress transport-based hybrid RANS/LES approach simulations under identical free-stream conditions for comparison. The sensitivity of the hybrid RANS/LES approach to the RANS models was observed for separated flow with surface curvature and adverse pressure gradient-induced separation. The RSM-based hybrid RANS/LES approach was found to provide a better prediction for the unsteady flows near the stern. That is because the effects of the streamline curvature and the strong interactions between individual stresses can be captured by the exact production terms in the RSM-based hybrid RANS/LES approach. These effects are important for predicting the development of turbulent boundary layers along the stern.

A fine drag coefficient model for hull shape of underwater vehicles

The increasingly diverse mission requirements result in challenges to the rapid design and functional diversification of underwater vehicles (UVs). The genealogical design of UVs shape is one of the key steps to deal with these challenges. In the present work, based on the hydrodynamic characteristics with different shape features of UVs, the hull shape genealogy including the spherical/spheroidal shape, slender axisymmetric shape, slender flat shape and round-dish shape was established according to the genealogy classification theory. Influence of the hull shape parameters and Reynolds numbers on hydrodynamics of the spherical/spheroidal shape, slender axisymmetric shape and slender flat shape for wide application in hull shape genealogy was investigated by using computational fluid dynamics (CFD) simulation method. The mathematical models of the drag coefficients of the hull shape genealogy with different shape parameters and Reynolds numbers were established by fitting the data obtained from CFD calculation. The accuracy of the established drag coefficient models was verified by the tank tests of “Petrel-L″ model developed by Tianjin University and SUBOFF model. Furthermore, the established drag coefficient models show higher accuracy than the traditional drag coefficient models, namely G&J model, VT model and MIT model, by comparing with the towing tank tests of SUBOFF model. The present research provides an important reference for realizing the rapid and efficient design of UVs with the same hydrodynamic shapes.

Designing Unmanned Underwater Vehicles through Biomimicry

Research vessels or small craft equipped for the specific purposes lead geological, geophysical and oceanographic research both at the coastal and offshore areas for constructing and surveying the engineering projects such as offshore structures, undersea pipelines, harbours, oil and natural gas extraction plants from resources etc., which require extensive labourship. However, this research method applied entails very high costs and can also include risks for occupational safety and property due to the harsh weather conditions at sea. In addition, high precision measurements cannot always function from the sea surface during such research projects above-mentioned. Thus, unmanned underwater vehicles (UUVs) have been designed and produced intensively in the last two decades. The aim of this study was to generate form design of a proper unmanned underwater surveying vehicle to be able to conduct research on the geological and geophysical structure of the sea bottom as well as oceanographic opinions. Therefore, computational fluid dynamics (CFD) analysis of the DARPA Suboff submarine model was created, and the validation of the results obtained in these analyses was provided with those of the experiments of this DARPA Suboff model performed by Liu and Huang [1]. After successful conformity of the simulations carried out using the commercial software STAR CCM+ (Simulation of Turbulent flow in Arbitrary Regions - Computational Continuum Mechanics, C++ based), the UUV forms were created by means of different way including biomimicry methodology. In this context, the models of a mature goose-beaked whale (ziphius cavirostris), a mature sperm whale (physeter macrocephalus) and two torpedo-shaped UUV models as well a hybrid model including parts of a whale and torpedo-forms with the same displacement were analysed, and the results obtained were discussed.

Vibro-acoustic characteristics of a coupled pump-jet – Shafting system – SUBOFF model under distributed unsteady hydrodynamics by a pump-jet

This study presents the unsteady hydrodynamics of the excitations from a pump-jet running in the wake of a SUBOFF model and the vibro-acoustic characteristics of a coupled pump-jet – shafting system – SUBOFF model under unsteady excitations. The excitations of the pump-jet are obtained by CFD and the vibro-acoustic responses are predicted by coupled FEM/BEM. A mapping method based on radial basis function is established between CFD and structural mesh to apply the distributed pulsation forces on the wet surface of the pump-jet. The effect of the method used to apply the excitation forces on vibro-acoustic responses is investigated by applying distributed or equivalent pulsation forces at different areas of the pump-jet. Modal shapes, transmitted forces, sound radiation power, radiation directivity pattern and panel contributions are reported to illustrate the characteristics of the vibro-acoustic responses and the influence of the method used to apply the excitation forces on the responses. It is seen that the shaft frequency and the longitudinal mode resonant frequency of the shafting system are the typical characteristic frequencies with large magnitude in vibro-acoustic responses of the coupled system. Compared with applying concentrated forces applied at hub or 0.7 R, the responses produced by applying distributed forces on the whole pump-jet present modes and excitation characteristics with more reasonable magnitudes, illustrating the importance of applying distribute forces instead of concentrated forces. Besides, the contribution of the distributed forces on rotor is dominant since the responses of applying distributed forces on the pump-jet and applying distributed forces on rotor are similar.

Experimental study on the maneuvering derivatives of a half-scale SUBOFF model in the vertical plane

The maneuverability of a submersible in the vertical plane is investigated. For this purpose, extensive experiments are performed on a half-scale SUBOFF model in the circulating water channel (CWC) with the planar motion mechanism (PMM). Oblique towing tests containing high attack angles, pure heave tests and pure pitch tests are involved. The validity of the experiments is identified by the accordance of the present hydrodynamic coefficients and the ones of Defense Advanced Research Projects Agency (DARPA) . In order to fit the measured data for cases with high attack angles, this paper develops the expression of dimensionless pitch moment M′ by the piecewise function. Besides, the blockage effects are corrected by Tamura equation. In addition, the uncertainty analysis (UA) is conducted referring to the recommended procedure 7.5-02-06-04 by 27th ITTC (2014b). The results show that, the developed equation turns out to be superior to the standard one. And the total uncertainties of the hydrodynamics are less than 2%, which reveals the accuracy and credibility of the experimental measurement.

Hull optimization of an underwater vehicle based on dynamic surrogate model

A parallel multidisciplinary optimization design is proposed for the lines design of an underwater vehicle. Resistance and energy consumption are concerned about in obtaining the optimized lines. Collaborative optimization strategy is adopted to construct the optimization framework. The simulated annealing algorithm in combination of the modified method of feasible directions are taken as the optimization algorithms. CFD based simulation is conducted to analyze the resistance of the underwater vehicle. To improve the optimization efficiency, a surrogate model is incorporated into the optimization framework to approximate the CFD calculation. Unlike the conventional static surrogate models used in the design of underwater vehicles, a dynamic surrogate based on trust region and lower confidence bound is proposed. An adaptive balance parameter is taken in the lower confidence bound approach to guarantee the tradeoff between the exploitation and exploration ability of the dynamic surrogate model. Optimal Latin hypercube algorithm is employed as the method of design of experiments. Radial basis function is employed to construct the surrogate model. SUBOFF model is used to demonstrate the proposed optimization scheme. Lines of the underwater vehicle are determined and optimization results of the resistance and energy consumption show the effectiveness of optimization scheme.

Stability and Manoeuvrability Simulation of a Semi-Autonomous Submarine Free-Running Model SUBOFF with an Autopilot System

On the basis of a full-appendage DARPA SUBOFF model (DTRC model 5470), a scale (λ = 0.535) semi-autonomous submarine free-running model (SFRM) was designed for testing its manoeuvrability and stability in the constrained water. Prior to the experimental tests of the SFRM, a six-degree-of-freedom (6-DOF) manoeuvre model with an autopilot system was developed by using logic operations in MATLAB. The SFRM’s attitude and its trim polygon were presented by coping with the changes in mass and trimming moment. By adopting a series of manoeuvring tests in empty tanks, the performances of the SFRM were introduced in cases of three sailing speeds. In addition, the PD controller was established by considering the simulation results of these manoeuvring tests. The optimal control gains with respect to each manoeuvring test can be calculated by using the PID tuner in MATLAB. Two sets of control gains derived from the optimal characteristics parameters were compared in order to decide on the most appropriate PD controller with the line-of-sight (LOS) guidance algorithm for the SFRM in the autopilot simulation. Eventually, the simulated trajectories and course angles of the SFRM would be illustrated in the post-processor based on the Cinema 4D modelling.

The Investigation of a Sliding Mesh Model for Hydrodynamic Analysis of a SUBOFF Model in Turbulent Flow Fields

A computational fluid dynamics (CFD)-based simulation using a finite volume code for a full-appendage DARPA (Defense Advanced Research Projects Agency) SUBOFF model was investigated with a sliding mesh model in a multi-zone fluid domain. Unsteady Reynolds Averaged Navier–Stokes (URANS) equations were coupled with a Menter’s shear stress transport (SST) k-ω turbulence closure based on the Boussinesq approximation. In order to simulate unsteady motions and capture unsteady interactions, the sliding mesh model was employed to simulate flows in the fluid domain that contains multiple moving zones. The pressure-based solver, semi-implicit method for the pressure linked equations-consistent (SIMPLEC) algorithm was employed for incompressible flows based on the predictor-corrector approach in a segregated manner. After the grid independence test, the numerical simulation was validated by comparison with the published experimental data and other numerical results. In this study, the capability of the CFD simulation with the sliding mesh model was well demonstrated to conduct the straight-line towing tests by analyzing hydrodynamic characteristics, viz. resistance, vorticity, frictional coefficients, and pressure coefficients.

Resistance and flow field of a submarine in a density stratified fluid

The research on resistance and wake of an underwater submarine in density-stratified fluid is very necessary. To study the hydrodynamic performances of an underwater submarine navigating in density-stratified fluid, a CFD model is established based on RANS equations, combined with the Realizable k - ε turbulence and Euler multiphase flow models. Meanwhile, the VOF method is used for capturing the interface. At first, the established CFD model is validated by comparing with experimental results. Thereafter, the resistance and flow field of a full-scale SUBOFF model in density-stratified and uniform fluids with different forward speeds, submerged positions and fluid densities are investigated. The results show that the forward speed and submerged depth significantly affect the hydrodynamic performances of the submarine. However, the density of the lower layer and the distance of submarine above the internal surface both have little effect. The influence of the density stratification on the resistance increases as the forward speed becomes faster and the position becomes closer to the free surface. The pattern of the internal wave when submarine navigates under the internal interface is obviously different from that when submarine navigates above the internal interface. In general, the pattern of the internal wave changes more slowly than that of the free surface.

Study of the Dynamic Characteristics of A Cone-Shaped Recovery System on Submarines for Recovering Autonomous Underwater Vehicle

National navies equip their submarines with Autonomous Underwater Vehicle (AUV) technology. It has become an important component of submarine development in technologically-advanced countries. Employing advanced and reliable recovery systems directly improves the safety and operational efficiency of submarines equipped with AUVs. In this paper, based on aerial refueling technology, a cone-shaped recovery system with two different guiding covers (closed structure and frame structure) is applied to the submarine. By taking the Suboff model as the research object, STAR-CCM was used to study the influence of the installation position of the recovery system, and the length of the rigid rod, on the Suboff model. It was found that when the recovery system is installed in the middle and rear of the Suboff model at the same velocity and the same length of the rigid rod, the Suboff model has the good stability and less drag. It experiences the largest drag when being installed in the front of the rigid rod. Moreover, when the recovery system is installed in the front and middle of the rigid rod, the drag increases as its length increases, and the lift decreases as its length increases. Compared with the closed structure guiding cover, the Suboff model will have less drag and better stability when the recovery system uses the frame structure guiding cover. Besides, the deflection and vibration of the rigid rod were also analyzed via mathematical theory.

Improving the hydrodynamic performance of the SUBOFF bare hull model: a CFD approach

The main aims of this study are to investigate the hydrodynamic performance of an autonomous underwater vehicle (AUV), calculate its hydrodynamic coefficients, and consider the flow characteristics of underwater bodies. In addition, three important parts of the SUBOFF bare hull, namely the main body, nose, and tail, are modified and redesigned to improve its hydrodynamic performance. A three-dimensional (3D) simulation is carried out using the computational fluid dynamics (CFD) method. To simulate turbulence, the k–ω shear stress transport (SST) model is employed, due to its good prediction capability at reasonable computational cost. Considering the effects of the length-to-diameter ratio (LTDR) and the nose and tail shapes on the hydrodynamic coefficients, it is concluded that a hull shape with bullet nose and sharp tail with LTDR equal to 7.14 performs better than the SUBOFF model. The final proposed model shows lower drag by about 14.9% at u = 1.5 m·s−1. Moreover, it produces 8 times more lift than the SUBOFF model at u = 6.1 m·s−1. These effects are due to the attachment of the fluid flow at the tail area of the hull, which weakens the wake region.