SUBOFF Model Research Articles

Influence of the stratification parameters on the wake field of an underwater vehicle in a two-layer stratified flow

Because the ocean exhibits different stratification states in different seasons and regions, the effect of different stratification parameters on a submarine wake must be studied. In particular, to investigate such effect in a two-layer stratified flow, this study established a computational fluid dynamics (CFD) model based on the URANS equations, shear-stress transport (SST) k-ω turbulence model, and Eulerian multiphase flow model. The volume of fluid (VOF) method was adopted to implement the free-surface capture method. The working conditions of the SUBOFF model under an infinite diving depth and near the free surface were compared with those of the experiments, and the convergence of the time step and grid number under stratified flow conditions was verified. These validation steps proved the feasibility of the CFD method. Finally, the effects of the stratification parameters on the free surface waves, internal waves, and turbulent kinetic energy behind the vehicle were observed by varying the stratification parameters to achieve different working conditions. The results show that when an underwater vehicle sails in a two-layer stratified flow, the interstratified density ratio and depth of the internal wave surface affect its wake field. When the underwater vehicle sails in the water layer, with an increase in the interstratified density ratio, the free-surface roughness increases and the turbulent kinetic energy decreases. When the underwater vehicle sails in the salt water layer, the result is just the opposite.

Numerical Investigation of Hydrodynamic Characteristics of a Rim-Driven Thruster Coupled with an Underwater Vehicle

In this paper, the hydrodynamic characteristics of a rim-driven thruster (RDT) behind the hull of an underwater vehicle are investigated. The studied underwater vehicle is the benchmark DARPA (Defense Advanced Research Projects Agency) suboff model, with and without full appendages. In order to verify and validate the numerical model, a grid sensitivity analysis is made for the AFF-1, AFF-8 and the ducted propeller cases, respectively. Then, the resistance and pressure distribution over the surface of the suboff with and without appendages are compared with available experimental measurements and good correlations were observed. As for the propeller, a well-studied ducted propeller, the 19A duct in combination with Ka-47 blades, is employed, and the numerical results exhibit a close relationship with the available experimental data under a wide range of advance coefficients. Afterwards, the self-propulsion characteristics of the suboff models propelled by RDTs using different duct configurations are studied, more specifically, the unsteady effects of the flow field induced by the interactions between propeller and hull under various working conditions. The results indicate that due to the influence of the hull, the RDTs operate in different working conditions compared to open water and exhibit distinct hydrodynamic characteristics. Moreover, the duct profile can have a significant effect on the unsteady pressure fluctuations in the flow field, especially in the vicinity of the propeller.

Numerical study of hydrodynamic drag effects of streamwise riblet structures on SUBOFF bare hull model

This study performs a full-scale numerical simulation on meter-scale underwater vehicles (UWVs) with micrometer-scale V-type streamwise riblet structures, focusing on hydrodynamic drag using the SUBOFF model. The periodic simulation along the azimuthal direction enables the full-scale analysis. Two main scenarios are discussed: (1) effect of riblet structure size on drag at a constant maneuvering speed and (2) effect of maneuvering speed on drag for representative riblet cases. In scenario (1), the average wall shear stress at the central region of the body exhibited significant reduction of approximately 50%. However, owing to larger effective area at the central surface of the body fixed with riblets, the friction drag may either decrease or increase depending on the riblet size. Consequently, R100 case showed maximum total drag reduction of ∼3%, whereas R500 case showed maximum increase in drag of ∼3%, where R100 and R500 indicate riblet-applied SUBOFF models with riblet heights of 100 and 500 μm, respectively. In scenario (2), friction drag in riblet-applied region exhibited smaller variations in R100 case, showing universal drag reduction effect at all speeds. In contrast, R500 case showed significant variations, with drag increments ranging from minimum of 1% to maximum of 12% at all speeds.

The comparative research on the VPMM experiment and numerical simulation of the SUBOFF model

To ensure optimal maneuvering performance of underwater vehicles, it is essential to obtain highly precise hydrodynamic coefficients for constructing the equations of motion. The principal approach to acquiring these coefficients is through the Planar Motion Mechanism (PMM) test. The subject of this paper is the SUBOFF full appendage model. To obtain hydrodynamic coefficients related to maneuvering motion, a Vertical Planar Motion Mechanism (VPMM) test is conducted in the circulating water tank. In addition, a numerical simulation of the typical pure heaving and pure pitching maneuverability test is performed for the model by programming its motion UDF using Fluent’s dynamic mesh technology. The test results and calculated hydrodynamic coefficients are compared to the data obtained from the Key Laboratory of Underwater Robotics Technology of Harbin Engineering University. The calculation results show high credibility of the test results of the circulating tank VPMM test. The corresponding numerical simulation has a certain practical engineering accuracy, which can break through the limitations of the test and also provides valuable experience and technical reference for the hydrodynamic prediction of other underwater vehicles.

Scale effects on the hydrodynamic performance of a submerged body

The increasing development and utilization of submerged bodies in ocean exploration have led to significant challenges due to their unique working environment. Conducting experiments with a full-scale submerged body is exceptionally difficult. To investigate scale effects on the hydrodynamic characteristics of submerged bodies, a three-dimensional numerical model for submerged bodies was established based on the Reynolds-averaged Navier–Stokes (RANS) equations and utilizing the Realizable k-ɛ turbulence model. The volume of fluid (VOF) method was used for interface capture. Initially, the reliability of the numerical model was verified through result validation and convergence analysis of the grid and time steps. Subsequently, SUBOFF model of different scales were adopted using the established numerical model considering the influence of the Froude number (Fr). The present study focused on investigating the effect of model scales on the wave pattern, resistance characteristics, and nominal wake fields. Research has shown that the scale effects on the frictional resistance and the wake field of the submerged body are significant. The numerical model used in the present study demonstrated good accuracy. A comprehensive analysis of the scale effects on submerged bodies was conducted, providing valuable insights for the application of model data to full-scale scenarios.

Numerical simulation of hydrodynamics of ocean-observation-used remotely operated vehicle

Remotely operated vehicle is the most widely used underwater robot and can work safely and steadily in complex environments compared to autonomous underwater vehicle and other types. It has obvious advantages in operation time and plays a significant function in marine engineering equipment. Hydrodynamic coefficients are the coefficients of ROV motion equation. In order to simulate the motion and predict the performance of a ROV, the hydrodynamic coefficients must be determined first. The motion mathematical model of remotely operated vehicles is also established, and the hydrodynamic dynamics of the vehicles are simulated using the finite volume method by combining overset mesh technology and governing equations. Finally, a simulation and verification of the standard model SUBOFF model and the calculation process’s dependability are also conducted. The primary hydrodynamic coefficients of the ROV were derived through the simulation data fitting process. The results showed that the ROV’s asymmetry results in an obvious disparity in pressure resistance between the forward and backward sailing, ascending and descending motions, and this disparity becomes significantly greater as the velocity increases. This method confirmed the accuracy of the hydrodynamic simulation computation of the remotely operated vehicle and served as a guide for the maneuverability and design of the vehicle.

Study on the wall-pressure fluctuation characteristics of a pump-jet propulsor duct in turbulent boundary layer

This study presents the distribution of excitation sources and pressure fluctuation characteristics on the duct wall of pump-jet propulsor. A modified submarine geometry based on the SUBOFF model is developed to provide a stern flow filed with turbulent characteristics for pump-jet. The Turbulent Boundary Layer (TBL) of outer wall is obtained by using Large Eddy Simulation (LES) numerical method to understand the velocity distributions. The characteristics of unsteady excitation force spectra are investigated respectively under different excitations excited by the internal and external flow field of duct. Besides, a spatial-temporal distribution of TBL pressure energy on duct wall excited by external flow field is emphatically revealed through the spectral analysis method. The results show that the force spectra excited by external flow field display an obvious broadband characteristic and discrete components appear at Blade Passing Frequency (BPF). While the force spectra excited by internal flow field present a remarkable discrete peak at BPF and its multiple. The BPF has a significant influence on the energy convergence range of excitation forces on outer wall of duct and a rapid decay trend starts to occur near BPF. The scale of vortices with most dominant energy contribution is determined by BPF and corresponding wavenumber, rather than the energetic wavenumber.

Measurements of steady manoeuvring forces and moments over an axisymmetric body with appendages in a wind tunnel

ABSTRACT Steady manoeuvring forces and moments over a generic submarine hull form (SUBOFF) at various angles of incidence were investigated through scaled model tests in the Wind Tunnel Facility at the Naval Science and Technological Laboratory, India. Measurements were performed for a range of angles of incidence in angle of attack (±18°) and drift (±18°) including control surface deflections (±15°) on a 1.372 m model at Reynolds number 1.7 × 106. The present study at angles of incidence is to gather steady-state force, and moment data and validate such load measurements performed in a Wind Tunnel. The particulars of the SUBOFF model, experimental facility and measurement technique, test results for load measurements and estimated hydrodynamic coefficients are highlighted in the paper. The measurement data generated from the above studies will complement the current research on hydrodynamic characteristics for submerged bodies both computationally and experimentally.

Assessment of Numerical Captive Model Tests for Underwater Vehicles: The DARPA SUB-OFF Test Case

During the early design stage of an underwater vehicle, the correct assessment of its manoeuvrability is a crucial task. Conducting experimental tests still has high costs, especially when dealing with small vehicles characterized by low available budget. In the current investigation, virtual towing tank tests are simulated using the open-source OpenFOAM library in order to assess the reliability of CFD methods for the prediction of hydrodynamic forces and moments. A well-known case study, the Defence Advanced Research Projects Agency (DARPA) SUB-OFF model, is used, and the outcomes are compared to the experimental results available in the literature. Five different configurations are investigated for pure drift tests, rudder tests and pure rotation in both vertical and horizontal plane. The results show an overall good agreement with the experimental data with a quite low demanding mesh arrangement of 3M cells, a favourable balance between accuracy and computational time. In more detail, the expected error in the most significant forces during manoeuvres is less than 2% for the fully appended configuration (the submarine real operative condition), whereas the accuracy is moderately reduced for the barehull configuration (a case not representative of a real hull) with an expected error of 15%. A possible reason for the differences observed could be attributed to the description of the two streamwise vortices generated when manoeuvring. Apart from the lateral force and yaw moment, the results of the longitudinal force are also presented, having a greater disparity when compared to the experimental data. Nevertheless, the longitudinal force has no important role for the purpose of making stability and control predictions. The study contributes to the validation and consolidation of CFD methods, offering insights into their accuracy and limitations for practical applications in underwater vehicles.

Hydrodynamic characteristics of an Asian sea bass-inspired underwater body

A novel underwater body geometry inspired from the morphology of Asian sea bass (Lates calcarifer) fish is developed, and its hydrodynamic coefficients for different combinations of angle of attack and drift angle are investigated using RANS-based CFD methods. The six degrees of freedom manoeuvring performance is evaluated by the calculation of surge, sway and heave forces and roll, pitch and yaw moments at Reynolds number 3.7 × 106. The numerical model is validated using experimental results for DARPA SUBOFF model. The sea bass sectional geometry has an inherent asymmetry about the horizontal plane, and this is observed in the force and moment coefficients. A detailed study for pressure coefficient and skin friction coefficient is presented, both along the length and circumferentially at specified locations for varying angles of attack and drift angles (-15° to 15°). Due to transverse pressure gradient, cross-flow structures are observed resulting in vortices from the mid-body that grow downstream, the strength depending on the angle of manoeuvre. It is observed that the sea bass-inspired body generates greater roll and pitch moments and heave force as compared to a symmetric tuna-inspired body for drift angles greater than 10°. The differences show the influence of asymmetry in underwater vessel geometry on its stability and performance during horizontal and vertical plane manoeuvres. The study of the hydrodynamic characteristics will aid the design of control surfaces and development of estimation of manoeuvring and propulsion performance for fish-inspired autonomous underwater vessels.

Intermittent behavior of a bypass transition of boundary layers over an axisymmetric body of revolution

A transitional boundary layer developing over the forebody of a SUBOFF model subject to high free-stream turbulence (FST) was experimentally measured using both laser induced fluorescence and time-resolved two-dimensional particle image velocimetry. Typical coherent structures in the bypass transition, including low-speed streaks, hairpin-like vortices and vortex packets, are clearly visualized. Continue wavelet transform analysis on the time-frequency characteristics of instantaneous Reynolds shear stress (RSS) indicates that the intermittent behavior is mainly confined in mild-to-high-frequency domain, whereas low-frequency component is attributed to the Klebanoff mode. A wavelet-based approach was proposed to detect multi-scale turbulent events from wavelet coefficients of RSS, and the spatial evolution of the multi-scale intermittency factor was then calculated. Furthermore, the conditional sampling of laminar and turbulence events provides a new perspective on the route of the disturbance growth in the bypass transition. It is clearly shown that FST-introduced disturbances with moderate-to-high frequency are unable to penetrate the near-wall region; meanwhile, multi-scale characteristics inside the boundary layer evolve spontaneously from the Klebanoff mode.

Direct simulation of flow field around SUBOFF in grid-generated turbulence with SWLBM

SWLBM is a Lattice Boltzmann Method (LBM) based software designed running on Sunway TaihuLight Supercomputer. In this paper, we introduce the simulation of flow over DARPA SUBOFF model in grid-generated turbulence with SWLBM. The simulation setting follows the Particle Image Velocimetry (PIV) measurement experiment done in the water tunnel of Beijing University of Aeronautics and Astronautics (BUAA) with a low speed of 0.25 m/s. The turbulent flow generated from a grid formed by 36 mm square mesh interlocking with 6 mm square bars was simulated directly, and the evolution of turbulence transition over the forebody surface of a scaled-down SUBOFF model with diameter of 200 mm was captured. The simulation results including velocity field and statistical variables were compared relatively well with experimental data. The simulation case has lattice size of 7.8billions and could finish 200,000 iterations within 24 h running on 2000 core groups of Sunway Taihulight supercomputer. This shows SWLBM is efficient tool for high resolution simulation for turbulent study.

An integrated dynamic modeling method for underwater vehicle with hull, propeller and rudder

An integrated motion simulation and dynamic modeling method is proposed, which takes into account the mutual influences of hull, propeller and rudder. Using Computational Fluid Dynamics (CFD) in a single simulation, all hydrodynamic coefficients, including those for the rudder and propeller, can be determined. Taking SUBOFF model as the object, the dynamics model was established based on the simplified equation of six degrees of freedom motion in space. The hydrodynamic coefficients were identified by using sliding time window and normal distribution methods. The results show that in comparison with the experimental hydrodynamic coefficients, the error of the predicted hydrodynamic coefficients is very small, and the force and moment curves fitted by dynamic equation are very close to those monitored by CFD simulation. This proves the reliability of the established dynamic model and the correctness of the parameter identification method.

Numerical Investigation of Self-Propulsion Performance and Noise Level of DARPA Suboff Model

Propulsion noise is an enduring problem of significant military and environmental importance. Hence, it is crucial to investigate propeller noise characteristics. In this study, the hydrodynamic performance and noise level of the DARPA (Defense Advanced Research Projects Agency) Suboff submarine with the E1619 propeller were analyzed. The hull resistance and propeller hydrodynamics were studied separately, and the numerical results were validated using available experimental values. The self-propulsion point was determined by matching the hull resistance and propeller thrust following ITTC (International Towing Tank Conference) convention. Based on hydrodynamics and acoustic Ffowcs Williams–Hawkings (FW–H) models, the underwater-radiated noise characteristics in the self-propulsion state were simulated. The calculations indicated that the contribution of the quadrupole term in the FW–H equation is not negligible in the high-frequency band. Compared with the noise of open-water propellers, the spectrum of the E1619 propeller in its self-propulsion state is more complex, and the upstream noise is amplified.

Similarity analysis of the flow-induced noise of a benchmark submarine

As one of the three major noise sources of submarines, flow-induced noise plays a key role for the stealth capability of submarines. Several research studies based on experiment or simulation have evaluated the sound radiation from the scale model; however, it is still a great challenge to efficiently evaluate the flow-induced noise of a large-scale prototype. In order to solve this problem, the flow-induced noise of different scale submarines is analyzed, and both the similarity law and the scale effect are discussed in the dimensionless frequency St = 10–1089. The fully appended DARPA SUBOFF, a famous benchmark submarine model, is used in our research. The relationship between the sound power, scale variables, and the speed and scale variables is obtained using the Buckingham Pi theorem. Then, the sound pressure level and the sound power level of the SUBOFF, with the scale ratio of 1:24 and 1:48 and the speed of 2, 4, and 8 m/s, are calculated based on the large-eddy simulation (LES)/Lighthill hybrid method. Finally, the scale effect between a hypothetical prototype (actually, a benchmark SUBOFF model with a scale ratio of 1:8) and its scale models are discussed at the same speed. The numerical results show that the submarine’s sound power level conforms to the similarity relationship of dipole source within the cut-off frequency St = 100. The error of the sound power level is about 20 lg (φ) caused by scale effect when the dimensionless frequency is greater than the cut-off frequency, where φ is the scale ratio from the hypothetical prototype to the model. The scale error of the sound pressure level at different position and different frequency exist differently when extrapolating from model results to prototype according to the similarity law based on dipole source.

Steady velocity measurements in the stern wake of submarine hull form at high angles of incidence

Velocity distribution in the stern region over a generic submarine hull form (SUBOFF) at high angles of incidence was investigated through steady velocity measurements in the Wind Tunnel Facility at Naval Science & Technological Laboratory, India. Velocity measurements were performed for a range of angles of incidence in angle of attack (+8° to - 40°) and drift (0°–28°) on 1.372 m model at Reynolds number 1.7 × 106. The present study for high angles of incidence is to depict a submarine motion during extreme manoeuvers in case of emergency situations. The particulars of the SUBOFF model, experimental facility and measurement technique, test results for axial, tangential and radial velocity measurements, non-uniformity and circumferential mean velocity, derived quantities for vorticity and circulation are highlighted in the paper. The velocity measurements data generated from the above studies will greatly complement the current research on turbulent flows over submerged bodies both computationally and experimentally.

Research on unsteady characteristics of different appendaged submarines flows based on dynamic mode decomposition

The unsteady flows around the submarines cause high-frequency fluctuating pressure and noise on the vehicle. Studying unsteady characteristics and establishing accurate prediction methods for Submarine unsteady flow is essential to understand flow separation-induced noise better. In this study, the SUBOFF model is taken as the research object for numerical analysis. In the speed range of 5.93kn∼16kn, the vortex distribution characteristics are analyzed. During the stable flow phase, the frequency and amplitude of vorticity at different modes are established based on the DMD method. The primary mode information is used to reconstruct the flow field of the submarine. The results show that at 5.93kn∼16kn speed, the unsteady characteristics of the bare hull model and sail appendages model are not prominent, and the stern appendages model and the fully appended model show regular vortex shedding and the frequency of little vortex shedding increases with increasing speed. The eigenfrequency modes reflect the unsteady characteristics of submarine winding. The mode analysis results of the fully appended model reflected the primary frequency vortex and frequency-doubled vortex are the main vortex forms of the whole appendages wake. The tail vortex of the sail merges with the stern wing vortex, making the flow field more stable.

Control mechanism of a vortex control baffle for the horseshoe vortex around the sail of a DARPA SUBOFF model

The horseshoe vortex generated in the attached part of a submarine could aggravate the vibration and noise of the submarine. The control mechanism of a vortex control baffle for the horseshoe vortex around the sail of a DARPA SUBOFF model is explored by the large eddy simulation (LES) model in this work. The variations of the distributions for streamwise vorticity, Q criterion and pressures are analyzed deeply. It is found that the arrangement of the vortex control baffle can interfere with the development process of horseshoe vortex. Due to the presence of the baffle, a strong reverse pressure gradient around the baffle would take place. Then a series of vortices are generated to weaken the intensity of the horseshoe vortex in the process of propagation and achieve the purpose of controlling the horseshoe vortex. The development characteristics of these vortices are discussed in detail. It is noted that the new induced vortex and the horseshoe vortex have opposite propagation direction, which would lead to the gradually decreasing intensity of the horseshoe vortex in the development process.

Numerical Simulation Analysis of Flow Noise Characteristics of Underwater Vehicle

Based on the standard large eddy simulation and k-epsilon model, the steady flow field and navigation flow noise of the underwater vehicle shell are simulated. In this paper, the underwater vehicle model is established by referring to classic SUBOFF model. The standard k-epsilon method is used to study the pressure distribution characteristic, velocity distribution characteristic, and velocity vector distribution characteristic on and around the shell of the underwater vehicle at different speeds. The results indicate that the pressure increases with the increase of the speed and in the bow of the shell is the largest. A large eddy simulation was used to analyze the noise characteristics of sailing flow at different speeds. The results show that the sound pressure level of navigation flow noise is higher in the low-frequency band, and will increase with the increase in speed.

To study the effectiveness of stern appendages (Cruciform & X Shaped configurations) for maneuverability of autonomous underwater vessel using computational fluid dynamics

Maneuverability is one of the main aspects to be considered during the early design stage of Autonomous Underwater Vehicles. Several captive tests such as Planar Motion Mechanism (PMM), Oblique towing test, and Rotating Arm tests were conducted to obtain the hydrodynamic derivatives of the Autonomous Underwater Vehicle (AUV). In this research, the effectiveness of control surfaces with Cruciform and X configurations has been predicted for the maneuverability of the DARPA SUBOFF model. Computational Fluid Dynamics (CFD) technique was applied and the k- ω turbulence model was used to estimate the flow characteristics in the boundary layer region. The Straight-Line Towing Tank test has been used to calculate the hydrodynamic derivatives due to linear velocity, while the Rotating Arm test with a moving reference frame has been used to estimate the hydrodynamic derivatives. All the tests were conducted at a speed of 6.5 knots and three angles of incidence (10,30,50) for Straight-Line motion and three radii (13m,18m,24m) for Rotating Arm test were adopted. The simulation results of hydrodynamic coefficients have been compared with experimental results for the Cruciform configuration and the error was below 15%. So, the same procedure was adopted for the X-form configuration, and the stability index was calculated for both configurations.