Underwater Propulsion System Research Articles

Review of diver propulsion vehicle: A review

The ocean serves as a vital arena for resource exploitation, scientific inquiry, and strategic endeavors. Among the array of underwater propulsion technologies, diver propulsion vehicles (DPVs) stand out for their exceptional integration, concealability, and maneuverability. They hold a pivotal role in both marine scientific exploration and military operations beneath the waves, thus carrying significant research implications. However, the existing literature on DPVs remains limited, lacking comprehensive examinations of their design processes and parameters. This review systematically surveys and assesses the current landscape of DPV development and research. Three key facets—propeller design, performance assessment, and equipment engineering—are scrutinized and analyzed. By consolidating essential data from ongoing studies, this review offers valuable insights. Additionally, it forecasts potential directions and emerging focal points in the evolution of underwater propulsion for frogmen, drawing from current advancements. The objective is to furnish foundational data to support the design and study of frogman underwater propulsion systems, thereby advancing engineering applications in this domain.

Investigating the Influence of Counterflow Regions on the Hydrodynamic Performance of Biomimetic Robotic Fish.

Biomimetic robotic fish are a novel approach to studying quiet, highly agile, and efficient underwater propulsion systems, attracting significant interest from experts in robotics and engineering. These versatile robots showcase their ability to operate effectively in various water conditions. Nevertheless, the comprehension of the swimming mechanics and the evolution of the flow field of flexible robots in counterflow regions is still unknown. This paper presents a framework for the self-propulsion of robotic fish that imitates biological characteristics. The method utilizes computational fluid dynamics to analyze the hydrodynamic efficiency of the organisms at different frequencies of tail movement, under both still and opposing flow circumstances. Moreover, this study clarifies the mechanisms that explain how changes in the aquatic environment affect the speed and efficiency of propulsion. It also examines the most effective swimming tactics for places with counterflow. The results suggest that the propulsion effectiveness of robotic fish in counterflow locations does not consistently correspond to various tail-beat frequencies. By utilizing vorticity maps, a comparative analysis can identify situations when counterflow zones improve the efficiency of propulsion.

Integrated Simulation Research of Multi-natural Energy-driven Unmanned Surface Vehicle

In response to the limitations of existing unmanned surface vehicles in complex marine environments, where they often fail to meet the requirements for extended endurance, broad operational range, offshore missions, and prolonged operational durations, an autonomous Unmanned Surface Vehicle (NUSV) has been designed and developed. The NUSV is propelled by wind, solar, and wave energy sources. Concerning wind energy, a micro wind energy storage mechanism based on a spiral spring has been designed to enable power generation in low-wind conditions. A wave energy transmission device, complete with a dynamic model, has been devised for wave energy. This device converts torque generated from wave action in different directions into a unified torque, providing continuous power to drive the generator. Solar energy is harnessed using Maximum Power Point Tracking (MPPT) algorithms to ensure optimal operation of the solar panels. The electrical energy generated from these three natural sources is stored using Pulse Width Modulation (PWM) and DC-DC Boost converters. A programmable logic controller manages and allocates this stored electrical energy for the NUSV’s equipment, including underwater propulsion systems, surface cameras, laser radar, GPS, and other electronic devices. This integration of wind, solar, and wave energy sources enables the NUSV to meet power demands, allowing for long-duration operations at sea, extended offshore missions, wide operational ranges, and prolonged mission durations.

A miniature swimmer actuated by a PZT ring ultrasonic underwater propulsion system

This study investigates a scheme utilizing a ring transducer for an acoustic underwater propulsion system. Acoustic underwater propulsion systems are well suited for the inspection and repair of underwater robots due to their small size, high power density, and simple structure. Previous research has focused on self-propelled swimmers utilizing disc transducers. However, the radial vibration component of disc transducers makes it difficult to provide propulsion for an acoustic underwater propulsion system driven by an acoustic driving force. Pure longitudinal vibration requires a greater thickness to achieve the same vibration area, resulting in higher impedance and reduced driving efficiency. In this paper, simulation, and measurements of vibration distribution demonstrate that a ring transducer exhibits a vibration distribution closely resembling pure longitudinal vibration. A prototype swimmer using a ring transducer was fabricated for experimental evaluation through measurements of admittance characteristics, zero-speed propulsion, and no-load speed in water.

An Experimental Study of the Effects of Asymmetric Pitching Motion on the Hydrodynamic Propulsion of a Flapping Fin

Aquatic organisms have evolved exceptional propulsion and even transoceanic migrating capabilities, surpassing artificial vessels significantly in maneuverability and efficiency. Understanding the hydrodynamic mechanisms of aquatic organisms is crucial for developing advanced biomimetic underwater propulsion vehicles. Underwater tetrapods such as sea turtles use fins or flippers for propulsion, which exhibit three rotational degrees of freedom, including flapping, sweeping, and pitching motions. Unlike previous studies that often simplify motion kinematics, this study employs a specially designed experimental device to mimic sea turtle fins’ motion and explore the impact of pitching amplitude, asymmetric pitching kinematics, and pausing time on lift and thrust generation. Force transducers and particle image velocimetry techniques are used to examine the hydrodynamic forces and flow field, respectively. It is found that boosting the fin’s pitching amplitude enhances both its lift and thrust efficiency to a certain extent, with a more pronounced effect on thrust performance. Surprisingly, the asymmetrical nature of the pitching angle’s pausing time within one flapping cycle significantly influences the lift and thrust characteristics during sea turtle swimming; extending the pausing time during the forward and upward flapping process improves lift efficiency; and prolonging the pausing time during the downward flapping process enhances thrust efficiency. Furthermore, the mechanism for high lift and thrust efficiency is revealed by examining the vortices shed from the fin during different motion kinematics. This research contributes to a more comprehensive understanding of the fin’s hydrodynamic characteristic, providing insights that can guide the design of more efficient biomimetic underwater propulsion systems.

A Novel Counter-Rotating AFPM Machine Based on Magnetic-Field Modulation for Underwater Propulsion System

The underwater propulsion system (UPS) with two counter-rotating propellers is very popular now. In tradition, it needs two electric machines or additional gears, which greatly increases the volume and weight of the system. Therefore, a novel counter-rotating axial-flux permanent magnet (AFPM) machine based on magnetic-field modulation (MFM) for the UPS is proposed. It contains a more pole (MP) rotor and a less pole (LP) rotor on two sides. The MFM is achieved through the split teeth on the stator. Based on the MFM, the two rotors can rotate in opposite directions. The principle of MFM and the relationships of rotary speeds between two rotors are analyzed. Furthermore, the effects of split tooth shape and dimensions on the performances of the proposed machine are also evaluated. The triangular skew fan-shaped split teeth are adopted in the counter-rotating AFPM machine. Finally, the prototype is manufactured and an experiment is conducted to validate the performance. The results show the consistency of analysis and experiment.

Enhancing efficiency and propulsion in bio-mimetic robotic fish through end-to-end deep reinforcement learning

Aquatic organisms are known for their ability to generate efficient propulsion with low energy expenditure. While existing research has sought to leverage bio-inspired structures to reduce energy costs in underwater robotics, the crucial role of control policies in enhancing efficiency has often been overlooked. In this study, we optimize the motion of a bio-mimetic robotic fish using deep reinforcement learning (DRL) to maximize propulsion efficiency and minimize energy consumption. Our novel DRL approach incorporates extended pressure perception, a transformer model processing sequences of observations, and a policy transfer scheme. Notably, significantly improved training stability and speed within our approach allow for end-to-end training of the robotic fish. This enables agiler responses to hydrodynamic environments and possesses greater optimization potential compared to pre-defined motion pattern controls. Our experiments are conducted on a serially connected rigid robotic fish in a free stream with a Reynolds number of 6000 using computational fluid dynamics simulations. The DRL-trained policies yield impressive results, demonstrating both high efficiency and propulsion. The policies also showcase the agent's embodiment, skillfully utilizing its body structure and engaging with surrounding fluid dynamics, as revealed through flow analysis. This study provides valuable insights into the bio-mimetic underwater robots optimization through DRL training, capitalizing on their structural advantages, and ultimately contributing to more efficient underwater propulsion systems.

Biomimetic Photothermal Actuator Exhibits Robust Motion and Omnidirectional Phototropism

Organisms in the natural world receive signals from the external environment and exhibit continuous propulsion. However, creating artificial systems that can rapidly track energy sources in real-time without human intervention and electronic circuit programming poses a significant challenge. Here, we have developed a soft actuator that mimics the phototropism of plants and possesses autonomous sensing and driving capabilities. The soft actuator is based on a thermo responsive PNIPAAM hydrogel, and its uniform incorporation of light-absorbing agents imparts photothermal response characteristics. Our preparation method involves a combination of ice templating and UV cryo-polymerization techniques, utilizing water as the primary solvent. This process results in the formation of substantial microscopic pores within the PNIPAAM hydrogel, greatly enhancing the rate at which water enters and exits the polymer network. The rapid response and recovery properties of the hydrogel soft actuator allow it to quickly track and align with a stimulus light source. Moreover, it can bend to 90° and undergo continuous oscillations, representing a substantial enhancement in performance compared to conventional composite PNIPAAM hydrogels. Furthermore, the soft actuator exhibits a low dependence on the type of incident light, as it can respond not only to pinpoint laser beams but also to broad-spectrum white light. This intelligent soft actuator holds promising applications in non-contact soft robotics, underwater propulsion systems, and various other fields.

Dual Network Co-Crosslinked HNBR Composites with Enhanced Tribological Properties under Water Lubrication

Water-lubricated bearings play a critical role in underwater propulsion systems but are often prone to failure due to mechanical wear and vibration, especially under high loads and prolonged friction. In response to this issue, our study introduces a novel approach: a dual network co-crosslinking strategy utilizing hydrogenated nitrile butadiene rubber (HNBR). This strategy connects the rubber network with the epoxy network through epoxidized Eucommia ulmoides gum. A comprehensive analysis was conducted to assess the resulting composite’s damping, tribological, and mechanical properties. The results show that the material has excellent mechanical, damping, and tribological properties relative to pure HNBR, with a 65.9% increase in the damping temperature domain, a 78.5% increase in tensile strength, a low coefficient of friction of 0.022, and a high resistance to abrasion of 3.87 × 10−6 mm3/Nm. The successful synthesis of HNBR-based composites via the dual network co-crosslinking strategy underscores their potential as a practical solution for improving the reliability and prolonging the service life of water-lubricated bearings.

A novel miniature swimmer propelled by 36° Y-cut lithium niobate acoustic propulsion system

Self-propelled swimmers using ultrasonic transducers have been proposed and studied. The screw and bionic propulsion systems have dominated underwater robots. However, the complex structures make miniaturization difficult. The swimmers with ultrasonic transducers have no moving parts such as fins or propellers and can be driven by the nanomechanical-vibration in the surface of the transducer. Simple structure and high driving frequency make it easy to miniaturize. A 36° Y-cut lithium niobate (LN) thickness-vibration-mode transducer is used to investigate an underwater acoustic propulsion system. LN transducers are manufactured to 10 × 10 mm, 7 × 7 mm, and 4 × 4 mm square plates. The zero-speed propulsion and no-load speed are investigated to evaluate the swimmer performance. A mm-order swimmer is studied with a 4 × 4 mm LN transducer. At last, the multi-degrees-of-freedom swimmer driven by three 7 × 7 mm LN transducers is demonstrated with the advance and sway locomotion. In the future, a small-scale and agile self-propelled swimmer with an acoustic propulsion system can be expected for narrow and complex environment tasks, such as seagrass-prone areas and narrow pipelines.

Acoustic underwater propulsion system based on ultrasonic disc PZT transducer

An autonomous swimmer with the acoustic propulsion system is proposed and evaluated in novel area of acoustofluidic actuation. The ultrasonic transducer has the same or higher propulsion power with the nanometer/micrometer vibration amplitude and the MHz/kHz frequency, compared with the locomotion of jellyfish body. The sound pressure and flow velocity measurements demonstrate the higher-pressure gradient and fluid downstream to investigate the acoustic propulsion force generated by pushing the surrounding water rearward. The performance of acoustic propulsion system is thus evaluated based on the sound pressure and flow velocity measurements equivalently. The vibration distributions in the transducer surface exhibit a ripple effect and gradually decayed from the center to the edge. The radial distributions of the sound pressure and flow velocity in water match the surface vibration amplitudes for thickness-vibration-mode. Based on the investigation of two thickness-vibration-mode PZT disc transducers, the longitudinal sound pressure and flow velocity in the normal direction result in ZSP force. The power radiated by the longitudinal vibration on the surface of the transducer is shown to be the main cause of the propulsion power. The propulsion calculation with vibration velocity is studied to evaluate the propulsion force. For an input power, the acoustic propulsion system with a higher Q-factor transducer has better performance. Autonomous underwater acoustic propulsion systems have a small size, high power density, and a simple structure, making them suitable for pipeline robots, endovascular microrobots, and underwater drones.

Soft Electromagnetic Motor and Soft Magnetic Sensors for Synchronous Rotary Motion.

To create fully-soft robots, fully-soft actuators are needed. Currently, soft rotary actuator topologies described in the literature exhibit low rotational speeds, which limit their applicability. In this work, we describe a novel, fully-soft synchronous rotary electromagnetic actuator and soft magnetic contact switch sensor concept. In this study, the actuator is constructed using gallium indium liquid metal conductors, compliant permanent magnetic composites, carbon black powders, and flexible polymers. The actuator also operates using low voltages (<20 V, ≤10 A), has a bandwidth of 10 Hz, a stall torque of 2.5-3 mN·m, and no-load speed of up to 4000 rpm. These values show that the actuator rotates at over two orders-of-magnitude higher speed with at least one order-of-magnitude higher output power than previously developed soft rotary actuators. This unique soft rotary motor is operated in a manner similar to traditional hard motors, but is also able to stretch and deform to enable new soft robot functions. To demonstrate fully-soft actuator application concepts, the motor is incorporated into a fully-soft air blower, fully-soft underwater propulsion system, fully-soft water pump, and squeeze-based sensor for a fully-soft fan. Hybrid hard and soft applications were also tested, including a geared robotic car, pneumatic actuator, and hydraulic pump. Overall, this work demonstrates how the fully-soft rotary electromagnetic actuator can bridge the gap between the capabilities of traditional hard motors and novel soft actuator concepts.

Ignition and combustion of Al and ammonium perchlorate-coated Al particles in a double oxidizing air–H2O–Ar atmosphere

Aluminum is a promising energy-containing material that can satisfy both aerospace and underwater thermodynamic propulsion systems. A high-temperature pressurized combustion chamber and a CO2 laser were adopted to investigate the ignition and combustion characteristics of 2, 5, and 40 μm aluminum particles in a 1.0 MPa air–H2O–Ar mixed atmosphere (with varying O2 and H2O concentrations). The combustion-promoting effect of the ammonium perchlorate-coated modification (namely, the AP@Al sample) was evaluated. The results show that Al melts and agglomerates into millimeter-sized droplets (covered with a gas-phase flame) after ignition, and its nonhomogeneous combustion is dominated by diffusion. The flame propagation is greatly enhanced by the AP combustion-promoting effect of oxygen/heat supply and particle dispersion, resulting in a significant decrease in the ignition delay time (ti, approximately 55–85%) and combustion time (tc, approximately 32–96%), whereas an extraordinary increase in the maximum combustion temperature (Tmax, approximately 170–224 K) and pressure changes (approximately 1.7–8.5 kPa, approximately 10.6–136.5 kPa/s) are also observed. Excess water vapor inhibits the flame propagation for AP@Al, viz. the flame area and number of ejected burning particles is decreased. The main factor affecting the combustion temperature is the oxidant rather than the particle size, which may be related to the small difference in the active aluminum content of the samples and the diffusion-controlled combustion mechanism. Due to the advantage of O2 for oxidation and the diffusion rate, the gradual replacement of H2O by air (similar to conversion to a fuel-lean system) leads to a decrease in ti, whereas Tmax is increased for all the samples, and the tc and self-sustaining combustion time are increased for the Al samples, whereas tc is decreased for AP@Al samples for both the equal-molar oxidizing gas concentration and equivalence ratio of O2–H2O control groups in this work. Since O2 creates a new pathway for AlO production, the ignition and combustion performance of Al and AP@Al samples is significantly improved when H2O is replaced by a small amount of air.

Investigation on heat transfer and flow of a sonic steam jet condensation in crossflow of water in pipes

Direct contact condensation is widely used in nuclear reactor thermal-hydraulic systems, underwater propulsion systems, liquid propellant rocket engines and other industrial fields due to its highly efficient heat and mass transfer. Understanding the distribution of the thermal-hydraulic parameters of steam jet condensation systems has become an urgent need for optimal design and safe operation of industrial processes. In this paper, the flow characteristics and heat transfer for the condensation of sonic steam jet in crossflow in a subcooled water pipe are numerically simulated based on experimental results. A two-resistance model under the framework of two-fluid model is employed to consider the mass and heat transfer across the steam-water interface. The steam plume transforms from contraction-cone into expansion-cone when steam mass flux rises from 500 to 900 kg/m2s, and the plume length gradually increases approximately linearly from 2.43de to 4.17de. When the steam mass flux is 900 kg/m2s, the expanding channel of the steam plume allows the jet to accelerate from sonic to supersonic with Ma = 1.43. At the transition of expansion to contraction, the steam dynamic pressure reaches a maximum and the steam total pressure decreases to a minimum. Because the water dynamic pressure is significantly higher than the steam dynamic pressure, the total pressure is maximum at the end of the two-phase region. Mass transfer is much higher in two-phase region than in other regions. The heat transfer coefficient calculated from the interface area decreases from 1.98 to 1.56 MW/m2K, and that calculated from the two-phase region volume increases from 3.37 × 103 to 4.05 × 103 MW/m2K.

Ignition and combustion experiments on Mg/AP composite fuels in different reaction environments

Magnesium is considered a viable fuel for underwater propulsion owing to its ignitability and the exothermicity of the Mg-water reaction. In this experimental study, we examine the ignition and combustion of Mg/AP composite fuels in three different reaction environments: air, water vapor, and argon. We consider Mg/AP composite fuels containing different Mg loadings and particle sizes. Using thermogravimetry and time-resolved optical imaging, we analyze the thermal oxidation, ignition delay time, steady-state gas-phase combustion processes, and flame temperature of the composite fuels. Our findings show that a water vapor environment can lower the initial oxidation temperature of Mg by 120 °C relative to a baseline air environment. Laser ignition tests reveal that the combustion intensity of Mg/AP composite fuels rises as the Mg loading increases and as the Mg particle size decreases. The ignition delay time of the Mg/AP composite fuels is found to be longer in a water vapor or argon environment than in an air environment. Owing to the intense reaction of Mg in air, the flame temperature is found to exceed that in argon and water vapor environments. The latent heat of evaporation of water tends to lower the flame temperature in the water vapor environment. From these empirical observations, we propose a phenomenological mechanism for the combustion of Mg/AP composite fuels. Overall, this study shows that the Mg loading, Mg particle size, and environmental composition can have profound effects on the ignition and combustion behavior of Mg/AP composite fuels. These findings could be used to steer the development of water-reactive metallic fuels, with a view to improving the design of underwater propulsion systems.

Multi-degrees-of-freedom swimmer using an ultrasonic longitudinal transducer

Acoustic underwater propulsion systems based on an ultrasonic transducer have been studied. In previous research, the self-propelled acoustic swimmer using thickness-vibration-mode transducer is evaluated widely. The thickness-vibration-mode transducer is excited in the thickness and radial direction. Because the acoustic propulsion system is based on the acoustic driving force, the vibration in the radial direction is hard to provide the propulsion thrust. In this study, a cylindrical transducer, the pure longitudinal vibrator, is evaluated for the acoustic underwater propulsion system. A prototype swimmer with multiple transducers is designed and fabricated. The admittance characteristics of the cylindrical transducer are investigated in air and in water. The zero speed propulsion and no load speed are measured in water. Multi-degrees-of-freedom swimmer with the multiple cylindrical transducers is evaluated. Because of the small size, high power density, simple structure and multi-degrees-of-freedom, self-propelled acoustic swimmer is suitable for applications such as inspection and repairment robots in a liquid environment.

Route to transition in propulsive performance of oscillating foil.

Transition in the propulsive performance and vortex synchronization of an oscillating foil in a combined heaving and pitching motion is numerically investigated at a range of reduced frequencies (0.16 ≤f^{*}≤ 0.64), phase offsets (0^{∘} ≤ϕ≤ 315^{∘}), and Reynolds number (1000≤Re≤16000). Focusing on the common case of Re=1000, the drag to thrust transition is identified on a ϕ-f^{*} phase map. Here, the range of 90^{∘} ≤ϕ≤ 225^{∘} depicted a drag-dominated regime for increasing reduced frequency. However, thrust-dominated regimes were observed for ϕ< 90^{∘} and ϕ> 225^{∘}, where increasing the reduced frequency led to an increased thrust production. The isoline-depicting drag-thrust boundary was further observed to coincide with transitions in the characteristic near-wake modes with increasing reduced frequency, which ranged from 2P+2S to 2P and reverse von Kármán modes. However, evaluation of the wake with changing phase offsets at individual reduced frequencies only depicted effects on the spatial configuration of the vortex structures, while the number of vortices shed in one oscillation period was unchanged. The existence of similar wake modes with significantly different propulsive performance clearly suggests that transitions of the wake topology may not always be a reliable tool for understanding propulsive mechanisms of fish swimming or development of underwater propulsion systems. We further assessed a possible route to drag production via investigation into the mean velocity fields at increasing phase offset and at intermediate reduced frequencies ranging from 0.24 to 0.40. This revealed bifurcation of a velocity jet behind the foil on account of the wake topology and dynamics of shed vortex structures. The changes posed by increasing ϕ on wake structure interactions further hints at potential mechanisms that limit the achievement of optimum efficiency in underwater locomotion.

Modeling and nonlinear dynamics of cantilevered pipe with tapered free end concurrently subjected to axial internal and external flows

The current study aims to investigate stability and nonlinear dynamics of a cantilevered pipe with a tapered free end concurrently subjected to axial internal and external flows, with the purpose of regulating and controlling the vibration behavior of the pipe. A nonlinear governing equation accounting for the effects of axial internal and external flows on the nonlinear dynamics of the pipe is first established based on the modified Hamilton’s principle. Then, a linear analysis is performed to investigate the stability region and instability mode, while considering the effect of length of the tapered free end. The results show that buckling, flutter and combined buckling-flutter behaviors can occur with varying the internal and external flow velocities. Subsequently, a nonlinear analysis is conducted to further explore vibration amplitude and oscillation shape of the pipe with a tapered free end. It is found that for different values of internal flow velocity, the pipe displays quite distinct vibration behaviors with increasing external flow velocity. This study is expected to be beneficial for optimizing an underwater propulsion system based on oscillations of an attached pipe through adapting the flow velocity and shape of the free end.

2Pb4-3 Acoustic Underwater Propulsion System via a 36° Y-cut LN Thickness-vibration-mode Transducer

2Pb4-3 Acoustic Underwater Propulsion System via a 36° Y-cut LN Thickness-vibration-mode Transducer

Evaluation method for acoustic underwater propulsion systems.

Acoustic underwater propulsion systems based on bulk acoustic waves and surface acoustic waves have been studied. In this study, an acoustic propulsion system that consists of a 2.065-MHz thickness-vibration-mode lead-zirconate-titanate ultrasonic transducer is evaluated. A prototype swimmer is designed and fabricated. The admittance difference of the transducer in water and air is investigated. The vibration amplitude of the transducer is measured to evaluate transducer performance. The acoustic radiation force is calculated to describe acoustic propulsion. The zero-speed propulsion (ZSP) force and no-load speed (NLS) are measured in water. Swimmer movement starts at a NLS of 6.1 mm/s and a ZSP force of 0.2 mN for an input voltage and input power of 12.4 V peak to peak and 0.4 W, respectively. Although the average efficiency of the acoustic propulsion system is 69% in water, the overall movement efficiency of the swimmer is less than 1% because of fluid resistance and wire traction. Based on admittance, acoustic propulsion calculations, ZSP force, NLS measurements, and efficiency analysis, an evaluation method is proposed for optimizing swimmers with an acoustic underwater propulsion system. Small size, high power density, and simple structure of an acoustic propulsion system with an ultrasonic transducer make such systems suitable for applications such as pipeline inspection and repair.