The exploration of the underwater world has always fascinated humankind, prompting scientists and engineers to develop advanced technologies for underwater vehicles. Unmanned underwater vehicles (UUVs), also known as underwater drones, are submersible vehicles that can navigate their way through various water systems with or without human intervention. Typically, UUVs fall into two categories, remotely operated underwater vehicles, or autonomous underwater vehicles.
Autonomous underwater vehicles (AUVs) are playing an ever-growing role in modern subocean operations, generating a demand for faster, more maneuverable designs capable of deployments of increasingly longer durations. In order to meet these demands, vehicle developers have been looking to biological aquatic animals for inspiration and innovative solutions.
Among these innovations, biomimetic propulsion systems have emerged as a game-changer. Drawing inspiration from nature’s own engineering marvels, biomimetic propulsion systems are revolutionizing the capabilities of Unmanned Underwater Vehicles (UUVs) like never before. In this article, we will delve into the fascinating world of biomimetic propulsion and explore its potential to transform underwater exploration.
The Power of Biomimetics:
Biomimetics, also known as biomimicry or bioinspiration, involves mimicking the designs, processes, and systems found in nature to solve complex human challenges. This interdisciplinary field draws upon principles from biology, engineering, materials science, and more. By observing and understanding the extraordinary adaptations of marine creatures, scientists and engineers are unlocking innovative propulsion solutions for UUVs.
Nature is a rich source of inspiration for robot development. Bio-inspired robotics is about studying biological systems, and look for the mechanisms that may solve a problem in the engineering field for example biosensors (e.g. eye), bioactuators (e.g. muscle), or biomaterials (e.g. spider silk).
Therefore, biomimetic systems that transform UUVs to BUUVs are attracting significant attention as they demonstrate higher propulsion efficiency, enhanced maneuverability, and quieter actuation than conventional UUVs equipped with axial propellors.
Navies are also interested in biomimetic systems for developing more efficient propulsion systems, stealthy submarines, UUV (unmanned undersea vehicle) and improved military armor. Many countries including the US, Japan, India and China are involved in research of bio-inspired systems.
Engineers have been able to mimic their behavior by constructing robotic imitations, with some considerable success. One of the best-known examples is Robotuna, an eight link tendon- and pulley-driven, whose external shape has the form of a bluefin tuna, capable of emulating the swimming motion of a live tuna. This project evolved into the Ghostswimmer, a prototype Navy vehicle that swims by manipulating its dorsal (back), pectoral (chest), and caudal (tail) fins.
Biomimetic underwater propulsion
Traditional underwater propulsion systems, such as screw-type axial propellors, convert torque into thrust; in other words, power from an engine turns the propellers and generates force by moving the flow of water downward and behind the blade.
For thrust, most modern human-designed propulsors utilize some sort of continuous rotation (think propellers), which is not a motion natural to biology. Fish and mammals such as dolphins and whales use fins and flukes to propel themselves in combined pitching and heaving motions, turtles use a paddling motion, while squid eject jets of fluid.
Animals have shown us that there are many more kinds of underwater locomotion, potentially offering unique benefits to robots. After evolving for millions of years, fish and cetaceans have developed fast efficient locomotion techniques, with levels of manoeuvrability that far outperform conventional engineered marine locomotion systems
Different types of swimmers and their propulsion mechanisms
In steady swimming, where there is no acceleration or deceleration, the thrust generated by the propulsive system of an underwater vehicle is precisely balanced by the drag acting on the vehicle, on average over time. The drag force can be broken down into two major components: friction drag and pressure (or form) drag. Friction drag is caused by the viscous shear stresses acting on the surface of the vehicle, while pressure drag is a result of pressure losses in the wake of the vehicle. In streamlined vehicles resembling fish, friction drag tends to dominate, whereas in more boxy shapes, such as bluff bodies, form drag becomes more significant.
There are four primary sources of thrust in underwater propulsion systems:
- Drag-based thrust: This type of thrust is generated by the drag force acting opposite to the direction of motion of the body. The propulsion system utilizes the reaction force from pushing against the water to generate forward motion. Examples of drag-based swimmers include humans, turtles, and ducks.
- Lift-based thrust: Lift is the force produced perpendicular to the direction of motion. Lift-based swimmers generate thrust by creating lift forces using their body shape or propulsive fins. Oscillatory and undulatory swimmers, like salmon, tuna, and dolphins, fall under this category.
- Added mass forces: These forces arise from the inertia of the water that is set in motion by the body of the swimmer or vehicle. They contribute to the overall thrust.
- Momentum injection: Swimmers like squid and jellyfish produce thrust by jetting fluid from their bodies. This expulsion of fluid creates a reaction force in the opposite direction, propelling the swimmer forward.
Different types of swimmers exhibit various performance characteristics, including swimming speed and energy efficiency (cost of transport). Most organisms, regardless of their size and swimming mechanisms, tend to swim at speeds between 0.5 and 1.5 body lengths per second, which is considered a typical cruising speed. The maximum swimming speed can vary significantly among different swimmer types. However, drag-based swimmers generally have a higher cost of transport compared to other types of swimmers. This is because drag-based swimmers have not solely adapted to aquatic environments and often engage in walking or flying as well.
The study of different types of swimmers and their propulsion mechanisms provides valuable insights for designing biomimetic propulsion systems for underwater vehicles. By understanding the principles employed by nature’s aquatic creatures, engineers can develop innovative and efficient propulsion systems that mimic the performance and capabilities of these swimmers.
Bio-inspired Propulsion Systems:
- Bionic Fins and Tails: Fish and marine mammals have evolved remarkable fins and tails that enable efficient and agile movement underwater. Biomimetic UUVs incorporate bionic fins and tails that replicate the natural movements of aquatic creatures. These biomimetic systems improve maneuverability, speed, and energy efficiency, allowing UUVs to navigate challenging underwater environments with ease.
- Jet Propulsion: Squids and octopuses are known for their extraordinary jet propulsion abilities. For example, Jellyfishes in nature propel themselves through their surroundings by radially expanding and contracting their bell-shaped bodies to push water behind them, which is called jet propulsion. By expelling water forcefully, these creatures can rapidly propel themselves through the water. Researchers have developed biomimetic jet propulsion systems that mimic this mechanism, enabling UUVs to achieve high speeds while maintaining precise control. Such biomimetic systems hold great promise for underwater exploration, surveillance, and research.
- Bio-inspired Coatings: Marine organisms have evolved unique surface properties that reduce drag and increase maneuverability. Biomimetic coatings inspired by shark skin, for instance, can minimize friction and turbulence, allowing UUVs to move swiftly through the water while conserving energy. These bio-inspired coatings also offer advantages such as resistance to biofouling, corrosion, and damage from harsh underwater environments.
As a source of inspiration, aquatic creatures such as fish, cetaceans, and jellyfish could inspire innovative designs to improve the ways that manmade systems operate in and interact with aquatic environments. Such vehicles have the potential to uncover new mission capabilities and improve maneuverability, efficiency, and speed.
Advancements and Demonstrations
For a deeper understanding of biomimetic propulsion and Applications please visit: Biomimetic Propulsion Systems for Underwater Exploration: Nature’s Blueprint for Efficient Locomotion
A team of researchers in Poland has developed a novel energy-saving propulsion system design for a biomimetic unmanned underwater vehicle (BUUV)
Traditional BUUV designs mimic the undulating movement of marine animals like fish, turtles, or seals to generate propulsion by pushing water against passing waves. However, the impact of tail oscillation on the fluid flow around the hull can create vortices, requiring the study of fluid-structure interaction and vortex structures.
Existing BUUV designs often have complex structures with numerous moving parts that couple rigid and flexible forms. This complexity poses challenges in terms of control, cost, and the risk of flooding electronic components. The researchers aimed to address these issues with their novel biomimetic propulsion system, which incorporates two fins fitted to a drive mechanism with fewer moving parts.
The movement of the fins is achieved by driving them inwards and pushing the water outwards in opposition to the BUUV’s body. The fins then return to their start position with reduced force generated by a positional shift. During the movement, the fins change their position from the vertical to the horizontal plane, reducing hydrodynamic drag and overall drag on the underwater vehicle.
The objective of this new propulsion system is to simplify design and control variables while demonstrating the difference between an innovative propulsion system and an undulating system. By increasing the velocity of the fins during thrust generation and decreasing it during the returning movement, the researchers aim to improve efficiency further.
The tests conducted on the propulsion system showcased higher thrust capabilities and improved net efficiency, particularly for low-speed movement, thus maintaining the biomimetic nature of the movement. The team plans to conduct further tests to evaluate the impact of the BUUV’s body on the propulsion system design and explore the incorporation of additional joints and flexible fins to accommodate various movement styles.
Overall, this novel biomimetic propulsion system design offers potential energy-saving benefits, simplifies the structure, and aims to improve the efficiency of BUUVs, paving the way for more effective and cost-efficient underwater exploration and research.
In May 2018, University of California reported Transparent eel-like soft robot can swim underwater without propulsion
In May 2018, engineers and marine biologists at the University of California developed a transparent eel-like robot that can swim underwater without the use of traditional propulsion systems. The robot, measuring about one foot in length, utilizes artificial muscles filled with water to propel itself. It is connected to an electronics board that remains on the water’s surface and is virtually transparent.
The development of this robot, described in the journal Science Robotics, represents a significant advancement towards a future where soft robots can swim alongside marine life without causing disturbance or harm. Unlike most existing underwater vehicles used for observing marine organisms, which are rigid and powered by noisy electric motors with propellers, this robot moves silently and gracefully.
One key innovation of this eel-like robot is the use of the surrounding saltwater to generate electrical forces that drive its movement. The robot is equipped with cables that apply voltage to the saltwater and pouches of water within its artificial muscles. By delivering negative charges to the water just outside the robot and positive charges inside the robot, the muscles are activated and produce an undulating swimming motion.
Importantly, the electrical charges used in the robot’s propulsion system carry very little current, ensuring the safety of nearby marine life. The concept of leveraging the environment as part of the robot’s design was a significant breakthrough for the researchers.
While there are still further advancements and refinements needed to create an efficient and untethered eel-like robot, the team has demonstrated that it is indeed possible. This development opens up possibilities for future underwater robots that can move in a more natural and inconspicuous manner, enabling enhanced exploration and observation of marine ecosystems.
Chinese Researchers developed bio-inspired transformable robotic fin reported in 2016
Researchers from the University of Science and Technology of China have developed a bio-inspired transformable robotic fin for unmanned underwater vehicles. The fin mimics the dynamic shape-changing motion of fish fins to enhance underwater propulsion. It features a multi-link frame and a flexible surface skin that can change shape using a purpose-designed mechanism. Experiments were conducted to study the performance of the fin, including the effects of controlling modes and kinematic parameters on thrust capability. The results demonstrate that the fin’s propulsive performance can be optimized by adapting its shape to changing environments. This research contributes to the development of high-performance bio-inspired underwater vehicles.
A jet-powered squid robot that can leap out of the water reported in June 2019
This “squid-like aquatic-aerial vehicle” from Beihang University in China is modeled after flying squids. Real squids, in addition to being tasty, propel themselves using water jets, and these jets are powerful enough that some squids can not only jump out of the water, but actually achieve controlled flight for a brief period by continuing to jet while in the air. The flight phase is extended through the use of fins as arms and wings to generate a little bit of lift. Real squids use this multimodal propulsion to escape predators, and it’s also much faster—a squid can double its normal swimming speed while in the air, flying at up to 50 body lengths per second.
The squid robot is powered primarily by compressed air, which it stores in a cylinder in its nose (do squids have noses?). The fins and arms are controlled by pneumatic actuators. When the robot wants to move through the water, it opens a value to release a modest amount of compressed air; releasing the air all at once generates enough thrust to fire the robot squid completely out of the water.
The robot squid can travel between 10 and 20 meters by jumping, whereas using its jet underwater will take it just 10 meters. At the moment, the squid can only fire its jet once, but the researchers plan to replace the compressed air with something a bit denser, like liquid CO2, which will allow for extended operation and multiple jumps. There’s also plenty of work to do with using the fins for dynamic control, which the researchers say will “reveal the superiority of the natural flying squid movement.”
A robotic scallop that moves just like the real thing reported in June 2019
In June 2019, researchers from EPFL’s Reconfigurable Robotics Laboratory introduced RoboScallop, a robotic scallop that mimics the swimming motion of real scallops. Real scallops generate forward thrust by opening and closing their shells to expel water. RoboScallop replicates this mechanism, making it a simple and robust design suitable for various applications, including large swarms of robots. The robot is safe to handle while in operation and is resistant to fouling, a common issue in marine environments. The prototype weighs 65 grams, can reach speeds of up to 16 centimeters per second, and can generate clapping motions at a frequency of over 2.5 Hz. Although steering is not yet implemented, the researchers envision RoboScallop being able to change direction by adjusting water jetting on one side. They also suggest potential gripper capabilities for the robot, which real scallops do not possess.
Indian Institute of Technology-Madras developing super-efficient propellers reported in Jan 2017
In January 2017, scientists at the Indian Institute of Technology-Madras were working on the development of super-efficient propellers inspired by aquatic animals such as penguins, turtles, and fish. These bio-inspired propellers have finlike blades that can function as both propellers and rudders, allowing for faster response to commands and improved maneuverability. The propellers are designed to minimize fuel consumption and create minimal disturbance in the water, making them difficult to detect. The researchers focused on the hydrodynamics of penguins and fish, which have efficient propulsion systems. The penguin-inspired system utilizes pressure differences between the upper and lower surfaces of the fins to generate propulsion, while the fish-inspired system incorporates side fins and a tail fin for increased thrust. The researchers conducted tests on ship models of varying sizes and speeds, evaluating parameters such as self-propulsion, thrust force, flapping amplitude and frequency, forward speed, lift, and drag. Certain propeller designs showed an efficiency of 80% compared to the average efficiency of 65%.
Learning from fish and flags to inform new propulsion strategies reported in April 2020
In April 2020, Andres J. Goza, an assistant professor in the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign, conducted research on learning from fish and flags to inform new propulsion strategies. The goal of the research was to understand the passive dynamics and fluid-structure interactions that occur when vehicles move through air or water in order to enhance performance and develop bio-inspired propulsion strategies.
Goza’s research focused on observing the efficient swimming of fish and the flight of birds, as well as the motion of flags in the wind, to gain insights into how these natural phenomena can be applied to engineered locomotion. He studied how the flexible wings of birds and the tails of fish induce passive motion when they swim or fly, and how the motion of air around a flag affects its flapping. By understanding these fluid-structure interactions, Goza aimed to design aircraft and submarines with novel locomotion capabilities.
One aspect of the research involved examining the resonant frequency of structures in the presence of fluid. Goza found that exciting a structure at its resonant frequency, taking into account the effect of the fluid, can lead to improved performance and thrust. He developed a new definition of resonance that incorporates the fluid’s impact, allowing for a better understanding of how the motion of structures is tied to the flow around them.
The research also investigated the role of amplitude in propulsion. While resonance is typically defined for small undulations, fish swim at larger amplitudes. Goza bridged the gap between small amplitude resonance and the larger motions observed in fish swimming, finding that resonance continues to play a significant role even at larger amplitudes. The results showed that resonance, both at small and large amplitudes, can contribute to peak thrust.
According to Goza, harnessing passive dynamics can potentially reduce the energy input required for propulsion. The research explored the possibility of utilizing passive motion induced by resonant frequencies to enhance propulsion efficiency. Future phases of the research aim to investigate modern active materials that can be tuned to have the desired resonant frequency and induce desired passive dynamics.
Overall, Goza’s research in April 2020 provided insights into using passive dynamics and fluid-structure interactions to inform new propulsion strategies. By studying natural phenomena and incorporating the concept of resonance, the research opens up possibilities for developing more efficient and bio-inspired propulsion systems.
Highly Maneuverable Robotic Fish Based on Biological Principles and Biomimetic Materials
This project, Sponsored by Office of Naval Research aims to develop highly maneuverable and efficient robotic fish by advancing biomimetic actuation and sensing materials, and by designing and controlling the robotic fish based on biological principles.
The research is concentrated on
Biomimetic actuation. “Inspired by fins of living fish, we are developing flexible artificial fins capable of complex 3D deformations based on electroactive polymers, seeking fundamental understanding of electro-mechano-hydrodynamics in fin-fluid interactions, and investigating biologically inspired maneuvering and propulsion strategies for biomimetic pectoral and caudal fins.”
Bioinspired flow sensing. “We are interested in developing micro flow sensors through novel microfabrication processes, characterizing and modeling the sensory response in a variety of flow conditions that are of biological and engineering interest, and exploring the use of arrays of such sensors as an artificial lateral line system for robotic fish.”
Feedback flow control. “We will investigate artificial lateral line-based feedback control of biomimetic fins to achieve maneuvers and schooling behaviors of robotic fish.”
Here are some of the latest breakthroughs in biomimetic propulsion systems for UUVs:
- In 2022, researchers at the University of California, Berkeley developed a biomimetic UUV called the “Flapper” that uses flapping fins to generate propulsion. The Flapper is able to swim at speeds of up to 2 meters per second, which is faster than many traditional UUVs. It is also more maneuverable than traditional UUVs, making it ideal for use in tight spaces.The Flapper is powered by a small battery and can operate for up to 12 hours on a single charge. It is equipped with a variety of sensors, including a camera, a sonar, and a GPS receiver. The Flapper can be used for a variety of applications, including ocean exploration, environmental monitoring, and military surveillance.
- In 2023, researchers at the Massachusetts Institute of Technology developed a biomimetic UUV that mimics the swimming motion of squid. The UUV was able to swim very efficiently and quietly, making it ideal for use in sensitive environments.The Squidbot is powered by a small electric motor and can operate for up to 6 hours on a single charge. It is equipped with a variety of sensors, including a camera, a sonar, and a magnetometer. The Squidbot can be used for a variety of applications, including ocean exploration, environmental monitoring, and military surveillance.
- In 2023, Researchers at at the University of Massachusetts Amherst reported Groundbreaking robotic fish has a twisted method of propulsion:The robotic fish is inspired by the movements and locomotion of real fish, which allows it to swim in a lifelike manner. The researchers employed a technique called multimaterial fiber fabrication to create the fish’s body, tail, and fins using a soft polymer material. The material is composed of two polymers that are twisted and coiled together, resulting in a structure that mimics the musculoskeletal system of a fish.The design of the robotic fish enables it to move with flexibility and agility, replicating the undulating motions of real fish. By controlling the movement of the fish’s body and tail, the researchers can steer the robotic fish in different directions and make it swim at various speeds.One significant advantage of the twisted and coiled polymer material is its lightweight and flexible nature, allowing the robotic fish to maneuver easily in water. The researchers believe that this technology has the potential to advance underwater exploration, surveillance, and environmental monitoring.In summary, the article highlights the development of a robotic fish made from a unique twisted and coiled polymer material. The innovative design allows the robotic fish to mimic the movements of real fish, providing enhanced maneuverability and flexibility. The technology has promising applications in underwater exploration and surveillance, contributing to advancements in the field of robotics and aquatic research.
- In 2023, Swiss multinational ABB unveiled a new form of electric propulsion called ABB DynafinTM. The technology, which mimics the movements of a whale tail, is expected to be commercially available soon and offers installation and operational benefits. Initially targeted at ferries, cruise ships, and offshore wind support vessels, the electric setup has the potential to reduce energy consumption by up to 22% compared to conventional shaft lines. The system is designed around a main electric motor that turns a large wheel rotating at 30-80 revolutions per minute. Individually controlled vertical blades extend from the wheel, creating both propulsion and positioning forces at the same time. The setup is a further development of ABB’s gearless power transmission.ABB Dynafin has a smaller footprint, fewer components, low maintenance costs, wider operational windows, and high open water efficiency. The power range is currently 1-4 MW, but it is expected to be developed for larger vessels in the future. ABB has a successful track record in electric propulsion with its Azipod systems, which have over 700 installations operating for more than three decades.
Benefits and Applications:
Biomimetic propulsion systems offer numerous benefits and open up exciting possibilities for UUV applications:
- Enhanced Performance: By imitating nature’s efficient propulsion mechanisms, biomimetic UUVs can achieve superior performance in terms of speed, agility, and maneuverability. This advancement translates into more effective underwater missions, whether it’s scientific research, ocean exploration, or military operations.
- Energy Efficiency: Biomimetic designs prioritize energy efficiency by emulating the streamlined forms and propulsion techniques of marine creatures. These systems enable UUVs to operate for longer durations, covering larger distances without the need for frequent recharging or refueling.
- Minimized Environmental Impact: Biomimetic propulsion systems can contribute to environmentally friendly underwater operations. By reducing energy consumption and optimizing propulsion efficiency, UUVs can minimize their carbon footprint and disturbance to delicate marine ecosystems.
- Scientific Research and Oceanography: Biomimetic UUVs are invaluable tools for studying marine life, underwater habitats, and oceanographic phenomena. Their enhanced maneuverability and stealth capabilities allow researchers to gather valuable data without disturbing the natural environment.
Biomimetic propulsion systems have unlocked a new era of innovation beneath the waves. By drawing inspiration from the remarkable adaptations of marine creatures, scientists and engineers are creating UUVs that possess superior performance, energy efficiency, and maneuverability. As this field continues to advance, we can expect to witness groundbreaking discoveries, improved underwater exploration, and a deeper understanding of our planet’s oceans. By collaborating with nature, we are paving the way for a more sustainable and efficient underwater future.