Aquatic Propulsion Research Articles

Harnessing tidal energy efficiency: A comprehensive analysis of tandem flapping hydrofoils for maximizing power generation from low-level currents

Harnessing natural water currents for renewable energy generation holds significant promise. Flapping foil hydrokinetic turbines (FHTs), inspired by aquatic organisms' propulsion mechanisms, leverage undulating motion to extract energy from flowing water. This study investigates the impact of hydrofoil shape and mechanical parameters on oscillating tandem hydrofoil systems' performance for energy harvesting, focusing on Persian Gulf tidal currents. Computational Fluid Dynamics (CFD) in a 2D domain is employed for analysis. Parameters include hydrofoil profiles (IFS, NACA 0015, HSVA, and flat plate), damping coefficient, and initial pitch angle optimized for low-level currents around 1 m/s, derived from HYCOM data. Numerical simulations highlight the superior hydrodynamic efficiency of the IFS hydrofoil. Through RANS equations in unsteady conditions, an optimal configuration with a 75-degree initial angle and damping coefficient of 4 achieves a maximum power efficiency of approximately 66.77%. Efforts have been made to ensure data accuracy, including rounding reported efficiencies appropriately and conducting thorough uncertainty analyses.

Underwater dynamic modeling and experiments of flexible structure with harmonic actuation of macro fiber composites

Flexible structures driven by smart actuators are promising alternatives for aquatic bionic propulsion vehicles. However, the hydrodynamic effects induced by viscous fluids on the dynamic response of flexible structures remain an ongoing challenge. Thus, this article presents a fluid-structure interaction dynamic model of a flexible underwater structure actuated by macro fiber composite actuators, and the underwater multimodal vibration characteristics are studied. The model is based on the extended Hamilton’s principle, which considers the effects of hydrodynamic forces. The segmented mode shape functions of the flexible structure are derived using the assumed mode method. The study shows that the first two mode shapes of the beam predicted by the model agrees with the experimental results. The underwater dynamic responses of the flexible structure in a board band covering the first two resonance frequencies are also investigated at different actuation levels. The underwater results indicate that the first two resonance frequencies are 1.05 and 6.1 Hz respectively, basically consistent with the simulation ones (1.07 and 6.61 Hz). Correspondingly, the maximum transverse deflections at the end of the beam are 4.81 and 1.31 mm respectively, which are slightly lower than the predicted values of 5.75 and 1.62 mm. Therefore, the validity of the proposed coupled dynamic model is verified, which can be utilized to predict the multimodal dynamic responses of underwater flexible structures actuated by MFC or other smart actuators.

Spinosaurus is not an aquatic dinosaur.

A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur Spinosaurus aegyptiacus when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture that S. aegyptiacus was the first fully aquatic dinosaur. The 'aquatic hypothesis' posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow-surface swimmer (<1 m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.

On the role of phase lag in multi-appendage metachronal swimming of euphausiids

Metachronal paddling is a common method of drag-based aquatic propulsion, in which a series of swimming appendages are oscillated, with the motion of each appendage phase-shifted relative to the neighboring appendages. Ecologically and economically important euphausiid species such as Antarctic krill (Euphausia superba) swim constantly by stroking their paddling appendages (pleopods), with locomotion accounting for the bulk of their metabolic expenditure. They tailor their swimming gaits for behavioral and energetic needs by changing pleopod kinematics. The functional importance of inter-pleopod phase lag (ϕ) to metachronal swimming performance and wake structure is unknown. To examine this relation, we developed a geometrically and dynamically scaled robot (‘krillbot’) capable of self-propulsion. Krillbot pleopods were prescribed to mimic published kinematics of fast-forward swimming (FFW) and hovering (HOV) gaits of E. superba, and the Reynolds number and Strouhal number of the krillbot matched well with those calculated for freely-swimming E. superba. In addition to examining published kinematics with uneven ϕ between pleopod pairs, we modified E. superba kinematics to uniformly vary ϕ from 0% to 50% of the cycle. Swimming speed and thrust were largest for FFW with ϕ between 15%–25%, coincident with ϕ range observed in FFW gait of E. superba. In contrast to synchronous rowing (ϕ = 0%) where distances between hinged joints of adjacent pleopods were nearly constant throughout the cycle, metachronal rowing (ϕ > 0%) brought adjacent pleopods closer together and moved them farther apart. This factor minimized body position fluctuation and augmented metachronal swimming speed. Though swimming speed was lowest for HOV, a ventrally angled downward jet was generated that can assist with weight support during feeding. In summary, our findings show that inter-appendage phase lag can drastically alter both metachronal swimming speed and the large-scale wake structure.

Optimized design and investigation about propulsion of bionic Tandem undulating fins I: Effect of phase difference

Motivated by the excellent maneuverability and high efficiency of the undulation fishes, a multi-fins propulsor was provided in this article and the propulsion performance was attempted to be improved by taking advantage of the interaction between the tandem fins. The effects of the wave-number and the phase difference were studied numerically. The results showed that the thrust force was enhanced significantly by introducing a fin with wave-numbers of 0.5 at the expense of propulsion efficiency. The vortices induced by the anterior and posterior fins were merged or offset, which depended on the phase difference between the tandem fins. The interaction of vortices led to the variation of thrust force with the phase difference in a sinusoidal curve. With the phase difference around the crest, the propulsion performance of tandem fins was improved clearly. Considering the thrust, lateral force and efficiency, the LL model where both the fin wave-numbers are set to be 1.0 is suitable for the cruising motion of the aquatic vehicle, and the SS model where the wave-numbers are adjusted to 0.5 is beneficial to the fast maneuver. The present results provided a theoretical basis for the design of aquatic multi-fin propulsion robots to meet different task requirements.

Cool your jets: biological jet propulsion in marine invertebrates.

Pulsatile jet propulsion is a common swimming mode used by a diverse array of aquatic taxa from chordates to cnidarians. This mode of locomotion has interested both biologists and engineers for over a century. A central issue to understanding the important features of jet-propelling animals is to determine how the animal interacts with the surrounding fluid. Much of our knowledge of aquatic jet propulsion has come from simple theoretical approximations of both propulsive and resistive forces. Although these models and basic kinematic measurements have contributed greatly, they alone cannot provide the detailed information needed for a comprehensive, mechanistic overview of how jet propulsion functions across multiple taxa, size scales and through development. However, more recently, novel experimental tools such as high-speed 2D and 3D particle image velocimetry have permitted detailed quantification of the fluid dynamics of aquatic jet propulsion. Here, we provide a comparative analysis of a variety of parameters such as efficiency, kinematics and jet parameters, and review how they can aid our understanding of the principles of aquatic jet propulsion. Research on disparate taxa allows comparison of the similarities and differences between them and contributes to a more robust understanding of aquatic jet propulsion.

The Role of the Tail or Lack Thereof in the Evolution of Tetrapod Aquatic Propulsion.

Secondary aquatic vertebrates exhibit a diversity of swimming modes that use paired limbs and/or the tail. Various secondarily aquatic tetrapod clades, including amphibians, reptiles, and mammals use transverse undulations or oscillations of the tail for swimming. These movements have often been classified according to a kinematic gradient that was established for fishes but may not be appropriate to describe the swimming motions of tetrapods. To understand the evolution of movements and design of the tail in aquatic tetrapods, we categorize the types of tails used for swimming and examine swimming kinematics and hydrodynamics. From a foundation of a narrow, elongate ancestral tail, the tails used for swimming by aquatic tetrapods are classified as tapered, keeled, paddle, and lunate. Tail undulations are associated with tapered, keeled, and paddle tails for a diversity of taxa. Propulsive undulatory waves move down the tail with increasing amplitude toward the tail tip, while moving posteriorly at a velocity faster than the anterior motion of the body indicating that the tail is used for thrust generation. Aquatic propulsion is associated with the transfer of momentum to the water from the swimming movements of the tail, particularly at the trailing edge. The addition of transverse extensions and flattening of the tail increases the mass of water accelerated posteriorly and affects vorticity shed into the wake for more aquatically adapted animals. Digital Particle Image Velocimetry reveals that the differences were exhibited in the vortex wake between the morphological and kinematic extremes of the alligator with a tapering undulating tail and the dolphin with oscillating wing-like flukes that generate thrust. In addition to exploring the relationship between the shape of undulating tails and the swimming performance across aquatic tetrapods, the role of tail reduction or loss of a tail in aquatic-tetrapod swimming was also explored. For aquatic tetrapods, the reduction would have been due to factors including locomotor and defensive specializations and phylogenetic and physiological constraints. Possession of a thrust-generating tail for swimming, or lack thereof, guided various lineages of secondarily aquatic vertebrates into different evolutionary trajectories for effective aquatic propulsion (i.e., speed, efficiency, and acceleration).

Aquatic Propulsion Mechanism Modeled on the Polychaete Worm's Swimming

Aquatic Propulsion Mechanism Modeled on the Polychaete Worm's Swimming

Fish-like aquatic propulsion studied using a pneumatically-actuated soft-robotic model

Fish locomotion is characterized by waves of muscle electrical activity that proceed from head to tail, and result in an undulatory pattern of body bending that generates thrust during locomotion. Isolating the effects of parameters like body stiffness, co-activation between the right and left sides of the body, and frequency on thrust generation has proven to be difficult in live fishes. We use a pneumatically-actuated fish-like model to investigate how these parameters affect locomotor force generation. We measure thrust as well as side forces and torques generated during propulsion. Using a statistical linear model we examine the effects of input parameter combinations on thrust generation. We show that both stiffness and frequency substantially affect swimming kinematics, and that there are complex interactive effects of these two parameters on thrust. The stiffer the backbone the more impact that increasing frequency has on thrust production. For stiffer models, increasing frequency resulted in higher values for both thrust and lateral forces. Large side forces reduce swimming efficiency but this effect could be mitigated by decreasing undulatory wavelength and allowing appropriate phasing of left and right body movements to reduce amplitudes of side force.

Self-propulsion of fishes-like undulating hydrofoil: A unified kinematics based unsteady hydrodynamics study

Unsteady flow characteristics and propulsive performance during self-propulsion of various types of fishes-like undulating 2D NACA0012 hydrofoil are studied using a unified kinematic model. The model considers a range of non-dimensional wavelength of undulation λ∗ (0.8−∞), with the smaller value corresponding to body undulation of anguilliform fishes and the larger value corresponding to the caudal-fin pitching motion of carangiform fishes. The study is conducted at various frequency based Reynolds number Ref500−1750 at a constant maximum amplitude of undulation of 0.1. A level-set function based immersed interface method is used for the numerical simulations. Comparative analysis of dynamic-transient flow and propulsion characteristics of different types of fishes-like self-propulsion is presented. Time-wise varying dynamic-transient results are presented for flow patterns, propulsion velocity, net thrust force, and power input. After the onset of pitching or undulation, the vorticity patterns shows a dipole formation along with a jet formation based momentum excess behind the foil that leads to a net thrust force coefficient CTmnet. The dipole and the momentum excess disappears at the dynamic-steady state leading to zero CTmnet. It was found that the pitching as compared to the undulating foil accelerates faster and reaches a larger propulsion velocity. Further, the pitching based propulsion results in a fluctuating velocity and larger lateral force coefficient leading to instability. Furthermore, for the pitching motion, a smaller lateral force in the initial time period leading to maximum stability and a slightly larger propulsive efficiency is found during the initial as compared to later time periods. This work presents a detailed analysis of the dynamic-transient state leading to the dynamic-steady state for different types of fishes-like self-propulsion − which can be used for designing aquatic propulsion systems.

Aquatic Propulsion Mechanism Modeled on the Polychaete Worm's Swimming

Aquatic Propulsion Mechanism Modeled on the Polychaete Worm's Swimming

A Soft Aquatic Actuator for Unsteady Peak Power Amplification

A soft hydraulic actuator is presented which uses elastic energy storage for the purpose of pulsed-jet propulsion of soft unmanned underwater vehicles. The actuator consists of a flexible membrane that can be inflated using a micropump and whose elastic potential energy may be released on demand using a controllable valve, in a manner inspired by the swimming of squids and octopuses. It is shown that for equivalent initial elastic energy, the drop in peak thrust is linearly proportional to the decrease in nozzle cross section. Peak hydraulic power amplification of the soft actuator of approximately 75% is achieved with respect to that of the driving pump, confirming that passive elasticity can be exploited in aquatic propulsion to replicate the explosive motion skills of agile sea-dwelling creatures.

Optimized kinematics enable both aerial and aquatic propulsion from a single three-dimensional flapping wing

Experiments on a prototype flapping robotic wing show that a single design can propel in both air and water, as do puffins and other auks, by varying wing kinematics. We optimize for each fluid with a force measurement feedback scheme and determine the unsteady wakes through dye visualization.

Hydrodynamic study of flapping foil propulsion system fitted to surface and underwater vehicles

ABSTRACTThe engineering translation of the aquatic animal propulsion systems and its appropriate application to marine vehicles help them to achieve movement with less power and hence resulting in the CO2 emission reduction. In this paper, the design of a robotic fish with pectoral and caudal fins, which operates at subsurface, is considered. The robofish body shape and fin geometrical parameters are also important with regard to the resistance and power aspects. Numerical studies are conducted with the robotic fish to determine its resistance in the bare hull and also for the case fitted with fins. Experimental studies carried out on a remotely operated surface ship with thunniform mode of fish propulsion, which operates at subsurface, are considered. Model tests are performed to analyse the propulsive performance of fishtail propulsion for the remotely operated surface ship in bollard pull condition and in self-propulsion mode.

SPH Analysis of suppression of reflected waves on aquatic propulsion mechanism by using vibrating elastic plate

SPH Analysis of suppression of reflected waves on aquatic propulsion mechanism by using vibrating elastic plate

Research of a Neutrophil’s Aquatic Propulsion Mechanism by Concentration Gradient of Cytokine

Research of a Neutrophil’s Aquatic Propulsion Mechanism by Concentration Gradient of Cytokine

Propulsion Characteristics of Bio-inspired Aquatic Propulsion Mechanism Using Fiber Composite Viscoelastic Fin Filled with Dilatant fluid

Propulsion Characteristics of Bio-inspired Aquatic Propulsion Mechanism Using Fiber Composite Viscoelastic Fin Filled with Dilatant fluid

Numerical analysis of suppression of reflected waves on aquatic propulsion mechanism by using wedge-shaped vibrating elastic plate

Numerical analysis of suppression of reflected waves on aquatic propulsion mechanism by using wedge-shaped vibrating elastic plate

On the role of nonlinear distortion in the theory of wave-like aquatic propulsion

Recent experimental investigations of small model boats propelled by propagating flexural waves carried out by the present author and his co-workers demonstrated viability of this type of propulsion as an alternative to a well-known screw propeller. Since the amplitudes of propagating flexural waves propelling the model boats are large enough, it is natural to consider the effect of nonlinear distortion of propagating localised flexural waves on generated thrust. This problem is explored in the present work by adding nonlinear harmonics of propulsive flexural waves to the well-known Lighthill's formula for generated thrust, which predicts a zero value of thrust in the case of linear flexural wave of constant amplitude. For simplicity, only the lowest (third) harmonic growing linearly with the distance of propagation is used. The resulting formula for the averaged thrust shows that, due to the effect of the third harmonic, the thrust is no longer zero, thereby demonstrating that nonlinear distortion of the propulsive flexural waves contributes positively to the generated thrust.

Undulatory Swimming Performance and Body Stiffness Modulation in a Soft Robotic Fish-Inspired Physical Model.

Undulatory motion of the body is the dominant mode of locomotion in fishes, and numerous studies of body kinematics and muscle activity patterns have provided insights into the mechanics of swimming. However, it has not been possible to investigate how key parameters such as the extent of bilateral muscle activation affect propulsive performance due to the inability to manipulate muscle activation in live, freely swimming fishes. In this article we extend previous work on passive flexible mechanical models of undulatory propulsion by using actively controlled pneumatic actuators attached to a flexible foil to gain insight into undulatory locomotion and mechanisms for body stiffness control. Two soft actuators were attached on each side of a flexible panel with stiffness comparable to that of a fish body. To study how bilateral contraction can be used to modify axial body stiffness during swimming, we ran a parameter sweep of actuator contraction phasing and frequency. Thrust production by the soft pneumatic actuators was tested at cyclic undulation frequencies ranging from 0.3 to 1.2 Hz in a recirculating flow tank at flow speeds up to 28 cm/s. Overall, this system generated more thrust at higher tail beat frequencies, with a plateau in thrust above 0.8 Hz. Self-propelled speed was found to be 0.8 foil lengths per second or ∼13 cm/s when actuated at 0.55 Hz. This active pneumatic model is capable of producing substantial trailing edge amplitudes with a maximum excursion equivalent to 1.4 foil lengths, and of generating considerable thrust. Altering the extent of bilateral co-contraction in a range from -22% to 17% of the cycle period showed that thrust was maximized with some amount of simultaneous left-right actuation of ∼3% to 6% of the cycle period. When the system is exposed to water flow, thrust was substantially reduced for conditions of greatest antagonistic overlap in left-right actuation, and also for the largest latencies introduced. This experimental platform provides a soft robotic testbed for studying aquatic propulsion with active control of undulatory kinematics.