Fin Ray Stiffness and Fin Morphology Control Ribbon-Fin-Based Propulsion.

High-fidelity numerical simulations with the spectral different (SD) method are performed to investigate the flow over a series of oscillating NACA 4-digit airfoils. Kinematic and airfoil thickness effects on the aerodynamic performances are highlighted. It is found that the combined plunging and pitching motion can outperform the pure plunging or pitching motions by sophisticatedly adjusting the airfoil gestures during the oscillation stroke. Also, the leading edge vortices (LEVs) manipulation during the gesture adjustment will vary with the airfoil thickness. The thin airfoil (NACA0006) better manipulates LEVs than the thick airfoil (NACA0030) does in our cases, and there exists an optimal thickness for large thrust generation with reasonable propulsive efficiency. At the same Strouhal number (0.3), the trends of the airfoil thickness effects on the thrust generation and propulsive efficiency are similar for the investigated reduced frequencies, namely 1 and 3.5. For all airfoil thicknesses, relatively low reduced frequency is conducive to the thrust production and propulsive efficiency. As the Strouhal number increases to 0.45, larger thrust can be generated with the same kinematics, but the propulsive efficiency will suffer if the kinematics is not optimal for the chosen aerodynamic parameters.

A generic kinematic model is presented for a unified hydrodynamics study covering the various types of motion found in real as well as hypothetical fish-like undulation. Undulating motion is presented here as a generic motion, considering chordwise forced flexibility—modelled by wavelength of undulation λ*—of a rigid NACA0012 hydrofoil in a free-stream flow. Using a level-set immersed boundary method-based in-house code, a non-dimensional study is presented for various wavelengths, λ* (0.8–8.0), and frequency of undulation, St (0.2–0.7), at a constant maximum amplitude of undulation of 0.1 and Reynolds number of 5000. A unified cause-and-effect-based analysis is presented with the help of flow patterns and propulsive performance parameters. Pressure contour demonstrates how the travelling wave distributes momentum in the streamwise direction and reduces the lateral force coefficient. Vorticity contours elucidate the mechanism of formation of reverse von Kármán vortex street and the secondary vortices. A correlation is proposed for thrust coefficient as a function of λ* and St. Good qualitative agreement is observed between the simulated results at smaller λ*-based undulating and larger λ*-based pitching foil and the published results for the anguilliform and thunniform fishes, respectively. The agreement with the real fishes is presented for the relative magnitude of thrust coefficient, propulsive efficiency, dynamic stabilization, and signal of the prey fish for the predator fish. Similar to propulsive performance of real fishes, larger (smaller) flexibility-based undulation (pitching) results in larger propulsive efficiency (thrust generation)—which can be used for the design of fish-like biomimetic propulsion system.

We present experimental results of thrust produced by a robotic propulsor, the design of which is inspired by the ribbon fin of the South American black ghost knifefish (Apteronotus albifrons). This remarkably nimble fish moves by oscillating its ribbon fin rays out of phase and thereby passing a traveling wave along the fin's length. Combinations of thrust from the ribbon fin and body rolls produced by the two pectoral fins enable the black ghost to swim in nearly any direction without bending its body. The fish's agile locomotor system is tightly integrated with its omnidirectional, active sensing system. The robotic ribbon fin has eight individually actuated metal rays which are linked by a thin latex sheet. The experimental results demonstrate the effect of varying the propulsive wave's frequency, amplitude and length on the robotic fin's thrust production. We found that thrust production peaks at particular combinations of the three variables and that the fin could produce steady forward thrust, despite the relatively small number of rays. The robotic ribbon fin has potential application as a propulsor for future underwater vehicles, in addition to being a valuable scientific instrument in understanding the swimming mechanics of the black ghost and similar fish

High fidelity numerical simulation of airfoil thickness and kinematics effects on flapping airfoil propulsion

Thrust generation and wake structure of wiggling hydrofoil

In this study, the thrust generation by simultaneous flapping airfoils in tandem configuration is parametrically studied with respect to flapping frequency, amplitude and relative location. Navier-Stokes solver with overset grid topology is employed to calculate the unsteady flowfields. The computation results indicate that when the two airfoils stroke in-phase - flapping phase lag is zero - the maximum propulsive efficiency and thrust can be obtained for most frequency and amplitude range. At a flapping amplitude of 0.2 chord and a reduced frequency of 0.75, the propulsive efficiency of aft airfoil is enhanced by about 37 % compared with that of forward airfoil. However, if flapping frequency exceeds some critical value, the strength of the leading edge vortex of aft airfoil is fortified by the trailing edge vortex of the forward airfoil, resulting in poor propulsive efficiency. It is also found that out-of-phase flapping has relatively low propulsive efficiency and thrust since vortical wake of the forward airfoil interacts with the leading edge vortex of aft airfoil in the unfavorable fashion. The total thrust and propulsive efficiency are shown to decrease with the horizontal miss distance of the aft airfoil. On the contrary, the vertical miss distance has little effect on the overall aerodynamic performance.

Jellyfish are a successful and diverse class of animals that swim via jet propulsion, with swimming performance and propulsive efficiency being related to the animal's feeding ecology and body morphology. The Rhizostomeae jellyfish lack tentacles but possess four oral lobes and eight trailing arms at the centre of their bell, giving them a body morphology quite unlike that of other free-swimming medusae. The implications of this body morphology on the mechanisms by which thrust is produced are unknown. Here, we determined the wake structure and propulsive efficiency in the blue-blubber jellyfish Catostylus mosaicus (order: Rhizostomeae). The animal is propelled during both bell contraction and bell relaxation by different thrust-generating mechanisms. During bell contraction, a jet of fluid is expelled from the subumbrellar cavity, which results from the interaction between the counter-rotating stopping (from the preceding contraction cycle) and starting vortices, creating a vortex superstructure and propulsion. This species is also able to utilise passive energy recapture, which increases the animal's swimming velocity towards the end of the bell expansion phase when the bell diameter is constant. The thrust produced during this phase is the result of the flexible bell margin manoeuvring the stopping vortex into the subumbrellar cavity during bell relaxation, enhancing its circulation, and creating a region of high pressure on the inner surface of the bell and, consequently, thrust. These mechanisms of thrust generation result in C. mosaicus having a relatively high propulsive efficiency compared with other swimmers, indicating that economical locomotion could be a contributing factor in the ecological success of these medusan swimmers.

We computationally study the propulsive performance of a two-dimensional elliptic foil undergoing interlinked pitching-heaving motion. This motion is realized by pitching the foil about an axis on its centerline outside the foil and by varying the distance between the pitching point and the leading edge. A distance of 0 and −∞ corresponds to leading edge pitching and pure heaving. An in-house fluid-structure interaction solver based on the sharp interface immersed boundary method is employed to resolve the flow field around the foil. We conducted simulations for different cases of the location of the pitching axis and pitching frequency at a Reynolds number of 100. The thrust generation is explained by the dynamics of leading-edge and trailing-edge vortices. The wake corresponding to thrust is either reverse von Kármán or a deflected reverse von Kármán vortex street. Analysis revealed the existence of an optimal pitching point for maximum thrust or propulsive efficiency at a given reduced pitching frequency. The optimal regions of the thrust and propulsive efficiency are quantified as a function of reduced pitching frequency and the location of the pitching axis. The pitching point for the maximum thrust and efficiency is found to be different. We discuss the fluid-mechanical reasons for the variation of propulsive performance with the location of the pitching point and the pitching frequency and corroborate our reasoning with the wake signatures.

A practical flapping wing micro air vehicle should have ability to withstand stochastic deviations of flight velocities. To design a flapping airfoil with this ability, it is necessary to evaluate the impacts of velocity deviations on the flapping performances numerically or analytically. In this paper, the responses of the time-averaged thrust coefficient and the propulsive efficiency with respect to a stochastic flight velocity deviation under Gauss distribution are numerically investigated using a classic Monte Carlo method. The response surface method is employed to surrogate the high fidelity CFD model to save computational cost. It is observed that both of the time-averaged thrust coefficient and the propulsive efficiency obey a Gauss-like but not the exact Gauss distribution. The effect caused by the velocity deviation on the time- averaged thrust coefficient is larger than the one on the propulsive efficiency.

Fish display remarkable swimming capabilities through the coordinated interaction of the body and caudal fin, yet the potential role of a passively pitching tail in enhancing hydrodynamic performance remains unresolved. In this work, we evaluate the performance of a carangiform swimmer equipped with either an actively pitching tail or a passively pitching tail. High-fidelity fluid–structure interaction simulations are employed to assess how variations in joint stiffness, damping, and inertia influence thrust generation, power demand, and overall stability at two representative Reynolds numbers, 500 and 5000. The results reveal that actively pitching tails tend to generate greater thrust, while passively pitching tails deliver improved outcomes in terms of power demand at the lower Reynolds number. Larger pitching amplitudes contribute positively only when associated with higher swimming frequency; when produced by reduced inertia or more flexible joints, they lead to unfavorable effects. At the higher Reynolds number, active tails consistently outperform passive ones, although a small subset of passive cases still achieve favorable performance. Across all cases, a recurring balance emerges, with thrust production and power expenditure varying inversely. These findings clarify the hydrodynamic consequences of passive versus active tail motion and establish design principles for bio-inspired underwater vehicles, in which smaller swimmers may benefit from passive tail pitching, whereas larger swimmers are better served by active control.

Experimental investigation of wing flexibility on vertical thrust generation and power consumption in hovering condition for a hovering Flapping-Wing Micro Air Vehicle, namely FlowerFly, weighing 14.5 g with a 3 g onboard battery and having four wings with double wing clap-and-fling effects, was conducted for several wing configurations with the same shape, area, and weight. A data acquisition system was set up to simultaneously record aerodynamic forces, electrical power consumption, and wing motions at various flapping frequencies. The forces and power consumption were measured with a loadcell and a custom-made shunt circuit, respectively, and the wing motion was captured by high-speed cameras. The results show a phase delay of the wing tip displacement observed for wings with high flexible leading edge at high frequency, resulting in less vertical thrust produced when compared with the wings with less leading edge flexibility at the same flapping frequency. Positive wing camber was observed during wing flapping motion by arranging the wing supporting ribs. Comparison of thrust-to-power ratios between the wing configurations was undertaken to figure out a wing configuration for high vertical thrust production but less power consumption.

It is well known that one drawback for stress-ribbon bridges can be that significant horizontal reactions at the abutment can be generated and that they have low flexural stiffness. The latter prevents them from being used as roadway bridges. A new hybrid structure of stress-ribbon deck and truss has been proposed and was adopted in the construction of Nozomi Bridge in Japan, a roadway bridge opened to traffic in 2003. Static and dynamic behaviours of the hybrid structure have been studied analytically and experimentally. The results show that the hybrid bridge has advantages over the stress-ribbon deck bridge as it generates much less horizontal force in suspension cables and has higher flexural stiffness, suitable for use as a roadway bridge, and that the hybrid bridge has advantages over the truss bridge since it can be constructed without extensive falsework and without large erection equipment. In this paper, at first, the hybrid structure is described, then analytical and test results are used to show its static and dynamic characteristics, and finally the construction of Nozomi Bridge is outlined.

The study of the mechanical properties of 3D printed food is crucial for food personalisation. Texture impacts the sensory experience along the oral food consumption. This work aimed to investigate the mechanical properties of post-processed printed food samples with different porosity topologies (triangled-shape/squared-shape). Image analysis showed changes in the structure before and after post-processing. The mechanical properties characterised via 3-point bending tests revealed that triangle-shaped topologies presented lower strength (20% less) and higher flexural stiffness (≈20%) when compared to the square-shaped topologies. A qualitative comparison of the porosity topologies and their role in the stiffness of structures under tensile and bending loads was performed via finite element models. The flexural stiffness varied between the triangle-shaped designs (13–35%) but remained almost constant for square-shaped designs (40–43%). The results presented in this work showed that the mechanical properties of 3D-printed food could be modified by the selection of porosity topology.

A two-dimensional time-domain constant potential panel method was used for analysis of chordwise flexible oscillating hydrofoils as oscillating propulsors. The oscillatory motions as well as the chordwise deflections were of large amplitude. Results are presented of propulsive thrust coefficient and efficiency over ranges of motion parameters including varied reduced frequency, pitch amplitude and feathering parameter; heave amplitude/chord ratio; pitching axis position; phase between pitching and heaving motions; and deflected shape. The ranges were taken about a favourable operating point for a flexible oscillating NACA 0012 foil. A peak in propulsive efficiency was found for each motion parameter variation studied. Propulsive efficiency varied strongly with changes in heave amplitude/chord ratio, pitch amplitude and phase between heaving and pitching motions, but less strongly for variations in pitching axis position and flexibility.

The propulsion system is the core component of unmanned underwater vehicles. The flapping propulsion method of marine animals' flippers, which allows for flexibility, low noise, and high energy utilization at low speeds, can provide a new perspective for the development of new propulsion technology. In this study, a new experimental flapping propulsion apparatus that can be installed in both directions has been constructed. The guide rail slider mechanism can achieve the retention of force in the direction of movement, thereby decoupling thrust, lift, and torque. Subsequently, the motion parameters of frequency-amplitude related to the thrust and lift of a bionic flapping-foil are scrutinized. A response surface connecting propulsion efficiency and these motion parameters is formulated. The highest efficiency of the flapping-foil propulsion is achieved at a frequency of 2 Hz and an amplitude of 40°. Furthermore, the impact of the installation mode and the aspect ratio of the flapping-foil is examined. The reverse installation of the swing yields a higher thrust than the forward swing. As the chord length remains constant and the span length increases, the propulsive efficiency gradually improves. When the chord length is extended to a certain degree, the propulsion efficiency exhibits a parabolic pattern, increasing initially and then diminishing. This investigation offers a novel perspective for the bionic design within the domain of underwater propulsion. This research provides valuable theoretical guidance for bionic design in the underwater propulsion field.