Wake Vortex Structure Characteristics of a Flexible Oscillating Fin

We compute the wake of a two-dimensional and three-dimensional flexible fin in an unsteady flow field with heaving and pitching motions using FLUENT. Deflexion mode is used for a non-uniform cantilever beam with non-uniformly distributed load. The effect of chordwise deflexion length on the characteristics of propulsion is discussed for two-dimensional flexible fin. The thrust coefficient decreases, propulsive efficiency increases and the intensity of turbulence attenuates gradually as the deflexion length increases. For a three-dimensional flexible fin, the intensity of the vortex in the plane of symmetry is higher than that in the plane at 3/4 span length of the caudal fin. But the propulsive perform.ance achieved is not what we expected with the given deflexion mode.     

Measurement of propulsive power and evaluation of propulsive performance from the wake of a self-propelled vehicle

Propulsive efficiency is a key indicator of propulsive performance, but it can be difficult to measure when the propulsion system is integrated into the vehicle body because the average rate of useful work done propelling the vehicle (Wu) and/or the average mechanical power expended propelling the vehicle (Pmech) is not known directly. A general approach would be to determine either or both of (Wu) and (Pmech) from the vehicle wake. The present discussion demonstrates that only (Pmech) can be determined from the flow crossing a plane a fixed (average) distance downstream of the vehicle. A method for measuring (Pmech) is presented using the observation that the power required to tow a permeable obstruction behind the vehicle depends on (Pmech). Several methods for evaluating propulsive performance using [Formula: see text] are proposed, including the definition of an equivalent jet velocity and corresponding Froude efficiency if the time-averaged mass flow rate through the propulsion system is known. If only (Pmech) is known, the recommended measure of propulsive performance is a power coefficient defined analogous to a drag coefficient.     

A long wavelength solution for a microorganism swimming in a channel

The hydrodynamics of a microorganism swimming in a channel is investigated. The microorganism is modeled as a two-dimensional sheet swimming at low Reynolds numbers between two rigid walls. The wavelengths of the propulsive waves passing down the sheet are assummed to be very large compared to the channel spacing, but the amplitude of the propulsive waves is arbitrary. Explicit analytical solutions for the propulsive velocity and the rate of energy dissipated in terms of the wave amplitude, channel spacing, wave number, and wave speeds are given.     

Swimming dynamics and propulsive efficiency of squids throughout ontogeny

Squids encounter vastly different flow regimes throughout ontogeny as they undergo critical morphological changes to their two locomotive systems: the fins and jet. Squid hatchlings (paralarvae) operate at low and intermediate Reynolds numbers (Re) and typically have rounded bodies, small fins, and relatively large funnel apertures, whereas juveniles and adults operate at higher Re and generally have more streamlined bodies, larger fins, and relatively small funnel apertures. These morphological changes and varying flow conditions affect swimming performance in squids. To determine how swimming dynamics and propulsive efficiency change throughout ontogeny, digital particle image velocimetry (DPIV) and kinematic data were collected from an ontogenetic range of long-finned squid Doryteuthis pealeii and brief squid Lolliguncula brevis swimming in a holding chamber or water tunnel (Re = 20-20 000). Jet and fin wake bulk properties were quantified, and propulsive efficiency was computed based on measurements of impulse and excess kinetic energy in the wakes. Paralarvae relied predominantly on a vertically directed, high frequency, low velocity jet as they bobbed up and down in the water column. Although some spherical vortex rings were observed, most paralarval jets consisted of an elongated vortical region of variable length with no clear pinch-off of a vortex ring from the trailing tail component. Compared with paralarvae, juvenile and adult squid exhibited a more diverse range of swimming strategies, involving greater overall locomotive fin reliance and multiple fin and jet wake modes with better defined vortex rings. Despite greater locomotive flexibility, jet propulsive efficiency of juveniles/adults was significantly lower than that of paralarvae, even when juvenile/adults employed their highest efficiency jet mode involving the production of periodic isolated vortex rings with each jet pulse. When the fins were considered together with the jet for several juvenile/adult swimming sequences, overall propulsive efficiency increased, suggesting that fin contributions are important and should not be overlooked in analyses of the swimming performance of squids. The fins produced significant thrust and consistently had higher propulsive efficiency than did the jet. One particularly important area of future study is the determination of coordinated jet/fin wake modes that have the greatest impact on propulsive efficiency. Although such research would be technically challenging, requiring new, powerful, 3D approaches, it is necessary for a more comprehensive assessment of propulsive efficiency of the squid dual-mode locomotive system.     

Kinematics of swimming garter snakes (Thamnophis sirtalis)

We investigate the kinematics of swimming garter snakes (Thamnophis sirtalis) using a novel nonlinear regression-based digitization method to establish quantitative statistical support for non-constant wavelengths in the undulatory pattern exhibited by swimming snakes. We find that in swimming snakes, the growth of the amplitude of the propulsive wave head-to-tail is strongly correlated (p < 0.005) with the head-to-tail growth in the wavelength. We investigate correlations between kinematic parameters and steady swimming speed, and find a very strong positive correlation between swimming speed and undulation frequency. We furthermore find a statistically well-supported positive correlation between swimming speed and both the initial amplitude of the propulsive wave at the head and the degree of amplitude growth from head to tail.     

Kinematics of swimming garter snakes (Thamnophis sirtalis).

We investigate the kinematics of swimming garter snakes (Thamnophis sirtalis) using a novel nonlinear regression-based digitization method to establish quantitative statistical support for non-constant wavelengths in the undulatory pattern exhibited by swimming snakes. We find that in swimming snakes, the growth of the amplitude of the propulsive wave head-to-tail is strongly correlated (p < 0.005) with the head-to-tail growth in the wavelength. We investigate correlations between kinematic parameters and steady swimming speed, and find a very strong positive correlation between swimming speed and undulation frequency. We furthermore find a statistically well-supported positive correlation between swimming speed and both the initial amplitude of the propulsive wave at the head and the degree of amplitude growth from head to tail.     

Locomotion of Gymnarchus Niloticus： Experiment and Kinematics

In addition to forward undulatory swimming, Gymnarchus niloticus can swim via undulations of the dorsal fin while the body axis remains straight; furthermore, it swims forward and backward in a similar way, which indicates that the undulation of the dorsal fin can simultaneously provide bidirectional propulsive and maneuvering forces with the help of the tail fin. A high-resolution Charge-Coupled Device （CCD） imaging camera system is used to record kinematics of steady swimming as well as maneuvering in G. niloticus. Based on experimental data, this paper discusses the kinematics （cruising speed, wave speed, cycle frequency, amplitude, lateral displacement） of forward as well as backward swimming and maneuvering. During forward swimming, the propulsive force is generated mainly by undulations of the dorsal fin while the body axis remains straight. The kinematic parameters （wave speed, wavelength, cycle frequency, amplitude） have statistically significant correlations with cruising speed. In addition, the yaw at the head is minimal during steady swimming. From experimental data, the maximal lateral displacement of head is not more than 1% of the body length, while the maximal lateral displacement of the whole body is not more than 5% of the body length. Another important feature is that G. niloticus swims backwards using an undulatory mechanism that resembles the forward undulatory swimming mechanism. In backward swimming, the increase of lateral displacement of the head is comparatively significant; the amplitude profiles of the propulsive wave along the dorsal fin are significantly different from those in forward swimming. When G. niloticus does fast maneuvering, its body is first bent into either a C shape or an S shape, then it is rapidly unwound in a travelling wave fashion. It rarely maneuvers without the help of the tail fin and body bending.     

Propulsive Velocity Optimization of 3-Joint Fish Robot Using Genetic-Hill Climbing Algorithm

Underwater robot is a new research field which is emerging quickly in recent years.Previous researches in this field focuson Remotely Operated Vehicles(ROVs),Autonomous Underwater Vehicles(AUVs),underwater manipulators,etc.Fish robot,which is a new type of underwater biomimetic robot,has attracted great attention because of its silence in moving and energyefficiency compared to conventional propeller-oriented propulsive mechanism.However,most of researches on fish robots have been carried out via empirical or experimental approaches,not based ondynamic optimality.In this paper,we proposed an analytical optimization approach which can guarantee the maximum propulsivevelocity of fish robot in the given parametric conditions.First,a dynamic model of 3-joint(4 links)carangiform fishrobot is derived,using which the influences of parameters of input torque functions,such as amplitude,frequency and phasedifference,on its velocity are investigated by simulation.Second,the maximum velocity of the fish robot is optimized bycombining Genetic Algorithm(GA)and Hill Climbing Algorithm(HCA).GA is used to generate the initial optimal parametersof the input functions of the system.Then,the parameters are optimized again by HCA to ensure that the final set of parametersis the"near"global optimization.Finally,both simulations and primitive experiments are carried out to prove the feasibility ofthe proposed method.     

Fish and chips: implementation of a neural network model into computer chips to maximize swimming efficiency in autonomous underwater vehicles

Recent developments in the design and propulsion of biomimetic autonomous underwater vehicles (AUVs) have focused on boxfish as models (e.g. Deng and Avadhanula 2005 Biomimetic micro underwater vehicle with oscillating fin propulsion: system design and force measurement Proc. 2005 IEEE Int. Conf. Robot. Auto. (Barcelona, Spain) pp 3312-7). Whilst such vehicles have many potential advantages in operating in complex environments (e.g. high manoeuvrability and stability), limited battery life and payload capacity are likely functional disadvantages. Boxfish employ undulatory median and paired fins during routine swimming which are characterized by high hydromechanical Froude efficiencies (approximately 0.9) at low forward speeds. Current boxfish-inspired vehicles are propelled by a low aspect ratio, 'plate-like' caudal fin (ostraciiform tail) which can be shown to operate at a relatively low maximum Froude efficiency (approximately 0.5) and is mainly employed as a rudder for steering and in rapid swimming bouts (e.g. escape responses). Given this and the fact that bioinspired engineering designs are not obligated to wholly duplicate a biological model, computer chips were developed using a multilayer perception neural network model of undulatory fin propulsion in the knifefish Xenomystus nigri that would potentially allow an AUV to achieve high optimum values of propulsive efficiency at any given forward velocity, giving a minimum energy drain on the battery. We envisage that externally monitored information on flow velocity (sensory system) would be conveyed to the chips residing in the vehicle's control unit, which in turn would signal the locomotor unit to adopt kinematics (e.g. fin frequency, amplitude) associated with optimal propulsion efficiency. Power savings could protract vehicle operational life and/or provide more power to other functions (e.g. communications).     

Experimental study of ATON stationary plasma thrusters

Results from experimental studies of integral characteristics of laboratory models of second-generation ATON stationary plasma thrusters are presented. Special attention is paid to high-voltage modes with a sufficiently high specific anode propulsive burn. Integral parameters of the thrusters were measured using a test bench with diffusion evacuation at the Moscow State Institute of Radioengineering, Electronics, and Automation and that with cryogenic evacuation at the Fakel Experimental and Design Bureau. The values of the thrust, specific propulsive burn, and efficiency measured in these test benches in the main operating mode coincide to within measurement errors. At a discharge power of 2 kW and voltage of about 700 V, the specific anode propulsive burn and anode thrust efficiency reach 3000 s and 60%, respectively. The experimental data show that the efficiency of the ATON stationary plasma thruster operating in a high-voltage mode is higher than that of other similar thrusters.     

Effect of Chord Flexure on Aerodynamic Performance of a Flapping Wing

Inspired by the fact that a high flexible wing in nature generates high aerodynamic performance, we investigated the aerodynamic performance of the flapping wing with different chord flexures. The unsteady, incompressible, and viscous flow over airfoil NACA0012 in a plunge motion was analyzed by using Navier-Stokes equation. Grid deformation, in which finite element and interpolation ideas are mixed, was introduced for computing large grid deformation caused by the chord flexures. We explored the optimal phase angle for thrust force and propulsive efficiency by varying the chord flexure from 0.05 to 0.7 when reduced frequency and plunge amplitude were fixed. Throughout parametric study on the phase angle and chord flexure amplitude, the maximum thrust force is achieved near at 0° in all given conditions, meanwhile, it is found that the optimal phase angle has dependency of chord flexure amplitude, which achieves higher aerodynamic performance compared to previous studies. These findings will provide a useful guideline for determining wing flexibility in design of a bio-mimetic air vehicle.     

Flipper bone distribution reveals flexible trailing edge in underwater flying marine tetrapods

Hydrofoil-shaped limbs (flipper-hydrofoils) have evolved independently several times in secondarily marine tetrapods and generally fall into two functional categories: (1) those that produce the majority of thrust during locomotion (propulsive flipper-hydrofoils); (2) those used primarily to steer and resist destabilizing movements such as yaw, pitch, and roll (controller flipper-hydrofoils). The morphological differences between these two types have been poorly understood. Theoretical and experimental studies on engineered hydrofoils suggest that flapping hydrofoils with a flexible trailing edge are more efficient at producing thrust whereas hydrofoils used in steering and stabilization benefit from a more rigid one. To investigate whether the trailing edge is generally more flexible in propulsive flipper-hydrofoils, we compared the bone distribution along the chord in both flipper types. The propulsive flipper-hydrofoil group consists of the forelimbs of Chelonioidea, Spheniscidae, and Otariidae. The controller flipper-hydrofoil group consists of the forelimbs of Cetacea. We quantified bone distribution from radiographs of species representing more than 50% of all extant genera for each clade. Our results show that the proportion of bone in both groups is similar along the leading edge (0–40% of the chord) but is significantly less along the trailing edge for propulsive flipper-hydrofoils (40–80% of the chord). Both flipper-hydrofoil types have little to no bony tissue along the very edge of the trailing edge (80–100% of the chord). This suggests a relatively flexible trailing edge for propulsive flipper-hydrofoils compared to controller flipper-hydrofoils in line with findings from prior studies. This study presents a morphological correlate for inferring flipper-hydrofoil function in extinct taxa and highlights the importance of a flexible trailing edge in the evolution of propulsive flipper-hydrofoils in marine tetrapods.     

Mass transfer enhancement in moving biofilm structures

Biofilms are layers of microbial cells growing on an interface and they can form highly complex structures adapted to a wide variety of environmental conditions. Biofilm streamers have a small immobile base attached to the support and a flexible tail elongated in the flow direction, which can vibrate in fast flows. Herein we report numerical results for the role of the periodical movement of biofilm streamers on the nutrient uptake and in general on the solute mass transfer enhancement due to flow-induced oscillations. We developed what to our knowledge is a novel two-dimensional fluid-structure interaction model coupled to unsteady solute mass transport and solved the model using the finite element method with a moving mesh. Results demonstrate that the oscillatory movement of the biofilm tail significantly increases the substrate uptake. The mass transfer coefficient is the highest in regions close to the streamer tip. The reason for substrate transfer enhancement is the increase in speed of tip movement relative to the surrounding liquid, thereby reducing the thickness of the mass transfer boundary layer. In addition, we show that the relative mass transfer enhancement in unsteady conditions compared with the rigid static structure is larger at higher flow velocities, and this relative increase favors a more flexible structure.     

Propulsion of micro-organisms by three-dimensional flagellar waves

The hydrodynamic effects of non-uniformities in cross-section and wavelength of three-dimensional flagellar waveforms are investigated. Estimates of propulsive velocity obtained by the use of mean constant wave parameters are close to the more precise calculations except where the wavelength varies more than twofold during wave propagation. Energy expenditures against external viscous forces are appreciably greater than the estimates based on mean wave parameter assumptions. Rotation of an inert head attached to a flagellum contributes a significant proportion of the total power dissipation. Energy requirements of an individual bull spermatozoon are greater than previous estimates. There is little difference between the energy supplies necessary to propel bacteria by rotating rigid flagellar helices or by propagation of helical waves.     

Effect of Cambering on the Aerodynamic Performance of Heaving Airfoils

In the present work,a parametric numerical study is conducted in order to assess the effect of airfoil cambering on theaerodynamic performance of rigid heaving airfoils.The incompressible Navier-Stokes equations are solved in their velocity-pressureformulation using a second-order accurate in space and time finite-difference scheme.To tackle the problem ofmoving boundaries,the governing equations are solved on overlapping structured grids.The numerical simulations are performedat a Reynolds number of Re=1100 and at different values of Strouhal number and reduced frequency.The resultsobtained show that the airfoil cambering geometric parameter has a strong influence on the average lift coefficient,while it hasa smaller impact on the average thrust coefficient and propulsive efficiency of heaving airfoils.     

A flexible domain is essential for the large step size and processivity of myosin VI

Myosin VI moves processively along actin with a larger step size than expected from the size of the motor. Here, we show that the proximal tail (the approximately 80-residue segment following the IQ domain) is not a rigid structure but, rather, a flexible domain that permits the heads to separate. With a GCN4 coiled coil inserted in the proximal tail, the heads are closer together in electron microscopy (EM) images, and the motor takes shorter processive steps. Single-headed myosin VI S1 constructs take nonprocessive 12 nm steps, suggesting that most of the processive step is covered by a diffusive search for an actin binding site. Based on these results, we present a mechanical model that describes stepping under an applied load.     

Unilateral ablation of trunk superficial neuromasts increases directional instability during steady swimming in the yellowtail kingfish Seriola lalandi

Detailed swimming kinematics of the yellowtail kingfish Seriola lalandi were investigated after unilateral ablation of superficial neuromasts (SNs). Most kinematic variables, such as tail‐beat frequency, stride length, caudal fin‐beat amplitude and propulsive wavelength, were unaffected but lateral amplitude at the tip of the snout (A₀) was significantly increased in SN‐disrupted fish compared with sham‐operated controls. In addition, the orientation of caudal fin‐tip relative to the overall swimming direction of SN‐disrupted fish was significantly deflected (two‐fold) in comparison with sham‐operated control fish. In some fish, SN disruption also led to a phase distortion of the propulsive body‐wave. These changes would be expected to increase both hydrodynamic drag and thrust production which is consistent with the finding that SN‐disrupted fish had to generate significantly greater thrust power when swimming at ≥1·3 fork lengths (L_F) s^?1. In particular, hydrodynamic drag would increase as a result of any increase in rotational (yaw) perturbation and sideways slip resulting from the sensory disturbance. In conclusion, unilateral SN ablation produced directional instability of steady swimming and altered propulsive movements, suggesting a role for sensory feedback in correcting yaw and slip disturbances to maintain efficient locomotion.     

Frequency tuning in animal locomotion

In locomotion that involves repetitive motion of propulsive structures (arms, legs, fins, wings) there are resonant frequencies f(*) at which the energy consumption is a minimum. As animals need to change their speed, they can maintain this energy minimum by tuning their body resonances. We discuss the physical principles of frequency tuning, and how it relates to forces, damping, and oscillation amplitude. The resonant frequency of pendulum-type oscillators (e.g. swinging arms and legs) may be changed by varying the mass moment of inertia, or the vertical acceleration of the pendulum pivot. The frequency of elastic vibrations (e.g. the bell of a jellyfish) can be tuned with a non-linear modulus of elasticity: soft for low deflection amplitudes (low resonant frequency), and stiff for large displacements (high resonant frequency). Tuning of elastic oscillations can also be achieved by changing the effective length or cross-sectional area of the elastic members, or by allowing springs in parallel or in series to become active. We propose that swimming and flying animals generate oscillating propulsive forces from precisely placed shed vortices and that these tuned motions can only occur when vortex shedding and the simple harmonic motion of the elastic elements of the propulsive structures are in resonance.     

Comparison of Efficency of Translation Between a Deformable Swimmer Versus a Rigid Body in a Bounded Fluid Domain: Consequences for Subcellular Transport

In this paper, we compare the translation efficiencies of a deformable circle that swims by means of low amplitude periodic tangential surface waves versus a rigid circle, moving in a bounded fluid domain. The swimmer is found to be much more efficient than the rigid body. We believe that this result gives some support to the active hypothesis of subcellular transport, where it is supposed that the organelle can generate by itself a propulsive flux, (by changes of form or metabolic activities) instead of just being carried by the motion of an external agent, like a molecular motor.