Half Stroke Research Articles

Performance Evaluation of Swim Fins under Zero Translation Speed

The aim of this study is the quantification of the efficiency of eight swim fins, measuring their thrust and transverse (vertical) force via a six-component piezoelectric balance, recording simultaneously their shape along the longitudinal axis. The fins were submerged in a water tank with zero free stream velocity, and were driven through a plunging and pitching mechanism. In the middle of half stroke the measured thrust varied for the examined fins between 11.3 N and 27.8 N and the corresponding vertical force between 55.8 N and 84.8 N. Fins with short, wide blades and stiff side rails proved to be more efficient than those with long, narrow blades of half the aspect ratio. Depending on the stiffness of the side rails and that of the fin blades, the performance of the low aspect ratio blades was very variable.

Hovering and intermittent flight in birds

Two styles of bird locomotion, hovering and intermittent flight, have great potential to inform future development of autonomous flying vehicles. Hummingbirds are the smallest flying vertebrates, and they are the only birds that can sustain hovering. Their ability to hover is due to their small size, high wingbeat frequency, relatively large margin of mass-specific power available for flight and a suite of anatomical features that include proportionally massive major flight muscles (pectoralis and supracoracoideus) and wing anatomy that enables them to leave their wings extended yet turned over (supinated) during upstroke so that they can generate lift to support their weight. Hummingbirds generate three times more lift during downstroke compared with upstroke, with the disparity due to wing twist during upstroke. Much like insects, hummingbirds exploit unsteady mechanisms during hovering including delayed stall during wing translation that is manifest as a leading-edge vortex (LEV) on the wing and rotational circulation at the end of each half stroke. Intermittent flight is common in small- and medium-sized birds and consists of pauses during which the wings are flexed (bound) or extended (glide). Flap-bounding appears to be an energy-saving style when flying relatively fast, with the production of lift by the body and tail critical to this saving. Flap-gliding is thought to be less costly than continuous flapping during flight at most speeds. Some species are known to shift from flap-gliding at slow speeds to flap-bounding at fast speeds, but there is an upper size limit for the ability to bound (∼0.3 kg) and small birds with rounded wings do not use intermittent glides.

Effects of flight speed upon muscle activity in hummingbirds

Hummingbirds have the smallest body size and highest wingbeat frequencies of all flying vertebrates, so they represent one endpoint for evaluating the effects of body size on sustained muscle function and flight performance. Other bird species vary neuromuscular recruitment and contractile behavior to accomplish flight over a wide range of speeds, typically exhibiting a U-shaped curve with maxima at the slowest and fastest flight speeds. To test whether the high wingbeat frequencies and aerodynamically active upstroke of hummingbirds lead to different patterns, we flew rufous hummingbirds (Selasphorus rufus, 3 g body mass, 42 Hz wingbeat frequency) in a variable-speed wind tunnel (0-10 m s(-1)). We measured neuromuscular activity in the pectoralis (PECT) and supracoracoideus (SUPRA) muscles using electromyography (EMG, N=4 birds), and we measured changes in PECT length using sonomicrometry (N=1). Differing markedly from the pattern in other birds, PECT deactivation occurred before the start of downstroke and the SUPRA was deactivated before the start of upstroke. The relative amplitude of EMG signal in the PECT and SUPRA varied according to a U-shaped curve with flight speed; additionally, the onset of SUPRA activity became relatively later in the wingbeat at intermediate flight speeds (4 and 6 m s(-1)). Variation in the relative amplitude of EMG was comparable with that observed in other birds but the timing of muscle activity was different. These data indicate the high wingbeat frequency of hummingbirds limits the time available for flight muscle relaxation before the next half stroke of a wingbeat. Unlike in a previous study that reported single-twitch EMG signals in the PECT of hovering hummingbirds, across all flight speeds we observed 2.9+/-0.8 spikes per contraction in the PECT and 3.8+/-0.8 spikes per contraction in the SUPRA. Muscle strain in the PECT was 10.8+/-0.5%, the lowest reported for a flying bird, and average strain rate was 7.4+/-0.2 muscle lengths s(-1). Among species of birds, PECT strain scales proportional to body mass to the 0.2 power (infinityM(b)(0.2)) using species data and infinityM(b)(0.3) using independent contrasts. This positive scaling is probably a physiological response to an adverse scaling of mass-specific power available for flight.

Stroke-averaged lift forces due to vortex rings and their mutual interactions for a flapping flight model

The stroke-averaged lift forces due to various vortex rings and their mutual interactions are studied using a flapping flight vortex model (Rayner,J. Fluid Mech., vol. 91, 1979, p. 697; Ellington,Phil. Trans. R. Soc. Lond.B, vol. 305, 1984b, p. 115). The vortex system is decomposed into the wing plane (wing-linked) vortex ring, a loop closed by the bound vortex and (arc-shaped) trailing vortex and the wake (the vortex rings shed previously). Using the vorticity moment theory (Wu,AIAA J., vol. 19, 1981, p. 432) we are able to identify the roles of vortex rings in lift production or reduction and express the lift as function of areal contraction or expansion of vortex rings. The wake vortex rings induce areal contraction of the trailing vortex, which should decrease the lift, but this decrease is exactly compensated by the inducing effect of the trailing arc on the wake. The wake reduces the lift through inducing a downwash velocity on the wing plane. The lift force is shown to drop to a minimum at the second half stroke, and then increases to an asymptotic value slightly below the lift at the first half stroke, in such a way following the experimental observation of Birch & Dickinson (Nature, vol. 412, 2001, p. 729). The existence of the negative peak of lift is due to the first shed vortex ring which, just at the second half stroke, lies in the close vicinity to the wing plane, leading to a peak of the wing plane downwash velocity.

The rowing-to-flapping transition: ontogenetic changes in gill-plate kinematics in the nymphal mayfly Centroptilum triangulifer (Ephemeroptera, Baetidae)

Comparative studies encompassing a wide range of aquatic animals have shown that rowing is exclusively used at low Reynolds numbers (Re 100, although few studies have been undertaken to document the transition in individual species that traverse the intermediate Re regime using a single set of appendages. Thus, it is not generally known whether a gradual increase in Re within a system results in a gradual or sudden shift between rowing and flapping. In the present study, we document ventilatory kinematics of a nymphal mayfly Centroptilum triangulifer that develops using a serial array of seven pairs of abdominal gill plates and operates at Reynolds numbers in the range 2–22 during ontogeny. We found that some kinematic variables (stroke frequency and metachronal phase lag) did not change during ontogeny but that others changed substantially. Specifically, gill kinematics in small instars used strokes with large pitch and stroke-plane deviations, whereas larger instars used strokes with minimal pitch and minimal stroke-plane deviation. Gills in larger instars also acquired an intrinsic hinge that allowed passive asymmetric movement between half strokes. Net flow in small animals was directed ventrally and essentially parallel to the stroke plane (i.e. rowing), whereas net flow in large animals was directed dorsally and essentially transverse to the stroke plane (i.e. flapping). The change in whole-gill kinematics from rowing to flapping occurred across a narrow Re range (3–8), which suggests a possible hydrodynamic demarcation between rowing and flapping. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 540–555.

2-D AERODYNAMIC MODEL OF INSECT FLYING MECHANISM FOR ROBOTIC APPLICATION

Two-dimensional aerodynamic models of unsteady flapping wing motions are analyzed with focus on advanced rotation and the wing's rotation axis to explain the force peak at the end of each half stroke. In this model, the translational velocity of the wing along the stroke plane is constant for most of the time except near end and the beginning of each stroke where it slows down to zero and then speeds up again for the next stroke. The flapping wing motions with various combination of controlled translational and rotational velocity of the wings along horizontal or inclined stroke planes with straight-line trajectory are investigated numerically through two-dimensional Navier-Stokes solutions. CFD programs based on Finite Volume Method are used to simulate the flow around a thin airfoil in combined translational and rotational motions. The simulation provides the flow patterns, pressure fields from which the aerodynamic forces and moments are obtained.

Stroking Characteristics during Time to Exhaustion Tests

Race analyses during swimming reveal how exercise duration affects both clean swimming speed (v), stroke rate (SR), and stroke length (SL). The aim of this study is to provide an explanation for the change of SL and SR during paced exercise swimming the front crawl through an analysis of intracycle changes in motor organization. Trained swimmers (N = 10) swam three times to exhaustion (TTE in seconds) at predetermined velocities corresponding to 95%, 100%, and 110% of the mean speed attained in a 400-m race (V 400). During TTE tests, SR, SL, durations of the glide + catch, pull, push, and recovery phases (s) were measured. Assessment of arm coordination was made through the calculation of the index of coordination (IdC). The time allotted to propulsion per distance unit was estimated (T prop). For all tested speeds, fatigue development induced a gradual increase of SR with concomitant decrease of SL. The duration of the nonpropulsive phases decreased, whereas the duration of the propulsive phases per stroke remained constant. The IdC increased reflecting a reduction of the lag time between two consecutive propulsive actions. Consequently, T prop increased. Fatigue development induced an increase of the SR to compensate for the reduced capacity to generate a propulsive impulse per stroke. The change in arm coordination allows a better chain of the propulsive actions and leads to a greater time allotted to propulsion per distance unit. Such motor adaptation ensures that the overall propulsive impulse remained constant whereas average propulsive force per arm stroke is reduced.

Two-Dimensional Aerodynamic Models of Insect Flight for Robotic Flapping Wing Mechanisms of Maximum Efficiency

In the “modified quasi-steady” approach, two-dimensional (2D) aerodynamic models of flapping wing motions are analyzed with focus on different types of wing rotation and different positions of rotation axis to explain the force peak at the end of each half stroke. In this model, an additional velocity of the mid chord position due to rotation is superimposed on the translational relative velocity of air with respect to the wing. This modification produces augmented forces around the end of each stroke. For each case of the flapping wing motions with various combination of controlled translational and rotational velocities of the wing along inclined stroke planes with thin figure-of-eight trajectory, discussions focus on lift-drag evolution during one stroke cycle and efficiency of types of wing rotation. This “modified quasi-steady” approach provides a systematic analysis of various parameters and their effects on efficiency of flapping wing mechanism. Flapping mechanism with delayed rotation around quarter-chord axis is an efficient one and can be made simple by a passive rotation mechanism so that it can be useful for robotic application.

When wings touch wakes: understanding locomotor force control by wake–wing interference in insect wings

Understanding the fluid dynamics of force control in flying insects requires the exploration of how oscillating wings interact with the surrounding fluid. The production of vorticity and the shedding of vortical structures within the stroke cycle thus depend on two factors: the temporal structure of the flow induced by the wing's own instantaneous motion and the flow components resulting from both the force production in previous wing strokes and the motion of other wings flapping in close proximity. These wake-wing interactions may change on a stroke-by-stroke basis, confronting the neuro-muscular system of the animal with a complex problem for force control. In a single oscillating wing, the flow induced by the preceding half stroke may lower the wing's effective angle of attack but permits the recycling of kinetic energy from the wake via the wake capture mechanism. In two-winged insects, the acceleration fields produced by each wing may strongly interact via the clap-and-fling mechanism during the dorsal stroke reversal. Four-winged insects must cope with the fact that the flow over their hindwings is affected by the presence of the forewings. In these animals, a phase-shift between the stroke cycles of fore- and hindwing modulates aerodynamic performance of the hindwing via leading edge vortex destruction and changes in local flow condition including wake capture. Moreover, robotic wings demonstrate that phase-lag during peak performance and the strength of force modulation depend on the vertical spacing between the two stroke planes and the size ratio between fore- and hindwing. This study broadly summarizes the most prominent mechanisms of wake-wing and wing-wing interactions found in flapping insect wings and evaluates the consequences of these processes for the control of locomotor forces in the behaving animal.

The mechanisms of lift enhancement in insect flight.

Recent studies have revealed a diverse array of fluid dynamic phenomena that enhance lift production during flapping insect flight. Physical and analytical models of oscillating wings have demonstrated that a prominent vortex attached to the wing's leading edge augments lift production throughout the translational parts of the stroke cycle, whereas aerodynamic circulation due to wing rotation, and possibly momentum transfer due to a recovery of wake energy, may increase lift at the end of each half stroke. Compared to the predictions derived from conventional steady-state aerodynamic theory, these unsteady aerodynamic mechanisms may account for the majority of total lift produced by a flying insect. In addition to contributing to the lift required to keep the insect aloft, manipulation of the translational and rotational aerodynamic mechanisms may provide a potent means by which a flying animal can modulate direction and magnitude of flight forces for manoeuvring flight control and steering behaviour. The attainment of flight, including the ability to control aerodynamic forces by the neuromuscular system, is a classic paradigm of the remarkable adaptability that flying insects have for utilising the principles of unsteady fluid dynamics. Applying these principles to biology broadens our understanding of how the diverse patterns of wing motion displayed by the different insect species have been developed throughout their long evolutionary history.

Gas–liquid mass transfer in a 15 cm diameter reciprocating plate column

AbstractData are reported on the absorption of oxygen by water under countercurrent flow conditions, in a 15 cm diameter reciprocating plate bubble column. Amplitude (half stroke) is set at 12.7 mm and reciprocation frequencies up to 5 Hz have been employed. Five different types of plate have been compared; two of the types are multiperforated and three of the types are ‘donut’ plates with a single central hole. When the comparison is made in terms of the volumetric mass transfer coefficient (kla) and power input per unit mass, the best of the five types of plate is a multiperforated plate with small holes of diameter 6.35 mm.© 2003 Society of Chemical Industry

Flow visualization and unsteady aerodynamics in the flight of the hawkmoth,Manduca sexta

The aerodynamic mechanisms employed durng the flight of the hawkmoth,Manduca sexta, have been investigated through smoke visualization studies with tethered moths. Details of the flow around the wings and of the overall wake structure were recorded as stereophotographs and high–speed video sequences. The changes in flow which accompanied increases in flight speed from 0.4 to 5.7 m s−1were analysed. The wake consists of an alternating series of horizontal and vertical vortex rings which are generated by successive down– and upstrokes, respectively. The downstroke produces significantly more lift than the upstroke due to a leading–edge vortex which is stabilized by a radia flow moving out towards the wingtip. The leading–edge vortex grew in size with increasing forward flight velocity. Such a phenomenon is proposed as a likely mechanism for lift enhancement in many insect groups. During supination, vorticity is shed from the leading edge as postulated in the ‘flex’ mechanism. This vorticity would enhance upstroke lift if it was recaptured diring subsequent translation, but it is not. Instead, the vorticity is left behind and the upstroke circulation builds up slowly. A small jet provides additional thrust as the trailing edges approach at the end of the upstroke. The stereophotographs also suggest that the bound circulation may not be reversed between half strokes at the fastest flight speeds.

Non-fibrillar muscles and the start and cessation of flight in the milkweed bug,Oncopeltus

1. The behavioural sequence at the start of flight inOncopeltus has been analysed by high speed photography (Fig. 1). The flight muscles (in particular the non-fibrillar ones) responsible for this behaviour have been identified by electrical recordings, ablation experiments and anatomical evidence. 2. The release of the wing base by the prothoracic lobe is assigned to the prothoracic dorsoventralis and mesosterni primus muscles on anatomical evidence. 3. The wing opening, consisting of unlocking and unfolding of the fore wings, is performed by a high frequency discharge of a motor neuron of the tergal promoter, remotor and tergotrochanteral muscles (Fig. 5). Non-flight wing opening is also similarly achieved (Fig. 6). 4. It is argued that the action of the leg muscles additionally triggers the oscillatory contractions of the myogenic motor by stretching the neurally activated dorsal longitudinal muscle. Thus the first half stroke in flight is a downstroke which serves to link the fore and hind wings. 5. At the start of flight the click mechanism in the wing articulation is put into operation by contraction of the tonic pleurofurcal muscle (Fig. 8) which adjusts the lateral stiffness of the thoracic box. 6. The take-off jump occurs after flight has started and is executed by a complex of trochanteral depressor muscles of the meso- and metathoracic legs (Fig. 9). 7. At the end of flight the wings are flexed and locked by the firing of a motor neuron in the episterno-alaris muscle (Fig. 10).

N/C Non-circular Gear Shaper by Hobbing Method

This gear generating machine is designed to cut the gear by the shaping method in which the cutter head having a hob cutter moves vertically. In this case, the hob makes an intermittent index rotation at every half stroke to move the rack in the direction of X axis and to lift the cutting edge in the returning stroke. Synchronously with this, the table is tape-controlled to make the relative angular displacement as well as the movement in the direction of Y axis and set up an automatic cycle. In this way, this machine generates non-circular gears of various dimensions. Consequently, the machine is not only made simple in construction but also makes it possible to meet the quantitative demand as well as the qualitative demand for improved accuracy and uniformity of the generated gears. The machine can be used to produce many kinds of non-circular gears more easily than a conventional machine.