Mean Forward Speed Research Articles

Contrasting thrust generation mechanics and energetics of flapping foil locomotory states characterized by a unified - scaling

Self-propelled flapping foils with distinct locomotion-enabling kinematic restraints exhibit a remarkably similar Strouhal number ( $St$ )-Reynolds number ( $Re$ ) dependence. This similarity has been hypothesized to pervade diverse forms of oscillatory self-propulsion and undulatory biolocomotion; however, its genesis and implications on the energetic cost of locomotion remain elusive. Here, using high-resolution simulations of translationally free and restrained foils that self-propel as they are pitched, we demonstrate that a generality in the $St$ - $Re$ relationship can emerge despite significant disparities in thrust generation mechanics and locomotory performance. Specifically, owing to a recoil reaction induced passive heave, the fluid's inertial response to the prescribed rotational pitch, the principal source of thrust in unidirectionally free and towed configurations, ceases to produce thrust in a bidirectionally free configuration. Rather, the thrust generated from the leading edge suction mechanics self-propels a bidirectionally free pitching foil. Owing to the foregoing distinction in the thrust generation mechanics, the $St$ - $Re$ relationships for the bidirectionally and unidirectionally free/towed foils are dissimilar and pitching amplitude dependent, but specifically for large reduced frequencies, converge to a previously reported unified power law. Importantly, to propel at a given mean forward speed, the bidirectionally free foil must counteract the out-of-phase passive heave through a more intense rotational pitch, resulting in an appreciably higher power consumption over the range $10 \leq Re \leq 10^3$ . We highlight the critical role of thrust in introducing an offset in the $St$ - $Re$ relation, and through its amplification, being ultimately responsible for the considerable disparity in the locomotory performance of differentially constrained foils.

Assessment of the Cutting Performance of a Robot Mower Using Custom Built Software

Before the introduction of positioning technologies in agriculture practices such as global navigation satellite systems (GNSS), data collection and management were time-consuming and labor-intensive tasks. Today, due to the introduction of advanced technologies, precise information on the performance of agricultural machines, and smaller autonomous vehicles such as robot mowers, can be collected in a relatively short time. The aim of this work was to track the performance of a robot mower in various turfgrass areas of an equal number of square meters but with four different shapes by using real-time kinematic (RTK)-GNSS devices, and to easily extract data by a custom built software capable of calculating the distance travelled by the robot mower, the forward speed, the cutting area, and the number of intersections of the trajectories. These data were then analyzed in order to provide useful functioning information for manufacturers, entrepreneurs, and practitioners. The path planning of the robot mower was random and the turfgrass area for each of the four shapes was 135 m2 without obstacles. The distance travelled by the robot mower, the mean forward speed, and the intersections of the trajectories were affected by the interaction between the time of cutting and the shape of the turfgrass. For all the different shapes, the whole turfgrass area was completely cut after two hours of mowing. The cutting efficiency decreased by increasing the time, as a consequence of the increase in overlaps. After 75 minutes of cutting, the efficiency was about 35% in all the turfgrass areas shapes, thus indicating a high level of overlapping.

Compliant legs enable lizards to maintain high running speeds on complex terrains

Substrate variations are likely to constrain animal performance in natural environments, as running over complex terrains challenges the dynamic stability of the body differently in each step. Yet, being able to negotiate complex terrains at top speed is a strong advantage for animals that have to deal with predators and evasive prey. Little is known on how animals negotiate such terrain variability at high speed. We investigated this in fast-running Acanthodactylus boskianus lizards, by measuring their 3D kinematics using four synchronised high-speed video cameras (325 Hz) on an adaptable racetrack. This racetrack was covered with four different substrates, representing increasing levels of terrain complexity. We found that the lizards deal with this complexity gradient by gradually adopting more erect parasagittal leg postures. Legs in a more-erect position are more compliant and are therefore highly adjustable on complex terrains. Additionally, the lizards stabilise their head, which facilitates vestibular and visual perception. Together, compliant legs and head stabilisation enable the lizards to minimise movements of the body centre of mass, even when running on highly irregular terrains. This suggests that the head and the centre of mass are the priority targets for running on uneven terrains. As a result, running performance (mean forward speed) decreases only slightly, and only on the most challenging substrate under investigation.

Harvesting and threshing of maize with combine harvester

This study was conducted to evaluate the performance of maize combines having different types of headers i.e. snap roll type header and cutter bar type header. The mean forward speed and mean field capacity for cutter bar type header type header was 2.10 km/hr and 0.30-0.40 ha/hr, respectively while mean forward speed and mean field capacity for snap roll type header was 1.50 km/hr and 0.20-0.30 ha/hr, respectively. Shattering loss in snap roll type header combine was 12.50 to 18.50 per cent while shattering loss in case of cutter bar type header was 5.00 to 10.50 per cent.

Performance and three-dimensional kinematics of bipedal lizards during obstacle negotiation

Bipedal running is common among lizard species, but although the kinematics and performance of this gait have been well characterized, the advantages in biologically relevant situations are still unclear. Obstacle negotiation is a task that is ecologically relevant to many animals while moving at high speeds, such as during bipedal running, yet little is known about how obstacles impact locomotion and performance. We examined the effects of obstacle negotiation on the kinematics and performance of lizards during bipedal locomotion. We quantified three-dimensional kinematics from high-speed video (500 Hz) of six-lined racerunners (Aspidoscelis sexlineata) running on a 3 m racetrack both with and without an obstacle spanning the width of the track. The lizards did not alter their kinematics prior to contacting the obstacle. Although contact with the obstacle caused changes to the hindlimb kinematics, mean forward speed did not differ between treatments. The deviation of the vertical position of the body center of mass did not differ between treatments, suggesting that in the absence of a cost to overall performance, lizards forgo maintaining normal kinematics while negotiating obstacles in favor of a steady body center of mass height to avoid destabilizing locomotion.

Modeling and Optimization Analysis of a Single-Flagellum Micro-Structure Through the Method of Regularized Stokeslets

Bacteria such as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rhodobacter</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sphaeroides</i> use a single flagellum for propulsion and change of orientation. These types of simple organisms have inspired microrobotic designs with potential applications in medicine, which motivates this work. In this paper, an elastic model for a single-flagellum micro-structure is presented and followed by an analysis of the system based on optimization. The model is based on the method of Regularized Stokeslets which allows for a discretization of the system into particles connected by spring forces. The optimization analysis leads to the design of an optimal elasticity distribution that maximizes the mean forward speed of the structure. These elasticity coefficients are obtained through the use of adjoint-based optimization. The results are illustrated through simulations showing improvement on the swimming pattern of the micro-structure.

Asymmetric Surging of Ships in Following Seas and its Repercussions for Safety

A comprehensive investigation of an asymmetric longitudinal motion of ships displayed in long and steep following waves is presented. The focus is on the strongly nonlinear response, on the verge of the so called surf-riding condition, where a ship could be performing large-amplitude longitudinal oscillations around a mean forward speed. Expressions, in closed form, describing transient surge motion on the phase-plane are derived. For steady-state in particular, is shown that it is possible to determine an explicit analytical solution in the time-domain. Simple prediction formulae for the higher limit of asymmetric surging (threshold of global surf-riding) are derived on the basis of two alternative methods. Also, we investigate the effect of asymmetric surging on a ship's tendency for capsize in the neighborhood of wave crests where restoring capability is often reduced. The paper includes, as an Appendix, a review of types of ship instability and a summary of the historical roots of the field.

A comparison of spontaneous and wind-evoked running modes in crickets and cockroaches

We studied the effects of repetitive wind pulses on the escape behaviour of tethered male cockroaches and crickets walking on a styrofoam ball (open-loop stimulation). The movements of this sphere were recorded to measure duration, forward and turning speed of walking and standing phases during the insect's intended locomotion. All tested parameters were quantitatively similar for spontaneous walking in both species. During stimulation crickets generated a sequence of running bouts very regular both in duration and forward speed. Interposed were standing phases the mean duration of which was inversely correlated with the wind puff frequency. The mean distance run per period varied only between 100 and 150 mm for the tested frequencies. Cockroaches often showed continuous high speed running for seconds, variable in duration and followed by variable standing phases. Both parameters together with the mean forward speed of walking phases depended on the stimulus frequency in a non-linear manner. Therefore, the longest distance per period was run during stimulation with medium frequencies. Crickets were much better than cockroaches in turning away from the wind source in the range of 5–10 Hz stimulation. In cockroaches, the escape reaction was partly retained after cercus ablation, but ceased completely when the styli were also removed.

Locomotor Performance and Energetic Cost of Sidewinding by the Snake Crotalus Cerastes

ABSTRACT We measured the performance (burst speed and endurance) and the energetic cost of sidewinding locomotion for the viperid snake Crotalus cerastes. The linear scaling regressions relating log mass to log burst speed and log endurance have slopes of 0.29 and 1.01, respectively. Maximal burst speed observed for an individual snake (SVL=41.9cm, SVL is snout-vent length) was 3.7 km h1. Adult snakes were able to match a tread speed of 0.5 km h−1 for times ranging from 33 to more than 180 min, and at 0.7 km h−1 endurance times ranged from 9 to 52 min. Rates of oxygen consumption increased linearly over a range of aerobically sustainable speeds (0.28−0.50 km h−1), with a resulting net cost of transport (NCT) of 0.408 ml O2g−1 km−1 for eight snakes with a mean mass of 110g. Sidewinding of C. cerastes involves periodic movements with a frequency that increases linearly with mean forward speed. At 0.50 km h-1, the mean (N=8) mass-specific energetic cost per cycle of movement was 0.28μl O2g−1cycle−1 for sidewinding. The NCT and the cost per cycle of movement of C. cerastes sidewinding are significantly less than those of similar mass snakes (Coluber constrictor) performing either terrestrial lateral undulation or concertina locomotion. The NCT of C. cerastes sidewinding is also significantly less than that predicted for the terrestrial limbed locomotion of lizards of similar mass. Mean C. cerastes (0.405 ml O2g−1h−1) is only about half that reported for C. constrictor; however, the mean endurance at 0.60 km h−1 (73 min) for sidewinding C. cerastes does not differ significantly from that reported for C. constrictor laterally undulating.

Turbulence measurements from a submersible

A Pisces submersible has been used to carry turbulence sensors through various mixing regimes in the coastal waters of British Columbia. Dissipation sales of kinetic energy and heat are suited to such a vehicles, as they are smaller than scales at which the submersible itself is moved by mean flows. In coastal waters, significant dissipation levels are associated with semi-deterministic flows (river plumes, tidal flows, internal hydraulic effects), which allow most efficient use of the limited time and space ranges typical of manned submersible operations. Major modifications necessary to stabilize the submersible for mid-water running, constraints on equipment, and present operational limitations are discussed. Advantages of a submersible over towed or self-propelled vehicles include lower vibrational contamination of velocity signals, slower mean forward speed, which places less strain on response of frequency-limited sensors, and extremely flexible operation, particularly close to the air-sea interface. These characteristics are documented with data obtained through an internal wave train and across the base of a river plume in coastal waters.

Saltation and suspension trajectories of solid grains in a water stream

This paper continues the investigation of the motion of solitary grains in a water stream, reported by Francis (1973)- The trajectories of solid grains are photographed by a multi-exposure technique as they are propelled by water streams along the bed of a laboratory channel. Many thousands of photographs were taken and analysed to determine the positions, velocities and accelerations of the grain. The technique does not take into account the possible effect, in multi-grain transport, of intergranular collisions. The three different modes of transport of grains were all observed — rolling, saltation and suspension, and the proportion of each found for a variety of transport stage w*/w*0. The development of suspension is much less rapid than the development of saltation from rolling, but even at the highest stage used, about 3.0, there is still a small amount of rolling. The trajectory dimensions and geometry are shown in relation to the stage which uniquely determines the geometry. Experiments where the grain is suddenly entrained from a stationary position show that several features of the subsequent trajectory are the same as those of a trajectory with a prior history of movement: thus it is inferred that the start of a trajectory is by way of hydrodynamic forces rather than by the conservation of momentum of previous trajectories. Impacts and trajectories were analysed for the coefficient of friction tan cc and for the height of the effective thrust. While tan a is shown to be rather larger than has been suspected in the past, the variation of yn throws light upon predominance of slow fluid near the bed rather than high speed inrushes of fast fluid. Better information is now available for finding the mean forward speed of grains compared to that presented in the earlier paper. There are grounds for believing the existence of a * shear-drift ’ force on grains when they are in a velocity gradient, giving a force opposing gravity: but there is no evidence of a proximity effect of the bed independent of the velocity gradient.

Experiments on the motion of solitary grains along the bed of a water-stream

Experiments are described in which heavy solid grains are observed as they are propelled one at a time by a water stream over a fixed, plane, bed of grains of the same sort. This is a simplification of a sediment flow in a river, and it allows the motions of a particular grain to be followed. Three modes of movement are observed - rolling, saltating and suspension, characteristics of each mode are described, and multi-exposure photographs of grains are obtained to show the trajectories involved. In the saltation mode, grains follow low, smooth trajectories. Since no random motions are detectable on them, they appear to be governed very largely by ballistic forces; in suspension, grains follow much longer, higher paths which, being wavy, show up the influence of the irregular turbulence in the stream. The change from one mode to the other is readily observed with this technique. The grains spin in all three modes. The mean forward speed of grains ͞U has been measured over a wide range of shear stress, shape and size of grain and of water depth. Two possible types of non-dimensional plots are presented, one of which is most appropriate to studies of the rolling mode, and the other for the suspension mode. Both methods satisfactorily correlate ͞U with various values of the settling speed V g in still water, for particles all of the same shape and size. There is a considerable difference in ͞U for different shapes of grains, angular ones always travelling more slowly than rounded ones. Relative to the mean stream speed ͞u , the grains travel faster at small depths than at large depths, for the same stress and V g . The trajectories of grains confirm that the change from saltation to suspension occurs near a stage when the vertical components of turbulent velocity are approximately equal to the settling velocity of grains. Multi-exposure photographs of trajectories are analysed to determine the angle of friction applicable to the process whereby the motion of grains is retarded by striking the bed. Some experiments have been done with spheres travelling over a bed consisting of cylinders of the same diameter. These travel faster than any natural grains. Their dynamics, when rolling in contact with the cylinders, have been investigated, and a theoretical model evolved to explain the conditions under which their forward motion just stops. This model is shown to give a useful approximation to the forward speed to all types of grain just before they suddenly stop moving; these grains will not travel at all at lower speeds. Finally, an experiment is described where one marked grain is observed while moving in the company of many other grains. Its speed is then reduced below that which it would have as a solitary grain in the same water stream, thus indicating how the friction of the grain load reacts upon the stream to give smaller speeds close to the bed. Appendix 1 gives details of a demonstration that saltation can occur in a laminar flow, with no turbulence present at all. Appendix 2 gives the experimental data, and appendix 3 gives the algebra of the analysis of rolling motion.

Computation of ship responses in waves using panel method

Hydrodynamic coefficients, Forces / Moments and Motions of a ship moving with a mean forward speed in six degrees of freedom are computed using Panel Method. In this study, an existing numerical model without speed consideration was modified by incorporating the speed parameters. Appropriate Green function was used to calculate the concern velocity potential. The accuracy of the developed numerical code employing the Panel Method has been validated by comparing the result with known/published results of a series 60 ship available in the literature. Based on the results presented in the paper, it can be concluded that the developed model is able to predict the responses of the ship with forward speed effect. Keywords: Motions, Green function, 3D Source distribution. Â doi: 10.3329/jname.v1i1.2037 Journal of Naval Architecture and Marine Engineering 1(2004) 35-46