Constant Velocity Phase Research Articles

Tribo-Charge of Diamond-Like Carbon (DLC) Contact at Slider Disk Interface and Tribo Current Flow Through Disk DLC

This paper reports an experimental investigation on tribo-charging of diamond-like carbon (DLC) at the slider-disk interface and tribo-current flow through disk DLC. Measurements of the tribo-current are in three regimes-slider flying and touch down during disk deceleration, slider dragging on disk surface, and slider takeoff and flying during disk acceleration. Comparisons are made for tribo-charge currents at different disk initial linear velocities. It is observed that the tribo-current appears earlier with larger amplitude for a disk with larger initial velocities. During disk constant velocity phase, tribo-current ripple was observed while the slider is dragging on the disk surface due to intermittent slider/disk interactions. Despite having the same constant velocity, the equilibrium level of the ripple is different and depends on the disk initial velocity. Additional tribo-voltage and acoustic emission measurements were performed and compared with the tribo-current measurement to provide more insight to the tribo-charging process.

Unsteady finite-depth effects during resistance tests on a ship model in a towing tank

This paper covers an extension of the study of Doctors et al. (J Ship Res 52(4):263–273, 2008) on oscillations in wave resistance during the constant-velocity phase of a towing-tank resistance test on a ship model to the case of relatively shallow water. We demonstrate here that the unsteady effects are very prominent and that it is essentially impossible to achieve a steady-state resistance curve in a towing tank of typical proportions for a water-depth-to-model-length ratio of 0.25. This statement is particularly true in the speed region near a depth Froude number of unity. However, on the positive side, we show here that an application of unsteady linearized wave-resistance theory provides an excellent prediction of the measured total resistance, when one accounts for the form factor in the usual manner. Finally, a simple application of the results to the planning and analysis of towing-tank tests is presented.

Experimental observations of water‐like behavior of initially fluidized, dam break granular flows and their relevance for the propagation of ash‐rich pyroclastic flows

The physics of ash‐rich pyroclastic flows were investigated through laboratory dam break experiments using both granular material and water. Flows of glass beads of 60–90 μm in diameter generated from the release of initially fluidized, slightly expanded (2.5–4.5%) columns behave as their inertial water counterparts for most of their emplacement. For a range of initial column height to length ratios of 0.5–3, both types of flows propagate in three stages, controlled by the time scale of column free fall ∼(h0/g)1/2, where h0 denotes column height and g denotes gravitational acceleration. Flows first accelerate as the column collapses. Transition to a second, constant velocity phase occurs at a time t/(h0/g)1/2 ∼ 1.5. The flow velocity is then U ∼ (gh0)1/2, larger than that for dry (initially nonfluidized) granular flows. Transition to a last, third phase occurs at t/(h0/g)1/2 ∼ 4. Granular flow behavior then departs from that of water flows as the former steadily decelerates and the front position varies as t1/3, as in dry flows. Motion ceases at t/(h0/g)1/2 ∼ 6.5 with normalized runout x/h0 ∼ 5.5–6. The equivalent behavior of water and highly concentrated granular flows up to the end of the second phase indicates a similar overall bulk resistance, although mechanisms of energy dissipation in both cases would be different. Interstitial air‐particle viscous interactions can be dominant and generate pore fluid pressure sufficient to confer a fluid‐inertial behavior to the dense granular flows before they enter a granular‐frictional regime at late stages. Efficient gas‐particle interactions in dense, ash‐rich pyroclastic flows may promote a water‐like behavior during most of their propagation.

Unsteady Effects During Resistance Tests on a Ship Model in a Towing Tank

It is known that there are oscillations in the wave resistance during the constant-velocity phase of a towing-tank resistance test on a ship model. In this work, the unsteady thin-ship resistance theory has been applied to this case. The results have been compared with experiment data obtained using a towing carriage the velocity history of which can be programmed. It is demonstrated here that generally excellent correlation exists between the theory and the experiments. In particular, one can predict the influence of Froude number, rate of acceleration, and type of smoothing of the acceleration on the characteristics of the oscillations. These characteristics include the amplitude, rate of decay, frequency, and phasing of the oscillations in the curve of wave resistance versus time.

Propagation and hindered settling of laboratory ash flows

The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3‐m‐long, horizontal lock‐exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with , like other types of dam‐break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s−1, aggradation rates were identical to those determined independently, at the same value of E, in quasi‐static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h0/x0 in the lock reservoir and (2) the ratio tsett/tgrav of two timescales: a particle settling time tsett and a gravitational acceleration time tgrav.

A comparison of sprinting kinematics on two types of treadmill and over‐ground

Conventionally motorized treadmills elicit different sprinting kinematics to the over-ground condition. Treadmills powered by a torque motor have been used to assess sprinting power; yet, the kinematics of sprinting on the torque treadmill are unknown. This study compares the sprinting kinematics, during the constant velocity phase, between a conventional treadmill, a torque treadmill and the over-ground condition to assess the suitability of each treadmill for sprinting studies and training. After familiarization, 13 recreationally active males performed multiple sprints at various experimental settings on each surface. Ninety sprints, which attained mean velocities over 7.0 m/s, had their lower-body sagittal plane joint angles during ground contact captured at 250 Hz. These data were low-pass filtered at 10 Hz, and compared with respect to surface, subject and velocity using an ANCOVA statistical model. Sprinting on the conventional treadmill elicited a longer ground contact time, a longer braking phase, a more extended knee at foot strike and a faster extending hip than the torque treadmill and over-ground (all P<0.05). The torque treadmill obtained an equivalent sprinting technique to the over-ground condition, with the exception of a less extended hip at toe-off, suggesting that it is more appropriate for laboratory sprinting analyses and training than the conventional treadmill.

Dynamic trunk stability is improved in paraplegics following kayak ergometer training

The purpose of the study was to assess whether postural stability in persons with spinal cord injury (SCI) could be affected by training. Ten post-rehabilitated persons with thoracic SCI performed 30 sessions of kayak ergometer training during a 10-week period. The ergometer was modified with a balance module adjustable in the medio-lateral direction. Before and after the training period, horizontal support-surface translations were presented randomly, either in the forward or backward direction, or to the side, while subjects sat in their own wheelchairs. The platform perturbation consisted of an unpredictable initial acceleration, followed by a constant-velocity phase and a predictable deceleration. Markers were applied on the trunk and movement data were recorded in 3D. Four kinematic responses of trunk angular and linear displacement were investigated. In general, postural stability was improved after training with smaller rotational and linear displacements of the trunk observed during both predictable and unpredictable translations in all directions. Thus, the training was able to improve the ability of persons with long-standing SCI to maintain an upright sitting posture in response to externally generated balance perturbations, which should imply an increased capacity to master similar challenges to balance in everyday life.

Bubble Acceleration in the Ablative Rayleigh-Taylor Instability

The highly nonlinear evolution of the single-mode Rayleigh-Taylor instability (RTI) at the ablation front of an accelerated target is investigated in the parameter range typical of inertial confinement fusion implosions. A new phase of the nonlinear bubble evolution is discovered. After the linear growth phase and a short constant-velocity phase, it is found that the bubble is accelerated to velocities well above the classical value. This acceleration is driven by the vorticity accumulation inside the bubble resulting from the mass ablation and vorticity convection off the ablation front. While the ablative growth rates are slower than their classical values in the linear regime, the ablative RTI grows faster than the classical RTI in the nonlinear regime for deuterium and tritium ablators.

A Novel Continuous Alternate Motion Mechanism With Two Input Wheels

Abstract This paper presents the design of a mechanism with the following specifications: continuous alternate motion, wide motion phases with constant angular velocity, parallel input and output shafts, and great strokes. Those specifications derive from a possible application in the textile field. The mechanism is composed of two star wheels properly coupled together: there are two counter-rotating input wheels, alternately coupling with slots first, then teeth at each side of the output wheel. As usual for star wheels, pins and slots handle the acceleration and deceleration phases, while the constant velocity phase is performed by coupling sectors of toothed gears. A proper design of pins and slots is performed, so that at the same time when a pin from one input wheel is releasing a slot, a pin from the other input wheel engages a slot on the other side of the output wheel, forcing the latter to an opposite motion. In this way the output wheel has a continuous and smooth alternate motion. By annihilating the arrest phases typical of star wheels, the proposed system eliminates the discontinuities in the acceleration diagram. The paper develops a complete parametrical analysis of the device, underlining the effect of the constraints on the shape of the motion laws with particular emphasis on the acceleration and deceleration phases. In this way the output wheel has a continuous and smooth alternate motion. With respect to an analogous mechanism realizing the same laws of motion, e.g., cams, this device is very compact and economical, also presenting parallel input and output shafts, and significantly reduces sliding and wear.

The front condition for gravity currents

Self-similar plane solutions for the inertial stage of gravity currents are related to the initial parameters and a coefficient that is determined by the boundary condition at the front. Different relations have been proposed for the boundary condition in terms of a Froude number at the front, none of which have a sound theoretical or experimental basis. This paper focuses on considerations of the appropriate Froude number based on results of lock-exchange experiments in which extended inertial gravity currents are generated in a rectangular cross-section channel. We use ‘top-hat’ vertical density profiles of the currents to obtain an ‘equivalent’ depth, defined by profiles having the same buoyancy at every position as the real profiles. As in previous work, our experimental results show that in the initial constant-velocity phase the Froude number can be defined in terms of the lock depth. However, as the current enters the similarity phase when the initial release conditions are no longer relevant, we find that the Froude number is more appropriately defined in terms of the maximum height of the head. Strictly speaking, the self-similar solution to the shallow-water equations requires a front condition that uses the height at the rear of the head. We find that this rear Froude number is not constant and is a function of the head Reynolds number over the range 400–4500.

二重予測制御で同期化した高速・高精度ＮＣタッピング

A new control algorithm for a numerically controlled tapping is proposed. This algorithm uses two predictive controls. The Z-axis servo has a predictive control and the spindle response is also predicted. By doing so we achieve precise synchronization between the spindle-axis and the Z-axis not only constant velocity phase but also while accelerating and decelerating. An experiment using this algorithm achieved 625 rev/s2 acceleration and deceleration, 6000 rpm maximum speed and 10μm precision.

Minimum time on-line joint trajectory generator based on low order spline method for industrial manipulators

In this paper, we propose the use of a low order polynomial method named 3-Cubic method to generate minimum time on-line joint trajectory for an industrial manipulator. A 3-Cubic Spline between two end points consists of a set of three cubic splines, which not only give an analytical solution to minimize travel time with velocity, acceleration and jerk constraints but also provides the continuity till second derivative of position (i.e. acceleration). This results in a position trajectory being dynamically balanced. This leads to the generation of a new type of trajectory named 3-Cubic Linear Segment with Trapezoidal Transition (3CLSTT) trajectory. In this trajectory generation, duration of constant acceleration/deceleration and constant velocity phases are calculated according to the requirements of maximum limiting values of velocity and acceleration thus traversing the trajectory in minimum possible time. Wandering (undershoot/overshoot) is completely eliminated in the resulting trajectories with limited computational effort.

Impulsive Motion of a Moving Circular Cylinder in a Viscous Flow by the Numerical Simulation

ABSTRACTAn innovative computation procedure is developed to solve the external flow problems for viscous fluids. The method is able to handle the infinite domain so that it is convenient for the external flows. The code is based on the projection method of the Navier-Stokes equations. We use the three-step explicit finite element method to solve the momentum equation by extracting the boundary effects from the finite computation domain. The pressure Poisson equation for the external field is treated by the boundary element method. The arbitrary Lagrangian-Eulerian (ALE) scheme is employed to incorporate the present algorithm to deal with the moving boundary, such as the motion of an impulsively moving circular cylinder in a viscous fluid. The model demonstrates that drag force is well predicted for a circular cylinder moving in a still viscous fluid starting from rest, to a constant acceleration, and then maintaining at a uniform velocity. In the constant acceleration phase, the drag force is closed to the added mass effect from the ideal flow theory. On the other hand, the drag force is equal to viscous flow theory in the constant velocity phase.

Interplanetary Spacecraft Controllers Using Thrusters

This paper is concerned with the design of attitude controllers for interplanetary spacecraft. The first controller is for Rhumb-line precession using almost periodic train of thruster pulses at a constant phase with the Sun. Earlier relationships are expanded and interlaced coherently. One such precession, along with its ungrowing nutation loops, is demonstrated on a Mercator plot. The second controller is for nutation damping. For spacecraft with products-ofinertia, a control scheme is formulated that involves bandpass filters for removing the constant biases from the transverse angular rates. Four phase angles, ninety degrees apart in a nutation cycle, are determined analytically whereat the positive or negative control impulses about the transverse axes are inducted to damp the nutations. The third controller is for spacecraft attitude control during aerobraking for planetary orbit insertion. The controller seeks and maintains a zero aerodynamic torque orientation of the spacecraft in that portion of the orbit where aerobraking is effective. Since this orientation may correspond to a constant, positive bias angle of attack measured from the instantaneous velocity vector turning at a nonuniform pitch rate, a pitch angular acceleration command profile is developed for parameterizing a thruster controller. The fourth, and the last, controller in the paper is for soft landing. It comprises (a) a longitudinal velocity controller employing a velocity-altitude reference trajectory, a gravity turn, and a constant velocity phase for soft impact; (b) a lateral velocity controller, requiring pitching the spacecraft at varying rates; and (c) a reaction jet controller for tracking the commanded pitch angle and rate until touchdown.

Patterning accuracy estimation during stage acceleration in the electron beam direct writing system EX-8D

We have evaluated patterning accuracy during stage acceleration of EX-8D in which a beam deflection control system has been installed to compensate a beam positional shift depending on both the stage velocity and position. From the fact that there is no difference in patterning accuracy between stage acceleration phase and constant velocity phase, it was found that the control system functions accurately. We have also analyzed beam position deviation. The analysis results showed that improvement of the laser interferometer resolution, stabilization of the objection lens, and elimination of the ambient magnetic field change are necessary to achieve 1 Gbit class ultra-large- scale integrated patterning of 0.15 μm within 10% beam position fluctuation. By adopting continuous writing during stage acceleration and deceleration while turning, the improvement of the throughput was also confirmed without reducing the patterning accuracy.

How fast can a crack go?

The assumption of a material specific relation between the energy dissipation at the edge of a crack propagating under small scale yielding conditions leads to upper limits of the crack velocity: the Rayleigh wave velocity for modes I and II, and theS wave velocity for mode III. If a mode II crack can pass the “forbidden” subsonic super-Rayleigh region, then the upper limit would be theP wave velocity. Experiments invariably show lower maximum speeds, typically less than 60% of theS wave velocity, but they also frequently show accelerations to a constant velocity, depending on the acceleration history, and increasing surface roughness, even during the constant velocity phase. Other experiments show that the energy dissipation at the crack edge usually is several times larger at very high than at very low velocities. These results indicate a considerable growth of the process region, so much that the intrinsic length parameter which determines its height at very low crack speeds becomes insignificant, and then the rationale for a material specific relation between energy dissipation and velocity disappears. This disappearance can contribute to the explanation of the experimental results.

Antagonist cocontraction of knee extensors during constant velocity muscle shortening and lengthening

Electromyography (EMG) was used to study the role of antagonist cocontraction of the quadriceps muscles of 24 normal subjects during constant velocity muscle performance tests on a KIN/COM dynamometer. The hypothesis tested the dependence of antagonist cocontraction on joint angle, limb velocity and type of contraction. Seated subjects performed two dynamic tests of resisted muscle shortening and lengthening of the knee flexors through a 65 ° range of knee motion under two constant velocity conditions in a single session. Each test consisted of four repetitions of maximum voluntary contractions (MVC) of constant velocity muscle shortening and lengthening of the knee flexors; one test was performed at 30 ° s −1, the other at 90 ° s −1. The sequence of velocity testing was randomized. Normalized (%Max) values of peak amplitude EMG of knee extensors were measured every 5 ° in the constant velocity phase of each test and compared in a paired manner for each angle, contraction type and velocity, by split-plot two-way analysis of variance (ANOVA). The level of antagonist quadriceps cocontraction was low, ranging from 5–8% Max between 70 ° and 15 ° of knee flexion for both velocities and contraction types. Antagonist quadriceps activity was found to be velocity dependent during agonist muscle shortening ( P < 0.02). However, no other effects of angle, contraction type, or velocity were found for antagonist quadriceps. It is concluded that antagonist quadriceps and the previously reported hamstrings 46 respond differently under typical constant velocity test conditions, and these differences need to be accounted for when intepreting muscle performance tests.

Motile kinetochores and polar ejection forces dictate chromosome position on the vertebrate mitotic spindle.

We argue that hypotheses for how chromosomes achieve a metaphase alignment, that are based solely on a tug-of-war between poleward pulling forces produced along the length of opposing kinetochore fibers, are no longer tenable for vertebrates. Instead, kinetochores move themselves and their attached chromosomes, poleward and away from the pole, on the ends of relatively stationary but shortening/elongating kinetochore fiber microtubules. Kinetochores are also "smart" in that they switch between persistent constant-velocity phases of poleward and away from the pole motion, both autonomously and in response to information within the spindle. Several molecular mechanisms may contribute to this directional instability including kinetochore-associated microtubule motors and kinetochore microtubule dynamic instability. The control of kinetochore directional instability, to allow for congression and anaphase, is likely mediated by a vectorial mechanism whose magnitude and orientation depend on the density and orientation or growth of polar microtubules. Polar microtubule arrays have been shown to resist chromosome poleward motion and to push chromosomes away from the pole. These "polar ejection forces" appear to play a key role in regulating kinetochore directional instability, and hence, positions achieved by chromosomes on the spindle.

Microtubule assembly and kinetochore directional instability in vertebrate monopolar spindles: implications for the mechanism of chromosome congression.

We have proposed previously a kinetochore motor-polar ejection model for chromosome congression to the metaphase plate where forces generated at the kinetochore are antagonized by away-from-the pole forces generated within each half-spindle on the chromosome arms. This model was based in large part on observations of the behavior of chromosomes on monopolar spindles. In these cells chromosomes typically become attached to the pole by only one kinetochore fiber. These mono-oriented chromosomes move to positions away from the pole even though they are pulled poleward at their kinetochores. Their arms are also ejected away from the pole when severed from the centromere. Here we have characterized further the properties of monopolar spindles in newt lung epithelial cells to determine the similarities between monopolar and bipolar spindles. We found no significant differences between monopolar and bipolar spindles over the parameters examined, which included: microtubule dynamics as measured by fluorescence redistribution after photobleaching; the ability of polar microtubule arrays to push chromosome arms away from the pole; the dependence of chromosome position relative to the pole on microtubule assembly; the number of kinetochore microtubules per kinetochore; and the directional instability of kinetochore motion during chromosome oscillations poleward and away-from-the-pole. As in bipolar spindles, kinetochore directional instability is characterized by abrupt switching between constant velocity phases of poleward and away-from-the-pole motion. From these data we conclude that the mechanism(s) responsible for chromosome positioning in monopolar spindles are fundamentally the same as those in bipolar spindles; only the geometry of the two spindle forms and the interplay between sister kinetochore directional instabilities are different. We also found no correlation in the kinetochore-to-pole distance with kinetochore microtubule number in monopolar spindles, but a strong qualitative correlation with microtubule density. This finding indicates that oscillations of mono-oriented chromosomes in both monopolar and bipolar spindles occur because chromosomes persist in poleward motion until they reach a density of polar microtubules sufficiently high to promote switching to away-from-the-pole motion. As the kinetochore and chromosome arms move away-from-the-pole, microtubule density decreases and the kinetochore switches to poleward motion, pulling the chromosome arms back into regions of higher microtubule density. The mechanism regulating kinetochore switching between poleward and away-from-the-pole motion is poorly understood, but may depend on tension at the kinetochore generated by pushing forces on the chromosome arms produced by the polar microtubule arrays.

Antagonist cocontraction of knee flexors during constant velocity muscle shortening and lengthening

Electromyography (EMG) was used to study the role of antagonist cocontration of the hamstring muscles of 24 normal subjects during constant velocity muscle performance tests on a KIN COM ™ dynamometer. The hypothesis tests whether antagonist cocontraction varies dependent on joint angle, limb velocity, and type of contraction. Seated subjects performed two dynamic tests of resisted quadriceps muscle shortening and lengthening through a 65° range of knee motion under two constant velocity conditions in a single session. Each test consisted of four repetitions of maximum effort constant velocity muscle shortening and lengthening of the knee extensors; one test performed at 30° s −1, the other at 90° s −1. The sequence of velocity testing was randomized. Normalized values of peak amplitude EMG of knee flexors were measured every 5° in the constant velocity phase of each test and compared in a paired manner between each angle, contraction type, and velocity, by split-plot 2-way ANOVA. Antagonist hamstrings cocontraction was found to be contraction type, and velocity dependent. Cocontraction was greater at larger angles of knee flexion and at higher velocity. These results do not support the hypothesis that such activity is a function of muscle moment-arm, but are consistent with the idea that antagonist muscle function is controlled by muscle spindles and perhaps Golgi tendon organs. Because antagonist cocontraction ranged up to a mean of 32% of maximum agonist activity in normal individuals, it should be taken into consideration when interpreting clinical muscle performance tests.