Tangential Acceleration Research Articles

Perception of tilt and ocular torsion of normal human subjects during eccentric rotation.

The authors hypothesized that a nonvisual measure of the subjective horizontal during eccentric rotation is a reliable method for evaluating the spatial orientation of normal subjects during such rotation. Eccentric rotation is a promising tool for the evaluation of otolithic function in healthy subjects and in patients. Although eye torsion is an objective measurement, it is also affected by angular acceleration/deceleration. Subjective horizontal is more accurately related to the changes in linear acceleration, but visual judgments of orientation are confounded by ocular torsion. 20 subjects were tested during eccentric yaw rotation in both directions, generating centripetal acceleration directed along the interaural axis ranging from 0.38 g to 1 g. Perception of body tilt in roll and in pitch was recorded in darkness using verbal reports and a somatosensory plate that the subjects maintained parallel to the perceived horizon. Torsion of the eyes was recorded by a video camera. Perceived roll tilt was close to the tilt of the gravito-inertial acceleration vector. However, there were differences in perceived roll and pitch tilt between facing-motion and back-to-motion headings, presumably related to the direction of the tangential acceleration. Ocular torsion was dependent on both angular and centripetal accelerations. Measurement of perceived roll and pitch tilt using a somatosensory plate is a reproducible method for quantifying the effects of linear accelerations during eccentric rotation. This method may prove useful for the diagnosis of otolithic dysfunction in dizziness and in patients with vestibular disorders.

Temporal aN and aT in Earthquake Ground Motion at a Single Point

This paper describes a continued study on three-dimensional temporal characteristics of earthquake ground motions at a single point. Based on an instantaneous tangential and normal acceleration decomposition of ground acceleration trajectory, a ground motion can be partitioned into a finite sequence of staggered time intervals of acceleration and deceleration. A formulation is developed to estimate speed and angular changes over a partitioned interval in terms of rates of positive and negative tangential and normal acceleration. Based on these concepts, general ground motion properties, peak ground acceleration, peak ground velocity, and peak ground displacement are examined. Several Northridge earthquake records are studied in detail. It is found that the highest peak of ground acceleration in these records corresponds to a high peak of deceleration, and a velocity maximum often precedes the peak of acceleration.

Sizing a Slingatron-Based Space Launcher

A slingatron is the name given to a propellantless mechanical means of launching a projectile. To date, slingatrons are only conceptual in nature, but their potential use as a ground-to-space launch mechanism for unmannedpayloads is under investigation. Slingatrons can be configured in a variety of geometries; one form consists of a spiral track (or launch tube) that gyrates at a constant frequency about a set radius. Under proper conditions (design parameters), a projectile entering the spiral at its small radius end will undergo nearly constant tangential acceleration before exiting. The differential equations governing the motion of the projectile within the spiral are highly nonlinear, making the optimum design solution nonintuitive. This paper briefly describes how the slingatron works, then uses the numerical integration procedures within the computer software environment of Simulink ® and MATLAB ® to search for the minimum-sized slingatron needed to launch a 1000-kg projectile into space.

MIXING, SETTLING, AND THE MOVEMENT OF THE INTERFACE BETWEEN THE MOBILE AND STATIONARY PHASES IN CCC

In a J-type centrifuge, wave mixing is the primary method of mass transfer between mobile and stationary phases. It is observed centered about the proximal key node, while settling is observed centered about the Distal key node. A hypothesis using the Kelvin-Helmholtz wave stability criteria gives an explanation of why mixing and settling occur centered around these key nodes. The assumption of constant retention means that the stationary phase has zero linear velocity for all positions within the coil, and that the mobile linear velocity is constant. Closer examination of the Kelvin-Helmholtz wave stability criteria shows that the relative flow of two phases is important in creating waves. The interfacial movement can be viewed as a relative pumping action between the phases that increases the relative linear velocity of the mobile phase at the proximal key node to cause wave mixing. The interface can move within one loop of a coil. This can cause wave mixing at the proximal key node and settling at the distal key node when the lower phase is selected as the mobile phase. The analysis presented, is based upon the following techniques: i) radial and tangential accelerations for helical wound coils on J-type centrifuges, ii) pressure gradient analysis, iii) Hagen- Poiseuille equation for laminar flow in circular bore tubing, iv) hydraulic mean depth to allow for two phase flow in circular bore tubing, v) computational numerical integration. A derived equation links the parameters: centrifuge radius (R), β value, rotational speed (ω), densities of both phases (ρL and ρU), viscosities of both phases (μL and μU), tubing bore diameter (d), and mobile phase volumetric flow rate (Qm). It is hoped, that the derived equation can be used to qualitatively predict the effects of changes to the above parameters on CCC.

テーパドラム回転式固液分離機に関する基礎研究 回転円筒容器内に投入された固体粒子運動

A new type of solid-liquid separator, called Tapered Drum Rotating Separator, was proposed by the authors. This system has two operation modes; one is a separating mode and another a discharging one. As a fundamental study on the separating mode, the numerical simulation of the motion of a solid particle thrown into the rotating cylindrical container was carried out by the Runge-Kutta numerical integration method. In was clarified that, while at the beginning of the motion, the smaller the diameter of the particle becomes, the larger the radial deflection, because of its very quick tangential acceleration. After a certain duration, the larger the diameter of the particle becomes, the larger the radial deflection. Then there exists the minimum reaching time at the wall for each particle size and the drum diameter. The numerical simulations were also carried out for different density of particle and show same characteristics of motion as those of different sizes qualitatively. The numerical results were compared with the experimental one obtained by the flow visualization technique. The numerical value, for example, the reaching time at the wall agrees well with the experimental one, except for very small particle and low drum revolution.

An Exact Solution for Steady Motion of an Extensible Belt in Multipulley Belt Drive Systems

The creep theory of multipulley belt drive systems is reexamined and the belt is modeled as an elastic extensible string. An exact solution of the nonlinear steady state equations of a string is obtained which includes, elastic extension, Coulomb friction, radial and tangential accelerations, and the power loss due to friction between the belt and the pulleys. Most importantly, the present solution uses the correct compatibility condition that ensures that the same belt is considered for all applied moments and speeds. An example of a two pulley system is considered which shows that existing approximations of the compatibility condition cause previous solutions for full slip to: underpredict the maximum transmitted moment; over predict the efficiency η; and underpredict both the low and high values of the tension. [S1050-0472(00)01503-8]

Synthesis of optimal feedback guidance law for homing missiles using neural networks

Most existing missiles are guided by proportional navigation guidance (PNG) law, but PNG is a particular case for LQ guidance rule with two main assumptions of small line-of-sight angles and negligible acceleration along the line-of-sight. However, most missile engagements exceed these limits because of high tangential and normal accelerations. Unfortunately, it is not possible to determine the feedback guidance law for non-linear systems such as homing missiles in real-time. We use artificial neural networks to synthesize feedback laws for homing missiles with non-linear state equations. We first obtain an open-loop optimal numerical solution for non-linear state equations and then use these data to train a feed-forward multilayer neural network in an off-line session. The network is then used effectively in a real-time for feedback guidance method. Simulation results show that this neural networks guidance method can efficiently produce an optimal feedback law in spite of relatively simple network architecture. Copyright © 2000 John Wiley & Sons, Ltd.

Mathematical modelling in geodesy based on quaternion Algebra

The main subject of the present paper is to show applications of quaternions in some general problems of geodesy and inertial navigation. Quaternionic descriptions of some generally employed geodetic reference frames connected with the earth and the sun are considered and quaternion form of relationships among them are presented. We present also different quaternion forms of the main kinematic equation of a rigid body motion in rotating reference frames. Using quaternions it is easy to see that such terms as tangential, Coriolis, and centripetal accelerations arise in kinematic equations very naturally. It is shown that mathematical modelling based on quaternions is a very elegant mathematical method which clarifies the essence of problems and is very useful for attacks of many complicated problems in different areas. Quaternions are convenient for computer modelling as far as they contain less parameters then usual matrices.

Disruption of postural readaptation by inertial stimuli following space flight*

Postural instability (relative to pre-flight) has been observed in all shuttle astronauts studied upon return from orbital missions. Postural stability was more closely examined in four shuttle astronaut subjects before and after an 8 day orbital mission. Results of the pre- and post-flight postural stability studies were compared with a larger (n = 34) study of astronauts returning from shuttle missions of similar duration. Results from both studies indicated that inadequate vestibular feedback was the most significant sensory deficit contributing to the postural instability observed post flight. For two of the four IML-1 astronauts, post-flight postural instability and rate of recovery toward their earth-normal performance matched the performance of the larger sample. However, post-flight postural control in one returning astronaut was substantially below mean performance. This individual, who was within normal limits with respect to postural control before the mission, indicated that recovery to pre-flight postural stability was also interrupted by a post-flight pitch plane rotation test. A similar, though less extreme departure from the mean recovery trajectory was present in another astronaut following the same post-flight rotation test. The pitch plane rotation stimuli included otolith stimuli in the form of both transient tangential and constant centripetal linear acceleration components. We inferred from these findings that adaptation on orbit and re-adaptation on earth involved a change in sensorimotor integration of vestibular signals most likely from the otolith organs.

Three-dimensional finite element analysis of subdural hematoma.

Head motion, an important factor in acute subdural hematoma (ASDH), can be broken down into translational and rotational elements. We used three-dimensional finite element analysis to examine the thresholds of angular and tangential acceleration required to tear bridging veins in humans during head impact. The lengths of midsagittal and parasagittal bridging veins were calculated first. To assess the effect of translational and rotational acceleration, the strain of each vein was then computed under three different motions. The threshold of ASDH was expressed in terms of tangential and rotational acceleration. Deformation-angle histories of the midsagittal and parasagittal bridging veins showed that veins that drain forward into the superior sinus at a 130-degree angle incurred the greatest stretch strain during occipital impact. In the midsagittal plane, pure rotation induced greater stretch strain on these veins (14.4%) than pure translation (2.5%) or combined translation and rotation motion (10.4%). A tangential acceleration of 3,912.9 G or an angular acceleration of 71.2 krad/s2 seemed to approximate the threshold for ASDH in the human midsagittal plane, whereas 5,010.9 G and 97.4 krad/s2 approximated the threshold in the parasagittal plane. Impact direction and orientation of bridging veins are both important factors in ASDH. Threshold criteria for ASDH can be expressed in terms of tangential and rotational acceleration.

PARTITIONING ACCELERATIONS TO REVEAL THE INFLUENCE OF THE CORIOLIS EFFECT

1508 In the analysis of joint dynamics often the motion of each segment is considered independent of the motion of its adjacent segment. What this approach fails to explicitly investigate is how the motion and more specifically the acceleration of the proximal segment are related to the accelerations of the distal segment. A term that reveals the effect of the motion of the proximal segment on the distal is called the Coriolis acceleration. Usually, the Coriolis acceleration is applied to humans only during full body rotation such as is the case with pilots in high-speed turns and participants in servo-electric chairs. Mathematically, the Coriolis acceleration is determined by taking the cross product of the angular velocity of the proximal segment and the velocity of the mass center of the distal segment, relative to the proximal segment. Since the Coriolis acceleration and therefore its corresponding force can act to either compress or distend the common joint, its consideration should help to explain mechanisms of injury. Isolating the Coriolis acceleration requires a partitioning of the accelerations that has yet to be noted in the literature. Specifically, the total acceleration of the mass center of the lower leg would be equal to the sum of terms involving the: linear acceleration of the hip, angular acceleration of the thigh, acceleration lower leg as a result of the thigh (Coriolis acceleration), and the relative acceleration of the lower leg with respect to the thigh (centripetal and tangential accelerations of the lower leg). While the total acceleration of the mass center is independent of the method of calculation, partitioning to include the Coriolis acceleration should enhance the understanding of the given motion and mechanisms of injury.

Pointing in 3D space to remembered targets

The accuracy of visually guided pointing movements decreases with speed. We have shown that for movements to a visually defined remembered target, the variability of the final arm endpoint position does not depend on movement speed. We put forward a hypothesis that this observation can be explained by suggesting that movements directed at remembered targets are produced without ongoing corrections. In the present study, this hypothesis was tested for pointing movements in 3D space to kinesthetically defined remembered targets. Passive versus active acquisition of kinesthetic information was contrasted. Pointing errors, movement kinematics, and joint-angle coordination were analyzed. The movements were performed at a slow speed (average peak tangential velocity of about 1.2 m/s) and at a fast speed (2.7 m/s). No visual feedback was allowed during the target presentation or the movement. Variability in the final position of the arm endpoint did not increase with speed in either the active or the passive condition. Variability in the final values of the arm-orientation angles determining the position of the forearm and of the upper arm in space was also speed invariant. This invariance occurred despite the fact that angular velocities increased by a factor of two for all the angles involved. The speed-invariant variability supports the hypothesis that there is an absence of ongoing corrections for movements to remembered targets: in the case of a slower movement, where there is more time for movement correction, the final arm endpoint variability did not decrease. In contrast to variability in the final endpoint position, the variability in the peak tangential acceleration increased significantly with movement speed. This may imply that the nervous system adopts one of two strategies: either the final endpoint position is not encoded in terms of muscle torques or there is a special on-line mechanism that adjusts movement deceleration according to the muscle-torque variability at the initial stage of the movement. The final endpoint position was on average farther from the shoulder than the target. Constant radial-distance errors were speed dependent in both the active and the passive conditions. In the fast speed conditions, the radial distance overshoots of the targets increased. This increase in radial-distance overshoot with movement speed can be explained by the hypothesis that the final arm position is not predetermined in these experimental conditions, but is defined during the movement by a feedforward or feedback mechanism with an internal delay.

Orbit Raising with Low-Thrust Tangential Acceleration in Presence of Earth Shadow

Orbit Raising with Low-Thrust Tangential Acceleration in Presence of Earth Shadow

Dynamics of Orbits Close to Asteroid 4179 Toutatis

We use a radar-derived physical model of 4179 Toutatis (975870) to investigate close-orbit dynamics around that irregularly shaped, non-principal-axis rotator. The orbital dynamics about this body are markedly different than the dynamics about uniformly rotating asteroids. The results of this paper are generally applicable to orbit dynamics about bodies in a non-principal-axis rotation state. The radar results support the hypothesis that Toutatis has a homogeneous density distribution, and we assume a density of 2.5 g/cc. The asteroid's gravity field is computed using a truncated harmonic expansion when outside of its circumscribing sphere and a closed-form expression for the potential field of an arbitrary polyhedron when inside that sphere. The complete equations of motion are time-periodic in the Toutatis-fixed frame due to the complex rotation of the asteroid. The system is Hamiltonian and has all the characteristics of such a system, including conservation of phase volume, but there is no Jacobi constant of the motion and zero velocity surfaces cannot be used to analyze the system's behavior. We also examine some of the close-orbit dynamics with the Lagrange planetary form of the equations of motion. Families of quasi-periodic “frozen orbits” that show minimal variations in orbital elements are found to exist very close to the asteroid; some of them are stable and hence can hold natural or artificial satellites. A retrograde family of frozen orbits is especially robust and persists down to semi-major axes of about 2.5 km, comparable to half of Toutatis' longest dimension. We identify families of periodic orbits, which repeat in the Toutatis-fixed frame. Due to the time-periodic nature of the equations of motion, all periodic orbits about Toutatis in its body-fixed frame must be commensurate with the 5.42-day period associated with those equations. Exact calculations of both stable and unstable periodic orbits are made. The sum of surface forces acting on a particle on Toutatis is time-varying, so particles on and in the asteroid are being continually shaken with a period of 5.42 days, perhaps enhancing the uniformity of the regolith distribution. A global map of the gravitational slope reveals that it is surprisingly shallow for such an elongated, irregularly shaped object, averaging 16° globally and less than 35° over 96% of the surface. A global map of tangential accelerations shows no values larger than 0.5 mm/s2, an average value of 0.2 mm/s2, and less than 0.25 mm/s2over 70% of the surface. A global map of the escape speed for launch normal to the surface shows that quantity to be between 1.2 and 1.8 m/s over most of the surface. Each of these mapped quantities has small periodic variations. We have found trajectories that leave the surface, persist in the region of phase space around a frozen orbit, and then impact the surface after a flight time of more than 100 days. Return orbit durations of years seem possible. Whereas a uniformly rotating asteroid preferentially accumulates non-escaping ejecta on its leading sides, Toutatis accumulates ejecta uniformly over its surface. We render a variety of close orbits in inertial and body-fixed frames.

Canal-otolith interactions in the squirrel monkey vestibulo-ocular reflex and the influence of fixation distance.

Natural head movements include angular and linear components of motion. Two classes of vestibulo-ocular reflex (VOR), mediated by the semicircular canals and otoliths (the angular and linear VOR, or AVOR and LVOR, respectively), compensate for head movements and help maintain binocular fixation on targets in space. In this study, AVOR/LVOR interactions were quantified during complex head motion over a broad range of fixation distances at a fixed stimulus frequency of 4.0 Hz. Binocular eye movements were recorded (search-coil technique) in squirrel monkeys while fixation distance (assessed by vergence) was varied using brief presentations of earth-fixed targets at various distances. Stimuli consisted of rotations around an earth-vertical axis and therefore always activated the AVOR. Horizontal and vertical AVORs were assessed when the head was centered over the axis of rotation and oriented upright (UP) and right-side-down (RD), respectively. AVOR gains increased slightly with increasing vergence in darkness, as expected given the small anterior position of the eyes in the head. Combined AVOR/LVOR responses were recorded when subjects were displaced eccentrically from the rotation axis. Eccentric rotations activated the AVOR just as when the head was centered, but added a translational stimulus which generated an LVOR component in response to interaural (IA) or dorsoventral (DV) tangential accelerations, depending on whether the head was UP or RD, respectively. When the head was eccentric and facing nose-out, the AVOR and LVOR produced ocular responses in the same plane and direction (coplanar and synergistic), and response magnitudes increased with increasing vergence. With the head facing nose-in, AVOR and LVOR response components were oppositely directed (coplanar and antagonistic). The AVOR dominated the response when fixation distance was far, and phase was compensatory for head rotation. As fixation distance decreased toward the rotation axis, responses declined to near zero, and when fixation distance approached even closer, the LVOR component dominated and response phase inverted. The same pattern was observed for both horizontal (head UP) and vertical (head RD) responses. The LVOR was recorded directly by rotating subjects eccentrically but in the nose-up (NU) orientation. The AVOR then generated torsional responses to head roll, coexistent with either horizontal or vertical LVOR responses to tangential acceleration when the subject was oriented head-out or right-side-out, respectively. Only the LVOR response components were modulated by vergence. A vectorial analysis of AVOR, LVOR, and combined responses supports the conclusion that AVOR and LVOR response components combine linearly during complex head motion.

Standing balance during internally produced perturbations in subjects with hemiplegia: validation of the balance scale.

To examine the concurrent validity of the Berg Balance Scale (BBS) in subjects with hemiparesis after stroke by correlating functional balance performance on the BBS with laboratory measures of balance (center of pressure [CP] and electromyographic [EMG] activity of leg musculature during rapid arm flexion and during quiet stance). Concurrent validation study. Consistency of performance was determined through repeat measures obtained from 13 subjects. Laboratory. Twenty-four volunteers with longstanding hemiparesis from cerebrovascular accident (CVA) were tested. BBS scores were compared with CP excursion speed during (1) self-initiated rapid unilateral arm flexion with the nonhemiplegic arm and (2) quiet stance. The peak tangential arm acceleration and the sequence of EMG events in the posterior leg muscles were monitored during the arm flexion movement. In the arm flexion condition, BBS performance was significantly correlated with arm acceleration and CP excursion speed (multiple regression R = .81). BBS performance was also related to the presence of anticipatory activation of ipsilateral hamstrings during the rapid arm flexion movement. BBS performance and CP excursion speed during quiet stance were also significantly related (r = .76). The initial and repeat measures were not significantly different in magnitude (paired tests of difference, p < .01). The BBS appears to reflect differing abilities to tolerate the internally produced perturbation to standing balance associated with forward arm flexion in individuals with hemiparesis from CVA.

Canal-otolith interactions driving vertical and horizontal eye movements in the squirrel monkey.

The vestibulo-ocular reflex (VOR) was studied in three squirrel monkeys subjected to rotations with the head either centered over, or displaced eccentrically from, the axis of rotation. This was done for several different head orientations relative to gravity in order to determine how canal-mediated angular (aVOR) and otolith-mediated linear (IVOR) components of the VOR are combined to generate eye movement responses in three-dimensional space. The aVOR was stimulated in isolation by rotating the head about the axis of rotation in the upright (UP), right-side down (RD), or nose-up (NU) orientations. Horizontal and vertical aVOR responses were compensatory for head rotation over the frequency range 0.25-4.0 Hz, with mean gains near 0.9. The horizontal aVOR was relatively constant across the frequency range, while vertical aVOR gains increased with increasing stimulation frequency. In the NU orientation, compensatory torsional aVOR responses were of relatively low gain (0.54) compared with horizontal and vertical responses, and gains remained constant over the frequency range. When the head was displaced eccentrically, rotation provided the same angular stimuli but added linear stimulus components, due to the centripetal and tangential accelerations acting on the head. By manipulating the orientation of the head relative to gravity and relative to the axis of rotation, the IVOR response could be combined with, or isolated from, the aVOR response. Eccentric rotation in the UP and RD orientations generated aVOR and IVOR responses which acted in the same head plane. Horizontal aVOR-IVOR interactions were recorded when the head was in the UP orientation and facing toward ("nose-in") or away from ("nose-out") the rotation axis. Similarly, vertical responses were recorded with the head RD and in the nose-out or nose-in positions. For both horizontal and vertical responses, gains were dependent on both the frequency of stimulation and the directions and relative amplitudes of the angular and linear motion components. When subjects were positioned nose-out, the angular and linear stimuli produced synergistic interactions, with the IVOR driving the eyes in the same direction as the aVOR. Gains increased with increasing frequency, consistent with an addition of broad-band aVOR and high-pass IVOR components. When subjects were nose-in, angular and linear stimuli generated eye movements in opposing directions, and gains declined with increasing frequency, consistent with a subtraction of the IVOR from the aVOR. This response pattern was identical for horizontal and vertical eye movements. aVOR and IVOR interactions were also assessed when the two components acted in orthogonal response planes. By rotating the monkeys into the NU orientation, the aVOR acted primarily in the roll plane, generating torsional ocular responses, while the translational (IVOR) component generated horizontal or vertical ocular responses, depending on whether the head was oriented such that linear accelerations acted along the interaural or dorsoventral axes, respectively. Horizontal and vertical IVOR responses were negligible at 0.25 Hz and increased dramatically with increasing frequency. Comparison of the combined responses (UP and RD orientations) with the isolated aVOR (head-centered) and IVOR (NU orientation) responses, indicates that these VOR components sum in a linear fashion during complex head motion.

A nonlinear finite element approach to kineto-static analysis of elastic beams

Recently the demand for higher speed in design of mechanical systems has led to an intensive research in the formulation of the elastic mechanisms. In this study it is shown that the kineto-static solution is close to full dynamic solution for typical dimensions of the links. The formulation presented here uses the Timoshenko Beam Model with geometric stiffening. The element matrices and nodal forces are reported as integral of product of shape functions. This gives a more generic form and can be easily applied to other shape functions. A new formalism is introduced for the nonlinear effect (geometric stiffening). Fundamental to this approach is the introduction of a tensor and its assembly which plays a similar role as element matrices of the linear terms. Several numerical simulations are conducted to quantify the effect of geometric stiffening, tip mass, and normal and tangential acceleration on the nodal displacements.

Soleus H-reflex; depression induced by ballistic voluntary arm movement in human

A biomechanical and neurophysiological analysis of anticipatory postural adjustments associated with the early phase of a voluntary arm movement was carried out in normal human subjects. Arm elevation, performed at maximal velocity, was studied with unilateral arm movement in freely and counterbalanced (suspended in a safety harness) standing humans. The ground reaction force of both legs, tangential acceleration of the shank and electromyographic activity (EMG) of the anterior deltoid (AD), biceps femoris (BF) and soleus (Sol) muscles were recorded. Sol H-reflexes of both legs were also elicited. To examine how the la inhibitory and presynaptic inhibitory pathways are related to anticipatory postural adjustments, additional Sol H-reflexes were elicited using the classical conditioning-test technique (time interval 2 and 15 ms, respectively). In this study, we systematically described biomechanical and EMG phenomena that precede and follow the onset of voluntary arm movements. Prior to and during the arm movement (i.e. AD activation), a sequence of EMG modifications occurred in ipsilateral BF and Sol muscles. Those modifications preceding BF activation included silent phases in Sol EMG and depression of Sol H-reflexes. By comparing EMG modifications with depressions of the Sol H-reflex, we conclude that Ia and presynaptic inhibitory pathways do not play a specific role in those Sol H-reflex depressions. Moreover, because Sol H-reflex depression occurs in both freely standing and counterbalanced postures, anticipatory postural adjustment appear to be preprogrammed in the central nervous systems unrelated to peripheral neural mechanisms. Since changes of EMG activities of ipsilateral BF and Sol appeared simultaneously and Sol H-reflex depressions were dependent on EMG activities of ipsilateral BF, control commands to anticipatory postural adjustment would excite BF and inhibit Sol.

Modeling Fluid Inertia Forces of Short Journal Bearings for Rotordynamic Applications

It has been recently suggested that fluid inertia may play an important role in the dynamic behavior of rotors supported on journal bearings. This paper presents a model for fluid inertia forces in short cylindrical journal bearings based on an energy approximation. The inertialess velocity profiles predicted by the solution of Reynolds’ equation are inserted in the axial momentum equation multiplied by the axial velocity profile and integrated across the film thickness, to obtain the pressure in short journal bearings including the fluid inertia effect. The pressure is then integrated to obtain the fluid inertia forces. It is shown that the inertia forces thus obtained are proportional to the usual radial, centripetal, tangential and coriolis accelerations of the journal, in addition to a nonlinear radial acceleration. Moreover, it is shown that the inertia forces contribute to the stiffness and damping characteristics of the journal bearings. The inertia coefficients of the bearings are obtained in cartesian and cylindrical coordinates, for both uncavitated and cavitated bearings, and are plotted versus the eccentricity ratio. The model thus obtained is an approximate analytical closed form model for fluid inertia forces in short journal bearings. Such a model is the most suitable for rotordynamic applications, particularly for time transient rotordynamic simulations.