Axial Acceleration Research Articles

Effect of tip clearance dimension on unsteadiness in a low-reaction aspirated transonic compressor rotor

A three-dimensional, multi-passage unsteady numerical study was conducted to enhance the understanding of unsteady flow phenomena in the tip region of highly loaded compressors. The first-stage rotor of a three-stage transonic low-reaction compressor was chosen as the computational model. Three different tip clearance sizes were calculated to demonstrate the effect of the tip clearance dimension on the unsteadiness in the rotor tip region. It was found that the unsteadiness existed at the vicinity of the stall point when the tip clearance size was larger than the design value. The unsteadiness in the tip region appeared as a “multi-passage structure” in the nine-passage unsteady simulation and it propagated along the circumferential direction. Tip leakage vortex breakdown was the source of unsteady flow behavior. Besides, special attention was paid to the difference between the conventional transonic rotor and the low-reaction rotor. The scale of the flow separation downstream of the shock wave was controllable for the low-reaction rotor even at near-stall conditions. The boundary layer would reattach to the blade surface due to local axial acceleration. Finally, attempts were made to study the stall mechanism of the low-reaction rotor.

Tibial Acceleration during Running Is Higher in Field Testing Than Indoor Testing

This study aimed to determine whether laboratory and field measures of tibial acceleration are comparable, and whether peak axial and peak resultant tibial acceleration are interchangeable. Nineteen healthy rearfoot striking runners between 18 and 45 yr of age participated. A precision accelerometer was aligned with the vertical axis of the distal tibia and firmly attached. Data were collected in the following conditions during running at 3.0 m·s ± 5%: traditional overground laboratory gait analysis contacting force plates, treadmill, outdoor grass, and outdoor sidewalk. Acceleration data were filtered and normalized to gravity. Peaks for variables of interest were extracted from the first 40% of stride for 10 trials per condition. Differences among conditions were determined. Peak positive acceleration was lower in laboratory and treadmill compared with grass and sidewalk conditions. However, laboratory and treadmill were similar in magnitude, as were grass and sidewalk. Peak resultant acceleration was consistently higher than peak axial acceleration, with the same pattern among conditions. Laboratory acceleration measures explained at best only half of the variance in the field conditions and did not explain the variance for grass. Tibial impact acceleration magnitude is influenced by testing procedures in runners. These findings support measuring tibial impact acceleration in the field to determine new metrics associated with injury.

Human Locomotion Classification for Different Terrains Using Machine Learning Techniques.

Gait analysis on healthy subjects was performed based on surface electromyographic and acceleration sensor signal, implemented through machine learning approaches. The surface EMG and 3-axes acceleration signals have been acquired for 5 different terrains: level ground, ramp ascent, ramp descent, stair ascent, and stair descent. These signals were acquired from the tibialis anterior and gastrocnemius medial head muscles that correspond to dorsiflexion and plantar flexion, respectively. After feature extraction, these signals are fed to 5 conventional classifiers: linear discriminant analysis, k-nearest neighbors, decision tree, random forest, and support vector machine, that classify different terrains for human locomotion. We compared the classification results for the above classifiers with deep neural network classifier. The objective was to obtain the features and classifiers that are able to discriminate between 5 locomotion terrains with maximum classification accuracy in minimum time by acquiring the signal from the least number of leg muscles. The results indicated that the support vector machine gives the highest classification accuracy of 99.20 (± 0.80)% for the dataset acquired from 15 healthy subjects. In terms of both accuracy and computation time, the support vector machine outperforms other classifiers.

Experimental study on the sEMG of "joint angle effect" of human muscle strength—Taking biceps brachii as an example

In the experiment, the author used wave plus wireless surface electromyography system (SEMs + 3-axis acceleration sensor) made in Italy and wave wireless EMG software system, high-definition high-speed camera and human joint angle measuring instrument. Taking human biceps brachii as an example, the static and dynamic isometric contraction of biceps brachii was completed surface electromyography. In the experiment, the surface electromyography of biceps brachii was measured at 30°, 60°, 90°, 120°, 150°, 180° and the surface electromyography of biceps brachii was measured at the same time when the biceps brachii was not loaded or when the biceps brachii was loaded. Secondly, the surface electromyography of biceps brachii was measured at the same time when the biceps brachii completed the whole process of flexion and extension of the elbow (centripetal and centrifugal). Finally, the paper combined with HD The effect of joint angle on the contraction of biceps brachii muscle was analyzed by camera technique. The results show that the static contraction force of biceps brachii is different when the elbow joint is at different angles; in addition, when the dynamic contraction, the contraction force of biceps brachii is inversely proportional to the angle of elbow joint.

Gait identification and authentication using LSTM based on 3-axis accelerations of smartphone

Abstract In this study, we apply recurrent neural networks and Long Short-Term Memory (LSTM) to 3-axis accelerations of walking acquired by a smartphone for gait identification and authentication. First, the accelerations during walking for 21 subjects are recorded in two holding situations, namely, while placing the smartphone in the pocket or looking at the screen of the smartphone. We then identify and authenticate the user by inputting the accelerations directly to the recurrent neural networks and LSTM. We have confirmed that the performance of this method is better than the conventional method of extracting features from the accelerations and using random forests.

Kinematic analysis and experimental verification of an oval noncircular bevel gears with rotational and axial translational motions

This paper proposes a new type of gear transmission based on the space engagement theory, which can achieve not only simple motion transmission between intersecting axes, but also the compound motion of the rotation and axial translation between them. An oval noncircular bevel gear engagement coordinate system was first established with parameter equations of the pitch cone described in spherical coordinates. Then, the spherical involute equations and formula for calculating displacement and axial translational velocity of the gears were derived. Experimental tests were carried out to validate the kinematic characteristics of oval noncircular bevel gear transmission calculated from theoretical models. The influences of eccentricity on the indicators, such as axial displacement, rotational angle, angular velocity and acceleration, periodicity of the line of contact and periodicity of each motion characteristic parameter, have been analyzed and discussed in last. Experimental verification and kinematic analysis indicate that the proposed gear design method is appropriate with great potential for industrial applications.

Radio-Telemetric Assessment of Cardiac Variables and Locomotion With Experimentally Induced Hypermagnesemia in Horses Using Chronically Implanted Catheters.

The objective of this study was to characterize the pharmacokinetics and pharmacodynamics of intravenous administration of magnesium sulfate to horses using a novel radio-telemetry system for physiologic signal capture. Five Horses were surgically implanted with a radio-telemetric carotid catheter. Implants were paired with a non-invasive telemetric unit which acquired a six lead ECG and 3-axis acceleration to assess activity acquired wirelessly in real-time for future analysis. Horses were exposed to a new stall environment before (baseline) and after 60 mg/kg (30 mL) of magnesium sulfate (MgSO4), or the same volume of 0.9% saline, administered intravenously in a blinded, random crossover design. Blood for pharmacokinetics, telemetric data, and body temperature were recorded serially for 24 h. Data were analyzed across time and between treatments. Significance was set at P < 0.05. Ionized magnesium concentration (Mg2+) increased and the Ca2+ to Mg2+ ratio decreased and persisted for 5 h after MgSO4 administration. Heart rate (HR) increased and mean arterial blood pressure (MAP) decreased for at least 6 h. Electrocardiogram (ECG) intervals (RR) decreased and (PR and QTc) increased in duration compared to controls indicating an increase in heart rate, and slower myocardial conduction in the MgSO4 group. Acceleration in all planes was less in the MgSO4 group compared to controls indicating decreased locomotion. This novel method permitted collection of physiologic signals without interference by handlers or animal restraint. An intravenous bolus of MgSO4 produced cardiac variable changes associated with the reduction of locomotion in these horses, and in a direction that may be causal. Locomotion was decreased when horses were first introduced into a new environment which reflects the calming effect desired in sport horses. Telemetric monitoring can be used as a model to elucidate the behavior and physiologic effects of other drugs. The administration of MgSO4 may be detected for regulatory purposes with the monitoring of Mg2+ and Ca2+ concentrations and their ratio.

Fall detection system with 3-axis accelerometer

In this work, the designed and developed process of a fall detection system using a NodeMCU microcontroller is described. A 3-axis acceleration sensor is used to obtain the data of the movement of humans and changes in acceleration are used to determine trends in falling. The ESP8266 microcontroller is used to the receive the data in real time and alert the family through the LINE application. Experimental results show that the system is capable of precisely producing alarms for falling actions and has a high detection precision and effectiveness.

Influence of the magnet aspect ratio on the dynamic stiffness of a rotating superconducting magnetic bearing

Rotating superconducting bearings promise great potential in applications due to their frictionless operation. However, these bearings show a lower dynamic stiffness and damping coefficient compared to ball bearings. In this paper we studied a bearing consisting of a fixed YBCO ring and a rotating magnet above the superconductor. The influence of the magnet aspect ratio on the dynamic stiffness of the bearing was investigated in order to find an optimized size. To change the aspect ratio, we kept the inner diameter of the ring constant and reduced the outer diameter while increasing the ring height. In addition to these magnets, one magnet with a reduced cross-sectional area was studied. The aspect ratio selection was based on preliminary magnetic flux density simulations, which compared the magnetic flux density distribution and the potential radial force for different aspect ratios.To conduct the measurements, the field-cooled magnets were displaced in a lateral direction and then released, resulting in a damped oscillation. The dynamic stiffness constants were calculated for each bearing from the relation of three axis acceleration measurements for different field cooling heights. The comparison of the stiffness constants for the different bearings revealed an optimal aspect ratio for the given YBCO ring. This optimum is almost independent from the cooling height. The comparison between the two magnet rings with similar diameters and different heights was similar for the bearing characteristics at a low cooling height, whereas a significant reduction of stiffness was observed with a larger cooling distance. The difference is bigger for the magnet with a reduced height. The optimal aspect ratio as well as the stiffness dependence on the cross-sectional area was confirmed by simulations of the magnetic flux density distribution.

Effects of Linear Acceleration on Passenger Comfort During Physical Driving on an Urban Road

In this exploratory study, we examined self-reports of physical discomfort among automobile passengers while being driven on an urban road. Eight adults participated as passengers in an automobile driven by a professional driver on a predetermined course through city traffic. Passengers were driven individually along the route. While underway, participants used a handheld device to indicate momentary feelings of discomfort arising from discrete vehicle motions. We continuously recorded vehicle motion in three axes of linear acceleration and 3 axes of angular velocity. We examined vehicle acceleration during the 3 s preceding each subjective response. We found that the maximum absolute acceleration required to elicit subjective discomfort was lower when vehicle acceleration changed sign (from + to −, or vice versa) than when acceleration was of a constant sign. In addition, participants’ reports of discomfort during the experiment were unrelated to their prior self-reports of generalized susceptibility to motion sickness. The results suggest that our method is valid, and has the potential to offer new insights into motion sickness causality.

Fault detection on fluid machinery using Hidden Markov Models

A fault detection method exploiting Hidden Markov Models (HMMs) is proposed for fluid machinery without adequate a priori information about faulty conditions. The method is trained only on data acquired during normal machine operation. For anomaly detection, typical quantities measured in monitoring fluid machines, namely 3-axis acceleration, electric power consumption, temperature, inlet and outlet pressure, are monitored. Principal Component Analysis is exploited for features extraction. Then, data is clustered and an HMM is trained. Finally, the trained model is employed together with a goodness-of-fit test to detect faulty states by processing online data. The method was tested and validated at CERN on screw compressors for cryogenic cooling.

Head and Neck Response of an Active Human Body Model and Finite Element Anthropometric Test Device During a Linear Impactor Helmet Test.

It has been proposed that neck muscle activation may play a role in head response resulting from impacts in American Football. The importance of neck stiffness and active musculature in the standard linear impactor helmet test was assessed using a detailed head and neck finite element (FE) model from a current human body model (HBM) compared to a validated hybrid III head and neck FE model. The models were assessed for bare-head and helmeted impacts at three speeds (5.5, 7.4, and 9.3 m/s) and three impact orientations. The HBM head and neck was assessed without muscle activation and with a high level of muscle activation representing a braced condition. The HBM and hybrid III had an average cross-correlation rating of 0.89 for acceleration in the primary impact direction, indicating excellent correspondence regardless of muscle activation. Differences were identified in the axial head acceleration, attributed to axial neck stiffness (correlation rating of 0.45), but these differences did not have a large effect on the overall head response using existing head response metrics (head injury criteria, brain injury criteria, and head impact power). Although responses that develop over longer durations following the impact differed slightly, such as the moment at the base of the neck, this occurred later in time, and therefore, did not considerably affect the short-term head kinematics in the primary impact direction. Though muscle activation did not play a strong role in the head response for the test configurations considered, muscle activation may play a role in longer duration events.

Identification of Changes in Mechanical Properties of Human Axial System Due to Stress and Relaxation Regime

The aim of this work was to verify the applicability of the TVS (Transfer Vibration through Spine) methodology for registration of changes in viscoelastic characteristics of the axial system. The detection method consists in applying &#947; pulse pulses with a half-width of 5ms and then a harmonic excitation of continuously varying from 5Hz to 160Hz on vertebrae C7 and L5. This ripple is transmitted along the axial system and acceleration of all the spinous processes of the vertebrae is sensed by accelerometer sensors to propagate the wave between C7 and S1. On the basis of measured data (input excitation and its sensed response on spinous processes), the change of total viscoelastic parameters of the spine was evaluated before and after monotonous or other spinal strain or before and after relaxation. Existing results confirmed the TVS method is suitable for detection of mechanical changes of the axial system. It was proven transmission of ripples through the axial system of a person also changes by the influence of alternations created by monotonous or physical strain or conversely relaxation.

Dynamic Modeling and Attitude Control System of Underactuated Fuel Slosh with Flexible Appendages

This paper investigates the dynamics and attitude control of the pitching motion of an axisymmetric liquid-filled satellite with flexible appendages under the conditions of zero gravity and constant thrust (fixed axial acceleration). Using Lagrange method, the dynamics model of the flexible liquid-filled satellite is established. The flexible appendages are considered as Euler–Bernoulli beams, and the sloshing liquid is modeled as a spring-mass model. Considering the motion of the satellite in the orbit plane and pitching attitude motion, a sliding mode controller is designed by decoupling the satellite dynamics equation into a typical underactuated system. The influence of the vibration of flexible appendages and the slosh of liquid fuel on the attitude control of the spacecraft is considered. The results show that the designed controller can effectively suppress the liquid sloshing and the vibration of the flexible appendages in such a manner as to achieve the desired result and control target.

A T-Shape Aluminum Nitride Thin-Film Piezoelectric MEMS Resonant Accelerometer

In this paper, we report a novel aluminum nitride (AlN) resonant micro-electromechanical systems (MEMS) accelerometer. The spring beams are T-shaped with two masses hanged at the end. This accelerometer is sensitive to the $z$ -axis acceleration due to a thin thickness. Different from the working mechanism of the ordinary MEMS resonant accelerometers, masses of this accelerometer are excited to resonate in-plane. In addition the stiffness of spring beams changes significantly when an out plane ( $z$ -axis) inertial force applied on the structure. Therefore, the resonant frequency of the structure will change with the out-plane inertial force. The resonant properties and sensitivities of this AlN accelerometer are simulated by COMSOL Multiphysics. The accelerometer is fabricated and tested. The size of the whole structure is $464\times 650\,\,\mu \text{m}^{2}$ . The resonant frequency is 16.10925 kHz at the static state. The sensing-axis sensitivity of this accelerometer is 1.11 Hz/g (i.e., 68.9 ppm/g) tested from −5g to +5g. The linearity of the accelerometer is 0.9954. The cross-axis sensitivities are 0.053 Hz/g ( $x$ -axis) and 0.048 Hz/g ( $y$ -axis) respectively. The temperature coefficient of frequency (TCF) of this accelerometer is 0.815 Hz/ °C (i.e., 50.6 ppm/°C), tested from 0 °C to 50 °C. [2019-0062]

Computational optimal launching control for balloon-borne solar-powered unmanned aerial vehicles in near-space.

The near-space solar-powered unmanned aerial vehicle has broad prospects in application owing to its high altitude long-endurance performance. Launching solar-powered unmanned aerial vehicle into the near-space with balloon-borne approach has advantages over the traditional sliding take-off methods, in that it is able to quickly and safely cross the turbulent zone. In this article, we investigate the control technology of balloon-borne launching for the solar-powered unmanned aerial vehicles. First, the motion of the launching process is divided into longitudinal and lateral-directional motion, with the longitudinal process and its equation addressed in detail. We then analyze the flight state and restriction conditions that the unmanned aerial vehicle should meet during the process. Second, the target variables and constraints are selected to formulate the optimization problem. The control variable parameterization method is applied to find the optimal pitch angle in the releasing-and-pulling process. More explicitly, a three-channel attitude stabilization controller is designed, in which the longitudinal channel takes the optimal pitch angle as the pitch instruction, the transverse channel carries out the zero control of the inclination angle, and the course channel takes the stabilization control, respectively. Numerical simulation results show that our proposed control design is capable of accelerating the solar-powered unmanned aerial vehicles from the vertical state and pulling them up to the horizontal cruising flight state, with the flight angle of attack, the maximum speed, and the maximum axial acceleration in the pulling process all within the designed range.

3D propagation of the shock-induced vibrations through the whole lower-limb during running

Shock-induced vibrations to the feet have been related to the feel of comfort, the biomechanical control of performance, and the risk of fatigue or injury. Up to recently, the complexity of measuring the human biodynamic response to vibration exposure implied to focus most of the research on the axial acceleration at the tibia. Using wireless three-dimensional accelerometers, this paper investigates the propagation of shock-induced vibrations through the whole lower-limb during running in the temporal and the spectral domains. Results indicated that the vibrations were not consistent across the lower-limb, showing various spatial and spectral distributions of energy. The amount of energy was not constantly decreasing from the distal to the proximal extremity of the runner’s lower-limb, especially regarding the lateral epicondyle of the femur. Vibrations in the transversal plane of the segments were substantial compared to the longitudinal axis regarding the distal extremity of the tibia, and the lateral epicondyle of the femur. Further, the spectral content was wider at the distal than at the proximal end of the lower-limb. Finally, to get a thorough understanding of the risks incurred by the runners, the need to account for shock-induced vibrations up to 50 Hz has been stressed when investigating three-dimensional vibrations. The overall study raises attention on the substantial importance of the transverse components of the acceleration, and their potential relation to shear fatigue and injury during running.

Direct Multiscale Analysis of Stability of an Axially Moving Functionally Graded Beam with Time-Dependent Velocity

In this study, the transverse vibration of a traveling beam made of functionally graded material was analyzed. The material gradation was assumed to vary continuously along the thickness direction of the beam in the form of power law exponent. The effect of the longitudinally varying tension due to axial acceleration was highlighted, and the dependence of the tension on the finite support rigidity was also considered. A complex governing equation of the functionally graded beam was derived by the Hamilton principle, in which the geometric nonlinearity, material properties and axial load were incorporated. The direct multiscale method was applied to the analysis process of an axially moving functionally graded beam with time-dependent velocity, and the natural frequency and solvability conditions were obtained. Based on the conditions, the stability boundaries of subharmonic resonance and combination resonance were obtained. It was found that the dynamic behavior of axial moving beams could be tuned by using the distribution law of the functional gradient parameters.

Vorticity dynamics in a spatially developing liquid jet inside a co-flowing gas

A three-dimensional transient round liquid jet within a low-speed coaxial outer gas flow is numerically simulated and analysed via vortex dynamics ($\unicode[STIX]{x1D706}_{2}$ analysis). Two types of surface deformations are distinguished, which are separated by a large indentation on the jet stem. First, there are those inside the recirculation zone behind the leading cap – directly affecting the cap dynamics and well explained by the local vortices. Second, deformations upstream of the cap are mainly driven by the Kelvin–Helmholtz (KH) instability, unaffected by the vortices in the behind-the-cap region (BCR), and are important in the eventual atomization process. Different atomization mechanisms are identified and are delineated on a gas Weber number ($We_{g}$) versus liquid Reynolds number ($Re_{l}$) map based on the relative gas–liquid velocity. In a frame moving with the liquid velocity, this result is consistent with prior temporal studies. A simpler and clearer portrait of similarity of the atomization domains is shown by using the relative gas–liquid axial velocity, i.e. $We_{r}$ and $Re_{r}$, and avoiding the widely used velocity ratio as a third key parameter. A detailed comparison of vorticity along the axis in an Eulerian frame versus a frame fixed to a surface wave reveals that the vortex development and surface deformations are periodic in the upstream region, but this periodicity is lost closer to the BCR. In the practical range of the density ratio and for early times in the process, axial vorticity is mainly generated by baroclinicity while streamwise vortex stretching becomes more important at later times and only at lower relative velocities when pressure gradients are reduced. The inertia, vortex, pressure, viscous and surface tension forces are analysed to delineate the dominant causes of the three-dimensional instability of the axisymmetric KH structure due to surface acceleration in the axial, radial and azimuthal directions. The inertia force related to the axial gradient of kinetic energy is the main cause of the axial acceleration of the waves, while the azimuthal acceleration is mainly caused by the pressure and viscous forces. The viscous forces are negligible in the radial direction and away from the nozzle exit in the axial direction. It is interesting to note that azimuthal viscous forces are important even at high $Re_{l}$, indicating that inertia is not totally dominant in this instability occurring early in the atomization cascade.

CFD investigation on thermodynamic characteristics in liquid hydrogen tank during successive varied-gravity conditions

In the present paper, a computational fluid dynamics (CFD) model is developed to simulate the successive varied-gravity operations for a liquid hydrogen (LH2) tank, and pressure evolution, physical field distributions and phase change amounts are obtained and analyzed. The results show that owing to the existence of helium accumulation in ullage, excessive pressure decline could be restrained, and instead a relatively stable pressure is maintained during the most time of the mission. The diffusion process of helium towards liquid surface is probably beneficial to the occurrence of mass transfer from liquid to vapor. However, the H2-He diffusion is nearly terminated within a relatively short time, so that a uniform H2-He mixture could be assumed in the researches on propellant performance in space. Moreover, acceleration level plays the dominant role in liquid-gas distribution. When an acceleration change from 3g0 to 10−3g0 occurs, no apparent interface deformation is observed. The liquid-gas interface maintains approximately horizontal distribution and only a slight liquid bending upwards is observed in near-wall region. When a zero-gravity environment is encountered, however, a complete coverage of ullage by liquid phase could be reached. Specially, it should be noted that a superheated ullage could exist in the tank center region for a long time even after its coverage by liquid phase. To meet secondary ignition requirement, orbit control thrust engines, producing axial acceleration of 10−3g0 and lasting over 100 s, could realize reliable liquid-gas separation. In addition, along with pressure reduction in the beginning stage of launching, a remarkable vapor condensation occurs. Coverage of ullage by liquid phase also yields a significant vapor condensation, while under space thrust effect, liquid evaporation mostly happens. Generally, the present study gives clear exhibitions on the pressure evolution and thermodynamic behaviors of cryogenic propellant tank, which is beneficial for the design and optimization of fuel tank system.