Small Relative Motion Research Articles

A Modified Aeromagnetic Compensation Method Robust to the Motion State of Aviation Platform Part

Aeromagnetic compensation is a signal-processing method to suppress the magnetic interference from the airborne platform that far exceeds the target magnetic signal. With the development of measurement technology, the traditional aeromagnetic compensation model in which the motion platform is considered as a rigidly connected whole cannot satisfy the request for high-quality aeromagnetic detection, and the magnetic interference generated by the small relative motion components needs to draw our attention. In this paper, the source of maneuver magnetic interference on the aircraft platform is split into two parts, i.e., the rigidly connected main body part (MBP) and moving part (MP), and their compensation models can be both built by referring to the Tolles–Lawson (TL) model. The key factor in these two models is how to obtain the direction cosine that describe the maneuvering actions of aircraft. In the MBP compensation model, the direction cosine can be directly obtained by the onboard fluxgate magnetometer. However, those in the MP model cannot be directly obtained because there is no any monitoring equipment for it. Therefore, a scheme for calculating the direction cosine of the MP compensation model is proposed. This scheme combines the direction cosine obtained by the onboard fluxgate magnetometer with the relative motion angles. And considering the influence of geomagnetic gradient magnetic interference, its compensation model is brought to form a new aeromagnetic compensation model, herein called the TLGM model. This model is different from the traditional TL model and is nonlinear since the three unknown relative motion angles, so the quantum genetic algorithm (QGA) is introduced to combine with recursive least-squares to estimate those unknowns. Some field experiments were conducted, and the results show that the proposed method reduces the standard deviations and figures of merit by over 42% and 39%, respectively, compared with those using the previous model. Also, the proposed compensation method requires no additional motion monitoring sensors and is more convenient for practical applications.

On turbulence and interfacial momentum transfer in dispersed gas-liquid flows: Contribution of bubbly flow experiments under microgravity conditions

The major focus of the present work is to develop an adequate closure model for the turbulent contribution of the interfacial momentum transfer term in turbulent bubbly flow. For this purpose, numerical simulations have been conducted on bubbly pipe flows under microgravity conditions. With no buoyancy effect, the average interfacial forces are negligible due to the small interphase relative motion. In these conditions, the distribution and the dynamics of bubbles are mainly controlled by the turbulence effects. Based on a phenomenological approach, a new closure for the turbulent interfacial momentum transfer is developed within Eulerian two-fluid modelling based on the averaging of the added mass force. The turbulent terms thus obtained comprises a non-linear term and a convective acceleration term associated with the drift velocity. The two-fluid model with the proposed interfacial momentum transfer closure and an original three-equation turbulence model for bubbly flows has been implemented in a 3D CFD code and applied to pipe bubbly flow under microgravity conditions. When compared to conventional two-fluid closures, the turbulent contribution of the added mass force proved to be a relevant closure that provides a substantial improvement in predicting the void fraction distribution and the bubbles’ dynamics.

Predicting Carpal Bone Kinematics Using an Expanded Digital Database of Wrist Carpal Bone Anatomy and Kinematics

The wrist can be considered a 2 degrees-of-freedom joint with all movements reflecting the combination of flexion-extension and radial-ulnar deviation. Wrist motions are accomplished by the kinematic reduction of the 42 degrees-of-freedom of the individual carpal bones. While previous studies have demonstrated the minimal motion of the scaphoid and lunate as the wrist moves along the dart-thrower's path or small relative motion between hamate-capitate-trapezoid, an understanding of the kinematics of the complete carpus across all wrist motions remains lacking. To address this, we assembled an open-source database of in vivo carpal motions and developed mathematical models of the carpal kinematics as a function of wrist motion. Quadratic surfaces were trained for each of the 42-carpal bone degrees-of-freedom and the goodness of fits were evaluated. Using the models, paths of wrist motion that generated minimal carpal rotations or translations were determined. Model predictions were best for flexion-extension, radial-ulnar deviation, and volar-dorsal translations for all carpal bones with R 2 > 0.8, while the estimates were least effective for supination-pronation with R 2 < 0.6. The wrist path of motion's analysis indicated that the distal row of carpal bones moves rigidly together (<3° motion), along the anatomical axis of wrist motion, while the bones in the proximal row undergo minimal motion when the wrist moves in a path oblique to the main axes. The open-source dataset along with its graphical user interface and mathematical models should facilitate clinical visualization and enable new studies of carpal kinematics and function. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2661-2670, 2019.

Roughness Effects on Fretting Fatigue

Fretting is a small oscillatory relative motion between two normal loaded contact surfaces. It may cause fretting fatigue, fretting wear and/or fretting corrosion damage depending on various fretting couples and working conditions. Fretting fatigue usually occurs at partial slip condition, and results in catastrophic failure at the stress levels below the fatigue limit of the material. Many parameters may affect fretting behaviour, including the applied normal load and displacement, material properties, roughness of the contact surfaces, frequency, etc. Since fretting damage is undesirable due to contacting, the effect of rough contact surfaces on fretting damage has been studied by many researchers. Experimental method on this topic is usually focusing on rough surface effects by finishing treatment and random rough surface effects in order to increase fretting fatigue life. However, most of numerical models on roughness are based on random surface. This paper reviewed both experimental and numerical methodology on the rough surface effects on fretting fatigue.

Energy harvesting using a flextensional compliant mechanism

This work presents a vibration-based electromagnetic energy harvester that relies on a mechanical motion amplification mechanism. The aim is to convert a small persistent base motion into larger oscillations in order to generate greater amounts of electric power. The device can be used in situations where a small cyclic relative motion occurs between two surfaces, and where a device can be fitted to generate energy from within. Unlike conventional amplification mechanisms that rely heavily on gears, we employ a compliant mechanism of the flextensional type for energy harvesting. Such a mechanism contains few mechanical parts and attains its mobility from the elasticity of the material, thereby reducing excessive clearance, friction, and power losses. To convert the amplified mechanical motion into electrical energy, a permanent magnet is attached to the output end of the mechanism and is designed to oscillate past a stationary coil. A quasi-static model is formulated in conjunction with a finite element model of the mechanism in order to evaluate the amplification ratio, internal stresses in the flexure joints, output voltage, and power in terms of the design parameters of the flextensional mechanism. The results are supported experimentally on a cam-driven polymer flextensional mechanism across a range of operating speeds and load resistance. A parametric study is conducted to investigate the effect of the various design parameters on the system performance. An effort is made to optimize the design parameters to achieve higher output power levels while minimizing the internal stresses generated in the mechanism.

Whistler emission in the separatrix regions of asymmetric magnetic reconnection

AbstractAt Earth's dayside magnetopause asymmetric magnetic reconnection occurs between the cold dense magnetosheath plasma and the hot tenuous magnetospheric plasma, which differs significantly from symmetric reconnection. During magnetic reconnection the separatrix regions are potentially unstable to a variety of instabilities. In this paper observations of the separatrix regions of asymmetric reconnection are reported as Cluster crossed the magnetopause near the subsolar point. The small relative motion between the spacecraft and plasma allows spatial changes of electron distributions within the separatrix regions to be resolved over multiple spacecraft spins. The electron distributions are shown to be unstable to the electromagnetic whistler mode and the electrostatic beam mode. Large‐amplitude whistler waves are observed in the magnetospheric and magnetosheath separatrix regions, and outflow region. In the magnetospheric separatrix regions the observed whistler waves propagate toward the X line, which are shown to be driven by the loss in magnetospheric electrons propagating away from the X line and are enhanced by the presence of magnetosheath electrons. The beam mode waves are predicted to be produced by beams of magnetosheath electrons propagating away from the X line and potentially account for some of the electrostatic fluctuations observed in the magnetospheric separatrix regions.

Fretting fatigue failure mechanism of automotive shock absorber valve

Fretting fatigue is a complex mechanical failure phenomenon, in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. This study proposes a methodology to analyze the fretting fatigue failure mechanism of automotive shock absorber valve by means of experimental and numerical approaches. A servo hydraulic test set-up is used to simulate fretting fatigue under real working conditions. Moreover, a 3-D finite element model is developed to analyze the contact status and stress distribution at contact interface between connected components, i.e. washer-disc contact. The experimental test results depict that fretting damage appears at contact interface between washer and disc, which causes the initial crack nucleation and advancing the crack up to the final fracture of valve disc. Stress field, obtained by numerical simulation, is used to monitor some fretting fatigue features such as the distribution of relative slip amplitude, contact pressure and different stress fields at contact interfaces. Eventually, the crack initiation site is estimated by monitoring variation of equivalent multiaxial damage stress at contact interface.

NUMERICAL MODELING OF FRETTING WEAR

In this paper, a review of some techniques proposed in the literature for modelling fretting wear is presented. Fretting wear occurs when surfaces are degraded and materials are removed due to a small relative oscillatory motion between two contact surfaces. Due to the continuous change in the shape of a structure during its lifetime, the design of engineering components subjected to fretting wear is a challenge to engineers and scientists. The available predictive tools in the literature, which make use of analytical solution of the contact pressure in the first cycle ignoring the wear effect and removal of materials in the subsequence cycles, provide inaccurate results. Numerical modelling techniques such as Finite Element Analysis (FEA) can be used to model any type of structures with configuration taking into account many complicated details such as large deformation, material non-linearity, change in geometry and time integration effect. Therefore, FEA is a very desirable technique for fretting wear analysis and lifetime prediction. This review covers modelling fretting wear using numerical modelling methods, which can be divided into three main techniques, namely the FEA method, Boundary Element Method (BEM) and Finite Discrete Element Method (FDEM).

Finite Element Analysis of Localized Plasticity in Al 2024-T3 Subjected to Fretting Fatigue

Fretting fatigue is a combination of two complex mechanical phenomena, namely, fretting and fatigue. Fretting appears between components that are subjected to small relative oscillatory motion. Once these components undergo cyclic fatigue load at the same time, fretting fatigue occurs. Fretting fatigue is an important issue in aerospace structural design. Many studies have investigated fretting fatigue behavior; however, the majority have assumed elastic deformation and very few have considered the effect of plasticity. The main goal of this study is to monitor the effect of different fretting fatigue primary variables on localized plasticity in an aluminum alloy (Al 2024-T3) test specimen. In order to extract the stress distribution at the contact interface under elasto-plastic conditions, a modified finite element contact model was used. The contact model was verified through comparison with an elastic analytical solution. Then, a bilinear elasto-plastic isotropic hardening model with a von Mises yield surface was implemented to simulate the material behavior of the aluminum alloy. The effect of different fretting fatigue primary variables, such as axial stress, contact geometry, and coefficient of friction, on localized plasticity was investigated. Finally, the relationship between the location of maximum localized plasticity and Ruiz fretting damage parameter with the crack initiation site is discussed.

Numerical Investigation into the Effect of Contact Geometry on Fretting Fatigue Crack Propagation Lifetime

Fretting fatigue is a phenomenon in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. There is a need to analyze the effects of contact geometry on crack propagation under fretting fatigue conditions. In this investigation, a finite element modeling method was used to study the effects of different contact geometries along with crack–contact interaction on crack propagation lifetime. Different contacts geometries—that is, cylindrical on flat and flat on flat—along with different contact span widths were analyzed. In addition, the effects of different contact spans on stress distribution at the contact interface were investigated. The computed crack propagation life was compared with experimental results. It was found that the crack initiated near the contact trailing edge for all contact geometries, which agreed with experimental observations. In terms of crack propagation for different contact spans, the fretting fatigue life for a two-based cylindrical pad was shorter than that for a two-based flat pad. By increasing the contact span width for both flat and cylindrical pads, the crack propagation lifetime increased. A comparison between the experimental and numerical results demonstrated a difference of about 18% in crack propagation lifetime.

Motion of the fibula relative to the tibia and its alterations with syndesmosis screws: A cadaver study

The motion of the fibula in relation to the tibia is coupled on the motion of the talus in the ankle joint. Several authors investigated this motion with different methods. An injury of the elastic fixation of the fibula to the tibia and its treatment with the syndesmotic set screw has an impact on this motion. The motion of the fibula relative to the tibia was measured in eight embalmed human above the knee amputated cadaver specimens using a 3D-motion analysis system. The relative motion was measured from 50° of plantar flexion to 30° of dorsiflexion. Experiments were performed in the following conditions: without fixation and intact ligaments, after sectioning of the four syndesmotic ligaments and the interosseous membrane, and application of either a tricortical screw, or a quadricortical screw or two quadricortical screws. Concordant movements of the lateral malleolus were a medial translation during plantar flexion, external rotation around the sagittal axis during plantar and dorsiflexion. The motion of the proximal fibula was smaller and more variable than in the distal part. After sectioning of the syndesmosis the range of motion, compared to the intact state increased, particularly in translation along the transversal (118%), sagittal (160%) and the longitudinal (136%) axis and in axial rotation (145%). Syndesmotic screws reduced the range of motion in transversal (p<0.006) and sagittal translation (p<0.011) and axial rotation. The small relative motion of the tibia and fibula is increased by syndesmotic injuries. Syndesmosis screws significantly limit this increased relative motion below physiologic values, which makes it necessary to remove the screws before flexion in the ankle joint is performed.

FEA of Fretting Fatigue: A Comparison between ANSYS and ABAQUS

Fretting fatigue occurs when two contact bodies undergo small oscillatory relative motion due to cyclic loading. It leads to fretting damage, which reduces the service life of the bodies in contact. This can be explained by the high stresses generated at the contact surface between the two bodies. Therefore, numerical analysis, such as Finite Element Analysis (FEA), would be useful to understand the fretting fatigue phenomenon and to investigate techniques to reduce its effect on fatigue lifetime of mechanical components. In this paper, FEA of fretting fatigue Aluminum specimen is carried out in order to study the stress distribution and crack initiation location. Two commercial FEA packages, namely ANSYS and ABAQUS are used to analyze the specimen. The stress distribution along the contact surface between the two bodies obtained using both codes is compared and analyzed.

The performance of structure-tuned liquid damper systems with different tank geometries

An equivalent mechanical model is employed to study structure-tuned liquid damper (TLD) systems and evaluate the performance of TLDs having rectangular, vertical-cylindrical or horizontal-cylindrical tank shapes, which are placed at the top of a structure. TLD performance is evaluated in terms of effective damping, efficiency and robustness. This theoretical study is limited to small response amplitude such that the natural frequency and the effective mass of the TLD can be assumed to remain constant, while the damping of the TLD can be expressed in terms of equivalent amplitude-dependent viscous damping. The tuning ratio, the structural response amplitude, the mass ratio and the liquid depth are varied to investigate their effects on TLD performance. Performance charts are developed and subsequently utilized to present findings from this study. It is found that small liquid depth ratio and large mass ratio lead to a more robust structure-TLD system with small relative motion occurring between the structure and the vibration absorber. Comparisons of TLD performance, corresponding to the different tank geometries investigated, are made. Copyright © 2008 John Wiley & Sons, Ltd.

Development of a fretting wear test rig and preliminary studies for understanding the fretting wear properties of steels

Rolling element bearings experience fretting wear type of failures due to small relative oscillatory motion existing between races and rolling elements of the bearing. Small oscillatory motion between contacting surfaces that occur due to overall machine or system vibration and unexpected loads in service causes accumulation of wear debris in the contact zone and accelerate the wear process. A fretting wear test rig is designed and developed in the laboratory for understanding the fretting wear behavior of bearing materials. The test rig is designed for testing at slip amplitudes ranging from 20 to 200 μm and test frequencies ranging from 1 to 30 Hz. Preliminary studies for understanding the fretting wear characteristics were conducted on hardened and tempered bearing steel, En31 and widely employed hardened and tempered structural steel, En24 at various normal loads. Coefficient of friction during fretting tests was high during the initial stages of fretting tests and reached a steady state. The coefficient of friction depends on the normal load. Wear scar morphology reveals the nature of contact exhibited by the fretting contacts.

New attitude motion following control algorithm for capturing tumbling object in space

A novel attitude control method to capture tumbling objects in space is discussed. The capturing procedure considered in this study is that the chaser satellite surrounds the target satellite and synchronizes its attitude to the target's one so that the capturing operation can be conducted with small relative motion between the chaser and the target. To realize this idea, a novel control algorithm called free motion path method (FMPM) has been developed, which enables us to completely ignore a nonlinearity effect in the dynamics by taking advantage of the conservative quantities of the system. The FMPM aims for on-line control and forms a closed-loop system. With FMPM, we can explicitly limit the maximum control torque and can also implement a certain kind of optimality. Some theoretical analyses and the results of computer simulations are shown in this paper.

The “pair” Fermi contour and high-temperature superconductivity

Superconducting pairing of holes with a large (on the order of doubled Fermi) total pair momentum and small relative motion momenta is considered taking into account the quasi-two-dimensional electronic structure of high-Tc cuprates with clearly defined nesting of the Fermi contour situated in an extended neighborhood of the saddle point of the electronic dispersion law (the momentum space region with a hyperbolic metric) and the arising of a spatially inhomogeneous (stripe) structure as a result of the redistribution of current carriers (holes) that restores regions with antiferromagnetic ordering. The superconducting energy gap and condensation energy were determined, and their dependences on the doping level were qualitatively studied. The energy gap was shown to exist in some hole concentration region limited on both sides. The superconducting state with a positive condensation energy appears in a narrower range of doping within this region. The reason for the arising of the superconducting state at a repulsive screened Coulomb interaction between holes is largely the redistribution of hole pairs in the momentum space related to the special features of the hyperbolic metric, which is responsible for the formation of the “pair” Fermi contour, and the renormalization of the kinetic energy of holes when the chemical potential changes because of the condensation of pairs. Hole pairs of the type under consideration exist not only in the condensate but also in the form of quasi-stationary states with very weak decay at temperatures substantially exceeding the superconducting transition temperature. The pseudogap region of the phase diagram of high-Tc cuprates is related to such states. The pairing mechanism under consideration allows not only the principal characteristics of the phase diagram but also key experimental data on high-Tc cuprate materials to be qualitatively explained.

Close approach and deformation of two viscous drops due to gravity and van der waals forces

The rate of approach and deformation of two small drops of different sizes in buoyancy-driven axi-symmetric motion is analyzed within the asymptotic limit of small capillary number, in which deformation becomes important only when the minimum separation between the drops is much smaller than either radius. Then, the drops translate with nearly equal velocities, and the small relative motion as the larger drop slowly catches up with the smaller drop can be described by an analysis which assumes a lubrication regime in the thin gap between them. When this lubrication squeeze flow in the gap exerts a sufficient tangential stress on the drop interfaces to drive an internal flow within the drops, then the coupling between these flows is analyzed by a boundary integral formulation. Numerical calculations are used to track the evolution of the shapes of the drops from a relatively undeformed state until the gap takes the form of a dimple and a long-time quasi-steady pattern is established. As the drop separation decreases, attractive van der Waals forces come into play and bring about the rupture of the fluid film between the drops. The rupture time is predicted, and a demarcation is observed between “nose rupture” for drops with small deformations and “rim rupture” for drops with large deformations and dimples.

Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception.

A black and white (positive) grating pattern was superimposed in exact register on its own photographic negative. Four operations were repetitively applied to this positive pattern so that it moved fractionally to the right, grew dimmer, moved back to the left, and grew brighter again. This sequence produced a strong illusion of continuous apparent motion to the right for as long as the cycle was repeated. The small relative motion between the two patterns generated two new illusory effects: enhanced real movement (ERM) and reversed real movement (RRM). The dimming and brightening phases gave rise to reversed apparent movement (RAM). All three effects are attributed to spatial filtering by neural mechanisms, which shifts the effective position of the positive-negative contours.

Analysis and Testing for Rotordynamic Coefficients of Turbulent Annular Seals With Different, Directionally-Homogeneous Surface-Roughness Treatment for Rotor and Stator Elements

A combined analytical-computational method is developed to calculate the transient pressure field and dynamic coefficients for high-pressure annular seal configurations which may be used in interstage and neck-ring seals of multistage centrifugal pumps. The solution procedure applies to constant-clearance or convergent-tapered geometries which may have different (but directionally-homogeneous) surface-roughness treatments on the stator or rotor seal elements. It applies in particular so-called “damper-seals” which employ smooth rotors and deliberately-roughened stator elements to enhance rotor stability. Hirs’ turbulent lubrication equations are modified slightly to account for different surface-roughness conditions on the rotor and stator. A perturbation analysis is employed in the eccentricity ratio to develop zeroth and first order perturbation equations. The zeroth-order equations define both the leakage and the development of circumferential flow due to shear forces at the rotor and stator surfaces. The first-order equations define perturbations in the pressure and axial and circumferential velocity fields due to small relative motion between the seal rotor and stator. The solution applies for small motion about a centered position and does not employ linearization with respect to either the taper angle or the degree of swirl, i.e., the difference between the circumferential velocity at the given axial position and the asymptotic circumferential-velocity solution. Test results for four different surface-roughness confirm the predicted net damping increase for “damper seals.” A round-hole-pattern stator yielded the highest net damping and lowest leakage of all seals tested. The seals are substantially stiffer than predicted, but the theory does an adequate job of predicting net damping.

Image registration by matching relational structures

We present a method of image registration where we use relational structures as representations of images and we register two images by finding a match between the two relational structures. It is better than traditional cross-correlation like techniques in several aspects: it is insensitive to scaling, rotation, gray level modification and small relative motion between the two images. The matching process makes use of star structures, which have several advantages over conventional node structures.