Asymmetric Compression Research Articles

High efficiency and broadband acoustic diodes

Energy transmission efficiency and working bandwidth are the two major factors limiting the application of current acoustic diodes (ADs). This letter presents a design of high efficiency and broadband acoustic diodes composed of a nonlinear frequency converter and a linear wave filter. The converter consists of two masses connected by a bilinear spring with asymmetric tension and compression stiffness. The wave filter is a linear mass-spring lattice (sonic crystal). Both numerical simulation and experiment show that the energy transmission efficiency of the acoustic diode can be improved by as much as two orders of magnitude, reaching about 61%. Moreover, the primary working band width of the AD is about two times of the cut-off frequency of the sonic crystal filter. The cut-off frequency dependent working band of the AD implies that the developed AD can be scaled up or down from macro-scale to micro- and nano-scale.

Position matching between the visual fields in strabismus.

The misalignment of visual input in strabismus disrupts positional judgments. We measured positional accuracy in the extrafoveal visual field (1°-7° eccentricity) of a large group of strabismic subjects and a normal control group to identify positional distortions associated with the direction of strabismus. Subjects performed a free localization task in which targets were matched in opposite hemifields whilst fixating on a central cross. The constant horizontal error of each response was taken as a measure of accuracy, in addition to radial and angular error. In monocular conditions, all stimuli were viewed by one eye; thus, the error reflected spatial bias. In dichoptic conditions, the targets were seen by separate eyes; thus, the error reflected the perceived stimulus shift produced by ocular misalignment in addition to spatial bias. In both viewing conditions, both groups showed reliable over- and underestimations of visual field position, here termed a compression of response coordinates. The normal group showed compression in the left periphery, regardless of eye of stimulation. The strabismic group showed a visual field-specific compression that was clearly associated with direction of strabismus. The variation in perceived shift of strabismic subjects was largely accounted for by the biases present in monocular viewing, suggesting that binocular correspondence was uniform in the tested region. The asymmetric strabismic compression could not be reproduced in normal subjects through prism viewing, and its presence across viewing conditions suggests a hemifield-specific change in spatial coding induced by long-standing ocular misalignment.

Stereoscopic Image Quality Assessment Based on Depth and Texture Information

With the increasing growth of multimedia applications over the networking in recent years, users have put forward much higher requirements for multimedia signals, particularly for image/video signal's quality of experience (QoE) than before. The objective approach of image quality assessment (IQA) plays an important role for the development of compression standards and various multimedia applications. However, the quality assessment of stereoscopic (3-D) images faces more new challenges, such as depth perception, virtual view synthesis, and asymmetric stereo compression. In this paper, we propose a new full-reference (FR) 3-D IQA method to measure the quality of the distorted images. The properties of the depth component, structure component, and gradient component are taken into account to establish the proposed metric. The experimental results show that the proposed metric is highly consistent with the subjective test scores compared with the existing related metrics. In addition, the main significance of the proposed metric is that it not only could effectively evaluate the quality of 3-D image but also has a satisfied effect for measuring the quality of 2-D image.

Asymmetric Compression of Representational Space for Object Animacy Categorization under Degraded Viewing Conditions.

Animacy is a robust organizing principle among object category representations in the human brain. Using multivariate pattern analysis methods, it has been shown that distance to the decision boundary of a classifier trained to discriminate neural activation patterns for animate and inanimate objects correlates with observer RTs for the same animacy categorization task [Ritchie, J. B., Tovar, D. A., & Carlson, T. A. Emerging object representations in the visual system predict reaction times for categorization. PLoS Computational Biology, 11, e1004316, 2015; Carlson, T. A., Ritchie, J. B., Kriegeskorte, N., Durvasula, S., & Ma, J. Reaction time for object categorization is predicted by representational distance. Journal of Cognitive Neuroscience, 26, 132-142, 2014]. Using MEG decoding, we tested if the same relationship holds when a stimulus manipulation (degradation) increases task difficulty, which we predicted would systematically decrease the distance of activation patterns from the decision boundary and increase RTs. In addition, we tested whether distance to the classifier boundary correlates with drift rates in the linear ballistic accumulator [Brown, S. D., & Heathcote, A. The simplest complete model of choice response time: Linear ballistic accumulation. Cognitive Psychology, 57, 153-178, 2008]. We found that distance to the classifier boundary correlated with RT, accuracy, and drift rates in an animacy categorization task. Split by animacy, the correlations between brain and behavior were sustained longer over the time course for animate than for inanimate stimuli. Interestingly, when examining the distance to the classifier boundary during the peak correlation between brain and behavior, we found that only degraded versions of animate, but not inanimate, objects had systematically shifted toward the classifier decision boundary as predicted. Our results support an asymmetry in the representation of animate and inanimate object categories in the human brain.

A kinematic hardening constitutive model for the uniaxial cyclic stress–strain response of magnesium sheet alloys at room temperature

A kinematic hardening constitutive model is presented, in which a modified form of von Mises yield function is adopted, and the initial asymmetric tension and compression yield stresses of magnesium (Mg) alloys at room temperature (RT) are considered. The hardening behavior was classified into slip, twinning, and untwinning deformation modes, and these were described by two forms of back stress to capture the mechanical response of Mg sheet alloys under cyclic loading tests at RT. Experimental values were obtained for AZ31B-O and AZ31B sheet alloys under both tension–compression–tension (T–C–T) and compression–tension (C–T) loadings to calibrate the parameters of back stresses in the proposed model. The predicted parameters of back stresses in the twinning and untwinning modes were expressed as a cubic polynomial. The predicted curves based on these parameters showed good agreement with the tests.

On the cause of two prompt shock-induced relativistic electron depletion events

Dayside interplanetary (IP) shock-induced injections are known to be a source of highly relativistic electrons in Earth's outer radiation belt, and are possibly the only source of >1 MeV electrons in the inner belt. The associated electron energization process is well understood and modeled. Recently, relativistic electron depletion echoes have also been associated with IP shocks, but the processes driving the depletions are less well understood. In this study, we investigate in detail two shock induced >1 MeV electron depletion events observed by the Van Allen Probes, March 17, 2015 and May 24, 2013, and draw similarities to night-side substorm related enhancements and depletions. Both events exhibit shock induced enhancements on one of the Van Allen Probes and depletions on the other in >1 MeV channels, such observations have not previously been reported. The depletion of >1 MeV electrons during the March 17, 2015 event is associated with enhancements in 10 s–100 s keV electrons on the same spacecraft. The depletion is consistent with the effects of a lack of seed electrons at larger radial distances combined with inward motion due to asymmetric compression by the shock impact. The immediate enhancements and depletions of 75 keV–2.6 MeV electrons are explained by the local phase space density radial profile. Observations of electron flux dynamics during the May 24, 2013 event can also be explained by a lack of a seed population at larger radial distances, supported by butterfly distributions observed during the event. The electron's inward radial motion can be attributed to the inward propagating impulse also associated with the >1 MeV electron enhancements observed on the complementary probe, rather than global asymmetric compression. This causal mechanism has parallels to substorm related depletions. Alternatively, evidence is provided to attribute the sudden depletions to losses due to a sudden but brief inward motion of the magnetopause.

Generation of high brightness electron beam by brake-applied velocity bunching with a relatively low energy chirp

Velocity bunching technique is a tool for compressing electron beams in modern high brightness photoinjector sources, which utilizes the velocity difference introduced by a traveling rf wave at a relatively low energy. It presents peculiar challenges when applied to obtain a beam with a very high current and a low transverse emittance in photoinjectors. The main difficulty is to control the emittance oscillations of the beam during high compression, which can be naturally considered as an extension of the emittance compensation process. In this paper, a brake-applied velocity bunching scheme is proposed, in which the electron bunch is injected into the accelerator with a low gradient at a deceleration phase, like “a brake is applied”, afterward slips to an acceleration phase. During the entire compression process, the energy chirp induced by the rf field is mostly linear, which retains a symmetric electron beam in the temporal distribution. The key point of the new scheme is a smaller energy chirp at a lower beam energy compared with the normal velocity bunching. Besides, the beam energy chirp before compression is dominated by the linear correlation due to a relatively short laser pulse. With a symmetric bunch compression, the transverse emittance could be compensated even if the compression factor is extremely high. As to our simulation results, the peak current of the compressed beam can be above 1.8 kA for the charge of 800 pC with a good emittance compensation.

Model Gas-Dynamical Problems Possessing Cylindrical and Spherical Symmetry and Their Solution with the Aid of Weno Schemes

The solution of a number of problems of gas dynamics that are associated with cylindrically and spherically symmetric nonstationary compression of an inviscid gas is considered. Discretization of the Euler equations is carried out with the aid of the method of finite volumes and difference schemes of the WENO type. The results of numerical simulation are compared with exact solutions and with the results of the calculations available in the literature. The solutions of two- and three-dimensional problems possessing cylindrical and spherical symmetry are given in comparison with the solutions obtained in one-dimensional formulation. The calculations with the use of WENO schemes make it possible to obtain an exact and monotonic solution of the problem in the presence of both weak and strong gas-dynamical discontinuities.

Fast ignition of asymmetrically compressed targets for inertial confinement fusion

It is shown that fast ignition can ensure the combustion of asymmetrically compressed targets for inertial confinement fusion with an efficiency close to the combustion of one-dimensionally compressed targets. This statement is valid not only for targets specially designed for fast ignition. Fast heating by an external energy source can ensure the ignition of a target designed for spark ignition, but where this ignition does not occur because inhomogeneities are formed in the temperature and density distributions owing to the development of hydrodynamic instabilities. The condition for ignition is the fast heating of the plasma in the combustion initiation region whose size is comparable with the sizes of compression inhomogeneities. Thus, fast ignition not only significantly reduces the ignition energy, but also is possibly a necessary stage in the inertial confinement fusion scheme when the spherically symmetric compression of a target requires very high engineering and financial expenses. The studies are based on the numerical simulation of the compression and combustion of inertial confinement fusion targets with one- and two-dimensional hydrodynamic codes.

Asymmetrically Compressed Stereoscopic 3D Videos: Quality Assessment and Rate-Distortion Performance Evaluation.

Objective quality assessment of stereoscopic 3D video is challenging but highly desirable, especially in the application of stereoscopic video compression and transmission, where useful quality models are missing, that can guide the critical decision making steps in the selection of mixed-resolution coding, asymmetric quantization, and pre- and post-processing schemes. Here we first carry out subjective quality assessment experiments on two databases that contain various asymmetrically compressed stereoscopic 3D videos obtained from mixed-resolution coding, asymmetric transform-domain quantization coding, their combinations, and the multiple choices of postprocessing techniques. We compare these asymmetric stereoscopic video coding schemes with symmetric coding methods and verify their potential coding gains. We observe a strong systematic bias when using direct averaging of 2D video quality of both views to predict 3D video quality. We then apply a binocular rivalry inspired model to account for the prediction bias, leading to a significantly improved full reference quality prediction model of stereoscopic videos. The model allows us to quantitatively predict the coding gain of different variations of asymmetric video compression, and provides new insight on the development of high efficiency 3D video coding schemes.

Particle simulation study on anisotropic pressure of electrons in laser-produced plasma interaction

Direct-drive inertial confinement fusion (ICF) requires a symmetric compression of the fuel target to achieve physical conditions for the ignition. The fast ignition scheme reduces the symmetry requirements for the target compression and the necessary driving energy, but symmetrically compressed target will certainly help improve the efficiency of the nuclear fuel burning. In this paper, with the particle-in-cell (PIC) simulation method, characteristics of the anisotropic pressure tensor of hot electrons are reported for the ultra intense laser pulse interaction with over dense plasmas, which mimics the scenario of the last stage when hot electrons are utilized to ignite the compressed fuel core in the ICF fast ignition scheme. A large number of hot electrons can stimulate pressure oscillations in the high density plasma. As the component parallel to the electron velocity dominates the pressure tensor, the electron density distribution perturbation propagates rapidly in this direction. In order to keep those hot electrons in the high density fuel plasma core for a period long enough for them to deposit energy and momentum, a magnetic field perpendicular to the electron velocity is used. The PIC simulation results indicate that the hot electrons can be trapped by the magnetic field, and the components of the anisotropic pressure tensor related to the parallel direction are significantly affected, thereby producing a high peak near the incidence surface. Since it is a relatively long process for the energy transfer from electrons to fuel ions and the nuclear interaction to be completed, the fluid effects take their roles in the fuel target evolution. The anisotropic electron pressure will deteriorate the fuel core symmetry, reduce the density, and achieve a lower efficiency of nuclear fuel burning and a lower gain of nuclear reaction than expected. The effects of the hot electron anisotropic pressure tensor in the fast ignition scheme should be considered as a factor in experiments where the nuclear reaction gain is measured to be much lower than the theoretical prediction.

Effects of asymmetric creep-ageing behaviour on springback of AA2050-T34 after creep age forming

Abstract This study investigates the effects of asymmetric creep-ageing behaviour on springback of aluminium alloys during creep age forming (CAF) process. An Al-Cu-Li alloy, AA2050, which shows an apparent asymmetric tension and compression creep-ageing behaviour is used for investigation. Several CAF trial tests to form singly-curved AA2050 plates with different thicknesses (3, 5 and 8 mm) are carried out with a four point bending setup. Meanwhile, two sets of finite element (FE) models of corresponding processes have been developed, in which either the conventional symmetric or the new asymmetric creep-ageing behaviour of the alloy was used. The asymmetric models can well predict the shape of formed plates for thicker materials (5 and 8 mm), while symmetric models provide over-prediction of final deflections. The results from asymmetric models indicate that more creep strain is generated in the tension stressed part of the forming plate than that in the other part with compressive stresses and therefore, resulting in an asymmetric distribution of the relaxed stresses through the thickness of the forming plates after creep-ageing. After springback, both top and bottom surfaces of the formed plates show significant compressive stresses while tensile residual stresses exist in the centre of the formed plates. The work in this study helps to understand the particular springback behaviour of AA2050 with asymmetric creep-ageing behaviour in CAF, and can be used to guide future industrial applications of the alloy in the CAF process.

Uniform irradiation of a direct drive target by optimizing the beam parameters

Laser driven fusion requires a high-degree uniformity in laser energy deposition in order to achieve the high-density compression required for sustaining a thermonuclear burn. Nowadays, uniform irradiation of capsule is still a key issue in direct drive inertial confinement fusion. The direct drive approach is to drive the target with laser light, by irradiating it with a large number of overlapping laser beams. In the direct drive scheme, the laser deposition pattern on the target can be decomposed into a series of Legendre spherical harmonic modes. The high mode (shorter wavelength) nonuniformity can lead to Rayleigh-Taylor instability, which may result in the failure of target compression. This nonuniformity can be suppressed by thermal conduction and beam conditioning technologies, such as continuous phase plate, smoothing by spectral dispersion and polarization smoothing. The low mode (longer wavelength) nonuniformity is related to the number, orientation and power balance of laser beams, which is hard to suppress by thermal conduction and beam conditioning technologies. Generally, the nonuniformity of laser irradiation on a directly driven target should be less than 1% (root mean square, RMS), to meet the requirement for symmetric compression. Several methods have been proposed to optimize the irradiation configuration in direct drive laser fusion, such as truncated icosahedron with beams at the 20 faces and 12 vertices of an icosaherdron, dodecahedron-based irradiation configurations, self-organizing electrodynamic method, etc. However, limited by the different parameters of incident beams, the irradiation uniformity is often not satisfactory. Therefore, it is necessary to find new way to improve the irradiation uniformity and make it more robust. According to the analytical result, the irradiation nonuniformity can be decomposed into the single beam factor and the geometric factor. Simulation results show that the single beam factor is mainly determined by the parameters of the incident beams, including beam pattern, beam width and beam wavelength. By analyzing and simulating the single beam factor with different incident beam parameters, and comparing the single beam factor with the geometric factor, a matching relationship between them is found by using the optimized parameters. Based on the simulation results, a method to optimize the incident beam parameters is proposed, which is applied to the 32-beam and 48-beam irradiation configurations. The results show that there is a set of optimal incident beam parameters which can attain the highest irradiation uniformity for a given configuration. The feasibility to achieve more uniform irradiation by optimizing the incident beam parameters is proved. When the single beam factor is optimized in a directly driven inertial confinement fusion system, the restrictions on the beam pointing error and power imbalance between incident beams can be relaxed. The results provide an effective method of designing and optimizing the uniform irradiation system of direct drive laser facility.

A unified constitutive model for asymmetric tension and compression creep-ageing behaviour of naturally aged Al-Cu-Li alloy

A set of unified constitutive equations is presented that predict the asymmetric tension and compression creep behaviour and recently observed double primary creep of pre-stretched/naturally aged aluminium-cooper-lithium alloy AA2050-T34. The evolution of the primary micro- and macro-variables related to the precipitation hardening and creep deformation of the alloy during creep age forming (CAF) are analysed and modelled. Equations for the yield strength evolution of the alloy, including an initial reversion and subsequent strengthening, are proposed based on a theory of concurrent dissolution, re-nucleation and growth of precipitates during artificial ageing. We present new observations of so-called double primary creep during the CAF process. This phenomenon is then predicted by introducing effects of interacting microstructures, including evolving precipitates, diffusing solutes and dislocations, into the sinh-law creep model. In addition, concepts of threshold creep stress σth and a microstructure-dependant creep variable H, which behave differently under different external stress directions, are proposed and incorporated into the creep model. This enables prediction of the asymmetric tension and compression creep-ageing behaviour of the alloy. Quantitative transmission electron microscopy (TEM) and related small-angle X-ray scattering (SAXS) analysis have been carried out for selected creep-aged samples to assist the development and calibration of the constitutive model. A good agreement has been achieved between the experimental results and the model. The model has the potential to be applied to creep age forming of other heat-treatable aluminium alloys.

Domain evolution and phase transition dynamics of BaTiO3 ferroelectric single crystal under high pressure with various loading methods: Phase field simulation

The microstructure and macroscopic properties of ferroelectric materials at high pressure are of great interest in both the engineering and scientific arenas. The effect ofthe pressure value, loading time (the time taken for the pressure to increase from atmospheric pressure to the highest pressure) and loading direction on the evolution of domains and the ferroelectric phase transition for a BaTiO3 single crystal was investigated using a phase field approach. It was found that under symmetrical compression loading the pressure loading time affected the phase transition path and rate but did not affect the phase transition pressure or the ultimate stable phase. For example, at room temperature, even when the loading time increased from 1ns to 10μs, the phase transition pressure remained stable at 2.1GPa, but the phase transition time was prolonged. At −70°C the orthorhombic–cubic phase transition was induced when the loading pressure was 5GPa and the loading time was 1ns, whereas the orthorhombic–tetragonal–cubic phase transition occurred when the loading time increased to 10μs. In addition, it was found that the application of symmetrical pressure tended to reduce the degree of ferroelectricity, while one-dimensional compression favored the ferroelectric phase.

Electron dropout echoes induced by interplanetary shock: Van Allen Probes observations

AbstractOn 23 November 2012, a sudden dropout of the relativistic electron flux was observed after an interplanetary shock arrival. The dropout peaks at ∼1 MeV and more than 80% of the electrons disappeared from the drift shell. Van Allen twin Probes observed a sharp electron flux dropout with clear energy dispersion signals. The repeating flux dropout and recovery signatures, or “dropout echoes”, constitute a new phenomenon referred to as a “drifting electron dropout” with a limited initial spatial range. The azimuthal range of the dropout is estimated to be on the duskside, from ∼1300 to 0100 LT. We conclude that the shock‐induced electron dropout is not caused by the magnetopause shadowing. The dropout and consequent echoes suggest that the radial migration of relativistic electrons is induced by the strong dusk‐dawn asymmetric interplanetary shock compression on the magnetosphere.

Reduction of a Three-Dimensional Problem of the Theory of Bending of Thick Plates to the Solution of Two Two-Dimensional Problems

We propose a new theory of bending of thick plates in the case where its stressed state is not described by the Kirchhoff–Love or Timoshenko hypotheses. It is assumed that the three-dimensional stress-strain state of the plate can be split into symmetric bending and symmetric compression. To describe symmetric bending, we use three harmonic functions. Integrating over the thickness of the plate, we express the bending and torsional moments and shear forces via two two-dimensional functions. The relations of the three-dimensional theory of elasticity are satisfied and a closed system of sixth-order partial differential equations for the introduced functions is constructed without using any hypotheses about the geometric character of deformation of the plate. The analytic and numerical methods for their solution are proposed.

Statistically motivated model of mechanisms controlling evolution of deformation band substructure

Deformation bands are interpreted as a spontaneously formed microstructure caused by anisotropy induced by the slip nature of plastic deformation. The gradient terms in the proposed constitutive equations have been derived by averaging the assembly of discrete dislocations. The rigid-plastic model points to the main constituents which control the fragmentation mechanism: anisotropy of the hardening matrix, anisotropic resistance of the boundaries to slip, and the bowing (Orowan) stress. For symmetric double slip compression, the model provides an explanation of the observed band orientation and band width, and of the significant change in structural morphology seen as the band reorientation occurs at large strains. The predictions are in favorable agreement with available observations.

Effects of Interplanetary Shock Inclinations on Nightside Auroral Power Intensity

We derive fast forward interplanetary (IP) shock speeds and impact angles to study the geoeffectivness of 461 IP shocks that occurred from January 1995 to December 2013 using ACE and WIND spacecraft data. The geomagnetic activity is inferred from the SuperMAG project data. SuperMAG is a large chain which employs more than 300 ground stations to compute enhanced versions of the traditional geomagnetic indices. The SuperMAG auroral electroject SME index, an enhanced version of the traditional AE index, is used as an auroral power (AP) indicator. AP intensity jumps triggered by shock impacts are correlated with both shock speed and impact angle. It is found that high AP intensity events typically occur when high speed IP shocks impact the Earths magnetosphere with the shock normal almost parallel to the Sun-Earth line. This result suggests that symmetric and strong magnetospheric compression leads to favorable conditions for intense auroral power release, as shown previously by simulations and observations. Some potential mechanisms will be discussed.

A grain based modeling study of mineralogical factors affecting strength, elastic behavior and micro fracture development during compression tests in granites

The influence of mineral size and distribution, and model discretization on elastic rock properties, rock strength, and types and numbers of micro cracks was studied numerically using a two-dimensional discrete element grain based modeling (GBM) approach. The model was calibrated to unconfined and confined compression tests and Brazilian tensile tests performed on Aue granite. Confined asymmetric compression tests were simulated for the same rock and compared to physical fracture paths. The GBM approach is able to reproduce all major laboratory observations including the correct ratio between compression strength and Brazilian tensile strength that cannot be captured with earlier contact models.