Identical Amplitude Research Articles

The role of magnetic dipoles and non-zero-order Bragg waves in metamaterial perfect absorbers

We develop a simple treatment of a metamaterial perfect absorber (MPA) based on grating theory. We analytically prove that the condition of MPA requires the existence of two currents, which are nearly out of phase and have almost identical amplitude, akin to a magnetic dipole. Furthermore, we show that non-zero-order Bragg modes within the MPA may consume electromagnetic energy significantly.

On the finite approximation of a Gaussian process and its effect on extreme value theory

The finite approximation of a Gaussian process is incorporated throughout a variety of engineering applications. Central to the marine industry is the reconstruction of a stochastic seaway using a finite number of Fourier components. When coupled with Monte-Carlo methods, a pragmatic approach is often taken to determine an appropriate number of Fourier components. In this paper, an analytical solution to the probability distribution of a finite summation of sinusoids with identical amplitudes is developed and then extended to the general case. The resulting probability distribution is then evaluated through the context of extreme value theory and compared to that of a true Gaussian process. As might be expected, the finite approximation underpredicts the theoretical value by an often significant margin. The relative magnitude of this deviation, however, provides a theoretical basis for readily determining a sufficiency condition for the number of Fourier components to be used in any specific application.

API 2W Gr. 50 강재 용접부의 피로균열전파거동의 거시적 및 미시적 관찰

It is well known that a considerable amount of scatter is shown in experimental results relating to fatigue crack growth even under identical and constant amplitude cyclic loading conditions. Moreover, flux cored arc welding (FCAW) is a common method used to join thick plates such as the structural members of large scale offshore structures and very large container ships. The objective of this study was to investigate the macro- and microscopic observations of the fatigue crack growth (FCG) behavior of the FCAWed API 2W Gr. 50 steel joints typically applied for offshore structures. In order to clearly understand the randomness of the fatigue crack growth behavior in the materials of three different zones, the weld metal (WM), heat affected zone (HAZ), and base metal (BM), experimental fatigue crack growth tests for each of five specimens were performed on ASTM standard compact tension (CT) specimens under constant amplitude cyclic loading. Special focus was placed on the fatigued fracture surfaces. As a result, a different behavior was observed at the macro-level, depending on the type of material property: BM, HAZ, or WM. The variability in the fatigue crack growth rate for WM was higher than that of BM and HAZ.

Experimental measurement of utricle dynamic response1

The utricle of the red-eared turtle was subjected to forced sinusoidal oscillations across various frequencies (10-125 Hz) and amplitudes (5-9 μm) to determine dynamic characteristics of the utricle under natural inertial stimulation. The utricle was maintained in physiologic solution during the entire experiment. Utricular specimens were prepared so that the Otoconial Layer (OL) crystals were exposed yet undisturbed, and the neuroepithelium was secured to a glass slide with dental floss strands. A piezoelectric-actuated platform, fitted to the stage of the microscope, created controlled sinusoidal displacement along the utricle's medial-lateral direction. The OL surface displacement was measured through the microscope with high-speed video at 1500 fps. A sub-pixel image registration algorithm was used to achieve displacement resolution ⩽ 15 nm. The Membranous Shelf (MS), that overlies the macula, was recorded with high-speed video under identical amplitude and frequency inputs and was used as a reference point. Maximum displacement amplitudes of the OL and MS were used to determine the Amplitude Ratio (AR) of the OL relative to the MS. ARs at various frequencies were fit to a single degree of freedom model of the utricle to determine the utricle's natural frequency of 363 Hz (95% confidence intervals: 328, 397) with a damping ratio of 0.96 (0.8, 1.12).

Periodic changes in fMRI connectivity

The first two decades of brain research using fMRI have been dominated by studies that measure signal changes in response to a presented task. A rapidly increasing number of studies are showing that consistent activation maps appear by assessment of signal correlations during time periods in which the subjects were not directed to perform any specific task (i.e. "resting state correlations"). Even though neural interactions can happen on much shorter time scales, most "resting state" studies assess these temporal correlations over a period of about 5 to 10 min. Here we investigate how these temporal correlations change on a shorter time scale. We examine changes in brain correlations to the posterior cingulate cortex (PCC) across a 10-minute scan. We show: (1) fMRI correlations fluctuate over time, (2) these fluctuations can be periodic, and (3) correlations between the PCC and other brain regions fluctuate at distinct frequencies. While the precise frequencies of correlation fluctuations vary across subjects and runs, it is still possible to parse brain regions and combinations of brain regions based on fluctuation frequency differences. To evaluate the potential biological significance of these empirical observations, we then use synthetic time series data with identical amplitude spectra, but randomized phase to show that similar effects can still appear even if the timing relationships between voxels are randomized. This implies that observed correlation fluctuations could occur between regions with distinct amplitude spectra, whether or not there are dynamic changes in neural connectivity between such regions. As more studies of brain connectivity dynamics appear, particularly studies using correlation as a key metric, it is vital to better distinguish true neural connectivity dynamics from connectivity fluctuations that are inherently part of this method. Our results also highlight the rich information in the power spectra of fMRI data that can be used to parse brain regions.

Autoparametric Resonances of Elastic Structures Coupled With Two Sloshing Modes in a Square Liquid Tank

Nonlinear vibrations of an elastic structure coupled with liquid sloshing in a square tank subjected to vertical sinusoidal excitation are investigated. In the theoretical analysis, the ratios of the natural frequencies of the structure and two sloshing modes satisfy 2:1:1. The equations of motion for the structure and seven sloshing modes are derived using Galerkin’s method while considering the nonlinearity of sloshing. The linear damping terms are then incorporated into the modal equations to consider the damping effect of sloshing. The frequency response curves are determined using van der Pol’s method. The influences of the liquid level, the aspect ratio of the tank cross-section, the deviation of the tuning condition, and the excitation amplitude are investigated. When the liquid level is high, and depending on the excitation frequency, there are three patterns of sloshing: (i) both (1,0) and (0,1) sloshing modes appear at identical amplitudes; (ii) these two modes appear at different amplitudes; and (iii) either (1,0) or (0,1) mode appears. Small deviations of the tuning condition may cause Hopf bifurcations to occur followed by amplitude modulated motion including chaotic vibrations. Bifurcation sets are also calculated to illustrate the influence of the system parameters on the response of the system. It is found that for low liquid levels, square tuned liquid dampers (TLDs) work more effectively than rectangular TLDs. Experiments were also conducted in order to confirm the validity of the theoretical results and were in good agreement with the experimental data.

Applications of parabolized stability equation for predicting transition position in boundary layers

The phenomenon of laminar-turbulent transition exists universally in nature and various engineering practice. The prediction of transition position is one of crucial theories and practical problems in fluid mechanics due to the different characteristics of laminar flow and turbulent flow. Two types of disturbances are imposed at the entrance, i.e., identical amplitude and wavepacket disturbances, along the spanwise direction in the incompressible boundary layers. The disturbances of identical amplitude are consisted of one two-dimensional (2D) wave and two three-dimensional (3D) waves. The parabolized stability equation (PSE) is used to research the evolution of disturbances and to predict the transition position. The results are compared with those obtained by the numerical simulation. The results show that the PSE method can investigate the evolution of disturbances and predict the transition position. At the same time, the calculation speed is much faster than that of the numerical simulation.

Combinational parametric resonance under imperfectly periodic excitation

Conditions for mean square stability of a two-degree-of-freedom system have been obtained for the case of sum combinational resonance due to sinusoidal parametric excitation with constant amplitude and white-noise random temporal variations in the instantaneous frequency. Analysis is based on the asymptotic Krylov–Bogoliubov averaging method. The resulting set of ten deterministic differential equations for second-order moments of the four new state variables (inphase and quadrature modal responses) was solved analytically for its neutral stability boundary. The imperfections in periodicity may lead to either stabilization or destabilization of the system and the corresponding conditions have been clearly established from the solution obtained. Furthermore, conditions for almost sure stability have been obtained for a special symmetric case of identical modal damping factors and identical modal excitation amplitudes.

Strength reduction factor demands for building structures under different seismic levels

SUMMARYDamage levels of building structures under a design earthquake are closely related to the assigned values of strength reduction factors. This paper is to investigate the strength reduction factor demands of building structures that were designed considering various earthquake ground intensity levels, soil ground types, and strength reduction factors. In the investigation, a huge number of rigorous nonlinear inelastic dynamic response analyses of various analytical models of five‐story and nine‐story frame structures were conducted under various generated ground motions with variations in phrase angles but identical response spectral acceleration amplitudes. Various scaled earthquake records were also considered for evidence of the investigation. The obtained results showed that when the same values of the strength reduction factors were used for determination of the design lateral seismic forces, the damage and reliability level demands of the structures designed for moderate seismic areas were much less than those for severe seismic ones. As a result, it is proposed that the strength reduction factor demands given in design codes can additionally be expressed in a linear relation of the maximum ground acceleration. Copyright © 2012 John Wiley & Sons, Ltd.

Peripheral and Central Inputs Shape Network Dynamics in the Developing Visual Cortex In Vivo

Spontaneous network activity constitutes a central theme during the development of neuronal circuitry [1, 2]. Before the onset of vision, retinal neurons generate waves of spontaneous activity that are relayed along the ascending visual pathway [3, 4] and shape activity patterns in these regions [5, 6]. The spatiotemporal nature of retinal waves is required to establish precise functional maps in higher visual areas, and their disruption results in enlarged axonal projection areas (e.g., [7-10]). However, how retinal inputs shape network dynamics in the visual cortex on the cellular level is unknown. Using in vivo two-photon calcium imaging, we identified two independently occurring patterns of network activity in the mouse primary visual cortex (V1) before and at the onset of vision. Acute manipulations of spontaneous retinal activity revealed that one type of network activity largely originated in the retina and was characterized by low synchronicity (L-) events. In addition, we identified a type of high synchronicity (H-) events that required gap junction signaling but were independent of retinal input. Moreover, the patterns differed in wave progression and developmental profile. Our data suggest that different activity patterns have complementary functions during the formation of synaptic circuits in the developing visual cortex.

Auditory streaming by phase relations between components of harmonic complexes: A comparative study of human subjects and bird forebrain neurons.

Auditory streaming describes a percept in which a sequential series of sounds either is segregated into different streams or is integrated into one stream based on differences in their spectral or temporal characteristics. This phenomenon has been analyzed in human subjects (psychophysics) and European starlings (neurophysiology), presenting harmonic complex (HC) stimuli with different phase relations between their frequency components. Such stimuli allow evaluating streaming by temporal cues, as these stimuli only vary in the temporal waveform but have identical amplitude spectra. The present study applied the commonly used ABA- paradigm (van Noorden, 1975) and matched stimulus sets in psychophysics and neurophysiology to evaluate the effects of fundamental frequency (f₀), frequency range (f(LowCutoff)), tone duration (TD), and tone repetition time (TRT) on streaming by phase relations of the HC stimuli. By comparing the percept of humans with rate or temporal responses of avian forebrain neurons, a neuronal correlate of perceptual streaming of HC stimuli is described. The differences in the pattern of the neurons' spike rate responses provide for a better explanation for the percept observed in humans than the differences in the temporal responses (i.e., the representation of the periodicity in the timing of the action potentials). Especially for HC stimuli with a short 40-ms duration, the differences in the pattern of the neurons' temporal responses failed to represent the patterns of human perception, whereas the neurons' rate responses showed a good match. These results suggest that differential rate responses are a better predictor for auditory streaming by phase relations than temporal responses.

How does spallation microdamage nucleate in bulk amorphous alloys under shock loading?

Specially designed plate-impact experiments have been conducted on a Zr-based amorphous alloy using a single-stage light gas gun. To understand the microdamage nucleation process in the material, the samples are subjected to dynamic tensile loadings of identical amplitude (∼ 3.18 GPa) but with different durations (83–201 ns). A cellular pattern with an equiaxed shape is observed on the spallation surface, which shows that spallation in the tested amorphous alloy is a typical ductile fracture and that microvoids have been nucleated during the process. Based on the observed fracture morphologies of the spallation surface and free-volume theory, we propose a microvoid nucleation model of bulk amorphous alloys. It is found that nucleation of microvoids at the early stage of spallation in amorphous alloys results from diffusion and coalescence of free volume, and that high mean tensile stress plays a dominant role in microvoid nucleation.

Acute and chronic methylphenidate modulates the neuronal activity of the caudate nucleus recorded from freely behaving rats

Methylphenidate (MPD) is currently one of the most prescribed drug therapies for attention deficit/hyperactivity disorder (ADHD) and moreover is abused for cognitive enhancement and used for recreation by the young and adults. Methylphenidate is used for prolonged periods of time and its mechanism of action on the brain is still unknown. The main action of MPD is known to act on the motive circuit of the brain, and one of these structures is the caudate nucleus (CN). The objective of this study was to investigate the neurophysiological properties of the CN neurons in response to acute and chronic administration of MPD in freely behaving animals, previously implanted with permanent semi microelectrodes. Twenty-six rats were permanently implanted with semi microelectrodes into the CN using general anesthesia. On experimental day one (ED1) the rat was placed into the testing chamber, and neuronal activity was recorded using a wireless (telemetric) headstage device following both a saline and a 2.5 mg/kg MPD injection. From ED2 to ED6 daily injections of 2.5 mg/kg MPD were administered without recordings to induce a chronic effect of the drug, preceded by three days of washout (ED7-ED9) where no injections were given. On ED10 rats were placed back into the testing chamber, the wireless headstage device was attached to skull cap and recordings were resumed for 1 h each following both a saline and re-challenge administration of 2.5 mg/kg MPD. Sixty-seven CN neuronal recorded units from twenty-six animals with identical shape and amplitude at ED1 and ED10 were evaluated. All the 67 CN units responded to MPD administration, 70% (47/67) CN units exhibited an increase in activity following initial 2.5 mg/kg MPD administration and 30% (20/67) exhibited a decrease in neuronal activity. On ED10 all the CN units showed a significant change in their firing rate baseline compared to ED1 baseline, 52% (35/67) exhibiting an increase in their ED10 baseline activity compared to ED1 baseline activity and 48% (32/67) of the CN units at ED10 exhibited decreasing activity. All the CN units responded significantly to MPD rechallenge at ED10, 57% (38/67) of the units exhibited increased neuronal activity while 43% (29/67) exhibited decreasing neuronal activity. The results indicate that the majority of the CN units exhibited neurophysiological sensitization.

Molecular dynamics study of the effect of pressure on the terahertz-region infrared spectrum of crystalline pentaerythritol tetranitrate

Terahertz infrared absorption spectra of crystalline pentaerythritol tetranitrate were obtained using classical molecular dynamics simulations for temperature T = 298 K and hydrostatic pressures P = 0, 1, 2, and 3 GPa. Two approaches were used to calculate the spectra; one based on combined normal-mode analysis and mode-relaxation calculations and the other on Fourier analysis of the dipole–dipole time autocorrelation function. The two methods yield similar, though not identical, positions and relative spectral amplitudes for all pressures studied. The predicted spectra exhibit an overall blue shift with increasing pressure, accompanied by a decrease in the integral absorption.

Crack propagation modeling on the interfaces of thermal barrier coating system with different thickness of the oxide layer and different interface morphologies

A finite element model (FEM) is developed to simulate the crack development in a typical plasma sprayed thermal barrier coatings system in consequence of the stresses induced by thermal cycling, the growth of the oxide layer and different interface morphologies. The thermo-mechanical model is designed to takes into account a non-homogenous temperature distribution and the effects of the residual stress generated during coating process. Crack propagation at the top-coat/oxide and oxide/bond-coat interfaces is simulated thanks to the contact tool “ Debond” present in the ABAQUS finite element code. Simulations are performed with a geometry corresponding to identical or dissimilar amplitude of asperity and for different thickness of oxide layer. The results show a significant difference between the case with and without presence of crack propagation and an important damage on the interfaces due to the growth of the oxide layer very close to the height of the interface asperities.

A melodic contour repeatedly experienced by human near-term fetuses elicits a profound cardiac reaction one month after birth.

BackgroundHuman hearing develops progressively during the last trimester of gestation. Near-term fetuses can discriminate acoustic features, such as frequencies and spectra, and process complex auditory streams. Fetal and neonatal studies show that they can remember frequently recurring sounds. However, existing data can only show retention intervals up to several days after birth.Methodology/Principal FindingsHere we show that auditory memories can last at least six weeks. Experimental fetuses were given precisely controlled exposure to a descending piano melody twice daily during the 35th, 36th, and 37th weeks of gestation. Six weeks later we assessed the cardiac responses of 25 exposed infants and 25 naive control infants, while in quiet sleep, to the descending melody and to an ascending control piano melody. The melodies had precisely inverse contours, but similar spectra, identical duration, tempo and rhythm, thus, almost identical amplitude envelopes. All infants displayed a significant heart rate change. In exposed infants, the descending melody evoked a cardiac deceleration that was twice larger than the decelerations elicited by the ascending melody and by both melodies in control infants.Conclusions/SignificanceThus, 3-weeks of prenatal exposure to a specific melodic contour affects infants ‘auditory processing’ or perception, i.e., impacts the autonomic nervous system at least six weeks later, when infants are 1-month old. Our results extend the retention interval over which a prenatally acquired memory of a specific sound stream can be observed from 3–4 days to six weeks. The long-term memory for the descending melody is interpreted in terms of enduring neurophysiological tuning and its significance for the developmental psychobiology of attention and perception, including early speech perception, is discussed.

Probing T-Tubular Electrophysiology by Random Access Two-Photon Microscopy in Cardiac Myocytes

In cardiac cells many membrane channels are heterogeneously distributed between Surface and T-tubule membranes. Simultaneous recording of membrane potential in the two sarcolemma domains can reveal potential peculiarities of T-tubule electrophysiology. This is not feasible with traditional electrophysiological techniques. Current optical techniques for recording membrane potential can potentially overcome the microelectrode limitation. However, most approaches to the optical recording of membrane potential events lack the spatial and temporal resolution needed for regional assessment of action potential (AP) profile. Here, we developed an ultrafast random access two-photon microscope capable of optically recording fast membrane potential transients in multiple positions of the cell membrane with µm spatial resolution. The random access microscope, in combination with a novel voltage sensitive dye, was used to simultaneously record AP in surface sarcolemma and T-tubules in isolated cardiac myocytes with sub-millisecond time resolution. We found that in myocytes, paced at 0.2 Hz, the AP in the T-tubule has identical amplitude and kinetics as in the surface sarcolemma, indicating that the tight electrical coupling between the two membrane domains prevails over the inhomogeneous distribution of membrane currents. Consistently, in myocytes that had been acutely detubulated by formamide-induced osmotic shock, T-tubule AP was absent, indicating a complete uncoupling from the surface sarcolemma. The electrophysiological properties of t-tubules may be altered in pathological conditions, when detubulation and T-tubule remodelling occur. To mimic a model of pathological detubulation, myocytes were cultured for 24-36 hours, thus obtaining a significant loss and disorganization of the T-tubular network. Membrane staining confirmed the loss and morphological alterations of T-tubules; however, the electrical activity in the remaining remodelled T-tubules was preserved, suggesting that remodelled T-tubules were still coupled to the surface sarcolemma.

Hetero-oligomerization of Neuronal Glutamate Transporters

Excitatory amino acid transporters (EAATs) mediate the uptake of glutamate into neuronal and glial cells of the mammalian central nervous system. Two transporters expressed primarily in glia, EAAT1 and EAAT2, are crucial for glutamate homeostasis in the adult mammalian brain. Three neuronal transporters (EAAT3, EAAT4, and EAAT5) appear to have additional functions in regulating and processing cellular excitability. EAATs are assembled as trimers, and the existence of multiple isoforms raises the question of whether certain isoforms can form hetero-oligomers. Co-expression and pulldown experiments of various glutamate transporters showed that EAAT3 and EAAT4, but neither EAAT1 and EAAT2, nor EAAT2 and EAAT3 are capable of co-assembling into heterotrimers. To study the functional consequences of hetero-oligomerization, we co-expressed EAAT3 and the serine-dependent mutant R501C EAAT4 in HEK293 cells and Xenopus laevis oocytes and studied glutamate/serine transport and anion conduction using electrophysiological methods. Individual subunits transport glutamate independently of each other. Apparent substrate affinities are not affected by hetero-oligomerization. However, polarized localization in Madin-Darby canine kidney cells was different for homo- and hetero-oligomers. EAAT3 inserts exclusively into apical membranes of Madin-Darby canine kidney cells when expressed alone. Co-expression with EAAT4 results in additional appearance of basolateral EAAT3. Our results demonstrate the existence of heterotrimeric glutamate transporters and provide novel information about the physiological impact of EAAT oligomerization.

Impacts of Genetic Algorithm Parameters on the Solution Performance for the Uniform Circular Antenna Array Pattern Synthesis Problem

In this paper, the uniform circular antenna array pattern synthesis problem is solved by means of the real coded genetic algorithm (GA). At the same time, the impacts of the mutation rate and the crossover position on the GAperformance are also investigated. For this purpose, a circular antenna array with uniformly spaced isotropic elements having identical excitation amplitudes is used as a model. Unlike the conventional GA (with fixed mutation rate and random crossover positions), typical GA implementations with variable mutation rate and restricted crossover position are considered for performance improvement. In conclusion, for the specific problem, decreasing mutation rate with negative derivative is observed to be outperforming the implementations with different mutation rate behaviors. Moreover, regarding the crossover technique, it is observed that imposing some restrictions on the crossover positions (rather than fully random position selection) yields better solutions.

A Comparison of Neuronal and Behavioral Detection and Discrimination Performances in Rat Whisker System

We used the rat whisker touch as a model system to investigate the correlation between the response function of cortical neurons and the behavior of rats in a sensory detection versus discrimination task. The rat whisker-barrel system is structurally well characterized and represents one of the main channels through which rodents collect information about the environment. In experiment 1, we recorded neuronal activity (n = 235) in the whisker area of the rat somatosensory cortex in anesthetized rats while applying vibrotactile stimuli of varying amplitudes to the whiskers. Neurons showed a characteristic sigmoidal input-output function, with an accelerating nonlinearity at low stimulus amplitudes and a compressive nonlinearity at high stimulus amplitudes. We further quantified the performance of individual neurons for stimulus detection and for discrimination across different stimulus pairs with identical amplitude differences. For near-threshold stimuli, the neuronal discrimination performance surpassed the detection performance despite the fact that detection and discrimination represented identical amplitude differences. This is consistent with the accelerating nonlinearity observed at low stimulus intensities. In the second stage of the experiment, four rats were trained to select the higher-amplitude stimulus between two vibrations applied to their whiskers. Similar to neuronal results, the rats' performance was better for the discrimination task compared with the detection task. The behavioral performance followed the same trend as that of the population of individual neurons. Both behavioral and neuronal data are consistent with the "pedestal effect" previously reported in human psychophysics.