Coherence Function Research Articles

Nonequilibrium Prethermal States in a Two-Dimensional Photon Fluid.

We report on the observation of a prethermal state in a nonequilibrium, two-dimensional fluid of light. Direct measurements of the first order coherence function of the fluid reveal the dynamical emergence of algebraic correlations, a quasi-steady-state with properties close to those of thermal superfluids. By a controlled increase of the fluctuations, we observe a crossover from algebraic to short-range (exponential) correlations. We interpret this phenomenon as a nonequilibrium precursor of the Kosterlitz-Thouless transition.

Coherence Function and Adaptive Noise Cancellation Performance of an Acoustic Sensor System for Use in Detecting Coronary Artery Disease.

Adaptive noise cancellation is a useful linear technique to attenuate unwanted background noise that cannot be removed using traditional frequency-selective filters. Usually, this is due to the signal and noise co-existing in the same frequency band. This paper tests a weighted least mean squares (WLMS) algorithm on a stethoscope system for use in detecting coronary artery disease in the presence of background noise. Each stethoscope is equipped with two microphones: one used to detect heart signals and one used to detect background noise. The WLMS method was used for four different sources of background noise whilst measuring a heartbeat, including a single tone, multiple tones, hospital/clinic noise, and breathing noise. The magnitude-squared coherence between both microphones was unity for the tone scenarios, resulting in complete attenuation. For the other background noise sources, a less-than-unity magnitude-squared coherence resulted in minor and no attenuation. Thus, the coherence function is a tool that can be used to predict the amount of attenuation achievable by linear adaptive noise-cancellation techniques, such as WLMS, as presented in this article.

Isotropy frequency-domain extension imaging and its application in detection of nanostructures

A wide-field high resolution microscopy with sub-micron resolution and high contrast image was demonstrated by using the isotropy frequency-domain extension imaging (IFEI), which was successfully applied to submicron-scale structure detection of fused silica wafer for the first time. Employing the frequency-domain extension and the isotropic period matching, the IFEI extended the bandwidth of coherent transfer function of the conventional wide-field microscopy, thereby improving the resolution to break the diffraction limit. In this method, a quantitative measurement of 200 nm was achieved using fused silica wafer with prefabricated etched structure on the surface. By imaging 81 nm gold nanoparticles, the system resolution calibration was achieved. And corresponding system’s lateral resolution of approximately 179.5 nm was obtained, which was 1.72 times higher than the conventional wide-field microscopy image. Moreover, the reconstructed IFEI images of ultra-precision processed fused silica optics were demonstrated to present the finer structures of sub-micron digs and scratches formed in the optics manufacturing process.

Mutual coherence function based topological charge detection in a Gaussian vortex beam optical communication system

We investigate a topological charge (TC) detection schema for an optical communication system employing Gaussian vortex beam (GVB). In this scenario, the transmitter maps the electrical message symbols to the TCs of GVBs. Thus obtained optical signal propagates in turbulent atmosphere arriving at the receiver, where a detection process is implemented to determine the TC of GVB by correlating the imaginary part of the mutual coherence function (MCF) of the incoming beam against the stored profiles. The feasibility of such a schema is firstly established by examining and comparing the analytical formulation of MCF and that of random phase screen setup. The latter is then used to explore the success rate and boundaries of this particular detection schema. Our results show that the proposed detection schema can operate with a error rate of 5% at a link length of 5.5 km and atmospheric turbulence structure constant of 10−14.

Investigation of Machining Stability Considering Thermal and Rotation Effect: Effectiveness of Impact Excitation for a Rotating Spindle

This study investigates machining stability considering the rotational and thermal effects of a spindle-bearing system. Conventional method for analysis of machining stability only has regarded the static response of spindle system, which does not represent actual machining conditions as some investigations mentioned. Bearing stiffness change caused by a contact angle change is applied to the analytical approach for a dynamics of spindle system. At the same time, in a frequency response function (FRF) of rotating spindle, disturbance-induced signals are included such as a slip between hammer and tool, a spindle run-out and a noise due to insufficient impulse force. In this paper, a conventional impact exciting method which has been popular for characterization is refined in order to obtain dynamic characteristics of rotating spindle-bearing system on the consideration of coherence function. Those disturbances in the output signal are remarkably reduced through finding both the optimal conditions in terms of impulse force and the filtering of false responses. The proposed refined impact exciting tests, which can manage both rotational and thermal effects under rotating conditions, lead to provide FRFs matching more accurately. Stability lobe diagrams (SLDs) derived from FRFs obtained by experiments were derived to evaluate the effectiveness of the dynamic characteristic change in bearing-spindle systems. As a result, the axial depth of cut limitations at chatter occurrence points were well matched to actual depth limits with a deviation of 3% or less. Chatter vibration generation conditions can be well estimated, which proved that SLDs considering the dynamic characteristic of spindle-bearing system reveal the actual cutting condition since it includes thermal and rotation effects of spindle-bearing system during milling process.

The probabilistic turbulence profiles of tropical cyclones in open and flat terrain

The fluctuating winds are usually characterized in the frequency domain by the power spectral density (PSD) function and the coherence function, in which the turbulence intensity and turbulence integral scale are key parameters to describe the gust characteristics of turbulent winds. These parameters estimated from tropical cyclone (TC) observations show significant scattering. Starting from a critical review of the state of the art, the Von Karman spectra are chosen for the TC winds. The turbulence intensity and the turbulence integral scale used in wind spectral functions are discussed in detail. By introducing randomness into the empirical models, the probabilistic longitudinal turbulence profiles of TCs are derived from a wide collection of field measurements with open and flat terrain in the literature. Moreover, the probabilistic models of the lateral and vertical turbulence components are obtained by assuming proportionality between these two components and the longitudinal one. The mean and standard deviation of the developed models for TCs are compared with those defined in the seven major wind loading codes worldwide as well as the model given by Solari to testify these models and illustrate their differences to synoptic winds.

Vibrations Affecting Stability and Edge Control of Snowboards

Background: During a carving turn, vibrations are induced at the heel of the snowboard through edge friction when the heel slips sideways and subsequently travel through and along the board to the shovel, which vibrates and affects the edge control. The purpose of this study was to find a method for assessing the edge grip with a laser vibrometer. Method: Two boards, loaded and tilted at four different angles, were placed on a soft surface, with a shaker connected to the heel at the hindmost edge point. The shovel and particularly the frontmost edge point were scanned with a Polytec laser vibrometer. The frequency response functions of coherence, average shovel displacement, and displacement of the foremost edge point were recorded, and the latter was integrated for obtaining an edge mobility measure (EMM) to quantify the edge control. Results: Of the two boards compared, the shovel of board A was stiffer in the 1st and in the 3rd torsional mode, and the one of board B was stiffer in bending modes. The 2nd torsional mode was responsible for large edge vibrations and therefore for a diminished edge control. Shovel B had a smaller EMM at greater tilt angles, that is, less amplitude of the vibrations at the frontmost edge point, and therefore a better edge control. Shovel A, however, had a smaller EMM at smaller tilt angles. Conclusion: The method developed in this study provides a reliable test for assessment of edge control of a snowboard under standardized test conditions.

Influence of Phases of Coherence Functions on the Wind Field Simulation Using Spectral Representation Method

The wave passage effect is a measure of the wave passage delay due to the apparent velocity of waves, which is one of spatially varying properties of multivariate random processes. The phase of coherence function reflects the wave passage effect of wind fields. In the wind field, simulation by the spectral representation method, the classical phase formula, is not rigorous. This may affect the accuracy of simulation results and even cause incorrect simulations. In this study, the influences of the phase on stationary and nonstationary wind field simulations are researched and discussed in detail. Two schemes containing the classical phase formula and the separated phase scheme are compared in four types of wind field simulation. The qualitative analysis based on theoretical correlation function formula is first made to study the influence of the phase. Then, four numerical examples are utilized to quantitatively study the magnitude of the influence on the sample time history and correlation function of the simulated wind field. Results show that the classical phase formula will result in considerable simulation error for all four types of wind fields because it cannot completely represent the phase angle of a complex number.

A Computational Model to Simulate Thunderstorm Downbursts for Wind Turbine Loads Analysis

Abstract The generation of wind fields is of interest in the study of the structural performance of wind turbines in critical events, such as thunderstorm downbursts. Various methods ranging from the use of empirical data to employing computational simulations are typically adopted to study the response of wind turbines in downburst flow fields. While the former approach is limited in the ability to account for accurate and spatially resolved details of the flow field, the latter is expensive and, therefore, has limitations in its use. As an alternative, in this work, we propose a paused downburst model in which a snapshot of a time-dependent computational fluid dynamics (CFD) simulation is used to generate “mean” wind fields during thunderstorm downbursts. The developed model for the mean wind field is validated against recorded downburst data in the literature. The turbulent component of the wind field is generated using computationally inexpensive techniques based on Fourier-based power spectral density functions and coherence functions. In an illustrative example, the combined mean and turbulence wind fields are generated and applied on a utility-scale wind turbine to study structural load characteristics during a downburst event.

An adaptive multi-taper spectral estimation for stationary processes

The multi-taper spectral estimation method is effective in reducing both bias and variance of the spectrum of stationary stochastic processes. The number of tapers (NoT) in the method controls the bias and variance trade-off of the spectrum. In general, the NoT is empirically determined and constant at all frequencies, limiting the adjustment of the local bias and variance. This paper develops an adaptive multi-taper approach with the varying NoT at each frequency point for estimating the power spectral density (PSD) and coherence function of multivariate stationary processes. An iterative procedure with a stopping criterion is proposed to optimize the NoT at each frequency without manual tunning. The sine taper with a simple analytical expression and satisfactory leakage protection is employed. The proposed adaptive multi-taper approach is applied to three numerical examples, the structural response of a building model, a wind speed process, and an autoregressive process, which exhibit different spectral characteristics. The results of all examples show that the proposed approach outperforms Welch’s and traditional multi-taper methods in estimating the PSD and coherence with a smaller bias and variance.

Holography with higher-order Stokes correlation

We present and experimentally demonstrate a holography technique with a higher-order Stokes correlation of the random light field. The proposed technique is capable to reconstruct the complex-valued object from the random light field, and overcome twin image issues in the higher-order correlation holography without angular shift, phase shifting, and iterations. A basic principle of the proposed technique is presented and its application is demonstrated by establishing a connection between the higher-order Stokes fluctuations correlations and the far-field complex coherence function of the random field in a lensless configuration. The viability of our technique is demonstrated by considering incoherent reconstruction of the hologram and reconstructing the complex-valued information encoded into the hologram as a distribution of complex coherence function.

Dynamic characteristics of wheel–rail collision vibration for high-speed train under crosswind

ABSTRACT This paper focuses on the dynamic characteristics of wheel–rail collision vibration for high-speed train under crosswind. The wind load model is established by using fast Fourier transform harmonic synthesis method and Davenport coherence function, the lateral displacement of wheelset of high-speed train under crosswind is calculated, the dynamic model of wheel–rail collision vibration system of high-speed train under crosswind is established. The effects of vibration frequency, wheel–rail clearance and lateral damping on the characteristics of wheel–rail lateral clearance impact vibration system of high-speed train are discussed. Numerical simulation results show that the impact velocity of wheelset collision on left and right rails increases with the increase of vibration frequency. When ω > 3.3, the system changes from chaos state to single period stable state through inverse doubling bifurcation; when b < 0.0033, it is in a chaos state, and with the increase of b, the inverse doubling bifurcation occurs until the hunting motion of the wheel–rail system is in a stable state; when , the system is in a stable single period operation state, and with the continuous increase of , it begins to have hop bifurcation, enters a multi-period and chaos state, and finally it is in a stable single period operation state.

Experimental comparison of three methods to measure electron source properties for synchrotron radiation

Measuring the electron source size is essential for determining the emittance of synchrotron radiation. Pinhole imaging is the most common technique which measures the source profile in all transverse directions. The grating interferometry technique measures the coherence function of the x-ray beam, which indirectly determines the source size. The newly developed phase-space beam position and size monitor (ps-BPM) system provide information on electron source properties: position, angle, size, and divergence. These three techniques were used to determine the source size at a bend magnet beamline at the Canadian Light Source. In this work, we compare experimental results from these measurements at the same time, which is used as a cross-calibration procedure for each method.

Transient Joule- and (ac) Josephson-like photon emission in one- and two- nucleon tunneling processes between superfluid nuclei: Blackbody and coherent spectral functions

Effective charged neutrons involved in one- and two- nucleon tunneling processes in heavy ion collisions between superfluid nuclei are expected to emit photons. Although the centroid, width and integrated energy area characterizing the associated gamma-strength functions are rather similar, the corresponding line shapes reflect the thermal equilibrated-like character of the quasiparticle transfer (1n-channel, blackbody spectral functional dependence), and the quantal coherent character of the Cooper pair transfer (2n-channel, Gaussian functional dependence) respectively. The predicted angular distributions, polarizations and analyzing power provide further insight into the profoundly different physics to be found at the basis of what can be considered a transient Joule-like and a (ac) Josephson-like nuclear processes

Nonlinear Computational Edge Detection Metalens

AbstractOptical image processing and computing systems provide supreme information processing rates by utilizing parallel optical architectures. Existing optical analog processing techniques require multiple devices for projecting images and executing computations. In addition, those devices are typically limited to linear operations due to the time‐invariant optical responses of the building materials. In this work, a single metalens with an illumination intensity dependent coherent transfer function (CTF) is proposed and experimentally demonstrated, which performs varying computed imaging without requiring any additional optical components. The metalens consisting of nanoantenna structures with a static geometric phase and a nonlinear metallic quantum well layer offering an intensity‐dependent dynamic phase results in a continuously tunable CTF. The approach allows for a weighted summation of two designed functions based on the metalens design, which potentially enables all optical computations of complex functions. The nonlinear metalens may lead to important applications in optical neural networks and parallel analog computing.

Extended aeroacoustic spanwise correction method for the aerodynamic noise prediction of large-span objects

In a numerical study, a shorter span extent than experiment is often used to save the computational resource. The predicted sound pressure level should be corrected before compared with the experimental results. This study concerns the extended aeroacoustic spanwise correction method for the noise prediction radiated from large-span objects. Four new types of spanwise correction models are derived based on the assumption of the spanwise coherence function taking the form of a rectangular function, a trigonometric function, a Laplacian function and a Gaussian function, respectively. The large eddy simulation (LES) combined with the acoustic analogy theory is used for the aerodynamic noise prediction. The predicted far-field sound levels are then corrected by the proposed spanwise correction methods for the large-span objects. Far-field acoustic measurements and near-field hot-wire measurements are also performed in an anechoic wind tunnel for validation purpose. The predicted aerodynamic and aeroacoustic results are found in good agreement with the experiments with the proposed spanwise correction method. The present models based on the Laplacian function and Gaussian function are unified models taking the advantage of that there is no need to compare the relative extent of the numerical length, experimental length, and coherence length. The results also indicate that although there is no significant difference between the various functions, corrections based on the Gaussian profile seem to perform better compared with other functions.

Quantum coherence on selectivity and transport of ion channels

Recently, it has been suggested that ion channel selectivity filter may exhibit quantum coherence, which may be appropriate to explain ion selection and conduction processes. Potassium channels play a vital role in many physiological processes. One of their main physiological functions is the efficient and highly selective transfer of K+ ions through the membranes into the cells. To do this, ion channels must be highly selective, allowing only certain ions to pass through the membrane, while preventing the others. The present research is an attempt to investigate the relationship between hopping rate and maintaining coherence in ion channels. Using the Lindblad equation to describe a three-level system, the results in different quantum regimes are examined. We studied the distillable coherence and the second order coherence function of the system. The oscillation of distillable coherence from zero, after the decoherence time, and also the behavior of the coherence function clearly show the point that the system is coherent in ion channels with high throughput rates.

Wave field in a layer with a linear background profile and multiscale random irregularities

The paper addresses the problem of determining the field of a reflected wave in a multiscale inhomogeneous medium, which consists of a background deterministic large-scale irregularity and multiscale random irregularities with scales both larger and smaller than the wavelength. A layer with a linear permittivity profile is taken as the background irregularity. The small-scale irregularities are responsible for wide-angle scattering including backscattering. The first approximation of the perturbation theory is used to account for scattering from these irregularities. As the zero approximation and Green’s function in determining the field backscattered by small-scale irregularities, we utilize the integral representation of the field, obtained in our earlier work by combining the method of double weighted Fourier transform (DWFT) and the Fock proper-time method. Asymptotic methods are used to reduce the sevenfold integral representation of a field to lower-order integrals. Conditions for validity of such representations are obtained. Formulas of the frequency coherence functions of waves reflected and backscattered from the turbulent plasma layer are given. The paper presents the results of the simulation of pulse sounding of a randomly inhomogeneous reflecting plasma layer demonstrating the effect of large-scale irregularities on backscattering by small-scale irregularities.

Computational Study of the Silicon Vacancy in 3C-SiC and Perspectives for Quantum Technologies

We theoretically study point defects in 3C-SiC for applications in Quantum Technologies, focusing on the neutral silicon vacancy, with an electron spin of 1, magnetically interacting with the SiC nuclear spin bath containing Si-29 and C-13 nuclei. Initially, the system's energetics are explored with ab-initio methods based on the Density Functional Theory. Thereon, we apply a Hahn-echo sequence on the electron spin and study the effects of the bath dynamics on the electron spin's coherence. The Electron Spin Echo Envelope Modulation (ESEEM) phenomenon, due to single nuclear spin flipping processes, and the overall decay, or decoherence, due to the electron spin's entanglement with the bath, are examined. We exploit the Cluster Correlation Expansion (CCE) theory for calculating an approximate version of the coherence function, at various orders of approximation, in order to associate the different coherence function behaviors to given n-body correlations within the bath.

Automated Dual-Channel Speech Enhancement Using Adaptive Coherence Function with Optimised Discrete Wavelet Transform

Voice quality enhancement is a significant method for any speech communication model. Speech Enhancement (SE) and noise reduction approaches can significantly improve the perceptual voice quality of a hands-free communication system and increase the recognition rates of automatic speech recognition systems. Speech communications in real-world cases require high-performance enhancement techniques for addressing the distortions, which can corrupt the intelligibility and quality of the speech signal. Recent portable devices generally incorporate several microphones that can be easily used for improving signal quality. This paper plans to present a novel dual-channel SE model using the coherence function and heuristic concepts. The adaptive coherence function relates to the dual-microphone SE approach suitable for smartphones with primary and reference microphones. With this improved signal, the enhancement is performed by optimising denoising using Discrete Wavelet Transform (DWT) by Adaptive wind speed-based Deer Hunting Optimization Algorithm (AWS-DHOA). The considered objective function depends on the quality measure called Perceptual Evaluation of Speech Quality (PESQ) score. From the results, the RMSE of the proposed model using AWS-DHOA is 39.8%, 45.5%, 53.8% and 45.5% minimised than GWO-CFD, WOA-CFD, CSA-CFD, and RDA-CFD, respectively, on considering the babble noise. Finally, the comparative analysis confirmed that the proposed method improves speech quality and intelligibility by comparing diverse algorithms when different noise types corrupt the speech.