Phase Difference Spectra Research Articles

Refined degree bond partitions, topological indices, graph entropies and machine-generated boron NMR spectral patterns of borophene nanoribbons

The advent of borophene sheets and their nanoribbons has inspired significant experimental and theoretical research on the 2D-sheets and nanoribbons of boron, wonder materials of the future with the potential to replace graphene. As a result of the electron deficient nature of boron compounds, borophenes exhibit multiple phases with contrasting network structures. The present study employs novel graph theoretical techniques based on neighborhood degree-sum parameters that provide refined bond partitions to characterize the different phases of borophene nanoribbons. We have applied these techniques to the β12 and χ3 phases of borophene nanoribbons. We have also employed computer-assisted combinatorial techniques to generate the boron NMR spectra of different phases of the borophene nanoribbons.

Stator ITSC Fault Diagnosis of EMU Asynchronous Traction Motor Based on apFFT Time-Shift Phase Difference Spectrum Correction and SVM

EMU (electric multiple unit) traction motors are powered by converters whose output voltage increases the voltage stress borne by the insulation system, making the ITSC (inter-turn short-circuit) fault more prominent. An index based on short-circuit thermal power is proposed in the article to evaluate the non-metallic ITSC faults extent. The apFFT (all-phase FFT) time-shift phase difference correction with double Hanning windows is used to calculate fault features to train the SVM (support vector machine) fault diagnosis model whose hyper-parameters C and g are optimized using grid search methods. The experimental verification was carried out on the EMU electric traction simulation experimental platform. According to the fault extent index proposed in this article, the experimental samples were divided into three categories, normal, incipient and serious fault samples. The ITSC fault diagnosis accuracy was 100% on the training dataset and 93.33% on the test dataset. There was no misclassification between normal and serious ITSC fault samples.

Research on the identification of asynchronous vibration parameters of rotating blades based on blade tip timing vibration measurement theory

The identification of rotating blade vibration parameters is crucial for diagnosing the health of the blades. While there are many rotating blade vibration measurement techniques, the tip timing vibration theory is currently used by most for non-contact rotating blade vibration measurements, which presents a number of challenges in its practical implementation. However, the traditional bladetip timing vibration measurement method is less involved in the calculation of vibration displacement and vibration data pre-processing and the frequency mixing problem brought on by undersampling is not fully analyzed. The majority of methods to increase measurement accuracy are studied in depth in data post-processing. The accuracy of identifying the spinning blade vibration parameters is lowered, and even identification error arises, as a result of measuring noise interference and the impact of frequency overlap. This work adjusts the calculation of blade tip vibration displacement based on the theory of blade tip timing vibration measurement and applies the Savitzky-Golay noise filtering technique to increase calculation accuracy. To combat the effect of frequency confusion brought on by undersampling and increase the precision of asynchronous vibration parameter detection of spinning blades, the synthetic phase difference spectrum correction method is adopted. In the experiments described in this work, a revolving blade is excited by an electromagnetic exciter to create vibrations at a single frequency. To determine the blade asynchronous vibration characteristics at various rotation speeds, rotating blade asynchronous vibration tests were carried out on a rotor blade table. The outcomes demonstrate that the blade asynchronous vibration parameters can be accurately identified using the parameter identification method, and the maximum vibration frequency identification error is 7.2%, demonstrating the accuracy of the blade asynchronous vibration parameter identification method.

Chromospheric Carbon Monoxide Formation around a Solar Pore

We present observations of NOAA AR 11159, obtained on 2011 February 14 in the 4.7 μm band of carbon monoxide (CO) and coordinated with spectroscopic imaging of three atomic lines (Na i 5896 Å, Fe i 7090 Å, and Ca ii 8542 Å) which sample heights from the mid-photosphere to the chromosphere. Phase-difference spectra between the observed spectral lines instead indicate that the CO lines form at z ≈ 530−650 km in the quiet Sun. During the two hours of observations, seven long-lived cooling events (“cold bubbles”) were observed in CO in the region surrounding a large pore, but were not visible in the three atomic lines. These events show self-similar temporal evolution with time scales consistent with the chemical formation rate of CO at z ≈ 1000 km. Due to the lack of such features in the surrounding quiet Sun, we hypothesize that the magnetic canopy field surrounding the pore, which suppresses the upward propagation of acoustic waves into the chromosphere and the subsequent formation of shocks, depresses the rate of acoustic heating and allows CO to condense and cool the atmosphere at those heights. These “cold bubbles” may be a source of the chromospheric CO that produces the unexpectedly high (z ≈ 1000 km) limb extensions seen in the stronger CO lines, and may provide a unique opportunity to study this enigmatic component of the solar atmosphere in spatially resolved observations.

Joint Angle Correction in Rheumatoid Arthritis and Its Reliability Analysis Based on Phase Dispersion Quantification

Background: Rheumatoid arthritis is a form of autoimmune disease characterized by synovitis that can ultimately cause joint deformities and impaired functioning. The cartilage destruction is one of the most important indicators for diagnosis and treatment of rheumatoid arthritis, and it is radiographically manifested as joint space narrowing. Issue: In the literature, the joint space narrowing progression between a baseline and its follow-up finger joint images can be quantified by using image registration algorithm. We found that the inconsistencies of joint angles may lead to characteristic mismatches and thus severely affect the accuracy of joint space narrowing quantifications. Methods: In this work, we introduce a rotation invariant phase only correlation in joint space narrowing quantification for the joint angle correction. Further, we propose a confidence index to quantify the quantification reliability of phase only correlation based on phase dispersion in phase difference spectrum. Conclusion: In our clinical experiments, the proposed quantification method can effectively overcome and manage the mismatch due to the inconsistency of joint angles. Additionally, the confidence index shows a high consistency with the joint space narrowing progression examinations manually done by a trained radiologist and one radiological technologist.

Long period ground motion simulation and its application to the seismic design of high-rise buildings

Increasing attention is being paid to the adverse effects of long period ground motion (LPGM) on high-rise buildings. However, it is hard to quantitatively define this special ground motion in seismic design, especially in the cases of regions in which sufficient seismic observations have not been made. To this end, this paper proposes an innovative LPGM simulation method. This method not only involves probabilistic seismic hazard analysis (PSHA), but also overcomes the limitations of the original empirical relation-based method. Firstly, a large number of data taken from typical sedimentary basins are utilized to determine the basin amplification factor model, by which an improved response spectra attenuation formula is developed based on the site amplification mechanisms in different periods. Then, based on the statistical regression analysis of the basin correction factors, the modified group delay time empirical formula is obtained. Furthermore, the conventional PSHA model is refined to determine the scenario earthquake. Finally, the design LPGM is generated by the phase difference spectrum method and its impact on high-rise buildings is embodied via a case study. The satisfactory result proves that the proposed method can effectively simulate LPGM and may have great potential in practice.

Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere.

There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Numerical determination of the cutoff frequency in solar models

Context. In stratified atmospheres, acoustic waves can only propagate if their frequency is higher than the cutoff value. The determination of the cutoff frequency is fundamental for several topics in solar physics, such as evaluating the contribution of the acoustic waves to the chromospheric heating or the application of seismic techniques. However, different theories provide different cutoff values. Aims. We developed an alternative method to derive the cutoff frequency in several standard solar models, including various quiet-Sun and umbral atmospheres. The effects of magnetic field and radiative losses on the cutoff are examined. Methods. We performed numerical simulations of wave propagation in the solar atmosphere using the code MANCHA. The cutoff frequency is determined from the inspection of phase-difference spectra computed between the velocity signal at two atmospheric heights. The process is performed by choosing pairs of heights across all the layers between the photosphere and the chromosphere to derive the vertical stratification of the cutoff in the solar models. Result. The cutoff frequency predicted by the theoretical calculations departs significantly from the measurements obtained from the numerical simulations. In quiet-Sun atmospheres, the cutoff shows a strong dependence on the magnetic field for adiabatic wave propagation. When radiative losses are taken into account, the cutoff frequency is greatly reduced and the variation of the cutoff with the strength of the magnetic field is lower. The effect of the radiative losses in the cutoff is necessary to understand recent quiet-Sun and sunspot observations. In the presence of inclined magnetic fields, our numerical calculations confirm that the cutoff frequency is reduced as a result of the reduced gravity experienced by waves that propagate along field lines. An additional reduction is also found in regions with significant changes in the temperature, which is due to the lower temperature gradient along the path of field-guided waves. Conclusions. Our results show solid evidence that the cutoff frequency in the solar atmosphere is stratified. The cutoff values are not correctly captured by theoretical estimates. In addition, most of the widely used analytical cutoff formulae neglect the effect of magnetic fields and radiative losses, whose role is critical for determining the evanescent or propagating nature of the waves.

A Statistical Model for Phase Difference Spectrum of Ground-Motion and Its Application in Generating Non-Stationary Seismic Waves

The intensity non-stationarity is one of the most important features of earthquake records. Modeling of this feature is significant to the generation of arti- ficial earthquake waves. Based on the theory of phase difference spectrum, an intensity non-stationary envelope function with log-normal form is proposed. Through a tremendous amount of earthquake records downloaded on Kik-net, a parameter fitting procedure using the genetic algorithm is conducted to obtain the value of model parameters under different magnitudes, epicenter distances and site conditions. A numerical example is presented to describe the procedure of generating fully non-stationary ground motions via spectral representation, and the mean EPSD (evolutionary power spectral density) of simulated waves is proved to agree well with the target EPSD. The results show that the proposed model is capable of describing the intensity non-stationary features of ground motions, and it can be used in structural anti-seismic analysis and ground motion simulation.

Leak Location Based on PDS-VMD of Leakage-Induced Vibration Signal Under Low SNR in Water-Supply Pipelines

Leak location in water-supply pipelines is of great significance in order to preserve water resources and reduce economic losses. Cross-correlation (CC) based leak location is a popular and effective method in water-supply pipelines (WSP). However, with a decrease of signal to noise ratio (SNR), the errors of time-delay estimation (TDE) based on CC will become larger making it almost impossible to determine a leakage position. Hence, this work proposes leak location based on a combination of phase difference spectrum and variational mode decomposition (PDS-VMD) of leakage-induced vibration signal under low SNR for WSP. Firstly, the leakage-induced vibration signal is decomposed into several intrinsic mode functions (IMFs) by VMD, where the characteristic frequency band is determined by PDS of cross spectrum of two leakage signals. Then, the energy ratio of leakage signal in characteristic frequency band serves as a guideline to select effective IMF components from the decomposed IMFs. Finally, the selective IMFs are reconstituted into a new signal which can be used to determine a leak position using CC based TDE. In order to verify the effectiveness of the proposed leak location algorithm, the method based on PDS-VMD is compared with that using CC, combination of CC coefficient and VMD (CCC-VMD) using both simulation and experiment. The simulation and experimental results demonstrate that the proposed PDS-VMD method is more suitable for leak location in WSP, which provides immunity to both broadband and narrow band noise under low SNR.

Numerical Simulation of Seismic Waves with Peak Arrival Time and Amplitude-Frequency Correlation

To improve the accuracy with which the non-stationary intensity characteristics of artificial acceleration-time-history curves are determined, a method of adjusting the frequency correlation between the artificial peak arrival time and amplitude spectrum is proposed. A calculation method for determining the peak arrival time to reflect the physical meaning of the phase-difference spectrum was first deduced. Subsequently, the amplitude-distribution characteristics corresponding to the peak arrival times for different durations of natural earthquakes were analyzed. In accordance with the distribution characteristics of the reference amplitude, the frequency distribution of the artificial peak arrival time was divided into four parts: the beginning, strong-earthquake, attenuation, and tail sections. By adjusting the arrival frequency of peak points for the different earthquake durations considered, the corresponding relationship between the artificial peak arrival time and amplitude spectrum was determined to be more consistent with the general law of natural earthquakes. The sample calculation described in this paper demonstrates that adjusting the frequency correspondence between the artificial peak arrival time and amplitude spectrum greatly improves the accuracy of the non-stationary intensity characteristics of artificial acceleration-time-history curves.

1-bit photon cross-correlation-based phase-modulation fluorometer using an OFDM signal-modulated excitation light source

We propose a phase-modulation fluorometer, which is a combination of a 1-bit photon cross-correlator constructed using a field-programmable gate array and an excitation light source modulated by an orthogonal frequency-division multiplexing signal. Such multiplexing allows us to use multiple modulation frequencies efficiently and selectively within a given baseband. Defined as the inverse of its fundamental frequency, the symbol length of the multiplexing is set equal to the width of the time window of the cross-correlator. Therefore, all temporal waveforms of the subcarriers in the baseband are connected smoothly at both ends of the symbol interval; hence, no spurious sidebands appear in the frequency domain of the subcarriers. The 1-bit cross-correlator brings about fluorescence lifetimes in the photon-counting realm. From a phase difference spectrum between the fluorescence and the reference signal, fluorescence lifetimes for each frequency may be obtained with precision simultaneously. The resolution time and the width of the time window of the cross-correlator are 2.0 ns and 256 ns, respectively. As a proof-of-principle experiment, we measured the fluorescence lifetimes of some standard samples.

The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

Abstract High spatial and temporal resolution images of a sunspot, obtained simultaneously in multiple optical and UV wavelengths, are employed to study the propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. Power spectra are generated from intensity oscillations in sunspot umbra, across multiple atmospheric heights, for frequencies up to a few hundred mHz. It is observed that the power spectra display a power-law dependence over the entire frequency range, with a significant enhancement around 5.5 mHz found for the chromospheric channels. The phase difference spectra reveal a cutoff frequency near 3 mHz, up to which the oscillations are evanescent, while those with higher frequencies propagate upward. The power-law index appears to increase with atmospheric height. Also, shorter damping lengths are observed for oscillations with higher frequencies suggesting frequency-dependent damping. Using the relative amplitudes of the 5.5 mHz (3 minute) oscillations, we estimate the energy flux at different heights, which seems to decay gradually from the photosphere, in agreement with recent numerical simulations. Furthermore, a comparison of power spectra across the umbral radius highlights an enhancement of high-frequency waves near the umbral center, which does not seem to be related to magnetic field inclination angle effects.

Significance of Differenced EGG Signal as a Spectrum in Phase Difference Computation for the Estimation of Glottal Closure Instants

Estimation of glottal closure instants (GCIs) from an electroglottograph (EGG) signal can aid in clinical applications involving the diagnosis and treatment of speech pathologies and can also serve as a ground truth to assess algorithms that estimate GCIs from speech signals. In this regard, the current work proposes a phase-difference-based approach that considers the symmetrized, differenced EGG (DEGG) signal to be the Fourier transform of an arbitrary even-signal, to estimate GCIs from EGG signals. The DEGG signal possesses sharp negative valleys at the GCIs and since the symmetrized DEGG is assumed to be a spectrum, these valleys correspond to zeros that lie outside the unit circle. The angular locations of these zeros, and in turn the locations of GCIs, can be derived from the phase-difference spectrum, since it possesses a value of around $$2\pi $$2? at these locations, the derivation of which is elaborated in the paper. The proposed algorithm is compared with the existing time of excitation generator, the high quality time of excitation algorithm, and the singularity in EGG by multiscale analysis algorithm, in terms of the identification, miss, and false alarm rates, and the identification accuracy, on normal and pathological EGG. The proposed algorithm is observed to outperform the rest with an identification rate of 98.28% in normal EGG and 96.90% in pathological EGG.

Co-prime Sensing-Based Frequency Estimation Using Reduced Single-Tone Snapshots

In the paper, we propose a co-prime sensing-based frequency estimator by combining a simplified procedure of the Chinese remainder theorem and a phase-difference spectrum corrector. This estimator can accurately extract the frequency information from two adjacent snapshots after initialization. Some theoretical deductions and performance analysis are also presented. Numerical results show that the proposed estimator is robust to insufficient snapshots and noise.

Seismic Responses of the Supporting Structure of FAST under Multiple-Support Excitations

Seismic ground motions are spatial incoherent partly due to uncertainty in fault mechanism and wave propagation. The spatial coherency and its effect can be important and need to be investigated for multiple-support structures. In this study, an investigation of seismic responses of a five-hundred-meter aperture spherical telescope (FAST) is investigated. The structure is a cable net structure with multiple supports. For the structural analysis, parameters of an existing coherency model for excitations along the same orientation but considering the multi-component excitations and spatially separated locations are developed using the records from Taiwan. This coherency model together with adopted target power spectral density function and phase difference spectrum model are used to simulate multi-component seismic ground motions at 338 grid points covering the FAST. The simulated records are used as the basis to interpolate the time history of the ground motions at 2395 supports for the considered structure. The time-history responses of the structure under incoherent multiple-support excitations are evaluated and compared to that under uniform excitations. The results show that the influence of coherency on the supporting structure of the FAST in three excitation directions differs. The analysis results show that as compared to uniform excitations, the consideration of the seismic coherency in seismic excitations leads to, on average, 87% of increase in stress for 94% of the main cables, and 22% of increase in stress for 44% of the tie-down cables. It was also noted that the maximum tensile stress of the cable by considering the coherency is 835 MPa while that under uniform excitations is 671 MPa. In both cases, they are smaller than the characteristic tensile strength of the cables used for design and construction of the FAST.

Comparion of seismic response of single-layer reticulated dome under unform and incoherence three-directional excitations

Seismic ground motions at structural supports are spatially varying and incoherent. The spatial incoherency effects should be considered for structures with large span and multiple supports. Two single-layer reticulated domes of 120m span, with and without substructures under incoherent multiple supports and uniform excitations were modeled and analysed in this study. Time-history responses of structures are obtained and compared for simulated ground motion records. For the simulation, use of existing phase difference spectrum models and coherence models were considered, and three-directional excitations (two horizontal directions and vertical direction) are considered. The analysis results show that the spatial coherency effect can influence the seismic responses of reticulated domes. The results also indicate that the seismic spatial coherency of all three directions can be important, especially for structures without flexible substructures.

Oscillatory behaviour of chromospheric fine structures in a network and a semi-active region

In the present work, we study the periodicities of oscillations in dark fine structures using observations of a network and a semi-active region close to the solar disk center. We simultaneously obtained spatially high resolution time series of white light images and narrow band images in the H$\alpha$ line using the 2D G\"ottingen spectrometer, which were based on two Fabry-Perot interferometers and mounted in the VTT/Observatorio del Teide/Tenerife. During the observations, the H$\alpha$ line was scanned at 18 wavelength positions with steps of 125 m\AA. We computed series of Doppler and intensity images by subtraction and addition of the H$\alpha$ $\pm$ 0.3 \AA\ and $\pm$ 0.7 \AA\ pairs, sampling the upper chromosphere and the upper photosphere, respectively. Then we obtained power, coherence and phase difference spectra by performing a wavelet analysis to the Doppler fluctuations. Here, we present comparative results of oscillatory properties of dark fine structures seen in a network and a semi-active region.

Compressing movement information via principal components analysis (PCA): Contrasting outcomes from the time and frequency domains

PCA has become an increasingly used analysis technique in the movement domain to reveal patterns in data of various kinds (e.g., kinematics, kinetics, EEG, EMG) and to compress the dimension of the multivariate data set recorded. It appears that virtually all movement related PCA analyses have, however, been conducted in the time domain (PCAt). This standard approach can be biased when there are lead-lag (phase-related) properties to the multivariate time series data. Here we show through theoretical derivation and analysis of simulated and experimental postural kinematics data sets that PCAt and, PCA in the frequency domain (PCAf), can lead to contrasting determinations of the dimension of a data set, with the tendency of PCAt to overestimate the number of components. PCAf also provides the possibility of obtaining amplitude and phase-difference spectra for each principal component that are uniquely suitable to reveal control mechanisms of the system. The bias in the PCAt estimate of the number of components can have significant implications for the veracity of the interpretations drawn in regard to the dynamical degrees of freedom of the perceptual-motor system.

Modeling of spatially correlated, site-reflected, and nonstationary ground motions compatible with response spectrum

Seismic risk analysis and mitigation of spatially extended structures require the synthesis of spatially varying ground motions in the response history analysis of these structures. These synthetic motions are usually desired to be spatially correlated, site reflected, nonstationary, and compatible with target design response spectra. In this paper, a method is presented for simulating spatially varying ground motions considering the nonstationarity, local site effects, and compatibility of response spectra. The scheme for generating spatially varying and response spectra compatible ground motions is first established for spatial locations on the ground surface with varying site conditions. The design response spectrum is introduced as the “power” spectrum at the base rock. The site amplification approach is then derived based on the deterministic wave propagation theory, by assuming that the base rock motions consist of out-of-plane SH wave or in-plane combined P and SV waves propagating into the site with assumed incident angles, from which tri-directional spatial ground motions can be generated. The phase difference spectrum is employed to model ground motions exhibiting nonstationarity in both frequency and time domains with different site conditions. The proposed scheme is demonstrated with numerical examples.