Nonstationary Sources Research Articles

Numerical Modeling of Dust Propagation in the Atmosphere of a City with Complex Terrain. The Case of Background Eastern Light Air

Micro-scale processes of dust distribution in the city of Tbilisi with very complex topography are modeled using a 3D regional model of atmospheric processes and numerical integration of the transport-diffusion equation of the impurity. The Terrain-following coordinate system is used to take into account the influence of a very complex relief on the process of atmospheric pollution. Modeling is carried out using horizontal grid steps of 300 m and 400 m along latitude and longitude, respectively. Cases of the stationary background eastern light air are considered. In the model, motor transport is considered as a nonstationary source of pollution from which dust is emitted into the atmosphere. Modeling of dust micro-scale diffusion process showed that the city air pollution depends on the spatial distribution of the main sources of city pollution, i.e. on vehicle traffic intensity, as well as on the spatial distribution of highways, and micro-orography of city and relief of the surrounding territories. It is shown that the dust pollution level in the surface layer of the atmosphere is minimal at 6 a.m. Ground-level concentration rapidly grows with the increase of vehicle traffic intensity and at 12 a.m. reaches maximum allowable concentration (MAC = 0.5 mg/m3) in the vicinity of central city mains. From 12 a.m. to 9 p.m. maximum dust concentration values are within the limits of 0.9 - 1.2 MAC. In the mentioned time interval formation of the high pollution zones, the slow growth of their areas and the value of ground-level concentrations take place. These zones are located in both central and peripheral parts of the city. Their disposition and area sizes depend on the spatial distribution of local wind-generated under the action of complex terrain, as well as on the processes of turbulent and advective dust transfer. From 9 p.m. to 24 p.m. reduction of dust pollution and ground-level concentration takes place. After midnight the city dust pollution process continues quasi-periodically.

On pressure perturbations caused by a moving heat source of the frontal type (hydrostatic mode)

The problem of perturbations of the surface pressure caused by a moving nonstationary frontal heat source (localized along one horizontal coordinate) is considered. Pressure disturbances are associated with internal gravity waves (IGWs). It is shown that when a source moves in a finite-height atmospheric layer (atmospheric waveguide) when a discrete set of vertical IGW modes is excited, there are three types of temporal variation of surface pressure at a fixed observation point. These types correspond respectively to the time signal with amplitude modulation, the signal with frequency modulated Doppler type and the signal that occurs only after passing through the source. Each type is implemented for specific values of the oscillation frequency of the source and the Mach number (the ratio of the speed of the source to the phase velocity of the IGW). At Mach numbers less than one, an oscillating source always excites wave precursors disturbances observed before the source arrives. The movement of the source in a semi-infinite atmosphere leads to additional excitation of waves that transfer energy to the upper layers of the atmosphere.

Simulation of Nonequilibrium Heat Transfer in an Anisotropic Semispace Under the Action of a Point Heat Source

Based on a novel analytical solution of the problem of wave heat transfer in an anisotropic semispace under the action of a nonstationary point source of thermal energy, a study was made of a nonequilibrium thermal state of this space originating as a result of its delay (for the time of heat flux density relaxation) from the temperature gradient on the semispace surface. Based on the extended Vernotte–Cattaneo–Luikov hypothesis, a new law of wave heat transfer has been formulated having the form of the Fourier law, and a wave equation has been derived for heat transfer of parabolic type with a lagging argument in time equal to the relaxation time derived.

On Pressure Perturbations Caused by a Moving Heat Source of Frontal Type (Hydrostatic Mode)

The problem of surface-pressure perturbations caused by a moving nonstationary heat source (localized along one horizontal coordinate and periodic along the other) is considered. Pressure perturbations are associated with internal gravity waves (IGWs). It is shown that, during the source movement throughout a finite-height atmospheric layer (atmospheric waveguide), when a discrete set of vertical IGW modes is excited, there may be three types of time surface-pressure variations at a fixed observation point. A time amplitude-modulated signal; a Doppler frequency-modulated signal; and a signal that occurs only after the passage of the source correspond to these three types, respectively. Each of these types is implemented for certain source oscillation frequencies and Mach numbers (ratio between the movement rate of source and the phase velocity of IGWs). At $$M < 1$$ , a nonstationary source always excites wave precursors—disturbances observed before the source arrival. The movement of the source in a semi-infinite atmosphere leads to additional generation of waves that transport energy into the upper atmospheric layers.

Solution of the task of planar geometric optics with a non-stationary source

In the present paper a geometric model of acquisition of receiver curve in the tasks of planar geometric optics with a non-stationary emitter is proposed. The model is based on cyclographic mapping of Euclidean space on a plane and significantly differs in terms of simplicity from the existing mathematical models applied in solution of similar tasks. Furthermore, the proposed model differs in applying uniform solution algorithms for direct and inverse planar geometric optics tasks. The variety of such tasks is dictated by variety of geometric forms of the sought element of the triad “emitter-reflector-receiver” as well as the initial conditions, where a single element of the triad is unknown and sought. The results of the present paper can find application in the area of metrology for optical profile control of machined cylindrical workpieces constituting reflector of the triad.

Highly reliable power sources for objects of agriculture and automation of the stages of their adjustment

The technique of experimental research of static, dynamic and thermal characteristics of elements of automation systems based on the use of traditional equipment and special algorithms of information processing is developed. The reliability of information determination is increased by the use of the method of synchronous signal detection in determining the frequency characteristics of the studied elements of automation. An experimental study of the input and output signals of the amplifier module for two modes of operation was carried out, which confirmed the prospects for using an adaptive power supply for the output stages of the amplifier module when working with non-stationary loads. The method of setting up two modes of operation of the elements of the control system for agricultural facilities with non-stationary power sources, including the choice of parameters for the elements of the system, the experimental determination of its parameters and the choice of parameters for corrective relationships, has been proposed.

Investigation of a nonlinear three-dimensional diffusion model of a porous medium

We study a general three-dimensional nonlinear diffusion model of porous medium with non-stationary source or absorption. For the model, admitting the widest group Lie of the transformations we found some invariant solutions of rank 1. These include the following solutions: the generalizations of “a layered circular pie”, “a layered plane pie”, a generalization of “a layered spiral pie”, “a layered conical pie”, a solution with a radial distribution of the pressure, “a layered spherical pie”. The obtained results can be used to study the processes associated with a underground fluid or gas flow, with water filtration, with the engineering surveys in the construction of the buildings, and also with shale oil and gas production.

Self-similar waves in a three-dimensional porous medium in the presence of non-stationary singular source or absorption

We study an important for applications a model of three-dimensional porous medium in the presence of non-stationary singular source or absorption. We obtained all invariant submodels of this model. These submodels are described by invariant solutions. We found that among the invariant solutions of rank 1, this model has only three types of essentially different (not connected by point transformations) self-similar waves. These waves include: a self-similar wave, propagating in a porous medium along one of the axes of coordinates, a plane self-similar circular wave and a self-similar spherically symmetric wave. We obtained containing arbitrary constant, integral equations describing these self-similar waves. We established the existence and uniqueness of self-similar waves for which at an initial instant of the time at a fixed point, or on a fixed circle, or on a fixed sphere, either a pressure and rate of its change, or a pressure and its gradient are given. The pressure distribution graphs, obtained as results of a numerical solution of these boundary value problem, are given. The obtained results can be used to study to describe the processes associated with a underground fluid or gas flow, with water filtration, with the engineering surveys in the construction of the buildings, and also with shale oil and gas production.

Robust adaptive beamforming for fast moving interference based on the covariance matrix reconstruction

Adaptive beamforming methods are sensitive to underlying assumptions on the environment, sources, or sensor array violation, especially when interferences are moving fast. In this study, the non-stationary interference source is estimated during the period in which snapshots are taken. Then, a new interference-plus-noise covariance matrix reconstruction is introduced which is derived from a simplified power spectral density function that can be used to shape the directional response of the beamformer. Finally, the beamformer is designed to impose nulls towards the regions of the moving interference based on the reconstructed covariance matrix. The essence of the proposed method is to express the inverse of the reconstructed covariance matrix which needs less computational complexity calculation. The effectiveness of the proposed method is demonstrated by numerical results.

Automatic rain and cicada chorus filtering of bird acoustic data

Recording and analysing environmental audio recordings has become a common approach for monitoring the environment. This has several advantages over other approaches, such as reducing costs by avoiding the need for experts to be present in the area of interest. A current problem with performing analyses of environmental recordings is interference from noise that can mask vocalisations of interest. This makes detecting these vocalisations more difficult and can require additional resources. While some work has been done to remove stationary noise from environmental recordings, there has been little effort to remove noise from non-stationary sources, such as rain, wind, engines, and animal vocalisations that are not of interest.This work addresses the challenge of filtering noise from rain and cicada choruses from recordings containing bird sound. The use of acoustic indices and Mel Frequency Cepstral Coefficients (MFCCs) with machine learning classifiers is investigated to find the most effective filters. Hyperparameters for several classification approaches are investigated to fine tune models to achieve the best results. The approach used enables users to set thresholds to increase or decrease the sensitivity of classification, based on the prediction probability outputted by classifiers. A novel approach to remove cicada choruses using bandpass filters is also proposed.A threshold-based approach (Multi-Layer Perceptron with Acoustic Indices and MFCCs) for rain detection is derived which achieves an AUC of 0.9911 and is more accurate than existing approaches when set to the same sensitivities. Cicada choruses are classified in the training set used with 100% accuracy using 10-fold cross-validation using a Support Vector Machine (SVM) classifier with MFCCs. The cicada filtering approach greatly increased the median signal to noise ratios of affected recordings from 0.53 for unfiltered audio to 1.86 to audio filtered by both the cicada filter and a common stationary noise filter.

Variable learning rate EASI-based adaptive blind source separation in situation of nonstationary source and linear time-varying systems

In the case of multiple nonstationary independent source signals and linear instantaneous time-varying mixing systems, it is difficult to adaptively separate the multiple source signals. Therefore, the adaptive blind source separation (BSS) problem is firstly formally expressed and compared with tradition BSS problem. Then, we propose an adaptive blind identification and separation method based on the variable learning rate equivariant adaptive source separation via independence (EASI) algorithm. Furthermore, we analyze the scope and conditions of variable-learning rate EASI algorithm. The adaptive BSS simulation results also show that the variable learning rate EASI algorithm provides better separation effect than the fixed learning rate EASI and recursive least-squares algorithms.

Fundamental and Generalized Solutions of the Equations of Motion of a Thermoelastic Half-Plane with a Free Boundary

A coupled thermoelasticity model is used to study the dynamics of a thermoelastic half-plane influenced by nonstationary body forces and heat sources. In space of Laplace transforms with respect to time, Green’s tensor of the boundary value problem for the half-plane with a boundary free of stresses and heat fluxes is constructed. The displacements and the temperature of the medium are determined for arbitrary body forces and heat sources.

Continuum Model and Method of Calculating for Dynamics of Inelastic Layered Medium

Mathematical modeling of the processes of wave propagation in a layered medium with viscoplastic slip conditions at contact boundaries was carried out, also as passing of waves through a fluid-containing layered massif. An improved model of a layered medium with nonlinear viscoplastic slip conditions at interlayer boundaries was constructed. A numerical solution method for the equations of a layered medium with viscoplastic interlayers for a power slip condition was developed. An example was given of a numerical calculation of the passing of a transverse elastic wave through a layered massif possessing effective anisotropic viscoplastic properties. A two-dimensional problem of the reflection from the buried layered massif was numerically solved for a system of waves excited by a nonstationary surface source. A comparison was made between the dynamics of the surface points for the elastic solution and the solution, taking into account the influence of the buried layered massif, as well as the effect of the thickness of the layers. The proposed models can be useful in solving of the dynamic problems of seismic survey and interpretation of wave patterns obtained in its course.

Automated Data Selection in the Tau–p Domain: Application to Passive Surface Wave Imaging

In the recent decades, passive surface wave methods have gained much attention in the near-surface community due to their ability to retrieve low-frequency surface wave information. Temporal averaging over a sufficiently long period of time is a crucial step in the workflow to fulfill the randomization requirement of the stationary source distribution. Because of logistical constraints, passive seismic acquisition in urban areas is mostly limited to short recording periods. Due to insufficient temporal averaging, contributions from non-stationary sources can smear the stacked dispersion measurements, especially for the low-frequency band. We formulate a criterion in the tau–p domain for selective stacking of dispersion measurements from passive surface waves and apply it to high-frequency (> 1 Hz) traffic noise. The criterion is based on the automated detection of input data with a high signal-to-noise ratio in a desired velocity range. Modeling tests demonstrate the ability of the proposed criterion to capture the contributions from the non-stationary sources and classify the passive surface wave data. A real-world application shows that the proposed data selection approach improves the dispersion measurements by extending the frequency band below 5 Hz and attenuating the distortion between 6 and 13 Hz. Our results indicate that significant improvements can be obtained by considering tau–p-based data selection in the workflow of passive surface wave processing and interpretation.

Melting of a Titanium Alloy Under the Action of Electrical Discharges of Different Duration

This work is dedicated to a study of the influence of the duration of an electrical discharge on the size of the melting zone formed at a cathode of VT20 titanium alloy. It is found that the melting zone possesses radial symmetry and is surrounded by a ring-shaped zone, whose surface is exposed to electrical erosion. The dependence of the radii of the melting region and the erosion zone, and of the volume of melted metal on the duration of the discharge pulse is determined in the interval from 0.05 ms to 1 ms. A mathematical model is proposed, allowing us to calculate the temperature distribution in a metal plate at different times during the discharge. The parameters of a nonstationary surface heat source arising in the region of the electrical discharge are found from the condition of coincidence of the position of the boundary of the solid–melt phase transition observed in experiments and determined by solving the heat transfer equation.

Non-stationary source localization using direction of arrival measurements from multiple in-air vector sensors

Vector sensors utilize a combination of pressure sensors, particle velocity sensors, or both, to determine the acoustic intensity magnitude and direction pointing toward an acoustic source. This acoustic intensity vector is often referred to as the Direction of Arrival (DOA). By combining DOA information from multiple vector sensor measurement locations, a sound source may be instantaneously localized. The majority of vector sensor research has been conducted for underwater applications. A few studies of in-air vector sensors, which utilize multiple microphones, have been conducted; however, the majority of them study stationary sound sources in a laboratory environment or non-real-world settings. The focus of this paper is to study in-air vector sensor capabilities when sensing non-stationary mechanical noise sources—specifically ground vehicles—in a non-laboratory environment where ambient noise may be present. The DOA measurements at multiple vector sensor locations are used to test the acoustic source localization potential for this method. Vector sensors utilize a combination of pressure sensors, particle velocity sensors, or both, to determine the acoustic intensity magnitude and direction pointing toward an acoustic source. This acoustic intensity vector is often referred to as the Direction of Arrival (DOA). By combining DOA information from multiple vector sensor measurement locations, a sound source may be instantaneously localized. The majority of vector sensor research has been conducted for underwater applications. A few studies of in-air vector sensors, which utilize multiple microphones, have been conducted; however, the majority of them study stationary sound sources in a laboratory environment or non-real-world settings. The focus of this paper is to study in-air vector sensor capabilities when sensing non-stationary mechanical noise sources—specifically ground vehicles—in a non-laboratory environment where ambient noise may be present. The DOA measurements at multiple vector sensor locations are used to test the acoustic sourc...

Automatic passive data selection in time domain for imaging near-surface surface waves

The passive surface wave method, as a valid supplement to the active survey method for the deeper imaging, has gained growing attentions from geophysicists and civil engineering communities, particularly in urban applications. The multi-channel analysis of passive surface waves (MAPS), as a newly developed method, combines seismic interferometry and multi-channel analysis of surface waves and shows high potential in extracting high-frequency surface-wave energy in urban environment. The unknown source-distribution of ambient noise and the complicated anthropogenic environment, however, make it difficult to obtain high-quality dispersion images of passive surface waves. We proposed a data selection technique in time domain for selective stacking of cross-correlations and applied it to improve the MAPS method. The proposed technique sorts the time segments with the defined acausal-to-causal ratio factor, and automatically detects the optimum time segments with the highest signal-to-noise ratio (SNR). Three real-world applications have demonstrated the feasibility of the proposed data selection technique. The results indicated that our method is able to preserve the time segments with coherent signals, and reject the time segments polluted by non-stationary noise sources and/or spatial aliasing. Compared with the conventional MAPS method, the improved one can broaden interested frequency band of surface waves and deblur the measured image of dispersion energy. Finally, we discussed the influence of velocity window parameter for SNR calculation on the performance of the proposed technique, and suggested that the velocity window should be appropriately wide to guarantee the accuracy of surface-wave imaging.

Including Non-Stationary Magnitude–Frequency Distributions in Probabilistic Seismic Hazard Analysis

We describe a first principles methodology to evaluate statistically the hazard related to non-stationary seismic sources like induced seismicity. We use time-dependent Gutenberg–Richter parameters, leading to a time-varying rate of earthquakes. We derive analytic expressions for occurrence rates which are verified using Monte Carlo simulations. We show two examples: (1) a synthetic case with two seismic sources (background and induced seismicity); and (2) a recent case of induced seismicity, the Horn River Basin, Northeast British Columbia, Canada. In both cases, the statistics from the Monte Carlo simulations agree with the analytical quantities. The results show that induced seismicity affects seismic hazard rates but that the exact change greatly depends on both the duration and intensity of the non-stationary sequence as well as the chosen evaluation period. The developed methodology is easily extended to handle spatial source distributions as well as ground motion analysis in order to generate a complete methodology for non-stationary probabilistic seismic hazard analysis.

Blind Identification of Noisy Non-stationary Sources Using a Binary Mask

In most functional blind source separation (BSS) applications, the observations contain additive noise that limits the performance of most existing BSS algorithms, especially in the case where the noise is not modeled by a random process (e.g., electromagnetic power supply noise). In this paper, we describe an algorithm where a new cost function is fed with a frequency profile of the noise. In this way, the coefficients of the separating matrix are identified without the bias introduced by the noise. The proposed cost function is based on a frequency domain binary mask and the coherence function. The binary mask selects the frequencies where the signal-to-noise ratio (SNR) is relatively high, while the coherence is minimized to obtain the inverse system. Moreover, any frequency noise profile, whether given a priori or estimated, can be applied to the binary mask to achieve the identification of the inverse matrix. Computer simulations show that the proposed algorithm exhibits better performance under different SNR scenarios compared to methods developed previously.

Far Fields of Internal Gravity Waves from a Nonstationary Source

The problem of far fields of internal gravity waves from a nonstationary source moving in a stratified ocean of finite depth is considered. It is shown that the wave pattern of the generated far fields of internal waves for certain generation parameters is a system of hybrid wave perturbations that simultaneously have the properties of two wave types: annular (transverse) and wedge-shaped (longitudinal). The features of the phase structure and wave fronts of the generated fields are studied. Uniform asymptotics of the solutions describing far hybrid internal waves from a nonstationary source are constructed.