Wave Perturbations Research Articles

Shear Flow Instability over a Finite Time Interval

Within the framework of a discrete quasi-geostrophic model with two vertical levels, the problem of linear stability of the flow of a stratified rotating fluid with constant vertical and horizontal velocity shifts is solved. It is shown that taking into account the horizontal shear leads to a qualitative change in the dynamics of unstable wave disturbances. The main feature is related to the effect of temporary exponential growth of unstable perturbations, i.e. growth over a finite time period. This effect manifests itself in the alternation of stages of smooth oscillating behavior (in time) with stages of exponential (explosive) growth of finite duration. A kinematic interpretation of the effect of temporal exponential growth is given, which is associated with the passage of a time-dependent perturbation wave vector through the region of exponential instability that exists in the absence of a horizontal shear. It is shown that mathematically this effect is described by solutions of a second-order differential equation containing turning points. Asymptotic solutions of the equation are given for weak horizontal shifts.

Dry air intrusions link Rossby wave breaking to large-scale dust storms in Northwest Africa: Four extreme cases

Large dust storms in the Saharan desert and the subsequent transport of airborne dust over large distances are a major meteorological hazard. Several mechanisms associated to dust emission, occurring on a range of scales, have been previously documented, notably involving Rossby wave breaking and a low-level jet. However, the mechanistic link between the different features and actual dust concentrations has not been coherently established. Here, using a Lagrangian approach, and the conceptual view of extratropical cyclone airstreams, the role of the dry intrusion airstream for translating the influence of the upper-tropospheric Rossby wave perturbation to near-surface flow conductive for the highest dust concentrations is examined. To this end, four large-scale dust storms that were accompanied by dry intrusions in west Africa are studied. Data from the Copernicus Atmospheric Monitoring Service (validated against AERONET station data) are combined with atmospheric data from reanalysis and objectively-identified Lagrangian trajectories of dry intrusions. Common, coherent structures highlighting the role of dry intrusions link the upper-tropospheric Rossby wave breaks with the lower-tropospheric dry and cold jets. These conditions favor dust uplift and transport along an arc-shaped cold front trailing from a Mediterranean cyclone. Consequently, the southwest side of the front is characterized by the highest near-surface dust concentrations ahead of the dry intrusion outflow, where the Intertropical Discontinuity shifts equatorward by 3 to 7°. The northeast part of the front is, however, accompanied by southerly, warm conveyor belt-like flow transporting the dust northward to the Mediterranean, Middle East and/or Europe at mid and upper-tropospheric levels. While case-to-case variations exists, the central role of dry intrusions in all cases calls for systematic investigation of its occurrence as a predictive tool for large dust transport events.

Investigation of the Intensified Chaotic Mixing and Flow Structures Evolution Mechanism in Stirred Reactor with Torsional Rigid-Flexible Impeller

This investigation addresses the development of a Möbius strip with torsional structure as the flexible connection pieces for rigid-flexible impellers, enhancing the macroinstability of the flow field structure, the fluid transfer efficiency, and the chaotic mixing degree in the stirred tank. The effects of intrinsic parameters, including the impeller types and length and width of the flexible connection piece among impellers on the laminar flow field were explored. In addition, the chaotic characteristic parameters, helicity distribution, vorticity distribution, flow field structure evolution, power consumption, and fluid velocity of different rigid-flexible impeller systems were investigated as well. The experiment and computational fluid dynamics calculations demonstrated that the DRFMSC-PBDT system with torsional structure has better mixing characteristics at a lower level for power consumption, being clearly superior to DRFC-PBDT and DRFTSC-PBDT in the LLE and NTm values. The torsional structure of the Möbius strip could facilitate strengthening to form the axial circulation of the flow field, decreasing mass transfer resistance and destroying the stability of the flow field structure while contributing to improving the mixing efficiency of the system. In addition, the enhanced superior proportion of the stagnant zone and the mixing zone is also attributed to the increased secondary flow, as well as the generated perturbation wave with larger amplitude and smaller frequency, which are responsible for mixing in laminar flows.

Thick-walled coating in Gunn diode for measuring of electromagnetic radiation power in microwave range

In the introduction, the analysis of the physical basis for the operation of the semiconductor Gunn diode is carried out and the issues that should be resolved in substantiation of the basic physical principles of this diode operation are stated. At applied voltages of up to 0.6 V to a gallium arsenide or indium phosphide Gunn diode, free electrons appear in the conduction band of aluminum as a result of ionization of negative ions of gallium or indium atoms. At voltages from 0.6 to 2.0 V, free electrons appear in the aluminum conduction band as a result of ionization of negative ions of arsenic atoms, and at voltages from 0.6 to 3.0 V — phosphorus. The formation of negative gallium or indium ions is caused by electron capture directly from the aluminum base, while the formation of negative arsenic or phosphorus ions is mainly due to spontaneous electron transfer from the aluminum conduction band and from the negative gallium or indium ions. Electric current flows only over the surface of a gallium arsenide or indium phosphide crystal. The frequency of electric current oscillation is defined by the voltage applied to the Gunn diode and the thickness of the gallium arsenide or indium phosphide crystal. Determination of the power of microwave signals by the Gunn diode is carried out by applying a constant voltage between the cathode and the anode, which provides the frequency of current oscillation equal to the frequency of the measured microwave signal. The maximum radiation power in microwave wavelengths measured by the Gunn diode is determined by the equality of the electromagnetic wave perturbation energy and the electron affinity energy of gallium or indium atoms in the vicinity of the aluminum conduction band.

Solitary wave solutions in time-fractional Korteweg-de Vries equations with power law kernel

<abstract><p>The non-linear time-fractional Korteweg-de Vries and modified Korteweg-de Vries equations are studied with Caputo's fractional derivative. The general higher-order solitary wave solutions are derived using a novel technique called the Aboodh transform decomposition method. To validate the obtained results, two examples of Caputo's fractional derivative with appropriate subsidiary conditions are illustrated. The accuracy and efficiency are confirmed by using numerical simulations and error analysis, where good agreements are obtained. The numerical analysis shows that, in comparison to the time-fractional Korteweg-de Vries solution, the solitary wave solution for the time-fractional modified Korteweg-de Vries equation is less stable against the oscillations. The variations in the temporal variable $ t $ enhance the strength of the wave solutions. Moreover, the wave perturbations taper off as $ t $ attains large values. The parameter $ \alpha $ signifies the fractional derivative influence on the wave dispersion and nonlinearity effects. This affects the amplitude as well as the spatial extension of the solitary waves. With a relatively small value of $ t $, the obtained solutions admit pulse-shaped solitons. Moreover, the wave's solutions suffer from oscillations when the temporal variable attains large values. This effect cannot be noticed in the soliton solutions obtained in the integer order systems.</p></abstract>

Резонансный ПИ-регулятор для судовой электрогидравлической стабилизированной платформы

The problem of stabilizing the angular position of the platform installed on the deck of a water vessel is considered. The platform is used to accommodate navigational equipment and radar antennas of communication systems that require stabilization of the platform's angular position in space under conditions of wave perturbations of the aquatic environment to work efficiently. The task under discussion is to eliminate the influence of wave perturbations on the platform's angle of inclination relative to the horizon. A mathematical model of a stabilized platform with an electric hydraulic drive is considered and a technique for synthesizing a control system that provides effective suppression of the influence of wave perturbations on the angular position of the platform is developed. The novelty of the proposed approach to the synthesis of the control system of the stabilized platform position consists in applying the time-scale separation method for calculating the resonant proportional-integral controller of the electrohydraulic drive of the platform. The results of numerical simulation of the control system under discussion are presented.

Применение к радиационной акустике метода Уитхэма для анализа волновых явлений в намагниченной плазме

The exact governing equation of radiative acoustics in radiating grey gas is derived in this work, taking into account the influence of the transverse magnetic field. The radiative MHD is described by three equations of hydrodynamics and two equations of radiation momentum with extensive use of the formalism of radiative thermodynamics. In order to describe the propagation of linear radiative magneto-acoustic perturbation waves with scattering and attenuation, radiation-thermal dissipation conditions, radiation drag force, and magnetic force and Joule heat are introduced into these equations. The Eddington approximation is used, which allows us to study the modes of radiative magneto-hydrodynamic waves in two asymptotic cases - optically thin and thick gas. The exact governing equation derived in this paper allows, using the heuristic Whitham method, to obtain a set of approximate governing equations of lower order, each of which is part of a reliable approximation to the exact equation in a certain region of the independent time variable. The relatively simple form of such equations allows, without formally solving the full problem, to investigate the physical processes occurring in each radiative magneto-hydrodynamic linear travelling wave.

Простые волны и малоамплитудные возмущения в радиационной газодинамике

The paper analyses one-dimensional simple waves and small-amplitude disturbances in radiating and scattering grey gas. The governing equation of radiation acoustics describing the dynamics of simple waves is derived. Radiation-thermal dissipation conditions and radiation resistance force are introduced into this equation to describe the propagation and attenuation of various radiation perturbation waves. To study non-equilibrium wave phenomena in a radiating medium, the phenomenological Whitham method is used. This method is an effective way to analyse fundamental modes when more than one velocity appears in the governing equation. The use of this method is demonstrated in the paper by considering the evolution of one-dimensional harmonic waves caused by a short-wave initial perturbation of the equilibrium state of the radiating and scattering medium. For all wave modes, analytical solutions have been obtained, which allow us to understand their physical significance. These solutions can be, in particular, an additional test for radiative hydrodynamic codes operating in the radiative acoustics regime. The general approach can be useful in the development of higher-order Godunov numerical schemes for radiation hydrodynamics problems.

Structural Demographic Waves of Russian Regions: Preliminary Analysis

The article shows how political, economic, environmental and other events, phenomena and processes (external factors) lead to sharp changes in the age structure of the population of the regions of Russia, creating structural waves, that is, steep changes in population by age. It is revealed how various components of the demographic balance (internal factors) in dynamics affect the perturbation and smoothing of age-related structural waves. Taking into account the experience of previous studies and on the basis of a large array of empirical data, the use of two generalizing meters of demographic structural waves of the regions of the Russian Federation is justified. The first indicator is the coefficient of unevenness of th7e age structure of the population regarding the structure of the calculated series of "Numbers living in a given age interval". The second indicator is a combination of two coefficients of structural differences (shifts) used in practice, A. Szalai and K. Gatev. The results of a comparative analysis of structural demographic waves in all regions of Russia for the last year before the start of the pandemic in 2019 are reflected. All regions of Russia are ranked according to the proposed coefficients of unevenness. Fourmain groups of territories have been identified from the regions according to the degree of unevenness of the age structure of the population. The main factors that caused this unevenness are revealed. The task of smoothing the structural demographic waves in the young population (0-45 years old) by building and implementing a time-emphasized and regionally accentuated concept of demographic policy in the field of fertility, interregional and international migration has been substantiated. This concept should be coordinated with the strategy of Russia's demographic development.

Experimental study on laser propagation characteristics in local air turbulence disturbed by coherent acoustic waves

The existence of atmospheric turbulence seriously restricts the application of laser technology. However, the acoustic wave will affect the air density during the propagation process, change the original structure of the air turbulence, and then affect the propagation of the laser in the air turbulence. To study the effect of disturbance of atmospheric turbulence by acoustic waves on laser propagation, the propagation characteristics of lasers in local atmospheric turbulence under coherent acoustic wave perturbation are investigated experimentally. First, a coherent acoustic wave generator and a closed local air turbulence simulation device are designed. The propagation experiments of laser in a closed local air turbulence environment disturbed by coherent acoustic waves are carried out. It is found that the coherent acoustic wave will interfere with the wave reflected from the inner wall of the closed local air turbulence simulation device, and the sound pressure forms a regular standing wave intensity distribution pattern. Moreover, the variations of laser beam drift fluctuation variance, scintillation index, and beam diameter with coherent acoustic wave frequency, power, and sound source array position are studied. The results show that the original structure of air turbulence will be changed in the process of coherent acoustic wave propagation, which will affect the beam drift fluctuation variance, scintillation index, and beam diameter changes of laser propagation in air turbulence. The beam drift fluctuation variance, scintillation index, and beam diameter change regularly with the change of the frequency and power of the sound source. Changes of the fluctuations with frequency are relatively gentle, whereas changes with power are severe. This paper explores the influence of coherent acoustic waves disturbing air turbulence on laser propagation characteristics, and the research results provide a basis for improving or disturbing laser propagation performance in atmospheric turbulent environments.

Stability Characteristics of Planar Rivlin–Ericksen Fluid Interface With Mass and Heat Transfer

Abstract The interface of viscous-Rivlin-Ericksen fluids is analyzed through the linear theory of stability analysis when mass and heat is transferring across the interface. The Rivlin-Ericksen fluid lies in the upper region while the lower region of the interface contains viscous fluid. The gravitational acceleration destabilizes the top-heavy arrangement and interface instability is governed by Rayleigh–Taylor instability. The two-dimensional interface is considered, and the viscous potential flow theory is employed to establish the relationship between perturbation's growth and wave number. This relationship is analyzed, and the perturbation's growth is plotted for various flow parameters. A marginal stability condition is obtained, and it is given in terms of heat transport coefficient Λ and wave number. The marginal stability criterion is analyzed using the well-known Newton–Raphson method. The heat and mass transfer phenomenon drives the unstable interface toward stability. It is pointed out that the viscoelastic coefficient λo influences the interface to be stable while the thickness of the viscoelastic fluid makes the interface unstable. Atwood numbers and Weber numbers show destabilizing behavior.

The essential spectrum of periodically stationary pulses in lumped models of short‐pulse fiber lasers

AbstractIn modern short‐pulse fiber lasers, there is significant pulse breathing over each round trip of the laser loop. Consequently, averaged models cannot be used for quantitative modeling and design. Instead, lumped models, which are obtained by concatenating models for the various components of the laser, are required. As the pulses in lumped models are periodic rather than stationary, their linear stability is evaluated with the aid of the monodromy operator obtained by linearizing the round‐trip operator about the periodic pulse. Conditions are given on the smoothness and decay of the periodic pulse that ensure that the monodromy operator exists on an appropriate Lebesgue function space. A formula for the essential spectrum of the monodromy operator is given, which can be used to quantify the growth rate of continuous wave perturbations. This formula is established by showing that the essential spectrum of the monodromy operator equals that of an associated asymptotic operator. Since the asymptotic monodromy operator acts as a multiplication operator in the Fourier domain, it is possible to derive a formula for its spectrum. Although the main results are stated for a particular experimental stretched pulse laser, the analysis shows that they can be readily adapted to a wide range of lumped laser models.

Heaving foil propulsion performance under combined base and perturbation signal inputs

The thrust variation, efficiency, and wake evolution of a flapping foil were investigated for a variety of base and perturbation frequencies. The study was inspired by the complex environment in which fish swim. In this numerical study, the waveform of the perturbation signal was adjusted by the waveform parameter k. The results demonstrate that the addition of perturbations increases thrust in the majority of cases, with square wave perturbations having the most substantial effect. However, the addition of a perturbation signal reduces the propulsion efficiency; a square wave perturbation significantly reduces the propulsion efficiency, while a sinusoidal or approximate sawtooth wave perturbation slightly decrease the propulsion efficiency. The change in the wake structure is closely related to the perturbation signal waveform, base frequency magnitude, and perturbation frequency magnitude, according to the wake structure analysis. The present work provides useful findings for optimizing underwater vehicle motion parameters.

Decay times of atmospheric acoustic–gravity waves after deactivation of wave forcing

Abstract. High-resolution numerical simulations of non-stationary, nonlinear acoustic–gravity waves (AGWs) propagating upwards from surface wave sources are performed for different temporal intervals relative to activation and deactivation times of the wave forcing. After activating surface wave sources, amplitudes of AGW spectral components reach a quasi-stationary state. Then the surface wave forcing is deactivated in the numerical model, and amplitudes of vertically traveling AGW modes quickly decrease at all altitudes due to discontinuations of the upward propagation of wave energy from the wave sources. However, later the standard deviation of residual and secondary wave perturbations experiences a slower quasi-exponential decrease. High-resolution simulations allowed, for the first time, for the estimation of the decay times of this wave noise produced by slow residual, quasi-standing and secondary AGW spectral components, which vary between 20 and 100 h depending on altitude and the rate of wave source activation and deactivation. The standard deviations of the wave noise are larger for the case of sharp activation and deactivation of the wave forcing compared to the steep processes. These results show that transient wave sources may create long-lived wave perturbations, which can form a background level of wave noise in the atmosphere. This should be taken into account in parameterizations of atmospheric AGW impacts.

Seismological footprint of an anomalous atmospheric activity registered in March 2021, in Baja California, Mexico

On 11 March 2021, a quite short strong signal was recorded by seismic stations of the CICESE Seismic Network, which cannot be associated with any regional or global earthquake. At the CICESE Campus and all along the city of Ensenada, in Baja California, Mexico, people reported vibration of the windows and even a short strong rumbling. Fortunately, houses and buildings did not report any damage. Due to the interaction between the atmosphere and the shallow earth surface, this anomalous atmospheric activity produced a special seismological footprint, with frequencies between 1 and 10 Hz. In this manuscript, we report on the observations of a multiparameter dataset, including seismic data along with wind velocity, wind density, temperature, humidity, atmospheric pressure, and THSW index. The atmospheric perturbation wave was strong enough to be clearly recorded by seismic stations within an area of almost 80 km and to produce some changes in the recorded meteorological parameters. The results from an FK analysis show that the atmospheric activity occurred to the south of Ensenada City and travelled to the north, as shown in the seismic records. We discuss the characteristics of the seismic signals in the frequency domain and the relation to the changes in the atmospheric parameters that could be related to this anomalous atmospheric activity.

Coupled Resonance Mechanism of Interface Stratification of Thin Coating Structures Excited by Horizontal Shear Waves

The coupled resonance mechanism of interface stratification of thin coating structures excited by horizontal shear waves is investigated by the forced vibration solution derived from the global matrix method, the integral transformation method, and the plane wave perturbation method. The interface shear stress reaches the peak at coupling resonance frequencies which are an inherent property of the structure, and decreases with the increase of coating thickness or the increase of shear wave velocity difference between the substrate and coating. At the coupling resonance frequency, the thin coating structure is more easily stratified at the interface. The result could provide a theoretical basis for the popularization and application of ultrasonic deicing/defrosting/de-accretion technology.

The deluge of peninsular India during Northeast Monsoon 2015: Observational aspects of thermodynamical, dynamical and microphysical features

During the period 1st November 2015 to 4th December 2015, Southern India especially the city of Chennai was battered by heavy rainfall owing to the passage of a Deep Depression during 8-11 November 2015 and low-pressure areas during 13-18 November 2015, 19-24 Nov. 2015 and 29 November 2015 - 4 December 2015. The deluge due to heavy rainfall associated with these systems resulted in enormous loss of life and property. A wave perturbation in deep zonal flow moved from east to west in November 2015. The observational, synoptic, dynamical and microphysical aspects of these systems have been analysed using in-situ surface observations, satellite observations and reanalysed datasets. 
 The results show the existence of a dominant trough in the easterlies off the southeast coast of peninsular India at 850 hPa and anomalous anticyclonic circulation over central parts of the country at 500 hPa extending up to the upper troposphere were conducive to the northward movement of these low-pressure systems and copious rainfall over NMR. The surface observational features of the deluge were explored using the Automatic Weather Stations (AWS), High Wind Speed Recording (HWSR) systems, Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites. The microphysical aspects of these low-pressure systems are also analysed using TRMM and ERA-Interim datasets.

CFD-aided study on transient wave-blockage interaction in a pressurized fluid pipeline

Blockages are commonly formed in fluid pipelines such as water supply systems, which may greatly affect the internal flow states and conveyance capacities. This paper investigates the transient behavior of a pressured water pipeline with blockage under different transient wave perturbations based on the Computational Fluid Dynamics (CFD) model. To this end, a water pipeline is modeled in a 2D axisymmetric geometry with refined mesh and the blockage is modeled as a small, constricted section. Both the low and high-frequency waves (LFW and HFW), in terms of radial fundamental wave frequency of a pipeline, ∼a/R, with a being acoustic wave speed and R being pipe radius, are injected for the numerical analysis. Through this CFD model, both the axial and radial transient waves have been observed for different frequency wave injections, which are firstly validated by datasets available from former studies. After validations, the local flow characteristics, such as the velocity field, the vorticity field, and its temporal, spatial evolution in the vicinity of blockage during transient wave processes, including before and after transient wavefront passing, are elaborated and analysed in this study. The results indicate the significant influence of blockages on transient behaviors, especially under high-frequency wave conditions.

Extraction of the gravitational potential and high‐frequency wave perturbation properties of nonlinear (3 + 1)‐dimensional Vakhnenko–Parkes equation via novel approach

The (3 + 1)‐dimensional Vakhnenko–Parkes mathematical model has a wide range of applications in science and engineering. In this paper, the model studied is investigated and analyzed by using two effective schemes such as sine‐Gordon expansion method and its newly developed version, rational SGEM. Moreover, many novel properties of model studied are extracted in detail. Furthermore, their wave distributions properties and graphs are also plotted under the strain conditions. Interactions of the gravitational potential and high‐frequency wave perturbation properties are also reported in a detailed manner.

Amplitudes meet cosmology: A (scalar) primer

Here, we review the most recent progress in our understanding of quantum mechanical observables in cosmology in the perturbative regime. It relies on an approach that considers them directly as functions of the data at the spacelike boundary at future infinity prescinding from the explicit time evolution. It takes inspiration from the on-shell formulation of perturbative scattering amplitudes developed in the past 20 years: starting with the requirement of consistency with some fundamental principles such as causality, unitarity and locality, it provides different ways of phrasing and extracting predictions. In this review, we aim to provide a pedagogical treatment of the most recent insights about the analytic structure of the perturbative quantum mechanical observables in cosmology, its relation to fundamental principles as well as physical processes, and how such observables and their features emerge from novel well-defined mathematical objects with their own first principle definition. The review is divided into three parts: Part 0 discusses the definition of quantum mechanical observables in cosmology and some general principles; Part I reviews the boundary approach to the analysis and computation of the perturbative wave function of the universe; Part II provides an introduction to the combinatorial-geometrical description of cosmological processes in terms of cosmological polytopes.