Stratified Medium Research Articles

Analytical Models of the Interaction of Convection with Intercepting Layers in the Atmosphere

The interaction of various forms of convection with stably stratified barrier layers in the atmosphere is theoretically investigated. Three different formulations of the problem are considered. 1) A linear problem with given perturbations of temperature and vertical velocity at the lower boundary of a stably stratified medium. 2) Penetration of an isolated turbulent thermal into such a medium. 3) Impact on the intercepting layer of an intense convective turbulent jet. Analytical solutions of the corresponding model problems are found. The possibility of connecting the considered models with signatures of intense convection, in particular, with dome-shaped protrusions above the anvil of a cumulonimbus cloud (or a cluster of a convective system), which represent the intrusion of a powerful updraft into a stably stratified medium, is discussed.

Analysis of the Electrically Large Target in Isotropic Medium by the Physical Optics Method With Adaptive Mesh Technique

In this work, the physical optics (PO) method based on multilayered medium theory is used to analyze high-frequency scattering characteristics of plasma sheath covering an aircraft. The ray intersection method is taken for occlusion judgment, but some errors occur on the edge between illuminated and shadowed regions of a target. Therefore, the iterative edge patch segmentation based on adaptive mesh technique is proposed to reduce this error, which is significant to enhance computational accuracy of the PO method. Furthermore, to simplify the computational process and validate effectiveness of adaptive mesh technique, the plasma sheath covering a RAM-C II aircraft is modeled with a five-layered isotropic medium. Compared with uniform fine segmentation, the proposed adaptive mesh technique markedly reduces the computational cost by an order of magnitude while maintaining accuracy of the PO method. Moreover, high-frequency scattering characteristics of plasma sheath covering a RAM-C II aircraft under different flight conditions are presented via the bistatic radar cross section (RCS) with observable attenuation effects. Hence, the PO method for multilayered media combined with the proposed lit region judgment method is suitable to analyze scattering features of electrically large coated target.

Contrast Source Inversion for Objects Buried into Multi-Layered Media for Subsurface Imaging Applications

Contrast Source Inversion for Objects Buried into Multi-Layered Media for Subsurface Imaging Applications

Determining light absorption changes in multilayered turbid media through analytically computed photon mean partial pathlengths

Determining light absorption changes in multilayered turbid media through analytically computed photon mean partial pathlengths

Insight into Hall current impact in the hybrid squeezing nanofluid flow amid two rotating disks in a thermally stratified medium

A hybrid nanofluid is an amalgamation of two or more types of nanoparticles in a customary and possesses a variety of industrial applications. This exploration examines the blend of copper (Cu) and gold (Au), and engine oil as a hybrid nanofluid. The model considers the Hall current, Cattaneo–Christov (C-C) heat flux, thermal stratification, nonuniform heat source, and nonlinear thermal radiative effects for heat analysis. Ordinary differential equations (ODEs) are obtained through a similarity transformation scheme, and the model is visualized using a MATLAB function bvp4c. Graphs are presented to demonstrate the impact of various parameters on velocities and temperatures, and the wall drag coefficient and wall heat transfer rate are calculated and tabulated for both disks. The results show that a stronger Hall effect leads to a decrease in tangential velocity, while nonuniform heat sources increase fluid temperature. Thermal radiation and stratification have opposite effects on liquid temperature. It is interesting to note that for the radiation parameter at Rd = 0.5 , a higher heat transfer rate occurred at the lower disk, i.e., ( N u 1 = − 3.63314 ) as compared to the upper disk ( N u 2 = − 3.63324 ) . Moreover, for the stratification parameter at S = 0.2 , it is observed that the heat transfer rate is lesser at the upper disk ( N u 2 = − 3.64374 ) than at the lower disk ( N u 1 = − 3.64274 ) . Verification of the truthfulness of the proposed model is also included in the article.

Multi-domain physics-informed neural networks for solving transient heat conduction problems in multilayer materials

In this paper, we aim to numerically resolve linear and nonlinear transient heat conduction problems in multilayer composite materials using a deep learning method called multi-domain physics-informed neural networks (MDPINNs). For this purpose, the multilayer media are first divided into independent sub-domains based on domain decomposition technique. The single-layer deep neural networks are first established, and each sub-domain has its corresponding sub-network. Then, each two sub-networks are connected by continuity conditions on the interface to form a MDPINNs’ framework that can directly solve the transient heat conduction problem in multilayer media. The temperature distribution in the computational domain can be obtained by training the MDPINNs, including the temperature values on the interface. A series of numerical experiments are carried out to verify that the proposed framework can achieve satisfactory accuracy, including in micrometer or even nanometer structures. Compared with conventional methods, the MDPINNs have the advantage of directly solving both linear and nonlinear heat conduction problems in multilayer materials in a unified and concise form.

Reflection of stepwise twisted stratified anisotropic optical media

Stratified optical films have fascinating optical properties and many important applications. Here we report a theoretical study of stepwise twisted stratified anisotropic optical media. We used Fourier transform to analyze the helical component of the dielectric tensor of the media. We then used the Berreman 4 × 4 method to calculate the reflection spectrum of the media. We discovered that right-handed and left-handed helices could coexist in stepwise twisted layers, which producing a simultaneous reflection of right-handed and left-handed circularly polarized light. This feature can be used to produce films with superior reflection properties, such as high reflectance, broad bandwidth reflection and short wavelength reflection.

Analytical layer element solution for layered porous media subjected to non-axisymmetric transient loadings

Transient horizontal loads (e.g., seismic, wind, and water waves) have a significant impact on the safety of tall structures and bridges. In this paper, the analytical layer-element solution for layered saturated porous media under non-axisymmetric horizontal transient loadings is derived. Starting from governing equations, the Laplace-Hankel integral transform and Fourier series expansion are used to establish the analytical layer elements of each saturated layer. By introducing the continuity and boundary conditions, the global matrices can be assembled by combining each single layer element. Then, the solution in the physical domain is derived by utilizing integral transform inversions. Finally, we conducted numerical examples to validate the proposed method and program and to investigate the effects of the load conditions and media parameters on the dynamic response of multilayered poroelastic media under horizontal transient circular loads.

From Serendipity to Sustainability: Exploring the Content Creation and Sharing Experiences of Local Travel Influencers in China

The study explored content creation and sharing experiences of local travel influencers in Meishan City, China. A qualitative phenomenological methodology was used to conduct in-depth interviews with 15 local travel influencers based on social exchange theory and its reciprocal principles. Thematic content analysis was used to analyse data. Study findings showed that being a local travel influencer was an emerging occupation requiring comprehensive competence in specific contextual and cultural backgrounds. Although travel influencers’ initial rise to popularity was often serendipitous, supportive policies from the local tourism community could assist travel influencers in achieving sustainable tourism. Local travel influencers face several challenges including staying authentic, monetizing content, dealing with the COVID-19 pandemic, avoiding homogenous content, and managing work load. Local travel influencers could serve as a significant liaison between tourists and the local tourism community by collaborating with local tourism stakeholders, building a local multi-layer media matrix, co-creating promotional content, and live-streaming e-commerce. Study findings have theoretical and practical implications for tourism stakeholders in integrating regional tourism community development with local influencer networks.

Time-domain Green’s function associated to the interface problem for the Klein–Gordon equation

In this work we present a representation formula for the Green’s function associated with the source or interface problem for the Klein–Gordon equation from a known result on solution for the non-dispersive wave equation. The results can extend to more general situations, in particular stratified media where the Cagniard-de Hoop method fails for reasons of non-homogeneity of the amplitude and the phase of the exponential in the Laplace transform. Examples and numerical applications are given in two dimensions for the interface problem associated to a dispersive wave equation where the reflected and incident fields are obtained. The calculation of the transmitted field is omitted for the sake of simplicity of the paper. Such a representation formula can be useful to study other qualitative properties of dispersive waves from those known on non-dispersive ones.

Dynamic Diffusion Model of Stray Current in DC Traction Power Supply System

The dynamic fluctuation of stray current is the main characteristic of the metro causing harm to surrounding buried metal structures. While establishing the equivalent circuit model of the return system, not only mainline but also metro depot should be considered. The diffusion models of the dynamic stray current of the DC traction power supply system (DTS) in multi-layer soil medium and massive texture soil medium are established. They are co-simulated with the DC traction power supply return system. The massive texture model (MTSM) is verified by an on-site experiment of ground potential distribution. In the case study, the direct grounding of the rail in the depot will deteriorate the ground potential gradient and stray current. When considering the impact of the river or lake, the stray current easily flows to the low resistivity texture soil, the maximum ground potential gradient increases.

Semi-analytical simulation of seismic waves generated by a point source in 1-D layered half-space of double-porosity media

SUMMARYIn this study, we present a semi-analytical method to simulate the propagation of seismic waves in horizontally stratified double-porosity media. We solve the governing equations in the frequency–wavenumber domain and then compute the time–space domain solutions by Hankel and fast Fourier transforms. We conduct numerical simulations to study the properties of the seismic waves in the double-porosity media. The results show the existence of three P waves, one fast P wave and two slow P waves. The two slow P waves are highly attenuated and can be observed by assuming a low fluid viscosity. By comparing the fast P wave in a single-porosity medium and that in a double-porosity medium, we find that the double-porosity model predicts much higher attenuation of the fast P wave due to the local fluid flow between the background medium and the inclusions. We also study the parameters affecting the attenuation, such as the radius of the spherical inclusions, the viscosity of the pore fluid, the permeability of the background medium and the porosity of the inclusions. Finally, we present a cross-well survey model to investigate the seismic responses in the double-porosity media and compare them with the responses in the single-porosity media. We find that both the amplitude and velocity of the fast P wave decrease with the increase of the volume ratio of the inclusions.

Numerical analysis for isolation of vibrations by pile rows in transversely isotropic saturated media due to moving loads

An analytical layer-element–boundary-element coupling approach is presented to explore the isolation effect of pile rows in stratified transverse isotropic saturated media for moving-load induced vibration. The analytical layer-element basic solution for the soils is derived. The soil flexibility matrices are obtained using boundary integral equations for pile-soil. Then, the pile is modeled as a Bernoulli–Euler beam utilizing the finite difference approach. The dynamic equations for the piles are achieved by considering the pile-soil displacement coordination condition. Finally, to obtain the amplitude reduction ratio, the displacement of an observation point is calculated with and without pile isolation, respectively. The results are in good consistency with the existing solutions. The impacts of the soils’ transverse isotropy, the pile’s size and physical properties on the isolation effect are evaluated using numerical examples. It is observed that when the pile length exceeds a certain value, the increment in the pile length shows no obvious improvement in the vibration isolation effect. A stiffer pile has a greater impact on vibration reduction. The material anisotropy is also an important parameter for the pile isolation problem.

Stiffness matrix method for modelling wave propagation in arbitrary multilayers

Natural and engineered media usually involve combinations of solid, fluid and porous layers, and accurate and stable modelling of wave propagation in such complex multilayered media is fundamental to evaluating their properties with wave-based methods. Here we present a general stiffness matrix method for modelling waves in arbitrary multilayers. The method first formulates stiffness matrices for individual layers based on the governing wave equations for fluids and solids, and the Biot theory for porous materials. Then it utilises the boundary conditions considered at layer interfaces to assemble the layer matrices into a global system of equations, to obtain solutions for reflection and transmission coefficients at any incidence. Its advantage over existing methods is manifested by its unconditional computational stability, and its validity is proved by experimental validations on single solid sheets, porous layers, and porous-solid-porous battery electrodes. This establishes a powerful theoretical platform that allows us to develop advanced wave-based methods to quantitatively characterise properties of the layers, especially for layers of porous materials.

Homogenizing fluid transport in stratified porous media using an elastic flow instability

Many key environmental, industrial and energy processes rely on controlling fluid transport within subsurface porous media. These media are typically structurally heterogeneous, often with vertically layered strata of distinct permeabilities – leading to uneven partitioning of flow across strata, which can be undesirable. Here, using directin situvisualization, we demonstrate that polymer additives can homogenize this flow by inducing a purely elastic flow instability that generates random spatio-temporal fluctuations and excess flow resistance in individual strata. In particular, we find that this instability arises at smaller imposed flow rates in higher-permeability strata, diverting flow towards lower-permeability strata and helping to homogenize the flow. Guided by the experiments, we develop a parallel-resistor model that quantitatively predicts the flow rate at which this homogenization is optimized for a given stratified medium. Thus, our work provides a new approach to homogenizing fluid and passive scalar transport in heterogeneous porous media.

Optimisation of stratified anisotropic media for security films

ABSTRACT Optical security films have gained a lot of attention due to its potential application in authentication services. Electrode-free optical devices are in demand along with electrode devices due to its secure access to the decryption key and ability to hide information from public access. Our concern in this study is the examination of electrode-free optical security film. In such devices, hidden information can be viewed in black/white contrast or controlled birefringent colours, which are produced by patterned phase plate and polariser. In particular, we develop the model of stratified anisotropic media for theoretical description of reflective characteristics of optical security films. Our theoretical results are compared with experimental observations. Blue and yellow birefringent colours of supertwisted nematic liquid crystal cell are achieved by using theory of optical characteristics in anisotropic media and partial coherence. This theoretical analysis suggests new openings in the optimisation of liquid crystal security films.

On the propagation of gravity waves in the lower solar atmosphere in different magnetic configurations

Gravity waves are generated by turbulent subsurface convection overshooting or penetrating locally into a stably stratified medium. While propagating energy upwards, their characteristic negative phase shift over height is a well-recognized observational signature. Since their first detailed observational detection and estimates of energy content, a number of studies have explored their propagation characteristics and interaction with magnetic fields and other waves modes in the solar atmosphere. Here, we present a study of the atmospheric gravity wave dispersion diagrams utilizing intensity observations that cover photospheric to chromospheric heights over different magnetic configurations of quiet-Sun (magnetic network regions), a plage, and a sunspot as well as velocity observations within the photospheric layer over a quiet and a sunspot region. In order to investigate the propagation characteristics, we construct two–height intensity - intensity and velocity- velocity cross-spectra and study phase and coherence signals in the wavenumber - frequency dispersion diagrams and their association with background magnetic fields. We find signatures of association between magnetic fields and much reduced coherence and phase shifts over height from intensity-intensity and velocity-velocity phase and coherence diagrams, both indicating suppression/scattering of gravity waves by the magnetic fields. Our results are consistent with the earlier numerical simulations, which indicate that gravity waves are suppressed or scattered and reflected back into the lower solar atmosphere in the presence of magnetic fields.

A half-space TD-BEM model for a seismic corrugated orthotropic stratum

In this paper, the model of a linear corrugated orthotropic stratum above a semi-infinite substratum is elaborated by a half-space time-domain boundary element method (TD-BEM) subjected to oblique incident plane SH waves. The ground surface and the interface are corrugated in a similar ratio using a Gaussian-shaped function. In the used approach, only the corrugated boundaries are required to be discretized. An incremental mesh scheme is applied to reduce the number of boundary elements in the large width of the interface. After developing the algorithm and solving a verification example, the seismic ground response is obtained and compared by assuming three cases of the model based on the isotropy factor, one isotropic case, and two orthotropic cases. Some effective parameters, including the shape ratio of the corrugation, frequency content, and wavefront propagation angle, are also considered to obtain the amplification pattern of the surface. Finally, by developing the method to multi-layered medium, its efficiency is investigated in the analysis of orthotropic heterogeneous models with more degrees of freedom. The results showed that both geometry and material factors were directly affected by the phenomenon of wave propagation so that the corrugation depth and the velocity of the wavefront in different directions could significantly control the response of the ground surface.

MHD Thermal and Solutal Stratified Stagnation Flow of Tangent Hyperbolic Fluid Induced by Stretching Cylinder with Dual Convection

The magneto-hydrodynamic dual convection stagnation flow pattern behavior of a Tangent Hyperbolic (TH) fluid has been reported in this study. The radiation, Joule heating, and heat generation/absorption impacts have also been analyzed. The flow-narrating differential equations, which are constrained by a thermal and solutal stratified porous medium, are transmuted into a system of nonlinear differential equations. To provide a numerical solution to the flow problem, a computational model is created. Numerical solutions are obtained using the fifth-order exactness program (Bvp5c), and for validation of the results, a comparison is also made with the methodology of the Runge–Kutta fourth order. The physical implications are appraised and depicted using diagrams or tables against flow-controlling parameters, such as Hartmann number, porosity parameter, solutal stratification, the parameter of curvature, temperature stratification, local Weissenberg number, Schmidt number, etc. It has been observed that in the appearance of Joule heating phenomena, the fluid temperature is a lowering function of thermal stratification. The findings are compared to the existing literature and found to be consistent with earlier research.

Transmission of an Acoustic Wave Through a Multilayer Medium Containing Layers of Bubble Liquid

Transmission of an Acoustic Wave Through a Multilayer Medium Containing Layers of Bubble Liquid