Velocity Factor Research Articles

Time dependent black holes and gravitational wave in Einstein–Gauss–Bonnet theory with two scalar fields

In this paper, we investigate the time dependent black holes in the frame of Einstein–Gauss–Bonnet theory having two scalar fields and investigate the propagation of the gravitational wave (GW). In the reconstructed models, there often appear ghosts, which could be eliminated by imposing some constraints. We investigate the behavior of high-frequency gravitational waves by examining the effects of varying Gauss–Bonnet coupling during their propagation. The speed of propagation changes due to the coupling during the black hole formation process. The propagation speed of gravitational waves differs when they enter the black hole compared to when they exit.

Shear wave speed, dispersion slope and anisotropy evaluation using cardiac shear wave elastography in patients with Fabry disease

Abstract Background Cardiac shear wave imaging has been proposed to investigate left ventricular myocardial alterations non-invasively. Dispersion slope and anisotropy are fundamental properties that have not been studied in Fabry disease (FD) patients yet. Purpose The aim of this study was to determine and compare shear wave propagation speed, dispersion slope and fractional anisotropy in FD patients and healthy controls. Methods We prospectively included 20 FD patients and 20 healthy controls (HC). Shear wave speed, dispersion slope and fractional anisotropy were measured non-invasively in anteroseptal basal segment using an ultrasound for cardiac shear wave imaging. Echocardiography, electrocardiogram and laboratory evaluations were performed. Results Shear wave propagation speed was higher in FD patients than in HC group (1.46 ± 0.12 m/s and 1.36 ± 0.17 m/s, p = 0.03). No difference was found in dispersion slope (11.4 m/s/kHz in FD group and 11.3 m/s/kHz in HC group, p = 0.6). Fractional anisotropy was 0.05 ± 0.04 in FD group and 0.05 ± 0.04 in HC group, p = 0.4. Conclusion Shear wave speed was higher in FD patients compared to control group. Dispersion slope and fractional anisotropy in FD group did not reveal significant difference to the control group.

Numerical study of bifurcation structures in reflected shock-wave/laminar-boundary-layer interaction within an end-wall tube

Abstract The primary aim of this study is to analyze the unsteady characteristics of the interaction between a reflected shock wave and a laminar boundary layer in an end-wall shock tube. Our direct numerical simulations at shock Mach numbers of Ms=1.9, 2.5, and 3.5 using a fifth-order WENO scheme and three-step Runge-Kutta time integration method revealed inhomogeneity and anisotropy in the shock bifurcation. Surprisingly, the upper and lower bifurcated structures maintain a notably asymmetric flow during the forward propagation of the reflected shock bifurcation. The inverse flow in the bifurcation resembles a crooked earthworm structure, exhibiting high-frequency oscillations indicative of instability. However, at higher shock intensities, the earthworm transforms into a stable strip-like configuration, facilitating the entrapment of inverse flow and leading to rapid bifurcation height growth and early convergence. Additionally, isolated islands with high density, temperature, and pressure emerge in the transitional region behind the bifurcated shocks, due to variations in wave propagation speed.

Probing the speed of scalar induced gravitational waves from observations

The propagation speed of gravitational waves is a fundamental issue in gravitational theory. According to general relativity, gravitational waves propagate at the speed of light. However, alternative theories of gravity propose modifications to general relativity, including variations in the speed of gravitational waves. In this paper, we investigate scalar-induced gravitational waves that propagate at speeds different from the speed of light. First, we analytically calculate the power spectrum of scalar induced gravitational waves based on the speed and spectrum of primordial curvature perturbations. We then explore several scalar power spectra, deriving corresponding fractional energy densities, including monochromatic spectrum, scale-invariant spectrum, and power-law spectrum. Finally, we constrain scalar-induced gravitational waves and evaluate the signatures of their speed from the combination of CMB+BAO and gravitational wave observations. Our numerical results clearly illustrate the influence of the speed of scalar-induced gravitational waves.

Study on ground-penetrating radar wave field characteristics for earth dam disease considering the medium randomness

Ground-Penetrating Radar (GPR) has been widely used for non-destructive testing of earth dam disease. However, the forward simulation of GPR for earth dam disease often employs layered homogeneous models, neglecting the influence of medium randomness on its wave field characteristics. Therefore, considering the randomness of the medium, a geoelectrical model for earth dam disease is established, which is based on the mixed-type autocorrelation function and the finite element time-domain method. The influence of random medium model parameters on the single-channel wave of GPR is analyzed. The electromagnetic wave propagation characteristics under different medium models are explored. The forward simulation of GPR for earth dam disease such as panel voiding, concentrated seepage, and loosening are performed. The differences in propagation characteristics for earth dam disease between uniform medium model and random medium model are compared. Compared to the calculation results of the uniform medium model, the propagation speed and amplitude of electromagnetic waves in the random medium model changes, and a number of diffraction waves are present. When performing forward simulation of GPR for earth dam disease, considering medium randomness can deepen the understanding of the GPR section view and help improve the accuracy of image interpretation.

Upstream influence of midlatitude jet stream biases in boreal summer

AbstractClimate models exhibit biases in the mean state and in variability across different regions of the Earth. For example, atmosphere‐only models have a poleward bias in summertime jet streams across the Northern Hemisphere (NH). This can result from many processes, including misrepresentation of Rossby waves that can propagate in different directions and thereby interact with jet streams. However, Rossby‐wave biases can result from biased background state of the climate system as well. The propagation speed of Rossby waves depends on jet stream strength, thus a poleward displacement of the jet stream can hinder westward propagation of Rossby waves at higher latitudes and displace eastward propagating Rossby waves (downstream development). These biases then impact other regions resulting in biased atmospheric circulation across the NH. Indeed, in this study we confirm this via regional nudging experiments within the Norwegian Earth System Model. Namely, nudged horizontal winds over the North Pacific can improve Rossby wave statistics and thereby atmospheric circulation over Eurasia (i.e., upstream). However, nudging over the North Atlantic has little effect on boreal summer atmospheric circulation. This implies that improving biases over the North Pacific is crucial for a better representation of modelled boreal summer circulation over Eurasia.

The modeling of acoustic black hole beam by spectral element method

A technique developed in recent decades for the passive control of vibrations and noise is the Acoustic Black Hole (ABH). The structural ABH effect is achieved by incorporating a local modification in the thickness of a thin structure, typically a beam or a plate, with a variation in thickness according to a spatial power law. The combination of the variation in the thickness power law and a local increase in damping, provided by the concomitant application of layers of viscoelastic material, produces a significant reduction in the wave propagation speed and an increase of attenuation properties. As an elastic wave propagates within an ABH, its propagation speed decreases smoothly and continuously, in theoretical case, when the structure's thickness disappears at the end of the ABH, the wave speed drops to zero. For practical applications, ABH is typically combined with dissipative materials to obtain significantly higher structural loss factors. This work aims to establish a numerical approach for modeling and designing a beam-type structural one-dimensional ABH device for vibration control device using the Spectral Element Method (SEM) and verified by the Finite Element Method (FEM). Examples are performed, and the results are compared with those found in the literature.

Estimation of ground vibration emission increase with respect to velocity in high-speed railways

During the design of a new line, it is often necessary to estimate railway vibration emissions at a given future speed from experimental data in similar circumstances, but at a different speed. Future levels are usually estimated according to a logarithmic evolution law with respect to speed, a typical one being that in USA FTA Report nº0123 "Transit Noise and Vibration Impact Assessment Manual", a de facto standard in the sector. Also, it is known that for high-speed lines a drastic increase in vibrations is observed when railway speed is higher than the propagation speed of surface Rayleigh waves. This phenomenon, known as trans-Rayleigh boom, produces a sharp discontinuity in the evolution of vibration emissions. However, vibration levels observed in the vicinity of a high-speed line have shown values well above the prediction rule suggested in FTA Manual, which could not be explained by a trans-Rayleigh boom. A more accurate extrapolation method based on local rail roughness measurements is capable of producing improved predictions compared to FTA rule of thumb. This contribution shows the details of the method suggested, and the improvement in the prediction (with respect to FTA extrapolation law) obtained in the specific case of a high-speed railway.

A Simple and Robust CryoSat‐2 Radar Freeboard Correction Method Dedicated to TFMRA50 for the Arctic Winter Snow Depth and Sea Ice Thickness Retrieval

AbstractCryoSat‐2 has been successful in observing sea ice thickness from space by providing ice freeboard information. The initial estimate of the ice freeboard, called radar freeboard, is obtained by analyzing the observed waveform using a retracker. A series of corrections are needed to convert the radar freeboard to the ice freeboard. Those are the physical effects (e.g., changes in wave propagation speed and the distribution of scattering at snow and ice surfaces, etc.) and the bias of the retracker; however, traditionally, only the wave speed correction has been applied due to lack of enough information to perform the complete correction. Here, an alternative correction method for the CryoSat‐2 radar freeboard derived using the Threshold First‐Maximum Retracker Algorithm with a 50% threshold (TFMRA50) is proposed. Snow depth was used as a predictor for the correction, similar to the traditional wave speed correction, but the coefficients were empirically determined by performing a direct comparison of the radar freeboard from CryoSat‐2 and the ice freeboard from airborne observations. Consequently, this new empirical correction treats the physical effects and the retracker bias as a whole, which have been difficult to separate in the retrieval process. In this paper, we demonstrate that the retrieval accuracy of snow and ice variables and the consistency of the two independent retrieval methods are improved when the new correction is applied. The result of this study emphasizes the importance of compatibility between the retracker and the freeboard correction method.

A cortical field theory – dynamics and symmetries

We characterise cortical dynamics using partial differential equations (PDEs), analysing various connectivity patterns within the cortical sheet. This exploration yields diverse dynamics, encompassing wave equations and limit cycle activity. We presume balanced equations between excitatory and inhibitory neuronal units, reflecting the ubiquitous oscillatory patterns observed in electrophysiological measurements. Our derived dynamics comprise lowest-order wave equations (i.e., the Klein-Gordon model), limit cycle waves, higher-order PDE formulations, and transitions between limit cycles and near-zero states. Furthermore, we delve into the symmetries of the models using the Lagrangian formalism, distinguishing between continuous and discontinuous symmetries. These symmetries allow for mathematical expediency in the analysis of the model and could also be useful in studying the effect of symmetrical input from distributed cortical regions. Overall, our ability to derive multiple constraints on the fields — and predictions of the model — stems largely from the underlying assumption that the brain operates at a critical state. This assumption, in turn, drives the dynamics towards oscillatory or semi-conservative behaviour. Within this critical state, we can leverage results from the physics literature, which serve as analogues for neural fields, and implicit construct validity. Comparisons between our model predictions and electrophysiological findings from the literature — such as spectral power distribution across frequencies, wave propagation speed, epileptic seizure generation, and pattern formation over the cortical surface — demonstrate a close match. This study underscores the importance of utilizing symmetry preserving PDE formulations for further mechanistic insights into cortical activity.

Estimating the In Situ Stratification via Remotely Sensed Internal Wave Speeds

Abstract This work tests a methodology for estimating the ocean stratification gradient using remotely sensed, high temporal and spatial resolution field measurements of internal wave propagation speeds. The internal wave (IW) speeds were calculated from IW tracks observed using a shore-based, X-band marine radar deployed at a field site on the south-central coast of California. An inverse model, based on the work of Kar and Guha, utilizes the linear internal wave dispersion relation, assuming a constant vertical density gradient is the basis for the inverse model. This allows the vertical gradient of density to be expressed as a function of the internal wave phase speed, local water depth, and a background average density. The inputs to the algorithm are the known cross-shore bathymetry, the background ocean density, and the remotely sensed cross-shore profiles of IW speed. The estimated density gradients are then compared to the synchronously measured vertical density profiles collected from an in situ instrument array. The results show a very good agreement offshore in deeper water (∼50–30 m) but more significant discrepancies in shallow water (20–10 m) closer to shore. In addition, a sensitivity analysis is conducted that relates errors in measured speeds to errors in the estimated density gradients. Significance Statement The propagation speed of ocean internal waves inherently depends on the vertical structure of the water density, which is termed stratification. In this work, we evaluate and test with real field observations a technique to infer the ocean density stratification from internal wave propagation speeds collected from remote sensing images. Such methods offer a way to monitor ocean stratification without the need for extensive in situ measurements.

Comprehensive analysis of wave interactions and resulting spall damage in layered heterogeneous medium under impact

The effect of dynamic spall damage on the response of layered medium subjected to one-dimensional impact involving an impactor and a multi-layered target is investigated. The study examines the damage caused by elastic waves below the Hugoniot Elastic Limit (HEL) during impact events. Analytical expressions of stress and particle velocity derived from mass and momentum conservation principles are utilized to solve the wave interactions within the impactor-target system, considering reflections, transmissions, and wave interactions between layers. These analytical expressions, integrated into a computational framework, facilitate the monitoring of material responses following each wave interaction, thereby analysing the impact response of the layered medium. Wave interactions, resulting in tensile stresses, that can cause damage to the target have been identified. A strain-based damage initiation and evolution law is incorporated into the developed computer program to predict damage in each layer of a medium. Damage alters the medium’s impact behaviour by decreasing stress magnitude and inducing temporal delays in the response due to attenuated wave propagation speed. The occurrences of tensile stress are explored, analyzing spatial and temporal variations of multiple damage incidents within the layered medium. The tension at the interface between two layers can induce damage in one or both adjacent layers, leading to debonding between layers. The effect of damage on the impact response of the medium is analysed by comparing the results for the damaged and undamaged target scenarios. The impact behaviour of a single-layer and a multi-layer target obtained from the present model, in terms of stress and particle velocity, is verified through Finite Element simulations of the identical impact problems, where the damage evolution criterion is incorporated using a VUMAT subroutine. The outcomes derived from this study align closely with Finite Element (FE) results.

Research on the dynamics of flame propagation and overpressure evolution in full-scale residential gas deflagration

To determine the effect of ignition height on indoor flame spread behavior and overpressure development, a comprehensive full-scale deflagration testing facility was established. Extensive experimental research was conducted within this facility. The findings indicate that indoor flame reignition and the occurrence of secondary explosions are most pronounced with intermediate ignition. Furthermore, the explosion overpressure generated during the reverse turn of shock wave propagation is greater than that produced by the forward turn. In comparison to the peak overpressure Pext in the master bedroom for top, middle, and bottom ignition, the peak overpressure Pext in the second bedroom increased by approximately 14.48 %, 15.04 %, and 19.20 %, respectively. When comparing middle ignition to top ignition, the propagation speed of shock waves in the kitchen balcony, restroom, second bedroom, and master bedroom was enhanced by 34.21 %, 40.85 %, 40.70 %, and 34.65 %, respectively. Furthermore, when comparing middle ignition to bottom ignition, the propagation speed of shock waves in these areas experienced a significant increase of 126.32 %, 124.39 %, 123.26 %, and 113.86 %, respectively. These research findings provide a theoretical foundation and empirical data to support the investigation and analysis of the causes of indoor gas explosion incidents.

Dark energy with a shift-symmetric scalar field: Obstacles, loophole hunting and dead ends

We discuss the possibility of a scalar field being the fundamental description of dark energy. We focus on shift-symmetric scalar-tensor theories since this symmetry potentially avoids some fine-tuning problems. We also restrict attention to theories satisfying that the propagation speed of gravitational waves is equal to the speed of light. These considerations lead us to investigate shift-symmetric Kinetic Gravity Braiding theories. Analysing the stability of scalar linear perturbations, we discuss the conditions that seems to be necessary to describe (super) accelerated cosmic expansion without introducing instabilities. However, it has been previously established that the linearised analysis does not guarantee the stability of this non-canonical scalar theory, as potentially dangerous interactions between dark energy fluctuations and tensor perturbations (essentially gravitational waves) appear at a higher order in perturbation theory. Indeed, although we shall point out that the standard proof of absence of dark energy stable braiding models due to this interaction has a possible way-out, we find general arguments suggesting that there are no dark energy stable solutions that can exploit this loophole. Thus, we discuss future research directions for finding viable fundamental descriptions of dark energy. We also provide a dictionary between the covariant version of the theory and that of the Effective Field Theory approach, explicitly computing the parameters in the latter formalism in terms of the functions appearing in the covariant version, and its derivatives. To the best of our knowledge, this is the first time these expressions are explicitly obtained up-to arbitrary order in perturbation theory.

A memory-type thermoelastic laminated beam with structural damping and microtemperature effects: Well-posedness and general decay

Abstract In previous work, Fayssal considered a thermoelastic laminated beam with structural damping, where the heat conduction is given by the classical Fourier’s law and acting on both the rotation angle and the transverse displacements established an exponential stability result for the considered problem in case of equal wave speeds and a polynomial stability for the opposite case. This article deals with a laminated beam system along with structural damping, past history, and the presence of both temperatures and microtemperature effects. Employing the semigroup approach, we establish the existence and uniqueness of the solution. With the help of convenient assumptions on the kernel, we demonstrate a general decay result for the solution of the considered system, with no constraints regarding the speeds of wave propagations. The result obtained is new and substantially improves earlier results in the literature.

Uso de ultrassom para análise das propriedades de uma haste cobreada de aterramento

The main objective of this work was to use ultrasonic wave values and density to analyze the elastic and physical properties of a copper-plated rod used for grounding. For this, an electronic ultrasound system with 2 MHz piezoelectric transducers and mathematical models that are a function of the propagation speeds of the ultrasonic waves was used. The ultrasonic speeds were determined with the transmission method using petroleum jelly paste as the couplant material. Thus, through the specific equations for isotropic materials, it was possible to estimate some elastic properties, such as modulus of elasticity, shear and compressibility, Poisson coefficient and anisotropy factor, in addition to the physical property of acoustic impedance. The properties of the grounding rod were evaluated before and after a chemical attack with nitric acid to remove the copper layer. As a result, the feasibility of using ultrasound as a non-destructive technique, capable of detecting small changes in the properties of the nail after chemical attack, was verified. The results were compared with the electrical resistivity test and discussed as a function of the values of physical and mechanical properties reported in the literature for the grounding rods.

VELOCITY AND EVOLUTION OF SHOCK WAVES IN SAND WITH INCREASING WATER CONTENT

Interest in the problems of shock wave propagation in saturated porous media is associated with studying seismic exploration issues, modeling the hummer effect during hydraulic fracturing, studying the water-gas impact on formations for the purpose of efficient oil production, etc. Experimental work on waves in bulk media is usually carried out in shock tubes. Traditionally, shock tubes are used to study the characteristics of the propagation of shock waves in various media, to analyze the wave properties of these media based on the speed of the shock waves and the change in the shape of the pulse along the passage of the first – main pulse. In a shock tube equipped with a section of bulk media, the wave is repeatedly reflected from the surface of the porous medium under study and the upper end of the tube. Re-reflected waves can be used to study changes in the environment that occurred under the influence of a shock wave, i.e. as probing pulses. In this article, sounding is understood as identifying acoustic properties (changing the shape of shock pulse diagrams and their velocities). This paper presents the results of experimental studies of the propagation of shock waves of small amplitude in sand with a volumetric content of liquid in the pore space from 0 to 40 %, and illustrates the influence of the presence of liquid in the pores on the shape of the main and probing pulses. The research was carried out using the Shock Tube installation, in the Laboratory of Experimental Hydrodynamics of the Mavlyutov Institute of Mechanics of Ufa Branch RAS. An increase in the speed of wave propagation was established with increasing water saturation of sand in the range from 0 to 40 %. An increase in the amplitude of reflected waves when passing through sand, the formation of peaks and stretching of pulses were discovered. The research results can be used in processing geophysical data to interpret seismograms of sandy deposits saturated with gas-liquid systems.

Propagation of sharp wave-ripple activity in the mouse hippocampal CA3 subfield in vitro.

Sharp wave-ripple complexes (SPW-Rs) are spontaneous oscillatory events that characterize hippocampal activity during resting periods and slow-wave sleep. SPW-Rs are related to memory consolidation - the process during which newly acquired memories are transformed into long-lasting memory traces. To test the involvement of SPW-Rs in this process, it is crucial to understand how SPW-Rs originate and propagate throughout the hippocampus. SPW-Rs can originate in CA3, and they typically spread from CA3 to CA1, but little is known about their formation within CA3. To investigate the generation and propagation of SPW-Rs in CA3, we recorded from mouse hippocampal slices using multi-electrode arrays and patch-clamp electrodes. We characterized extracellular and intracellular correlates of SPW-Rs and quantified their propagation along the pyramidal cell layer of CA3. We found that a hippocampal slice can be described by a speed and a direction of propagation of SPW-Rs. The preferred propagation direction was from CA3c (the subfield closer to the dentate gyrus) toward CA3a (the subfield at the boundary to CA2). In patch-clamp recordings from CA3 pyramidal neurons, propagation was estimated separately for excitatory and inhibitory currents associated with SPW-Rs. We found that propagation speed and direction of excitatory and inhibitory currents were correlated. The magnitude of the speed of propagation of SPW-Rs within CA3 was consistent with the speed of propagation of action potentials in axons of CA3 principal cells. KEY POINTS: Hippocampal sharp waves are considered important for memory consolidation; therefore, it is of interest to understand the mechanisms of their generation and propagation. Here, we used two different approaches to study the propagation of sharp waves in mouse CA3 in vitro: multi-electrode arrays and multiple single-cell recordings. We find a preferred direction of propagation of sharp waves from CA3c toward CA3a - both in the local field potential and in sharp wave-associated excitatory and inhibitory synaptic activity. The speed of sharp wave propagation is consistent with the speed of action potential propagation along the axons of CA3 pyramidal neurons. These new insights into the dynamics of sharp waves in the CA3 network will inform future experiments and theoretical models of sharp-wave generation mechanisms.

Impact of rupture disk morphology on self-ignition during pressurized hydrogen release: A numerical simulation study

In the study of self-ignition during pressurized hydrogen release, the morphology of the rupture disk alters the flow and mixing processes of the gas jet, as well as the development of shock waves, thereby affecting the occurrence of self-ignition. This study employs the LES method based on the RNG-SGS model, EDC model, and NUIMech1.1 H2 combustion mechanism to investigate the flow mixing, shock wave development, and self-ignition phenomena under three different the rupture disk morphologies. The research findings indicate that as the growth trend of the rupture disk rupture ratio increases, the propagation speed of shock waves within the tube and the compression speed of the air and hydrogen-air mixing region accelerate, resulting in an earlier occurrence of self-ignition. A more complex rupture morphology leads to higher mixing rates between hydrogen and air and increased complexity of shock waves. Self-ignition initially occurs in the fuel-lean region of the boundary layer, followed by ignition at the center of the tube. The characteristic number of rupture disk is proposed to describe its impact on the self-ignition time. The competitive relationship between the two elementary reaction pathways of oxygen influences the development of the flame.

Pressure Characteristics and Secondary Ignition Effects of Gas Produced in RDE Using Lignite and Anthracite/CH4 Fuel.

This study aims to address the energy efficiency release and environmental impact of coal dust in rotating detonation engines (RDEs) by monitoring the detonation characteristics of lignite and anthracite in methane gas-solid mixtures using high-precision sensor technology. Experimental results indicate that the peak detonation pressure of anthracite is 1.4% higher than that of lignite, and its detonation wave propagation speed is at least 5.5% faster, suggesting that anthracite exhibits more stable and efficient fuel energy release characteristics during detonation. Additionally, the sensor technology enabled detailed recording of pressure fluctuations and gas composition changes during the detonation process, providing accurate data for assessing and controlling emissions of harmful gases such as carbon monoxide and carbon dioxide. These data are crucial for designing more efficient and environmentally friendly coal dust detonation systems, enhancing mine safety and environmental protection. The outcomes of this research not only advance technological progress in the field of energy science but also address efficiency and environmental issues in industrial applications, offering significant support for achieving safer and more sustainable energy production methods.