Velocity Factor Research Articles

Ballistic impact induced wave propagation and dislocation of three-dimensional graphene origami/copper nanocomposites

Graphene (Gr)-reinforced nanocomposites have great potential as impact protection materials due to their exceptional mechanical properties. However, the intrinsic brittleness of Gr originating from its two-dimensional (2D) geometry considerably hinders its application. In this work, a three-dimensional (3D) graphene origami (GOri) reinforced copper (Cu) nanocomposite is designed and its wave propagation characteristics under a high-velocity ballistic impact are studied using the molecular dynamics (MD) method. It is found that the intrinsic brittleness of Gr can be alleviated by transforming its original 2D shape into 3D origami shape and embedding GOri in Cu matrix can lead to effectively increased out-of-plane wave propagation speed thereby boost the dissipation of impact energy by promoting rapid dislocation propagation within the Cu matrix during the initial stage of impact.

Meta-Analysis of Randomized Clinical Trials on the Speed of Pulse Wave Propagation

Meta-Analysis of Randomized Clinical Trials on the Speed of Pulse Wave Propagation

Particle simulations of ion-extraction process from a decaying plasma assisted by radio-frequency plasma heating

In this study, two-dimensional kinetic particle simulations were employed to examine the potential of radio-frequency (rf) plasma heating in enhancing ion extraction efficiency in a decaying plasma with the configuration of parallel plates. The numerical results suggest that the application of rf power based on the direct current electrostatic method leads to a remarkable increase in the ion extraction flux, thereby reducing the time required for ion extraction. The increase in the ion extraction flux is attributed to the enhancement of the penetration ability of the rf electric field into the plasma, especially in cases of high rf frequencies, which can elevate the bulk electron temperature to approach 10 eV. The propagation speed of ion rarefaction waves is enhanced by the increased electron temperature, speeding up the process of ion extraction. The study also found that an increase in rf voltage causes more intense plasma oscillations to screen out the rf disturbance, further increasing the electron temperature. Furthermore, as ion extraction continues, the heating effect of rf frequencies was found to be enhanced due to the decay of plasma density.

An improved artificial compressibility method for aeroacoustics at low Mach numbers

This work presents an improved artificial compressibility method that enables correct sound propagation behaviour in low Mach number flows. As an extension of incompressible approaches, it offers the advantage of low computational costs since compressible flow equations are simplified under the isentropic assumption, and the coupling of flow and sound is preserved. The non-uniform speed of sound is considered in this method, and eigenvalue analysis of the modified governing equations reveals that the propagation speed of pseudo waves restores to that of physical acoustic waves. The effect of Mach number on the dissipation and dispersion error is then studied using a two-dimentional monopole example with uniform background flow. The results suggest that the proposed method can provide a satisfactory prediction of sound propagation when the Mach number is below 0.3. Lastly, a direct noise computation of the aerofoil trailing-edge noise problem is performed to assess its capability to deal with the interaction between flow and sound. It shows that the present method can well capture the multiple-tone phenomenon.

Natural shear wave measurements detect changes in operational stiffness induced by mechanical ventilation in sheep

Abstract Shear wave elastography (SWE) has the potential to non-invasively measure myocardial stiffness, based on the propagation speed of shear waves traveling through the myocardium after valve closure. However, shear wave propagation speed (SWS) is related to the operational stiffness of the heart, which is defined by the slope of the end-diastolic stress-strain relationship at the operating volume/pressure. Therefore, operational stiffness can be altered by both changes in intrinsic myocardial stiffness and myocardial loading conditions. We aimed to investigate whether SWE can detect differences in operational stiffness induced by loading changes during mechanical ventilation, and whether SWE combined with respiratory hold manoeuvres could detect an evolution in stiffness during the course of ischemic cardiomyopathy (ICM). We assessed SWS in 8 sedated and mechanically ventilated sheep: 1 healthy control, 3 within 8 weeks, and 4 after at least 16 weeks following left coronary artery ligation (early vs. late ICM). Left ventricular parasternal long-axis views were acquired with a state-of-the-art ultrasound scanner with high frame rate technology (> 1100 Hz). Shear waves after mitral valve closure were considered during an inspiratory hold at 30 cmH2O and 15 cmH2O and expiratory hold at 0 cmH2O (atmospheric pressure). We used linear regression to assess the trend of the SWS during ventilation manoeuvres per animal and a two-tailed Student’s T-test to assess changes between ICM stages and their respective response to ventilation manoeuvres. In all but one sheep, SWS declined with higher inspiratory pressures (see fig. 1), possibly due to a reduction in preload at higher inspiratory pressures resulting in a shift towards a reduced slope on the myocardial stress-strain relation. The sheep with a positive slope was excluded from further analyses. SWS during expiratory hold at atmospheric pressure was 3.4 m/s in the healthy control and was significantly lower in early vs. late ICM (mean±SD 4.0±0.6 vs. 9.0±1.7 m/s; p=0.007). Furthermore, the change in SWS was larger in late vs. early ICM (slope of regression line -0.16 vs. -0.05 m/s/cmH2O; p=0.03). These observations suggest loading on a steeper part of the end-diastolic stress strain relationship in late ICM. In conclusion, our results suggest that SWE is able to detect differences in operational stiffness induced by mechanical ventilation. Secondly, SWE appears to distinguish between different time points in the course of ICM based on the magnitude of the SWS during expiratory hold at atmospheric pressure, but also on the magnitude of the SWS change during loading alterations induced by mechanical ventilation. Further work should assess whether SWE combined with respiratory hold manoeuvres could contribute to differentiate intrinsic from hemodynamic stiffening, and to exploit its full potential as a non-invasive bedside method for the assessment of operational stiffness.

Low voltage and low wave speed are rarely present outside the left atrium-pulmonary veins junction in paroxysmal atrial fibrillation but frequently present in persistent forms

Abstract Background and aim Multielectrode high-density catheters have enabled acquisition of comprehensive and dense maps of electrogram’s amplitude and timing. However, automated activation and voltage mapping are flawed by catheter orientation in relation to the wavefront activation. Omnipolar mapping technology (OT) uses both unipolar and bipolar signals to obtain OT signals and increases the accuracy of automatic point acquisition allowing for a high- density map in a quick and efficient way. The aim of this study was to automatically assess with OT, the left atrium (LA) voltage and wave speed propagation map in sinus rhythm (SR) according to the type of atrial fibrillation (AF). Methods We studied 14 consecutive patients referred for ablation of AF, either paroxysmal (PAF) (n=6) or persistent (Pers-AF) (n=8) using catheters with OT. Patients with Pers-AF were cardioverted, and a SR map was obtained before ablation in all patients and voltage and propagation wave speed maps were obtained. The cut-off for low voltage areas (LVAs) was less than 0.1 mV and for low wave speed (LWS) was less than 0.4 mm/ms. Results were compared according to the type of AF. Results The results are depicted in the Table. The median duration of AF in Pers-AF was 18 (5-24) months. Patients with Pers-AF had larger LA volumes and displayed presence of LVAs and LWS areas in 100% of cases in comparison with 17% and 33% of cases respectively for PAF patients (p=0.003 and p=0.015). Also, the total LVAs and LWS areas in cm2 were significantly higher, (p=0.001 and p=0.005)). The LVAs and the LWS areas were more frequently located outside the PV-LA junction in Pers-AF than in PAF, respectively 75% vs 0% and 88% vs 0%, p=0.010 and p=0.005 (Figure). Conclusions Patients with PAF never presented LVAs or LWS outside the PV-LA junction. On the contrary, patients with Pers-AF had frequently anatomical and electrophysiological abnormalities in the LA body. These findings may suggest the need for a wider ablation strategy in Pers-AF.Normal voltage in purple ; LWS in white

Theory of Magnetic Switchbacks Fully Supported by Parker Solar Probe Observations

Magnetic switchbacks are rapid high-amplitude reversals of the radial magnetic field in the solar wind that do not involve a heliospheric current sheet crossing. First seen sporadically in the 1970s in Mariner and Helios data, switchbacks were later observed by the Ulysses spacecraft beyond 1 au and have been recently discovered to be a typical component of solar wind fluctuations in the inner heliosphere by the Parker Solar Probe spacecraft. While switchbacks are now well understood to be spherically polarized Alfvén waves thanks to Parker Solar Probe observations, their formation has been an intriguing and unsolved puzzle. Here we provide a simple yet predictive theory for the formation of these magnetic reversals: the switchbacks are produced by the distortion and twisting of circularly polarized Alfvén waves by a transversely varying radial wave propagation velocity. We provide an analytic expression for the magnetic field variation, establish the necessary and sufficient conditions for the formation of switchbacks, and show that the proposed mechanism works in a realistic solar wind scenario. We also show that the theoretical predictions are in excellent agreement with observations, and the high-amplitude radial oscillations are strongly correlated with the shear of the wave propagation speed. The correlation coefficient is around 0.3–0.5 for both encounter 1 and encounter 12. The probability of this being a lucky coincidence is essentially zero with p-values below 0.1%.

One-dimensional haemodynamic model of a vascular network with fractional-order viscoelasticity

Abstract We propose a computational framework for a one-dimensional haemodynamic model with the arterial walls described by the fractional-order viscoelastic material constitutive law. This framework is used to compare blood flow characteristics for simulations with elastic and fractional-order viscoelastic walls. We use three well-established benchmark tests: a single pulse wave in a long vessel, flow in a 37-segment network of elastic tubes, and flow in anatomically detailed arterial network consisting of 61 arterial segments. All results for elastic model are in a good agreement with analytical solutions, in vitro data and other well-established approaches. Fractional-order model demonstrates noticeable differences in pulse wave propagation speed and minor differences in pressure and flow profiles. Differences in profiles are negligible in major vessels, but more profound in vessels beyond the third or fourth generation.

Exponential stability and exact controllability of a system of coupled wave equations by second‐order terms (via Laplacian) with only one non‐smooth local damping

The purpose of this work is to investigate the exponential stability of a second‐order coupled wave equations by Laplacian with one locally internal viscous damping. The strong stability is established by combining a unique continuation result and a Carleman estimate. Besides, the exponential stability is proved under (PMGC) condition on the damping region without any restriction on wave propagation speed (i.e., whether the two wave equations propagate at the same speed or not). The proof of this result relies on the frequency domain method combined with the multiplier techniques.

Continuous shear wave measurements for dynamic cardiac stiffness evaluation in pigs

Ultrasound-based shear wave elastography is a promising technique to non-invasively assess the dynamic stiffness variations of the heart. The technique is based on tracking the propagation of acoustically induced shear waves in the myocardium of which the propagation speed is linked to tissue stiffness. This measurement is repeated multiple times across the cardiac cycle to assess the natural variations in wave propagation speed. The interpretation of these measurements remains however complex, as factors such as loading and contractility affect wave propagation. We therefore applied transthoracic shear wave elastography in 13 pigs to investigate the dependencies of wave speed on pressure–volume derived indices of loading, myocardial stiffness, and contractility, while altering loading and inducing myocardial ischemia/reperfusion injury. Our results show that diastolic wave speed correlates to a pressure–volume derived index of operational myocardial stiffness (R = 0.75, p < 0.001), suggesting that both loading and intrinsic properties can affect diastolic wave speed. Additionally, the wave speed ratio, i.e. the ratio of systolic and diastolic speed, correlates to a pressure–volume derived index of contractility, i.e. preload-recruitable stroke work (R = 0.67, p < 0.001). Measuring wave speed ratio might thus provide a non-invasive index of contractility during ischemia/reperfusion injury.

Wave propagation through a stationary field of clouds: A homogenisation approach

AbstractThe effect of a subgrid‐scale cloud field on the propagation of long atmospheric waves is investigated using a new scale‐consistent formulation based on the asymptotic theory of homogenisation. A key aim is to quantify potential model errors in wave propagation speeds, introduced by using averaged fields in place of the fully resolved circulation, in the setting of a simple stratified Boussinesq midlatitude ‐channel model. The effect of the cloud field, represented here by a random array of strongly nonlinear axisymmetric circulations, is found to appear in the large‐scale governing equations through new terms which redistribute the large‐scale buoyancy and horizontal momentum fields in the vertical. These new terms, which have the form of nonlocal integral operators, are linear in the cloud number density and are fully determined by the solution of a linear elliptic equation known as a cell problem. The cell problem in turn depends on the details of the nonlinear cloud circulations. The integral operators are calculated explicitly for example cloud fields and then dispersion relations are compared for different waves in the presence of clouds at realistic densities. The main finding is that baroclinic Rossby waves are significantly slowed and damped by the clouds, whilst inertia–gravity waves are affected almost exclusively by damping, most strongly at the lowest frequencies. In contrast, all waves with a barotropic structure are found to be almost unaffected by the presence of clouds, even at the highest realistic cloud densities. An important consequence of this study is a new approach to the closure of subgrid‐scale cloud fields in the parameterisation of convection in large‐scale atmospheric models.

Mapping the Unresolved Tidal Resource in Estuaries

Estuaries are ideal locations for extracting tidal energy and yet the global resource appears poorly mapped. Estuaries typically have high tidal ranges and strong tidal currents, due to amplification processes; and this resource is juxtaposed to industrial/residential areas with electricity demand [1]. For example, 22 of the 32 largest cities in the world are adjacent to estuaries [2], and UK estuaries collectively worth over £5.5b to UK economy with &gt;1/6th of the population and ~1b tonnes of cargo traded at their ports [3]. &#x0D; Mapping the tidal energy resource is challenging due to the sensitivity of ocean-model simulated currents to the model resolution [4]. Both tidal amplitude and currents are heavily modified in estuaries [5]. Whilst many estuary-specific resource modelling studies have been achieved (e.g. [6]), no large-scale future tidal energy resource mapping project has been undertaken – likely due to computational cost (e.g. [4]). It is therefore unrealistic to hydrodynamically model all global estuarine systems, to resolve current and future changes to the tidal energy resource; instead we aim to apply a simplified analytical technique that could be calibrated and validated by the new NASA SWOT satellite mission (https://swot.jpl.nasa.gov/), as well as citizen science. Estuarine tidal dynamics have been observed to change rapidly due to changes in river-flow climatology and bathymetry (e,g. dredging); see [7]. Indeed, climate change driven impacts to tidal dynamics (sea-level rise and altered riverine climatology), alongside anthropogenic driven morphodynamical changes and interactions with future tidal dynamics, could increase the future estuary tidal resource [1].&#x0D; Three physical processes drive mean tidal amplification excluding over-tides): (1) Funnelling - concentration of the tidal energy flux with reducing width; (2) Resonance - when estuary length-scales are close to the natural period of the basin; and (3) Shoaling (reduction in the propagation speed of the tidal wave resulting in an increase in amplitude). Each of these three processes are analytical solved, and a 1D analytical model applied to demonstrate the resource. Sea-Level Rise (SLR) scenarios are included to show SLR can modify estuarine tidal dynamics through both estuarine geometry (e.g. increasing resonance) and boundary conditions (global mean sea-level rise increasing boundary conditions). A normalised result of the analytical approach is shown in Figure 1, which demonstrates a computationally efficient analytical solution that includes future changes to tidal dynamics. &#x0D; Our research will apply this analytical solution to the Bristol Channel, a region soon-to-be heavily instrumented as part of the “Cal-Val phase of the NASA SWOT project (https://projects.noc.ac.uk/swot-uk/swot-uk-bristol-channel). Finally, we will develop a technique to bring together digital observations from citizens and sensor networks (see https://digitalenvironment.org/), to validate our simplified approach and resolve the tidal energy resource.

FREE AND BOUND WAVES IN THE COASTAL ZONE: FIELD, LABORATORY AND NUMERICAL EXPERIMENTS

Traditionally we look on storm waves as on periodic in space and time motion. Main feature of wave transformation in the coastal zone over inclined bottom is the higher harmonic generation that occurs so fast (during the one wavelength) that we can’t consider the wave motion as periodic in space. Spatial measurements of the wave profile are difficult in the coastal zone and are replaced by point chronograms of free surface elevations and then interpreted using traditional wave theories to calculate the practically important the wave energy and speed of wave propagation. These interpretations very often lead to paradoxical results, such as unexpected spatial fluctuations of the wave energy and propagation velocities, which are usually interpreted as shortcomings in the calibration of gauges, but our experience told that is the result of simultaneous existing of free and bound waves. Many researchers consider the free and bound second and higher harmonics of waves on the intermediate water as “parasitic” or “spurious” and try to avoid it in laboratory and in numerical experiments, but second harmonic are practically important. We will try to figure out what is the matter on the basis of field, laboratory and numerical experiments consider the waves in both time and space domains.

The perturbed Riemann problem for the Chaplygin pressure Aw–Rascle model with Coulomb-like friction

In this paper, we are concerned with the Riemann problem and the perturbed Riemann problem for the Chaplygin pressure Aw–Rascle model with Coulomb-like friction, which can also be seen as the nonsymmetric Keyfitz–Kranzer system with Chaplygin pressure and Coulomb-like friction. For the Riemann problem, we show that it explicitly exhibits two kinds of different structures and the delta shock wave appears in some certain situations. The generalized Rankine–Hugoniot conditions of the delta shock wave are established and the exact position, propagation speed and strength of the delta shock wave are given explicitly. Unlike the homogeneous case, it is shown that the Coulomb-like friction term makes contact discontinuities and the delta shock wave bend into parabolic shapes and the Riemann solutions are not self-similar anymore. For the perturbed Riemann problem with delta initial data, not only the delta shock wave but also the delta contact discontinuity are found in solutions and the friction term makes them bent. Under the generalized Rankine–Hugoniot conditions and the entropy condition, by taking variable substitution, we constructively obtain the global existence of generalized solutions which explicitly exhibit four kinds of different structures. The results in this paper yield a way of studying the wave interaction involving the delta shock wave for conservation laws with source terms and will give us some insights into the research on the Chaplygin pressure Aw–Rascle model, the pressureless Euler equations and the Chaplygin Euler equations with various kinds of source terms.

Contributed Session II: Detecting subclinical keratoconus by mapping corneal biomechanics using wave-based optical coherence elastography.

The detection of subclinical keratoconus in human corneas remains a challenging task. It is hypothesized that the focal reduction in biomechanical properties of the cornea would be the primary initiating event of keratoconus before changes in topography and pachymetry are manifested. We propose to use an air-couple ultrasonic optical coherence elastography system to map the elasticity of the corneas of 15 patients with a clinical diagnosis of keratoconus (KC) in one eye, and subclinical keratoconus (SK) in the fellow eye. The fully non-contact system excites the corneal apex to produce Lamb wave propagation and measures wave speed and average corneal thickness along 16 semi-meridians. Measurements in 30 human healthy subjects (control group: CG, N = 60 corneas) defined a baseline of normal biomechanics in two biomarkers: Spatial Anisotropy of Wave Speed (SAWS < 0.207), and the Speed-Thickness Index (STI > 0). SAWS was significantly higher in KC (0.353, p < 0.001) and SK (0.249, p < 0.001) corneas when compared to the CG. Moreover, STI maps show abnormal elasticity in SK (STI = -1.22) and KC (STI = -0.375) compared to the CG. Our results show important biomechanical differences in SAWS and STI between normal, subclinical, and advanced stages of keratoconus, suggesting those as potential biomarkers to identify "at-risk" corneas before changes in topography and pachymetry become evident.

Research on Impact Load Localization Based on Piezoelectric Fibers with Metal Cores

Using a metal core piezoelectric fiber (MPF) as a sensing element and utilizing its directional characteristics, a shock load localization method based on the MPF flower structure is proposed. This method does not need to know the propagation speed of stress waves in the structure. At the same time, experimental research on impact load positioning of plate structures was conducted using this method. The research results showed that the method based on MPF flower shaped composite structure can be used for impact load positioning, and the positioning accuracy is high.

Propagation characteristics of tsunami waves in shear flow based on the Boussinesq model with constant vorticity

Propagation of tsunami wave in linear shear flow with constant vorticity is studied in this paper. The nonlinear and dispersive Boussinesq equations for water waves in shear flow are derived by introducing uniform vorticity into the Euler equations. Generally, the transoceanic propagation of tsunami can be well simulated by the weakly nonlinear Boussinesq model. Thus, the nonlinear and dispersive terms are retained to the first-order and second-order (ϵ,μ2) respectively in the Boussinesq equations with uniform vorticity in the present work, which satisfies the simulation accuracy especially the dispersion effects of the long-distance propagation of tsunamis and does not require huge computational cost for the simulation. The set of equations is solved with the total variation diminishing (TVD) Lax–Wendroff scheme which is second-order both in space and time. This TVD scheme is applicable to handle the dispersion effects of the N-waves and undular bores. Different patterns of tsunami wave in the water with presence of vorticity are simulated by the numerical model. Wave height and wavelength are affected by the current with vorticity. The pattern of solitary wave is significantly changed in upstream and downstream conditions. The wave deformation and propagation speed of N-waves and undular bores in the shear flow are also studied. The vorticity has impacts on the leading crest amplitude and the maximum wave appearance time for N-wave and changes the wave amplitude and the following water level for undular bores.

Exponential stability for porous system of thermoelasticity-type III with past history and distributed delay term

As a continuity to the study by Ouchenane et al. [On the Porous-elastic system with thermoelasticity of type III and distributed delay: Well-posedness and stability, J. Funct. Spaces (2021) 1–12], we consider a one-dimensional linear thermoelastic system of porous type with past history and distributed delay term. This model deals with dynamics of engineering structures and non-classical problems of mathematical physics. It includes new results and their proofs with a help of the energy method combined with Lyapunov functional, we discuss the stability of system for the case of equal speeds of wave propagation.

Exact decay rates for weakly coupled systems with indirect damping by fractional memory effects

AbstractThis paper deals with the asymptotic behavior of a weakly coupled system of two equations in which one of them has a dissipative mechanism given by a memory term. This term depends on the fractional operator with exponent . We show that strong solutions of the system decay polynomially with a rate that depends on both the exponent θ and wave propagation speeds. Optimal decay rates are found and the results show a surprising aspect: More regular damping does not necessarily imply a faster decay.

Large displacements and rotations of a local linear elastic rod

AbstractThe Local Linear Timoshenko (LLT) model for the planar motion of a rod that undergoes flexure, shear and extension, was recently derived in Van Rensburg et al. (2021). In this paper we present an algorithm developed for this model. The algorithm is based on the mixed finite element method, and projections into finite dimensional subspaces are used for dealing with nonlinear forces and moments. The algorithm is used for an investigation into elastic waves propagated in the LLT rod. Interesting properties of the LLT rod include the increased propagation speed of elastic waves when compared to the linear Timoshenko beam, and the appearance of buckled states or equilibrium solutions for compressed LLT beams. It is also shown that the LLT rod is applicable to large displacements and rotations for a wide range of slender elastic objects; from beams to highly slender flexible rods.