Velocity Limit Research Articles

Appraising scattering theories for polycrystals of any symmetry using finite elements.

This paper uses three-dimensional grain-scale finite-element (FE) simulations to appraise the classical scattering theory of plane longitudinal wave propagation in untextured polycrystals with statistically equiaxed grains belonging to the seven crystal symmetries. As revealed from the results of 10 390 materials, the classical theory has a linear relationship with the elastic scattering factor at the quasi-static velocity limit, whereas the reference FE and self-consistent (SC) results generally exhibit a quadratic relationship. As supported by the results of 90 materials, such order difference also extends to the attenuation and phase velocity, leading to larger differences between the classical theory and the FE results for more strongly scattering materials. Alternatively, two approximate models are proposed to achieve more accurate calculations by including an additional quadratic term. One model uses quadratic coefficients from quasi-static SC velocity fits and is thus symmetry-specific, while the other uses theoretically determined coefficients and is valid for any individual material. These simple models generally deliver more accurate attenuation and phase velocity (particularly the second model) than the classical theory, especially for strongly scattering materials. However, the models are invalid for the attenuation of materials with negative quadratic coefficients. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'.

Strain rate effect analysis of hybrid composites under the high-velocity impact

In this paper, the damage behavior of fiber metal laminate (FML) is studied under high-velocity impact (HVI) at different strain rates (1 s−1, 100 s−1, and 1000 s−1) for flat, hemisphere, and sharp nosed projectiles. A finite element model has been used to examine the damage modes and failure processes of carbon fiber reinforced aluminium laminates (CRALLs) under HVI. This study is based on continuum damage mechanics. The damage criteria for aluminium (Al) and carbon fiber composite layers are identified by Johnson-Cook (J-C) and Hashin damage models, respectively. To analyse the inter-laminar de-lamination caused by the HVI, a cohesive surface regulated by bi-linear traction-separation is applied. The CRALL ballistic limit is analysed for different projectiles, which strike at initial velocity ranges from 60 m/s to 150 m/s to the CRALL. The obtained numerical results are in good agreement with the literature experimental results, which indicates the model is permissible for further analysis. The obtained results show that if strain rate varies from 1 s−1 to 1000 s−1 then CRALL ballistic limit velocity increases for all projectiles. The CRALL ballistic limit is the highest for flat nosed projectiles, followed by hemisphere and sharp nosed projectiles.

Nonadiabatic Correction and Adiabatic Criteria of Noninteracting Quantum Dot Systems

We theoretically study nonadiabatic corrections for charge pumping in a noninteracting electron model of a single-level quantum dot. We derive a formula for the velocity limit of parameter driving to realize adiabatic pumping and illustrate its features in the wide-band limit. We discuss the effect of the nonadiabatic corrections in terms of a typical velocity limit, which is defined by averaging the velocity limits on the parameter contour. We also show that the typical velocity limit vanishes when the adiabaticity breakdown occurs in a quantum dot coupled to reservoirs with a band edge.

Influence of gas expansion on the velocity and stability limits of stationary curved flames in channels

ABSTRACT The influence of gas expansion on the velocity of freely propagating curved stationary premixed flames in channels with free-slip adiabatic walls is studied utilizing 2D computational simulations, with the focus on the strongly nonlinear effects of the Darrieus-Landau instability on the stationary flame shape and velocity. The complete system of unsteady compressible Navier–Stokes equations is solved, including the transport properties (heat conduction, diffusion, and viscosity), and the chemical kinetics in the form of a one-step irreversible Arrhenius reaction. The parametric study focuses on the gas expansion, with a wide yet realistic range of expansion coefficients employed, and on the channel width. It is shown that the stationary curved flame velocity increases with gas expansion coefficient, in line with previous studies. The second critical channel width, at which the stationary curved flame velocity exceeds the one predicted by weakly nonlinear analyses and starts to grow rapidly with channel width, is identified with a sufficient accuracy for a wide range of gas expansion coefficients. The critical channel width for the Darrieus-Landau instability development depends on the gas expansion coefficient non-monotonically. The second critical channel width is found to decrease with the gas expansion coefficient, in a qualitative agreement with previous theoretical predictions.

Hip Torque Is a Mechanistic Link Between Sprint Acceleration and Maximum Velocity Performance: A Theoretical Perspective.

Sprinting performance is critical for a variety of sports and competitive activities. Prior research has demonstrated correlations between the limits of initial acceleration and maximum velocity for athletes of different sprinting abilities. Our perspective is that hip torque is a mechanistic link between these performance limits. A theoretical framework is presented here that provides estimates of sprint acceleration capability based on thigh angular acceleration and hip torque during the swing phase while running at maximum velocity. Performance limits were calculated using basic anthropometric values (body mass and leg length) and maximum velocity kinematic values (contact time, thigh range of motion, and stride frequency) from previously published sprint data. The proposed framework provides a mechanistic link between maximum acceleration and maximum velocity, and also explains why time constant values (τ, ratio of the velocity limit to acceleration limit) for sprint performance curves are generally close to one-second even for athletes with vastly different sprinting abilities. This perspective suggests that specific training protocols targeted to improve thigh angular acceleration and hip torque capability will benefit both acceleration and maximum velocity phases of a sprint.

Development of a Coated Fabric Armour System of Aramid Fibre and Rubber

Ballistic impact tests were conducted on polymer-coated multi-layer fabric targets using 0.357 SIG projectiles. The effect of fatigue loading on the yarn pull-out force of coated Twaron fabrics was studied for the first time. A validated mesoscale numerical modelling approach was developed to simulate the ballistic impact. The elastomeric coating significantly improved impact resistance. The coating is stable in a wide range of ambient temperatures and performs well upon being subjected to fatigue. A coated target having 6.6% less areal density and 20% less thickness than its neat counterpart has shown similar ballistic limit velocity.

High-velocity projectile impact response of rubber-coated aramid Twaron fabrics

The impact response of aramid Twaron CT709 fabrics thinly coated with a commercially available spray-on rubber material has been studied for the first time. Both neat and rubber-coated fabric targets were subjected to impact using 12 mm, 7.05 g spherical steel projectiles. The impact velocities ranged from 150 to 250 m/s. Each target consisted of three plies of neat or coated fabrics. Residual velocities, impact velocities and transverse deformation profiles were recorded using a high-speed camera, and the mechanism of failure and energy absorption were studied. It was observed that the ballistic limit velocity (BLV) increased by 18% in the rubber-coated specimens compared with neat fabric specimens. A critical velocity (Vc) was established below which the coated targets absorbed higher energy than their neat counterparts. The rubber coating was found to promote a membrane effect with global deformation to failure leading to higher energy absorption than the neat counterparts. However, the coated samples absorbed lower energy for impact velocities greater than Vc. At these velocities, the rubber-coated specimens showed localised failure leading to lower energy absorption than the neat counterparts. A novel numerical modelling approach was proposed to simulate coated fabrics, and the models predicted the BLV with high accuracy (error < 4%).

A lattice hydrodynamic model integrating the velocity limit effect under V2X environment

The velocity limit signal often occurs because of bad traffic road infrastructure. Therefore, a novel lattice hydrodynamics model is provided by combining the velocity limit effect under V2X environment. The linear stability analysis reveals that the stability conditions are closely related to the velocity limit effect. Moreover, the evolution of density and hysteresis loop have been deeply explored in this paper via simulation. Interestingly, it is valuable to correctly consider the impact of velocity limit on traffic flow, which can play a positive role on traffic stability.

Experimental Tests and Numerical Simulations on the Ballistic Impact Response of a Highly Inhomogeneous Aluminium Foam.

A sandwich structure is a composite material consisting of thin skins encapsulating a cellular core. Such structures have proven to be excellent energy absorbents and are frequently found in various types of protection. Even so, few studies exist in the open literature on the response of the core material itself under extreme loadings such as blast and impact. Since a blast load is usually accompanied by numerous fragments, it is important to understand and be able to predict the ballistic impact resistance of the often highly inhomogeneous cellular core materials in design. In this study, the ballistic impact response of an aluminium foam with a complex cell structure has been investigated both experimentally and numerically. First, an extensive material test program involving compression tests on cubic specimens loaded in the thickness direction of the foam was carried out to reveal the mechanical properties of the material. In addition, several of the specimens were scanned before testing using X-ray Micro Computed Tomography (XRMCT) to map the multi-scale topology and morphology of the material. These data were later analysed to extract density-variation plots in many different material orientations. Second, ballistic impact tests were conducted using a gas gun where rigid spheres were launched towards aluminium foam plates, and the ballistic limit velocity and curve of the foam material were established. Finally, numerical simulations of both the material tests and the ballistic impact tests were carried out using LS-DYNA and different modelling approaches based on the XRMCT data. It will be shown that, independent of the modelling strategy applied, good agreement between the experimental impact tests and the numerical predictions can be obtained. However, XRMCT data are important if the final goal is to numerically optimise and improve the behaviour of inhomogeneous foams with respect to energy absorption, thermal isolation, or similar properties.

Kinematic Analyses of Metallic Plate Perforation by Penetrators with Various Nose Geometries

This study analyses kinematics of a metallic plate perforation by a penetrator with truncated ogive nose geometry to find solutions also to blunt, conical, ogive, and hemi-spherical nosed penetrators. Plugging, ductile hole enlargement, dishing, and petal forming failure modes are used in the analyses. Acceleration throughout perforation is calculated by using the related failure mode, analytical model, and the target-penetrator interaction geometry. Depending on the failure model; back lip and front lip formation during ductile hole enlargement, plug formation during plugging, and deflection of target plate during dishing is also analysed. Analyses are based on projectile’s equation of motion, momentum and energy equations, and projectile-target plate interactions. The analyses results for selected cases, with the impact velocity range 215-863 m/s, are compared with the test data. The residual velocity estimation for a strike velocity is close to the related test data with an error of 0.3-2.2 %, except for conical nosed penetrators at impact velocities approaching the ballistic limit velocity

Tunable optical topological transition of Cherenkov radiation

Approaches to generate and manipulate Cherenkov radiation (CR) are challenging yet meaningful. Optical topological transition (OTT) in novel materials and metamaterials is also promising for modern photonics. We study the OTT of CR in graphene-based hyperbolic metamaterials (GHMs) for the first time. In GHMs, conventional and hyperbolic CR can be switched when crossing the topological transition frequency. This frequency can be altered by metamaterial components and external optical elements. For instance, external ultrafast optical pumps cause an ultrafast OTT from the elliptical to the hyperbolic state. Then, hyperbolic CR can be excited by low-energy electrons by leveraging the excellent photothermal properties of graphene. Hyperbolic CR vanishes when the GHM returns to its original state. Furthermore, graphene nonlocality occurs when the electron velocity is low enough, corresponding to a large wave vector. Concretely, when the electron velocity approaches the Fermi velocity of graphene, a nonlocality-induced OTT modifies the plasmonic properties of the GHM and brings a new lower velocity threshold of hyperbolic CR. Therefore, hyperbolic CR can only be induced in a limited velocity range. These findings pave the way for understanding CR properties in active plasmonic metamaterials and may be applied to complex photonic and polaritonic systems.

Propagation dynamics of the solid–liquid interface in Ge upon ns and fs laser irradiation

Monitoring the laser-induced melting and solidification dynamics of Ge upon laser irradiation is an enormous challenge due to the short penetration depth of its liquid phase. In this work, real-time pump-probe experiments in combination with finite element calculations have been employed to investigate the melting and solidification dynamics of germanium upon ns and fs laser pulse irradiation (λ = 800 nm). Excellent agreement between experiments and simulations allowed us to indirectly determine additional time- and depth-dependent information about the transformation dynamics of germanium, including the thickness evolution of the molten layer, as well as its melting and solidification velocities for the two pulse durations for different fluences. Our results reveal considerable differences in the maximum thickness of the molten Ge superficial layers at sub-ablative fluences for ns and fs pulses, respectively. Maximum melt-in velocities of 39 m s−1 were obtained for ns pulses at high fluences, compared to non-thermal melting of a thin layer within 300 fs for fs pulses already at moderate fluences. Maximum solidification velocities were found to be 16 m s−1 for ns pulses, and up to 55 m s−1 for fs pulses. Weak signs of amorphization were observed for fs excitation, suggesting that the lower limit of solidification velocities for a complete amorphization is above 55 m s−1. In addition, we show high precision measurements of the melt-in velocities over the first 20 nm by means of fs microscopy with sub-ps temporal resolution. Here, differences of the melt-in process of several orders of magnitude were observed, ranging from virtually instantaneous melting within less than 2 ps even for a moderate peak fluence up to 200 ps for fluences close to the melting threshold.

Campo del Cielo modeling and comparison with observations: I. Atmospheric entry of the iron meteoroid

AbstractThis paper presents an attempt to reconstruct the Campo del Cielo (CdC) impact event, that is, to estimate the preatmospheric mass and velocity of the iron meteoroid and pre‐impact parameters of its fragments allowing formation of funnels and impact craters. The goal of this study is to improve the understanding of the effects small‐scale iron meteoroids can have on the Earth's surface. We model the meteoroid's atmospheric flight taking deceleration, ablation, and fragmentation into account, and then compare the results with available observations. We found that a fragment's velocity near the surface should be &lt;1 km s−1 in order to form a funnel with an intact meteorite inside. The estimates of preatmospheric (at an altitude of 100 km) parameters of the CdC impact event are as follows: minimal mass of 7500–8500 t, which corresponds to a diameter range of 12.2–12.8 m; maximum entry angle above the atmosphere of ~16.5° and velocities of 14.5–18.4 km s−1, which is close to the one most frequently reached by Near‐Earth objects (NEOs). Near the surface, the largest fragments with a mass of 400–1500 t and velocities of 4–7 km s−1 form impact craters whereas fragments with a mass &lt;31 t and velocities &lt;1 km s−1 form funnels. Masses &lt;3 t are not included in our simulations. Their total mass is 280–460 t at the point of disruption but &lt;110 t on the Earth's surface. These numerous small fragments are dispersed over a large area and are very popular among meteorite hunters and dealers. In spite of all the observed crater location/size data and impactor velocity limits from the models, there are far more free parameters than constraints. As a result, any values for preatmospheric mass, velocity, and entry angle are merely representative or limitative as opposed to true values.

Behavior Decision Model With Situation Assessment for Intelligent Vehicles Based on Vehicle-to-Everything Information

Under the application of cooperative vehicle infrastructure technology, vehicles can drive more safely and efficiently in urban traffic environments. This paper presents a behavior decision model composed of a situation assessment module and decision process module to make a decision on lane selection and velocity control. Different common traffic elements are considered, such as surrounding obstacles, traffic lights, lane lines, and road velocity limits, to ensure the model outputs safe and legal behaviors. A green-wave passage strategy is also proposed to allow vehicles to pass through traffic intersections at a smoother velocity and to improve driving efficiency. To verify the correctness and rationality of the model, a Prescan/Simulink co-simulation platform including multiple common urban driving scenarios is established. The results show that the model can output correct behaviors with safety and legitimacy in different scenarios and realize vehicles meeting at intersections, vehicle-following, queuing at intersections with traffic lights, lane-changing for obstacle avoidance, and so forth. Compared with the general decision model without the green-wave passage strategy, the driving time is reduced by an average of 8.53%, and the standard deviation of the driving velocity is reduced by an average of 65.42%, which means that the model can reduce the variation of velocity and improve driving efficiency. The model has a certain practical application value.

Features and aspects of radioactive flow and slippage velocity on rotating two-phase Prandtl nanofluid with zero mass fluxing and convective constraints

The communication aims to analyze the conduct of rotating Prandtl nanofluid with the inclusion of effects like MHD, variable species diffusivity, nonlinear warm radiation, convective limit conditions, velocity slip and chemical reaction on fluid moving along a stretchable surface. Buongiorno model is implemented in orde to catch the Brownian movement and thermophoresis impacts by the presence of nanofluids. The effect of viscous dissemination and Joule heating is used in the current review. Using suitable similarity variables, the governing differential conditions with respect to momentum, energy, and concentration are improved into customary, ordinary differential equations and these equations numerically with the shooting technique. The ranges of different dimensionless parameters used in our study are Prandtl fluid parameter 0 ≤ α ≤ 3.5, elastic parameter 1 ≤ β ≤ 7, thermal radiation 1 ≤ Rd ≤ 2, temperature ratio parameter 1 ≤ θw ≤ 1.6, Eckart number 0.3 ≤ Ec ≤ 0.7, velocity slip parameter 0 ≤ γ1 ≤ 1, thermophoresis parameter 0.1 ≤ Nt ≤ 4, Brownian motion parameter 0.1 ≤ Nb ≤ 1.5, Prandtl number 1.7 ≤ Pr ≤ −2.6, magnetic parameter 0.1 ≤ M ≤ 0.8, chemical reaction parameter 0.2 ≤ δ ≤ 1, convection parameter 0.3 ≤ γ2 ≤ 1.1, rotational parameter 0.1 ≤ λ ≤ 0.7, Schmidt number 0.1 ≤ Sc ≤ 0.7, variable species diffusivity 0.1 ≤ ϵ ≤ 0.5 which shows up during mathematical arrangement are shown as tables and charts. These results are enhanced with the data for the heat transfer rate and skin friction coefficients. It is seen that the temperature profile enhanced on account of an increase in temperature convection boundary and mass fraction field raises due to an improvement in species diffusivity parameter.

An improvement on the Florence analytical equation for predicting the ballistic limit velocity of ceramic–aluminum targets and its development to ceramic–aluminum–polyurea panels

An improvement on the Florence analytical equation for predicting the ballistic limit velocity of ceramic–aluminum targets and its development to ceramic–aluminum–polyurea panels

The spring-mass model and other reductionist models of bipedal locomotion on inclines.

The spring-mass model is a model of locomotion aimed at giving the essential mathematical laws of the trajectory of the center of mass of an animal during bouncing gaits, such as hopping (one-dimensional) and running (two-dimensional). This reductionist mechanical system has been extensively investigated for locomotion over horizontal surfaces, whereas it has been largely neglected on other ecologically relevant surfaces, including inclines. For example, how the degree of inclination impacts the dynamics of the center of mass of the spring-mass model has not been investigated thoroughly. In this work, we derive a mathematical model which extends the spring-mass model to inclined surfaces. Among our results, we derive an approximate solution of the system, assuming a small angular sweep of the limb and a small spring compression during stance, and show that this approximation is very accurate, especially for small inclinations of the ground. Furthermore, we derive theoretical bounds on the difference between the Lagrangian and Lagrange equationsof the true and approximate system, and discuss locomotor stability questions of the approximate solutions. We test our models through a sensitivity analysis using parameters relevant to the locomotion of bipedal animals (quail, pheasant, guinea fowl, turkey, ostrich, and humans) and compare our approximate solution to the numerically derived solution of the exact system. We compare the two-dimensional spring-mass model on inclines with the one-dimensional spring-mass model to which it reduces under the limit of no horizontal velocity; we compare the two-dimensional spring-mass model on inclines with the inverted-pendulum model on inclines towards which it converges in the case of high stiffness-to-mass ratio. We include comparisons with historically prevalent no-gravity approximations of these models, as well. The insights we have gleaned through all these comparisons and the ability of our approximation to replicate some of the kinematic changes observed in animals moving on different inclines (e.g. reduction in vertical oscillation of the center of mass and decreased stride length) underlines the valuable and reasonable contributions that very simple, reductionist models, like the spring-mass model, can provide.

Fetal weight projection model to define growth velocity and validation against pregnancy outcome in a cohort of serially scanned pregnancies.

ABSTRACTObjectiveFetal growth assessment is central to good antenatal care, yet there is a lack of definition of normal and abnormal fetal growth rate which can identify pregnancies at risk of adverse outcome. The aim of this study was to develop and test a model for defining normal limits of growth velocity which are specific to the fetal weight measurement interval.MethodsThe cohort consisted of 102 138 singleton pregnancies which underwent at least two third‐trimester measurements of ultrasound estimated fetal weight (EFW), usually carried out because routine early‐pregnancy risk assessment had indicated an increased risk of fetal growth restriction. We projected the EFW from the first of each consecutive measurement pair along its own centile rank to the gestational age of the second scan. Normal growth was defined as the second EFW being within a weight range based on limits derived by partial receiver‐operating‐characteristics‐curve (pROC) analyses for small‐for‐gestational‐age (SGA; < 10th centile) and large‐for‐gestational‐age (LGA; > 90th centile) birth weight. The limits were measurement‐interval specific and calculated for a fixed false‐positive rate (FPR) of 10%. The resultant normal, slow and accelerated growth rates calculated from consecutive EFW pairs were evaluated against the following predefined perinatal outcome measures: stillbirth, neonatal death, SGA and LGA at birth, 5‐min Apgar score < 7 and admission to the neonatal intensive care unit. Slow growth based on the last two scans was compared with SGA fetal weight (EFW < 10th centile) at the last scan and association with stillbirth risk was assessed, expressed as relative risk (RR) with 95% CI.ResultsThe optimal cut‐off limits for normal growth rate between consecutive scans varied according to the length of the measurement interval, with an average of –8.0% for slow growth and + 9.3% for accelerated growth at a fixed FPR of 10%. Slow growth between random consecutive scan pairs was associated significantly with all predefined outcome measures including stillbirth (RR, 2.19; 95% CI, 1.84–2.53) and neonatal death (RR, 2.28; 95% CI, 1.60–3.13). Accelerated growth was associated with LGA at birth (RR, 2.15; 95% CI, 2.10–2.20), while normal growth was protective of all adverse outcome measures. Slow growth between the last two scans (which were performed at a median gestational age of 33 + 1 to 36 + 4 weeks) and SGA at the last scan were each predictors of stillbirth, and stillbirth risk was highest when both were present (RR, 2.65; 95% CI, 1.67–4.20). However, 66.2% of pregnancies with slow growth were not SGA at the last scan and these cases also had an increased risk of stillbirth (RR, 2.07; 95% CI, 1.40–3.05).ConclusionFetal growth velocity defined by projected, measurement‐interval specific fetal weight limits is associated independently with perinatal outcome and should be used for antenatal surveillance in addition to assessment by fetal size. © 2022 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Recurrent neural networks as kinematics estimator and controller for redundant manipulators subject to physical constraints

Redundant manipulators could be efficient tools in industrial production as a result of their dexterity. However, existing kinematic control methods for redundant manipulators have two main disadvantages. On one hand, model uncertainties or unknown kinematic parameters may degrade the performance of existing model-based control methods subject to joint limits. On the other hand, existing model-free control methods ignore the existence of joint limits although they do not need to know kinematic models of redundant manipulators. In this paper, a quadratic programming (QP) scheme is elaborated to achieve the primary tracking control task of redundant manipulators as well as joint limits avoidance task. Besides, a gradient neurodynamics (GND) model is utilized to estimate the kinematics of redundant manipulators. Then, a primal dual neural network, which is employed to solve the QP problem, and the GND model are integrated towards developing a model-free control method constrained by joint angle and velocity limits for redundant manipulators. The visual sensory feedback is fed to the two neural networks. The efficacy of the proposed control method is demonstrated by extensive simulations and experiments, and the merits of the proposed method are also substantiated by comparisons.

Asynchronous Double-Frame-Exposure Binocular-Camera With Pixel-Level Pipeline Architecture for High-Speed Motion Tracking

The synchronous double-frame exposure (S-DFE), also called frame-straddling, has been widely applied for commercial video-based velocity measurements to provide high-speed velocimetry-flow images. Nevertheless, no low-cost, high-speed cameras offer the video quality that researchers, scientists, and industry demand. This brief presents an asynchronous DFE (A-DFE) method with a binocular camera for motion estimation, which captures double frames with the scalable time interval of the dual generic cameras. This time interval can significantly improve the upper limit of displacements of the motion, which is commonly solved by an expensive high-speed camera. A dedicated hardware architecture is designed to implement the perspective transformation and Wallis filter to eliminate the two cameras’ position mismatch and brightness variance. We build a high-speed Lucas-Kanade optical flow estimation system with A-DFE on the XILINX Spartan-7 FPGA platform with MT9V034 CMOS image sensors. The shortest time interval can reach 1.06 ns, which means a significant improvement of the upper limit of the measurement velocity. Except for the image sensor, the compact, fully parallel architecture can attain energy efficiency with 3.23 nJ/pixel on a low-cost FPGA device.