Acceleration Stage Research Articles

ARIA—A VUV Beamline for EuPRAXIA@SPARC_LAB

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN. The electron beam driving the FEL will be delivered by an X-band normal conducting LINAC followed by a plasma wakefield acceleration stage. It will be characterized by a small footprint and will deliver ultra-bright photon pulses for experiments in the water window to the user community. In addition to the soft-X-rays beamline already planned in the project, we propose the installation of a second photon beamline with seeded FEL pulses in the range between 50 and 180 nm. Here, we will present the FEL generation scheme, the layout of the dedicated beamline and the potential applications of the FEL radiation source in this low energy range.

Demonstration of kilohertz operation of hydrodynamic optical-field-ionized plasma channels

We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated \SI{1}{ms} apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of \SI{0.4}{kHz} for a period of 6.5 hours without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.

The Effect of Acceleration on the Separation Force in Constrained-Surface Stereolithography

Constrained-surface-based stereolithography has recently attracted much attention from both academic and industrial communities. Despite numerous experimental, numerical and theoretical efforts, the fundamental need to reduce the separation force between the newly cured part and constrained surface has not yet been completely solved. In this paper, we develop a fluid dynamics approach, proposed in our previous work, to theoretically model the separation force in 3D printing of a cylindrical part for flat and patterned windows. We demonstrate the possibility of separation force reduction with an accelerated movement of the printing platform. In particular, we investigate behaviors of transient parameter, its reduction rate, and separation force reduction with respect to elevation speed and time. The proposed approach involves deceleration and acceleration stages and allows to achieve the force reduction for the entire printing process. Finally, we provide implicit analytical solutions for time moments when switching between the stages can be done without noticeable increase of separation force and explicit expression for separation force in case of patterned window.

Analysis of Swarm Satellite Magnetic Field Data for the 2015 Mw 7.8 Nepal Earthquake Based on Nonnegative Tensor Decomposition

A non-negative tensor decomposition (NTD) approach has been developed to analyze the ionospheric magnetic field data of the Swarm Alpha and Charlie satellites for the Mw7.8 2015 Nepal earthquake. All available satellite data were analyzed regardless of geomagnetic activity. We used the amplitude time-frequency spectra of the two-satellite data to build third-order tensors and decomposed them into three components. One of these components seems to be more affected by seismicity. In particular, the cumulative number of anomalous tracks of this component displays accelerated growth that conforms to a Sigmoid fit from 60 to 40 days before the mainshock. Subsequently, until 10 days before the earthquake, it shows a weak accelerating trend that obeys a power-law behavior and then resumes linear growth after the mainshock. Moreover, the cumulative anomaly was indicated not to be caused by geomagnetic activity, solar activity or other non-seismic factors. An investigation of the foreshocks around the epicenter reveals that the cumulative Benioff strain also exhibited two accelerated growths before the mainshock, which is consistent with the cumulative result of ionospheric anomalies. In the first acceleration stage, seismicity appeared in the region surrounding the epicenter, and most of the ionospheric anomalies were offset away from the epicenter. During the second acceleration stage, some foreshocks occurred closer to or on the mainshock fault, and ionospheric anomalies also appeared near two faults around the epicenter. Furthermore, the correspondence between the ionospheric anomalies and the anomalies in different geo-layers can be explained by the lithosphere–atmosphere–ionosphere coupling model.

Numerical and Experimental Study on Vertical Takeoff and Separation of the Compound Aircraft

The traditional technical solution to achieve vertical take-off of the fixed-wing aircraft is adding power components, which will inevitably increase the weight of the aircraft. This paper proposes a compound flight scheme to achieve vertical take-off for the fixed-wing aircraft, using the carrier aircraft with vertical take-off capability to carry the target aircraft. Compared with the vertical take-off performance of a single aircraft, the present scheme enables the target aircraft to have a larger carrying capacity and longer flight distance. In order to verify the feasibility of the current scheme, this paper first investigates the steady aerodynamic performance of the compound aircraft and the dynamical separation process by computational fluid dynamics simulation. The steady lift coefficient of the compound aircraft during the acceleration stage and the unsteady aerodynamic force during the separation stage are obtained and analyzed. Next, an unmanned aerial vehicle (with vertical take-off and horizontal acceleration capabilities) is designed specifically, and the flight experiments of the vertical take-off and separation processes are conducted. Successful separations are achieved many times in the flight tests, which validate the feasibility of the present scheme.

Fracture mechanism of tight sandstone under high and complex 3-D stress compression: Insights from acoustic emission

Tight sandstone gas (TSG) reservoirs are developed in large buried depths and complex geo-stress field environments. During exploitation, the surrounding rock of a drill well is often damaged or can collapse under artificial engineering disturbances. Understanding the failure mechanism of tight sandstone under high and complex three-dimensional (3-D) stress states is essential for the safe and efficient exploitation of TSG. In this study, using the stress Lode angle ( θ σ ) as a variable, failure experiments of low porosity sandstone specimens under various 3-D stress paths are performed, and the mechanical responses (e.g., stress–strain behavior, strength, fracture pattern, and acoustic emission characteristics) are analyzed. The results show that as θ σ increases, the strength of the specimen as well as the deviatoric stress required its failure decrease linearly, whereas its brittleness increases. The failure of the specimens is primarily due to numerous micro tensile cracks and a few macro shear cracks. θ σ significantly affects the cracking mode during failure. Acoustic emission (AE) parameters show that the failure process can be categorized into three stages within the time-to-failure window, among which Stage 2 (acceleration stage) can be regarded as the precursor stage of the ultimate failure of the specimen. The descriptive statistical results of AE energy show that uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on the damage mode of the specimens. Under the high 3-D stresses, the multiple fracture surfaces formed inside the specimen are intertwined to present several “X”-shaped fracture pairs. These findings facilitate the understanding of the failure mechanism of rock mass surrounding the wellbore, and of significance in stability designing in well trajectory of TSG reservoir actual development. • Stress Lode angles significantly affect cracking mode of tight sandstone. • Uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on damage mode of tight sandstone. • Precursor stage of tight sandstone ultimate failure under complex three-dimensional stress conditions can be identified through acoustic emission signals.

EVIDÊNCIAS DA DISCRIMINAÇÃO SALARIAL DE GÊNERO EM DIFERENTES CONTEXTOS MACROECONÔMICOS NO BRASIL

This article aims to identify wage inequalities and the effect of gender discrimination in the acceleration (2004-2008) and economic deceleration phases (2011-2014) in Brazil. Based on the data from the PNAD, the counterfactual decomposition of income is performed, and the results show a reduction in the income gap between men and women between 2004 and 2014, however, the wage differences and discrimination against women persist and occur independently of the economic performance of the country.  In the stage of economic acceleration to wage divergences and discrimination against women they are resurgence; the wage convergence and reduction of discrimination against women are evidenced in the economic slowdown. Among white workers, economic discrimination prevails, compared to non-white workers, with a sharp reduction in skin color discrimination in the period of slowdown in economic growth in the country.

Impacts of the Lewis and Markstein numbers on premixed flame acceleration in channels due to wall friction

The effects of flame stretch as well as that of thermal and molecular diffusion on the scenario of flame acceleration in channels are quantified by means of computational and analytical endeavors. The analytical formulation incorporates the internal transport flame properties into the theory of flame acceleration due to wall friction by means of the Markstein number, which characterizes the flame response to curvature and stretch. Being a positive or negative quantity and a function of the thermal-chemical combustion parameters, such as the thermal expansion ratio as well as the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes flame acceleration. While the Markstein number may provide a substantial impact on the flame acceleration rate in narrow channels, this effect diminishes with increase in the channel width. The analytical formulation is accompanied by extensive computational simulations of the reacting flow equations, which clarify the impact of the Lewis number on flame acceleration. It is noted that for Lewis numbers below a certain critical value, at the initial stage of flame acceleration, a globally convex flame front splits into two or more finger-like segments, accompanied by a drastic increase in the flame front surface area and associated enhancement of flame acceleration. Later, however, these segments of the flame front meet, promptly consuming cavities and pockets, which substantially decreases the flame surface area and moderates acceleration. Eventually, this dynamics results in a single, globally convex flame, which keeps accelerating. Overall, the thermal-diffusive effects substantially facilitate flame acceleration, thereby advancing a potential deflagration-to-detonation transition.

Tensile damage evolution of 2D SiC(f)/BN(i)/(SiC-B4C)(m) composites based on acoustic emission pattern recognition

The tensile test of 2D SiC (f) /BN (i) /(SiC-B 4 C) (m) , with acoustic emission (AE) online monitoring, was carried out to study the mechanical properties and failure mechanism. Using the pattern recognition technique to find out the relationship between AE signals and damage mechanisms, an improved sentry function was proposed by choosing the AE energy of fiber breakage instead of the AE energy in its definition. Combined with the cumulative trend of each class of the AE signals, the damage evolution process was identified. It is found that the entire loading process includes four stages: (a) the linear elastic stage, there have sporadic AE events; (b) the initial damage stage, the matrix cracking contributes 80.4% of the AE energy and leads to the non-linear behavior of the stress-displacement curve; (c) the damage development stage, in which all types of damage continuously happen; and (d) the damage acceleration stage, dominated by fiber breakage that accounts for 60.1% of the AE energy. The improved sentry function has a decreasing trend during the fourth stage, which provides an early warning before failure, and gives a reliable ultimate strength.

DIMENSIONS OF URBANIZATION AND POPULATION TRENDS IN NAGAUR CITY

This paper attempts to analyse the trends of urbanization based on three-decade census data during 1991, 2001, and 2011 in Nagaur city. Various dimensions of urbanization have been studied through charts and diagrams. For this purpose, the urban parameters such as the growth rate of the urban population, gender dynamics, literacy rate, density, work, and economic structure of the Nagaur city have been examined. Results show that in these last three decades the urban structure of Nagaur city has been changed due to rapid industrialization and rural to urban migration. Urbanization in Nagaur has been relatively slow compared to many developing megacities. As per data released by Govt. of India, Nagaur is an Urban Agglomeration coming under the category of Class I UAs/Towns. According to trends, Nagaur is at the acceleration stage of the process of urbanization. Rapid urbanization raises many issues that might have both positive and negative impacts on the environment. In this situation, monitoring urbanization is vital for planners, management, government, and non-government organizations for implementing policies to optimize the use of natural resources and accommodate development at the same time minimizing the impact on the environment.

Role of the radiative stage for cosmic ray acceleration in SNRs

ABSTRACT We consider diffusive shock acceleration in supernova remnants throughout their evolution including a radiative stage. It is found that a more efficient acceleration and fast exit of particles at the radiative stage results in the hardening of the source cosmic ray proton and electron spectra at energies ∼100–500 GeV. The effect is stronger for cosmic ray electrons.

Improving temporal stability of stable cavitation activity of circulating microbubbles using a closed-loop controller based on pulse-length regulation

Stable cavitation (SC) has shown great potential for novel therapeutic applications. The spatiotemporal distribution of the SC activity of microbubbles circulating in a target region is not only correlated with the uniformity of treatment, but also with some undesirable effects. Therefore, it is important to achieve controllable and desirable SC activity in target regions for improved therapeutic efficiency and biosafety. This study proposes a closed-loop feedback controller based on pulse length (PL) regulation to improve the temporal stability of SC activity. Microbubbles circulating in a physiological flowing phantom were exposed to a 1 MHz focused transducer. The SC signals produced were initially received by another 7.5 MHz plane transducer, followed by high-speed signal acquisition and real-time processing. Based on the real-time-measured SC intensity excited by the current acoustic pulse, the proposed closed-loop feedback controller used three proportional coefficients to regulate the peak negative pressure (PNP) and PL of the next acoustic pulse during the acceleration and stable stages, respectively. The results show that the rise time and the temporal stability of the SC intensity of the microbubbles circulating in these two stages were improved significantly by the optimized proportional coefficients used in the proposed controller. Importantly, when compared with the traditional closed-loop feedback controller based on PNP regulation, the proposed closed-loop feedback controller based on PL regulation reduced the probability of a transition between stable and inertial cavitation, thus avoiding the risk of disadvantageous bioeffects in practical applications. These results demonstrate the effectiveness of the proposed PL-based closed-loop feedback controller and provide a feasible strategy for realization of controllable cavitation activity in applications.

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.

A Survey on Internet of Vehicles Applications, Architecture, Communication Models and Challenges

To solve the problems of long shifting time and low transmission efficiency of motor transmission in unmanned electric vehicles, the performance test of motor transmission is carried out. The EMT transmission system is selected as the test object of the motor transmission of an unmanned electric vehicle. The gear number of the transmission is determined with the aim of reducing the speed requirement of the motor when the motor runs at high speed and working in the high-efficiency area near the base speed as far as possible. According to the gear speed and torque of the load motor, the driving moment and torsion moment of the drive motor in the acceleration stage are determined based on the total inertia of the drive motor and other parameters. Finally, the test bench of the motor transmission towing is set up and the test is carried out. The experimental results show that when the rated torque of the motor transmission is 100% at 1600r/min shift speed point and 150% at 1800r/min shift speed point, the minimum shift time of the motor transmission is 425ms. Meanwhile, the transmission efficiency of the motor transmission of the unmanned electric vehicle is between 90% and 92% and the transmission performance is high.

Experiment on hydration exothermic characteristics and hydration mechanism of sand-based cemented paste backfill materials

Cemented paste backfill technology can effectively control overburden movement, protect surface buildings and ecological environment. In this study, the early hydration exothermal characteristics, compressive strength evolution and hydration mechanism of sand-based cemented paste backfill materials (SCPB) were studied by hydration heat tests, uniaxial compressive strength tests, x-ray diffraction, scanning electron microscope and energy dispersive spectrometer tests; the hydration model of SCPB and fly ash were also constructed. The results show that: (1) the early hydration exothermic process of the SCPB can be divided into five stages: Dissolution stage, Induction stage, Acceleration stage, Deceleration stage and Slow deceleration stage. The heat release rates in the five stages are 5.64, 2.09, 3.63, 3.16 and 1.07 J·(g·h)-1, respectively; (2) the mechanical strength and elastic modulus of the SCPB increase with the increase of curing age; when the curing age of 7 d, the elastic modulus of the SCPB reaches 67.92% of the final value; (3) the primary hydration products of the SCPB are calcium silicate hydrate, ettringite, calcium hydroxide, Calcium carbonate and Calcium aluminate hydrate; (4) the hydration process of the SCPB can be divided into Dissolution stage, Hydration stage and Hardening stage, and the hydration process of fly ash experienced Adsorption stage and Hydration stage.

Simulation of the plume of a magnetically enhanced plasma thruster with SPIS

A three-dimensional fully kinetic particle-in-cell (PIC) simulation strategy has been implemented to simulate the acceleration stage of a magnetically enhanced plasma thruster (MEPT). The study has been performed with the open-source code Spacecraft Plasma Interaction Software (SPIS). The tool has been copiously modified to simulate properly the dynamics of a magnetized plasma plume. A cross-validation of the methodology has been done with Starfish, a two-dimensional open-source PIC software. Two configurations have been compared: (i) in the absence of a magnetic field and (ii) in the presence of a magnetic field generated by a coil with maximum intensity of 300 G at the thruster outlet. The results show a reduction of the plume divergence angle, an increase of ion speed and an increase of the specific impulse in the presence of the magnetic nozzle. The simulations presented in this study are representative of the operative conditions of a 50 W MEPT. Nonetheless, the methodology adopted can be extended to handle the magnetized plasma plume of several other types of thrusters such as electron cyclotron resonance and applied field magnetoplasmadynamic thrusters.

Deformation damage and acoustic emission characteristics of red sandstone under fatigue–creep interaction

Most engineering rock masses are actually under the interaction of fatigue load and creep load. Currently, the understanding of the fatigue or creep characteristics of rock is relatively mature, but the damage and failure characteristics of rock under fatigue–creep interaction (FCI) are rarely studied. In this study, FCI loading tests were carried out on red sandstone with prefabricated cracks at different angles. The results showed that the elastic modulus tended to be stable with the loading. While the deformation modulus and secant modulus decreased nonlinearly, and a relatively significant decrease occurred after the creep loading. The strain evolution could be divided into deceleration stage (P1), stable stage (P2) and acceleration stage (P3). Creep loading would increase the energy dissipation of the sample, thereby accelerating the accumulation of rock damage. A macro–micro coupling damage model was established, which reflected the damage evolution and strength variation of samples. In addition, acoustic emission (AE) was mainly generated in the fatigue loading stage, and the AE level increased with the increase of the crack angle. The crack propagation process of samples with different angles of crack was analyzed by b value of AE, and the failure modes of samples were further discussed. The research results have certain theoretical reference value for understanding the damage and failure characteristics of fractured rock mass under interaction.

The influence of gas pore pressure in dense granular flows: numerical simulations versus experiments and implications for pyroclastic density currents

We investigate the influence of gas pore pressure in granular flows through numerical simulations on horizontal and low-angle inclined surfaces. We present a two-phase formulation that allows a description of dam-break experiments considering high-aspect-ratio collapsing columns and depth-dependent variations of flow properties. The model is confirmed by comparing its results with data from analogue experiments. The results suggest that a constant, effective pore pressure diffusion coefficient can be determined in order to reproduce reasonably well the dynamics of the studied dam-break experiments, with values of the diffusion coefficient consistent with experimental estimates from defluidizing static columns. The discrepancies between simulations performed using different effective pore pressure diffusion coefficients are mainly observed during the early acceleration stage, while the final deceleration rate, once pore pressure has been dissipated, is similar in all the studied numerical experiments. However, these short-lasting discrepancies in the acceleration stage can be manifested in large differences in the resulting run-out distance. We also analyze the pore pressure at different distances along the channel. Although our model is not able to simulate the under-pressure phase generated by the sliding head of the flows in experiments and measured beneath the flow-substrate interface, the spatio-temporal characteristics of the subsequent over-pressure phase are compatible with experimental data. Additionally, we studied the deposition dynamics of the granular material, showing that the timescale of deposition is much smaller than that of the granular flow, while the time of the deposition onset varies as a function of the distance from the reservoir, being strongly controlled by the surface slope angle. The simulations reveal that an increment of the surface slope angle from 0° to 10° is able to increase significantly the flow run-out distance (by a factor between 2.05 and 2.25, depending on the fluidization conditions). This has major implications for pyroclastic density currents, which typically propagate at such gentle slope angles.

Alternative approaches to the reaction rate modelling in gas explosion simulation using STOKES

Computational fluid dynamics (CFD) has been increasingly applied to process safety studies involving scenarios of accidental gas explosions. By offering a deeper understanding of the underlying physics of flame propagation, CFD can improve existing consequence analysis and risk assessment, particularly when confinement or obstructions are present. In the numerical modelling of gas explosions, the reaction rate is a quantity of fundamental importance because it ultimately leads to the values of flame speed and peaks of overpressure. In this work, we investigate the reaction rate models implemented in the CFD tool STOKES, which has been designed for the simulation of gas explosions in complex geometries. STOKES counts on the Bray–Moss–Libby (BML) reaction rate model, where the effects of flame stretch are neglected in its original version, which may lead to an inaccurate representation of the flame behaviour. Also, the length scale of wrinkling is often calculated as a function of the velocity fluctuations via empirical correlations that depend on adjustable constants. In a first approach, we propose an expression for calculating the stretch factor dynamically, based on the local divergence of velocity. In a second approach, a hybrid reaction rate model is presented, incorporating the well-known fractal approach into the BML model. The hybrid approach also counts on the calibration of parameters of the initial laminar burning model. Despite some improvement, the proposed dynamic stretch factor leads to unexpectedly enlarged flame contours. Results using the hybrid approach on the other hand improved agreement with experimental data, particularly in the early stages of flame acceleration when combustion chambers of relatively small sizes are considered. However, applications in real-scale semi-confined geometries indicate that the initial stages of flame development require further work.

Experimental Study on the Relationship between the Velocity of Surface Movements and Tilting Rate in Pre-Failure Stage of Rainfall-Induced Landslides.

With the development of deformation measuring technology at slope surfaces, prediction methods for rainfall-induced landslides based on the surface movements and tilting of slopes in the pre-failure stage have been recognized as a promising technique for risk reduction of slope failure triggered by rainfall. However, the correlation and possible mechanism for these prediction methods were rarely discussed. In this study, the comparison between the prediction methods of slope failure based on the time history of surface displacements and tilting in the acceleration stage was carried out by conducting a series of laboratory tests and field tests under rainfall, in which the movements and tilting behaviors at the slope surface were measured. The results show that the predicted failure time of tested slopes obtained by different prediction methods is consistent, and the correlation between these landslide prediction methods were also detected. A proportional relationship between the velocity of surface displacements and tilting rate was observed, and a possible mechanism for the sliding behavior was discussed based on this linear relationship as well. In addition, an expression for the linear relationship between the rate of the surface tilting and displacement was also established in this study, and the results could have significance for the understanding of the sliding behavior in the failure process in rainfall-induced landslides.