Velocity Spread Research Articles

Design and experiment of an inverse magnetron injection gun for a Ka-band gyro-TWT with non-superconducting magnet

A triode inverse magnetron injection gun (IMIG) for a Ka-band gyrotron traveling wave tube (gyro-TWT) with non-superconducting magnet is proposed in this paper. Nowadays, the applications of gyro-TWT are intended to expand to moveable and fast-startup systems, so room-temperature magnet and system miniaturization are necessary. For a Ka-band gyro-TWT, to adapt to a room-temperature magnet with 20 mm bore radius, the IMIG form is adopted. Compared with conventional MIG, the gun maximum radius is reduced by 44%. On one hand, a curved emitter and cathode steps are utilized for better beam quality. On the other hand, isolation gaps and a cooling structure are designed to suppress stray electrons. A velocity ratio of 1.14 and a transverse velocity spread of 2.43% are obtained finally. The stray electrons are analyzed, and the IMIG tolerance is also evaluated. Finally, the cathode is fabricated, and its surface morphology is tested. It is then assembled into a gun shell, and the cathode temperatures are measured under both no cooling and cooling conditions. When the temperature of the emitter reaches 1050 °C, the heat powers are 84 and 115 W, respectively. The temperatures of the inner and outer electrodes are low enough to reduce the proportion of stray electrons.

Influence of Wind Direction on Fire Spread on High-Rise Building Facades

In order to study the influence of wind direction on fire spread on building facades, a three-dimensional model of a high-rise building was built with PyroSim software. Numerical simulations were conducted with five wind directions (0°, 45°, 90°, 135° and 180°) and a reference wind speed of 3 m/s. The results show that the fire spread on the building facade was most significant on the leeward side, followed by the upwind condition. The horizontal spread of fire was promoted by crosswinds at 45°, 90° and 135°, while its vertical spread was inhibited. Among these, the inhibitory effect of the 45° crosswind was the greatest. The fire spread area under the leeward condition was the greatest at approximately 341.3 m2, accounting for 17.1% of the facade, and the vertical spread velocity under the condition of no wind was the fastest at 0.075 m/s. In this study, the motion characteristics of facade fire spread under different wind directions were determined, providing supporting evidence to enhance fire safety prevention and control measures in high-rise buildings.

Effective and environment-friendly oil removal with microbubble jet

Effective removal of oil contaminants from solid surfaces is crucial for various applications. However, traditional cleaning methods often involve the use of hazardous chemicals. Because of their stability and ability to adsorb pollutants, microbubbles are considered as a promising alternative for environmentally friendly cleaning. This study explores the enhancement of oil cleaning efficiency using a microbubble water jet. The properties of the microbubbles generated via ultrasonic agitation are examined under varying water temperatures, ultrasonic amplitudes, and surfactant concentrations. The microbubble jet demonstrates approximately 29 % enhanced cleaning efficiency compared with a single-phase water jet. Visualization techniques, including particle image velocimetry, reveal that this enhancement is driven by the behavior of microbubbles consistently adsorbing oil through hydrophobic interactions as well as the instability and increased turbulence intensity of the microbubble jet. Additionally, the higher (by 7 %) spreading velocity of the microbubble jet indicates the free surface of bubbles results in a drag reduction. Consequently, this enables the jet to cover wider regions to be cleaned effectively. The present findings demonstrate the potential of microbubble jets as an effective and eco-friendly alternative for oil removal and offer insights for their practical implementation in diverse fields utilizing microbubbles.

Signal detection

Acoustic emission (AE) occurring in concrete fracture process is a combination of concrete fracture at local material level and the propagation of induced elastic waves at structural level. Simulation of the fracture-induced AE in concrete needs a model that is able to address the fracture behavior and the wave propagation at the same time. However, to date, no available modelling tools can simulate the whole fracture-induced AE process. Among others, the lattice models have the potential to achieve the simulation of the whole fracture-induced AE process. However, the lattice models are currently restricted to statistical analysis of AE events in fracture process. This paper investigates the feasibility and limitation of lattice model for the simulation of elastic wave propagation process. Several simulation cases are performed considering the propagation of both plane waves and point-source waves. The simulation results, in terms of wave velocity and geometric spreading loss of wave amplitudes, agree well with the analytical solutions. This study represents the first step for the simulation of complete fracture-induced AE waves in concrete.

Pressure and wall shear stress from high-speed droplet impact

We report on experimental and numerical results of the impact of millimeter sized droplet with high-speed projectile. The impact velocity Vimp ranges between 70m/s and 245m/s and is sufficiently high that the compressibility of the liquid becomes important. High-speed images reveal non-axisymmetric lamella spreading, hydrodynamic and secondary cavitation and the ejection of a thin jet from the distal side of the droplet. The spreading velocity is supersonic for the impact velocities tested. Secondary cavitation at the distal side of the droplet is found for Vimp≳ 120m/s. The experiments are compared and analyzed further with a CFD model based on a compressible volume of fluid (VoF) method. Excellent agreement of the early droplet and lamella dynamics is obtained with the experimental data. Additionally, we provide quantitative data for the pressure loading and the shear stress on the projectile using spatio-temporal maps.

Support Vector Machine Framework for Detecting Ambiguous Contours of Space Debris Cloud

Operational spacecraft are prepared for collisions with space debris by installing protective walls (bumpers). A group of vaporized and liquefied microparticles generated by debris collisions is called a debris cloud. To evaluate the protective performance of bumpers, it is important to measure the spread velocity and angle of the debris cloud. However, debris cloud images exhibit ambiguous contours of debris clouds. Consequently, measurement errors in the spread velocity and spread angles occur because of the observer’s subjectivity. Commonly used derivative-based methods can detect steep contours; however, they cannot detect ambiguous contours such as those of debris clouds. In this study, a method for uniquely detecting the ambiguous contours of a debris cloud based on supervised machine learning and a continuous wavelet transform (CWT) is proposed. Feature points that indicate candidate points of the debris cloud contour are extracted using a method based on CWT. Four types of specific features for debris cloud contours were assigned to the feature points. Supervised machine learning using a support vector machine can detect the debris cloud contours. The proposed method enables the evaluation of the ambiguous contours of a debris cloud and the protection performance of a bumper without observer subjectivity.

Enhanced propagation of free films with fast spread-out phenomena under the influence of megahertz surface acoustic waves

In this study, a novel approach for enhancing the rapid spreading of free liquid film on lithium niobate (LiNbO3) substrate under megahertz (MHz) surface acoustic wave (SAW) excitation is presented by treating it with surfactants. Through the design of a specific interdigital transducer structure, it was discovered that exciting the SAW at a frequency of 32.3 MHz can achieve optimal spreading performance for water droplets on the surface of surfactant-treated LiNbO3 substrate. The maximum average velocity reaches 1.76 mm/s at position P2 = 1250 μm in the water film front, and the stable film spreading speed shows a 204.9% increase compared to the existing research. Simultaneously, through the investigation of the spreading experiment phenomenon of silicone oil and de-ionized water droplets at varying frequencies, we have discovered the dynamic mechanism of “reverse phase” propagation in liquid film for the first time. This entails that the advancing edge of the wetting film demonstrates a spreading motion law that is opposite to the traditional spreading phenomena, with the spreading velocity in the central exceeding that on both sides. Our research demonstrates that this microfluidic device developed by SAWs enhances the spreading efficiency of the free films, enabling rapid expansion of the target liquid to form a high-surface area film layer. This advancement holds promise for overcoming the limitations of low sensitivity and short response time in the field of rapid pathological diagnosis in contemporary medicine.

Decay characteristics and application predictions of far-field high-pressure water: Jet velocity and volume fraction

In the field of deep mining engineering, high-pressure water jets progress toward larger diameters and higher velocities to enhance their impact performance. Understanding the decay characteristics as well as their implications remains challenging. In this article, the jet spread coefficients (k and c) within the empirical model of jet diffusion are determined by series of lab experiments, while simulations using the Eulerian method are conducted by implementing the modified diffusion model as a subroutine. The effects of spread coefficient, jet velocity, and nozzle diameter on the degree of jet decay are studied. The results show that the jet spread coefficient increases logarithmically with increasing jet velocity. With the increase in nozzle diameter, the growth of spread coefficient k of the jet gradually decreases. Increasing the nozzle diameter and spread coefficient k can effectively reduce the decay degree of jet axial velocity and water volume fraction. Notably, although the increase in jet velocity does not impact the decay of axial velocity, it exacerbates the decay of water volume fraction. Similarly, the increase in spread coefficient c has no effect on the reduction of water volume fraction, but it intensifies the decay of jet axial velocity. The combined effects of increased jet velocity and jet spread coefficient weaken the degree of jet decay. The research presents a comprehensive and innovative study of jet diffusion and attenuation phenomena. These insights not only expand the application of jet diffusion models but also provide theoretical support for understanding and optimizing the application of jets.

Confirming the explosive dispersal outflow in DR21 with ALMA

We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum and CO (2–1) line emission observations toward the high-mass star formation region DR21. Five new continuum sources are found. We identify 18 outflow streamers detected in CO emission that radially arises from a common origin. The velocity spread of the outflow streamers ranges between −100 and +70 km s−1. The radial velocities of each outflow roughly follow linear gradients (Hubble–Lemaître–like expansion motions). Using the CO emission of the whole ensemble of streamers, we estimate a total outflow mass of 120−210 M⊙. Additionally, we derived the dynamical age (8600 yr), momentum (~103 M⊙ km s−1), and kinetic energy (~1048 erg) of the outflow. The morphology and kinematics presented by the CO outflow streamers confirm an explosive dispersal outflow at the heart of DR21. Five dispersal explosive outflows associated with massive star-forming regions have been confirmed in our Galaxy (Orion BN/KL, G5.89-0.39, S106-IR, IRAS 16076-5134, and IRAS 12326-6245). However, their occurrence frequency in the Galaxy and their origin are still uncertain.

Effects of Fuel Bed Slope and Wind Direction on Flame Spread Characteristics Over a Bed of Pinus Silvestris Needles

ABSTRACT Important factors influencing the flame characteristics over a bed of dry pine needles from Siberian Boreal forests (Pinus silvestris) in lab-scale is provided. Factors such as slope of the fuel bed, wind velocity, and direction of spread (concurrent flow or opposed flow), which play significant roles in determining the flame spread rate, have been investigated. Systematic lab-scale experiments have been carried out in a wind tunnel with varying air velocities and on an inclined fuel bed in still air. Numerical simulations have been carried out using a three-dimensional physics-based flame spread model available in Fire Dynamics Simulator, and the results are validated against a wide range of measured flame characteristics. The validated numerical model is then used to simulate concurrent flow and opposed flow flame spread over a sloping fuel bed in the presence of wind. Gas phase temperature on and above the bed surface and total and radiative heat flux measurements for concurrent flow and opposed flow flame spread under the influence of wind as well as for upslope flame spread in still air are presented. A stationary flame propagation mode is observed for a flat fuel surface. Predicted temperature, flow, and species concentration fields have been reported for selected cases to understand the flow and flame dynamics in gas phase and within the fuel beds. These data serve as a validation of the numerical model used for simulating the experimental cases. Further, it can be used to validate popular fire models such as Fire Foam and Fire-Star 3D.

Large eddy simulation of round jets with mild temperature difference

Understanding the behaviour of hot jets is crucial for various engineering and environmental applications. The present work studies the influence of heat transfer on the dynamics of horizontal round hot jets through Large Eddy Simulations (LES). Our focus lies on trajectory development, large-scale coherent structures, and turbulent kinetic budget analysis in the near-field and intermediate-field regions. LES of two horizontal round hot jets with Reynolds numbers (3934 and 5100) and corresponding Froude numbers (32.98 and 17.07) were carried out using buoyantPimpleFoam solver in OpenFOAM, and the simulation on an isothermal jet was also performed as a baseline for comparison. The results reveal that the jet core temperature decays faster in the streamwise direction but more slowly in the radial direction, indicating a wider temperature spread than velocity, and the maximum difference between the temperature and velocity spread is about 0.5D. Moreover, the energy associated with the large-scale coherent structure decreases with increasing initial jet temperature. The energy of the first two modes of snapshot Proper Orthogonal Decomposition (POD) and extended POD dropped by 12% and 14%, respectively. The coherent motion with the greatest correlation between the temperature and velocity fluctuations is identified as four pairs of Q1 and Q3 events, which are Reynolds shear stress dominant events. Furthermore, compared with the isothermal jet, the turbulent kinetic energy budgets of the hot jets indicate that the diffusion and generation terms are both reduced by approximately 50%, suggesting a transfer of more kinetic energy into potential energy rather than turbulence. The finding highlights the potential of heightened temperatures to mitigate instabilities associated with large-scale motions in hot jets. This study fills the gap on a comprehensive analysis of heat transfer effects on jet dynamics, and quantitative insights into the large-scale coherent structures are provided, contributing to a better understanding of hot jet behaviour.

Neutral beam microscopy with a reciprocal space approach using magnetic beam spin encoding

The emerging technique of neutral beam microscopy offers a non-perturbative way of imaging surfaces of various materials which cannot be studied using conventional microscopes. Current neutral beam microscopes use either diffractive focusing or pin-hole scanning to achieve spatial resolution, and are characterised by a strong dependence of the imaging time on the required resolution. In this work we introduce an alternative method for achieving spatial resolution with neutral atom beams which is based on manipulating the magnetic moments of the beam particles in a gradient field, and is characterised by a much weaker dependence of the imaging time on the image resolution. The validity of the imaging approach is demonstrated experimentally by reconstructing one dimensional profiles of the beam which are in good agreement with numerical simulation calculations. Numerical simulations are used to demonstrate the dependence of the signal to noise on the scan resolution and the topography of the sample, and assess the broadening effect due to the spread of velocities of the beam particles. The route towards implementing magnetic encoding in high resolution microscopes is discussed.

Fission and fusion of heavy nuclei induced by the passage of a radiation-mediated shock in BNS mergers

ABSTRACT We compute the structure of a Newtonian, multi-ion radiation-mediated shock (RMS) for different compositions anticipated in various stellar explosions. We use a multifluid RMS model that incorporates electrostatic coupling between the different plasma constituents as well as Coulomb friction in a self-consistent manner, and approximates the effect of pair creation and the presence of free neutrons in the shock upstream on the shock structure. We find that under certain conditions a significant velocity separation is developed between different ions in the shock downstream and demonstrate that in fast enough shocks ion–ion collisions may trigger fusion and fission events at a relatively high rate. Our analysis ignores anomalous coupling through plasma microturbulence, which might reduce the velocity spread downstream below the activation energy for nuclear reactions. A rough estimate of the scale separation in RMS suggests that for shocks propagating in binary neutron star (BNS) merger ejecta, the anomalous coupling length may exceed the radiation length, allowing a considerable composition change behind the shock via inelastic collisions of $\alpha$ particles with heavy elements at shock velocities $\beta _\mathrm{ u}\gtrsim 0.25$. A sufficient abundance of free neutrons in the shock upstream, as expected during the first second after the merger, is also expected to alter the ejecta composition through neutron capture downstream. The resultant change in the composition profile may affect the properties of the early kilonova emission. The generation of microturbulence due to velocity separation can also give rise to particle acceleration that might alter the breakout signal in supernovae and other systems.

Analytical solutions for airborne droplet trajectory: Implications for disease transmission

Airborne droplets containing viruses from infected persons can present long range disease transmission risks. In this study, we examine the trajectory of an airborne droplet based on a point force model. The interplay between gravity, drag, inertia and deformation are factored in, for drop sizes ranging from micrometer to millimeter size. In particular, we propose an expression for the drag force which enables analytical solutions for cases of practical interest, such as the transient velocity behavior and spreading distances. This allows us to obtain physical insights which are not obvious from direct numerical simulations. Effects such as droplet deformation, breakup and evaporation are also considered. Our analytical solutions compare favorably with numerical and experimental data. The evaporation rate was determined experimentally with a levitating device and some experimental drop velocity versus time data were obtained with falling millimeter sized droplets.

Analysis of the ion collection model in the ExB probe

An ExB probe is a pass-band velocity filter for an ion beam. The ExB probe measurement is related to the ion velocity distribution function (IVDF); however, the finite pass-band filter window size leads to differences in the true IVDF and measured ExB probe spectrum. We derive for the first time an analytical ExB probe transmittancy matrix and use it to examine differences in the IVDF and the probe spectrum. The probe spectra are compared to a synthetically defined test IVDF to study how probe geometry and ion species affect the differences. It is found that the difference in the probe spectrum and the true IVDF is dependent on the ion species, and the deviation is larger for heavier ions. The peak velocity of the spectrum was shifted by up to 13%, and the velocity spread was broadened by up to 276%. The relative ion species fraction is calculated from the probe spectra by two different methods and compared to the true fraction. Using the approach of this work, direct integration of the spectrum resulted in a 2% difference, while a more common approach from literature overestimated one ion species by 184%.

GA-NIFS: The core of an extremely massive protocluster at the epoch of reionisation probed with JWST/NIRSpec

The SPT0311-58 system resides in a massive dark-matter halo at z sim 6.9. It hosts two dusty galaxies (E and W) with a combined star formation rate (SFR) of sim 3500 mostly obscured and identified by the rest-frame IR emission. The surrounding field exhibits an overdensity of submillimetre sources, making it a candidate protocluster. Our main goal is to characterise the environment and the properties of the interstellar medium (ISM) within this unique system. We used spatially resolved low-resolution ($R$=100) and high-resolution ($R$=2700) spectroscopy provided by the JWST/NIRSpec Integral Field Unit to probe a field of sim 17 times 17 kpc$^2$ around this object, with a spatial resolution of sim 0.5 kpc. These observations reveal ten new galaxies at zsim 6.9 characterised by dynamical masses spanning from sim 109 to 1010 and a range in radial velocity of sim 1500 in addition to the already known E and W galaxies. The implied large number density (phi sim $) and the wide spread in velocities confirm that is at the core of a protocluster immersed in a very massive dark-matter halo of sim (5 pm 3) times 1012 and therefore represents the most massive protocluster ever found at the epoch of reionisation (EoR). We also studied the dynamical stage of its core and find that it is likely not fully virialised. The galaxies in the system exhibit a wide range of properties and evolutionary stages. The contribution of the ongoing Halpha -based unobscured SFR to the total star formation (SF) varies significantly across the galaxies in the system. Their ionisation conditions range from those typical of the field galaxies at similar redshift recently studied with JWST to those found in more evolved objects at lower redshift, with OIII varying from sim 0.25 to 1. The metallicity spans more than 0.8 dex across the FoV, reaching nearly solar values in some cases. The detailed spatially resolved spectroscopy of the E galaxy reveals that it is actively assembling its stellar mass, showing inhomogeneities in the ISM properties at subkiloparsec scales, and a metallicity gradient (sim 0.1 dex/kpc) that can be explained by accretion of low metallicity gas from the intergalactic medium. The kinematic maps also depict an unsettled disc characterised by deviations from regular rotation, elevated turbulence, and indications of a possible precollision minor merger. These JWST/NIRSpec IFS observations confirm that is at the core of an extraordinary protocluster, and reveal details of its dynamical properties. They also unveil and provide insights into the diverse properties and evolutionary stages of the galaxies residing in this unique environment.

Influence of network structure on spreading dynamics via tie range

Abstract There are various phenomena of malicious information spreading in the real society, which cause many negative impacts on the society. In order to better control the spreading, it is crucial to reveal the influence of network structure on network spreading. Motifs, as fundamental structures within a network, play a significant role in spreading. Therefore, it is of interest to investigate the influence of the structural characteristics of basic network motifs on spreading dynamics. Considering the edges of the basic network motifs in an undirected network correspond to different tie ranges, two edge removal strategies are proposed, short ties priority removal strategy and long ties priority removal strategy. The tie range represents the second shortest path length between two connected nodes. The study focuses on analyzing how the proposed strategies impact network spreading and network structure, as well as examining the influence of network structure on network spreading. Our findings indicate that the long ties priority removal strategy is most effective in controlling network spreading, especially in terms of spread range and spread velocity. In terms of network structure, the clustering coefficient and the diameter of network also have an effect on the network spreading, and the triangular structure as an important motif structure effectively inhibits the spreading.

Impact of pilot diesel injection timing on performance and emission characteristics of marine natural gas/diesel dual-fuel engine

In diesel-ignited natural gas marine dual-fuel engines, the pilot diesel injection timing (PDIT) determines the premixing time and ignition moment of the combustible mixture in the cylinder. The PDIT plays a crucial role in the subsequent development of natural gas flame combustion. In this paper, four PDITs (− 8 °CA, − 6 °CA, − 4 °CA, and − 2 °CA) were studied. The results show that the advancement of PDIT increased the engine's power, thermal efficiency, and natural gas flame spread velocity, and increased NO emissions and CH4 emissions of the marine engine. The PDIT affected the ignition delay period and the rapid combustion period to a greater extent than the slow combustion period and the post combustion period. With each 2 °CA advancement of PDIT, the engine's power increased by 69.87 kW, thermal efficiency increased by 0.42%, radial flame spread velocity increased by 2 m/s, axial flame spread velocity increased by 1.7 m/s, NO emissions increased by 6.1%, and CH4 emissions increased by 3.75%.

Smoke diffusion mechanism and mitigation design for fire accidents in a tunnel-groove structure of a large hydropower station

In this study, field tests and numerical simulations were conducted to investigate the smoke diffusion mechanism and mitigation design for a fire accident occurring in a special hybrid structure, i.e., the tunnel-groove structure (TGS) of a large hydropower station. It is found that the characteristics of the smoke spread in the TGS include the smoke temperature attenuation, incomplete chimney effect and smoke entrainment. The combustion of the polyurethane flame-retardant material is controllable and extinguishes immediately after the fire source is removed. However, there is a fire risk in a relatively closed space under the conditions of strong winds, especially at the connection port with the external environment, due to the wind tunnel and chimney effects, which result in rapid smoke diffusions. During the fire accident, the smoke first fills in the entire tunnel (89 m long), then propagates upward along the groove, and finally reaches the dam top (115 m above the tunnel) in 10 s. The heat release rate (HRR) was determined as approximately 50 MW and the maximum velocity of the smoke spread along the dam surface could reach 10 m/s. As a result, nearly no time was available for the workers in the groove to evacuate when the fire accident occurred. Sensitivity analyses were further conducted to study the effects of different parameters, such as the HRR, the air inflow velocity and the radial gate opening, on the smoke diffusion in the TGS. A horizontal smoke plate (HSP) was proposed for the temporary smoke control to prevent life from losses due to the smoke diffusion in the TGS. The safe and dangerous zones near the dam surface were identified by analyzing the characteristics of the smoke temperature and visibility. It was finally concluded that the HSP can mitigate the smoke and heat along the groove. The results from this study may serve as a useful reference for the smoke control design in the TGS or other similar structure configurations.

On how structures convey non-diffusive turbulence spreading

We report on comprehensive experimental studies of turbulence spreading in edge plasmas. These studies demonstrate the relation of turbulence spreading and entrainment to intermittent convective density fluctuation events or bursts (i.e. blobs and holes). The non-diffusive character of turbulence spreading is thus elucidated. The turbulence spreading velocity (or mean jet velocity) manifests a linear correlation with the skewness of density fluctuations, and increases with the auto-correlation time of density fluctuations. Turbulence spreading by positive density fluctuations is outward, while spreading by negative density fluctuations is inward. The degree of symmetry breaking between outward propagating blobs and inward propagating holes increases with the amplitude of density fluctuations. Thus, blob-hole asymmetry emerges as crucial to turbulence spreading. These results highlight the important role of intermittent convective events in conveying the spreading of turbulence, and constitute a fundamental challenge to existing diffusive models of spreading.