Velocity Spread Research Articles

A numerical study on the packing quality of fibre/polymer composite powder for powder bed fusion additive manufacturing

ABSTRACT A discrete element model has been developed to simulate the packing process of fibre/polymer composite powder for powder bed fusion additive manufacturing. The geometric shapes of polymer powder particles and fibres are represented by multi-sphere particles and individual cylinders with round ends, respectively. The numerical model can help to understand the flow dynamics of composite powder particles and the formation mechanisms of voids in powder packing processes. The numerical model has been utilised to analyse the effects of packing parameters on the packing quality of the powder bed. The simulation results suggest that the increase of the powder layer thickness is beneficial to the increase in the packing density and the decrease in the surface roughness of the powder bed. A high roller spreading velocity degrades the packing quality of the powder bed. Furthermore, a small number of fibres in the composite powder particles are in favour of the packing quality, but a further increase in the fibre number reduces it.

A unicorn in monoceros: the 3 M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate

ABSTRACT We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($d\sim 460\, \rm pc$), bright (V ≃ 8.3 mag), evolved (Teff, giant ≃ 4440 K, and Lgiant ≃ 173 L⊙) red giant in a high mass function, f(M) = 1.72 ± 0.01 M⊙, nearly circular binary (P = 59.9 d, e ≃ 0). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey, Kilodegree Extremely Little Telescope, and Transiting Exoplanet Survey Satellite light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities, and stellar temperature give an inclination of $87.0^{\circ ^{+1.7^\circ }}_{-1.4^\circ }$, a mass ratio of q ≃ 0.33 ± 0.02, a companion mass of Mcomp = 3.04 ± 0.06 M⊙, a stellar radius of Rgiant = 24.9 ± 0.7 R⊙, and a giant mass of Mgiant = 1.00 ± 0.07 M⊙. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of ${\sim }63{{\ \rm per\ cent}}$ and ${\sim }24{{\ \rm per\ cent}}$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion discs. The X-ray luminosity of the system is $L_{\rm X}\simeq 7.6\times 10^{29}~\rm ergs~s^{-1}$, corresponding to L/Ledd ∼ 10−9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the ‘mass gap’.

Post-Impact Characteristics of a Diesel-in-Water Emulsion Droplet on a Flat Surface Below the Leidenfrost Temperature

Droplet impingement on solid surfaces takes place in a variety of industrial and environmental applications. However, there are still some areas that are not fully comprehended; emulsion droplet impact on a heated surface is one of these areas that require further comprehension. Hence, the present work represents an experimental exploration for spread characteristics of diesel-in-water (DW) emulsion droplet impacting a heated flat plate. Three different emulsions in which water concentration is set to 10%, 20%, and 30% of the overall emulsion content by volume have been tested in addition to the neat diesel. The temperature of the flat plate is varied over the range 20, 40, 60, and 80ºC respectively. Magnified high speed direct imaging and shadowgraphy have been used simultaneously for tracking droplet spread over the heated surface post impact. Droplet spread rate, maximum diameter, rebound height and velocity represent the main evaluated parameters. The results show that the maximum spread diameter is proportional while spread rate is inversely proportional to the increase in plate temperature for all diesel concentrations including the neat diesel. Whereas, droplet rebound height and velocity are found to be more responsive to the variation in diesel concentration than the variation in plate temperature, so they are both minimum in the case of neat diesel and are increasing by the decrease of diesel concentration in the emulsions.

A CONCEPTUAL STUDY OF A LIQUID METAL ALLOY IN A DISK-SHAPED MAGNETOHYDRODYNAMICS CONVERSION SYSTEM

The use of solar-heated liquid metal in a magnetohydrodynamics (MHD) generator provides an alternative and direct conversion method for electric power generation. This prompted the present study to conduct a three-dimensional numerical analysis for a liquid Ga68In20Sn12 flow exposed to several uniform magnetic field intensities (Bo of 0.5 T, 1T and, 1.41 T) within a disk channel geometric boundary. The aim is to study the influence of the external magnetic fields on the generator performance and the fluid flow stability at a high Reynolds number (Re) and Hartmann number (Ha) using the Ansys Fluent software. The simulation results show that at Re of ≈ 2.44e6, the fluid velocity decreases inside the generator regardless of Bo. When Bo of 1T and 1.41T are applied, the velocity magnitude decreases and spreads within the disk channel and walls due to high Ha values (5874 and 8282). The fluid pressure increases from the nozzle pipe inlet to the disk channel and decreases towards the outlet. The induced current density in the radial direction, jx, increases within the disk channel and near the inner electrode edge as Bo increases. A significant observation is that the current densities obtained for Bo of 1T and 1.41T cases are higher than in other cases. The numerical analysis obtained in this study showed that the Bo of either 1T or 1.41T is needed to achieve the required flow stability, current density, and output powers.

Linear and Nonlinear Characteristics of a Low-Voltage Gyrotron Backward-Wave Oscillator in the Subterahertz Range

Gyrotrons operating at low voltage are highly attractive for subterahertz applications because such an operation can greatly reduce the size and cost of gyrotron systems. Among the oscillator versions of gyrotrons, the gyrotron backward-wave oscillator (gyro-BWO) features continuous frequency tuning in a broadband. This study investigates the characteristics of the beam-wave interaction in a low-voltage gyro-BWO in the subterahertz wave range. Operation at lower voltages has the advantage of a broader frequency tuning range and the suppression of negative $k_{z}$ competing modes. The phenomenon of a sharp increase in the start-oscillation current near the lower limit of the operating voltage is investigated. In addition to the linear characteristics, the dependence of the efficiency and tuning bandwidth on voltage is analyzed. The potential performance of a low-voltage gyro-BWO is predicted using the existing beam parameters: beam voltage of 2 kV, beam current of 0.2 A with a velocity ratio of 1.0, and axial velocity spread of 20%. Although the gyro-BWO operating at 2 kV exhibits the problem of low efficiency (an efficiency of 5.6%), it has a broad frequency tuning range of 3.0 GHz. The efficiency can be enhanced to 15% (a peak power of 60 W) by tapering the waveguide radius. Possible competing modes in low-voltage operation are considered and a stability analysis reveals stable operation.

How Dispersal Evolution and Local Adaptation Affect the Range Dynamics of Species Lagging Behind Climate Change.

AbstractAs climate changes, species' ability to spatially track suitable climate depends on their spread velocity, a function of their population growth and dispersal capacity. When climate changes faster than species can spread, the climate experienced at species' expanding range edges may ameliorate as conditions become increasingly similar to those of the range core. When this boosts species' growth rates, their spread accelerates. Here, we use simulations of a spreading population with an annual life history to explore how climatic amelioration interacts with dispersal evolution and local adaptation to determine the dynamics of spread. We found that depending on the timing of dispersal evolution, spread velocity can show contrasting trajectories, sometimes transiently exceeding the climate velocity before decelerating. Climatic amelioration can also accelerate the spread of populations composed of genotypes best adapted to local climatic conditions, but the exact dynamics depends on the pattern of climatic adaptation. We conclude that failing to account for demographic variation across climatic gradients can lead to erroneous conclusions about species' capacity to spatially track suitable climate.

Fire hazard potential of non-metallic powder layers induced by deposit surfaces

Combustible dust may result in a catastrophic dust explosion with much larger accident consequences than that of combustible gas or vapors. By considering the significant contribution of dust layer fire to dust explosion, fire hazards of dust layer were investigated to clarify the possible worsening situation induced by the properties of deposit surface. The results showed that the effect of thermal conductivity of deposit surface on layer fire hazard was complex even for the same powder material, which depended on the contribution of conductivity to the layer flame spreading. Good permeability of the deposit surfaces (e.g., metal gauze) could contribute to a bottom fire with high flame height and temperature. For whatever deposit surfaces (i.e., ceramic and metal plate, or metal gauze), the inclination angle of the dust layer could result in a high flame spread velocity (FSV). As a result, good permeability deposit surfaces coupled with a large inclination angle would significantly enhance the fire hazard of the dust layer, which should be taken enough consideration in the powder process industries or public activities where combustible powders were used.

Immune Profile in Patients With COVID-19: Lymphocytes Exhaustion Markers in Relationship to Clinical Outcome.

The velocity of the COVID-19 pandemic spread and the variable severity of the disease course has forced scientists to search for potential predictors of the disease outcome. We examined various immune parameters including the markers of immune cells exhaustion and activation in 21 patients with COVID-19 disease hospitalised in our hospital during the first wave of the COVID-19 pandemic in Slovakia. The results showed significant progressive lymphopenia and depletion of lymphocyte subsets (CD3+, CD4+, CD8+ and CD19+) in correlation to the disease severity. Clinical recovery was associated with significant increase in CD3+ and CD3+CD4+ T-cells. Most of our patients had eosinopenia on admission, although no significant differences were seen among groups with different disease severity. Non-survivors, when compared to survivors, had significantly increased expression of PD-1 on CD4+ and CD8+ cells, but no significant difference in Tim-3 expression was observed, what suggests possible reversibility of immune paralysis in the most severe group of patients. During recovery, the expression of Tim-3 on both CD3+CD4+ and CD3+CD8+ cells significantly decreased. Moreover, patients with fatal outcome had significantly higher proportion of CD38+CD8+ cells and lower proportion of CD38+HLA-DR+CD8+ cells on admission. Clinical recovery was associated with significant decrease of proportion of CD38+CD8+ cells. The highest AUC values within univariate and multivariate logistic regression were achieved for expression of CD38 on CD8+ cells and expression of PD1 on CD4+ cells alone or combined, what suggests, that these parameters could be used as potential biomarkers of poor outcome. The assessment of immune markers could help in predicting outcome and disease severity in COVID-19 patients. Our observations suggest, that apart from the degree of depletion of total lymphocytes and lymphocytes subsets, increased expression of CD38 on CD3+CD8+ cells alone or combined with increased expression of PD-1 on CD3+CD4+ cells, should be regarded as a risk factor of an unfavourable outcome in COVID-19 patients. Increased expression of PD-1 in the absence of an increased expression of Tim-3 on CD3+CD4+ and CD3+CD8+ cells suggests potential reversibility of ongoing immune paralysis in patients with the most severe course of COVID-19.

Discrete Element Method Analysis of the Spreading Mechanism and Its Influence on Powder Bed Characteristics in Additive Manufacturing.

Laser powder bed fusion additive manufacturing is among the most used industrial processes, allowing for the production of customizable and geometrically complex parts at relatively low cost. Although different aspects of the powder spreading process have been investigated, questions remain on the process repeatability on the actual beam–powder bed interaction. Given the influence of the formed bed on the quality of the final part, understanding the spreading mechanism is crucial for process optimization. In this work, a Discrete Element Method (DEM) model of the spreading process is adopted to investigate the spreading process and underline the physical phenomena occurring. With parameters validated through ad hoc experiments, two spreading velocities, accounting for two different flow regimes, are simulated. The powder distribution in both the accumulation and deposition zone is investigated. Attention is placed on how density, effective layer thickness, and particle size distribution vary throughout the powder bed. The physical mechanism leading to the observed characteristics is discussed, effectively defining the window for the process parameters.

Botany is the root and the future of invasion biology.

Botany is the root and the future of invasion biology.

Methods for online measurement and control of section deviations during hot rolling of wire rod and bars

In the joint project PIREF, the metal forming group of the University of Duisburg-Essen has collaborated with the University of Applied Sciences Ruhr-West Mülheim (Ruhr), the University of Siegen, EMG Automation GmbH and SMS group GmbH to develop sensors, for an online measurement of material velocity and cross section as well as control models for the rolling process of wire rod and bars. The University of Duisburg-Essen provided a metal forming process model for the rolling process to assess the influencing parameters on the rolled section precision. A technique was found to segregate height- from width- influencing parameters from a measured cross-sectional area and actual roll gap. With this measuring technology and with help of the process model, rules for control of the rolling process to achieve close tolerances were obtained. The modelling was accompanied by rolling trials on a laboratory rolling mill at the University of Duisburg-Essen, where a typical Round-OvalRound pass sequence was used for validation of the rolling model concerning lateral spread, inlet and outlet velocity as well as rolling force and torque calculation. The present paper shows how the material flow and the distribution of the velocity in the roll gap can be described. In subsequent rolling of bar and rod in a continuous rolling mill the dimensions can be influenced by application of longitudinal stresses and screwdown. The application of stress can be achieved by an inter-stand velocity mismatch. With the developed models the necessary velocity mismatch can be calculated.

MAGNUM survey: Compact jets causing large turmoil in galaxies

Context. Outflows accelerated by active galactic nuclei (AGN) are commonly observed in the form of coherent, mildly collimated high-velocity gas directed along the AGN ionisation cones and kinetically powerful (≳1044 − 45 erg s−1) jets. Recent works found that outflows can also be accelerated by low-power (≲1044 erg s−1) jets, and the most recent cosmological simulations indicate that these are the dominant source of feedback on sub-kiloparsec scales, but little is known about their effect on the galaxy host. Aims. We study the relation between radio jets and the distribution and kinematics of the ionised gas in IC 5063, NGC 5643, NGC 1068, and NGC 1386 as part of our survey of nearby Seyfert galaxies called Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM). All these objects host a small-scale (≲1 kpc) low-power (≲1044 erg s−1) radio jet that has small inclinations (≲45°) with respect to the galaxy disc. Methods. We employed seeing-limited optical integral field spectroscopic observations from the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope to obtain flux, kinematic, and excitation maps of the extended ionised gas. We compared these maps with archival radio images and in one case, with Chandra X-ray observations. Results. We detect a strong (up to ≳800–1000 km s−1) and extended (≳1 kpc) emission-line velocity spread perpendicular to the direction of the AGN ionisation cones and jets in all four targets. The gas excitation in this region of line-width enhancement is entirely compatible with shock ionisation. These broad and symmetric line profiles are not associated with a single coherent velocity of the gas. A ‘classical’ outflow component with net blueshifted and redshifted motions is also present, but is directed along the ionisation cones and jets. Conclusions. We interpret the observed phenomenon as due to the action of the jets perturbing the gas in the galaxy disc. These intense and extended velocity spreads perpendicular to AGN jets and cones are indeed currently only observed in galaxies hosting a low-power jet whose inclination is sufficiently low with respect to the galaxy disc to impact on and strongly affect its material. In line with cosmological simulations, our results demonstrate that low-power jets are indeed capable of affecting the host galaxy.

Spreading Dynamics of a Precursor Film of Ionic Liquid or Water on a Micropatterned Polyelectrolyte Brush Surface.

Time evolution of the microscopic wetting velocity of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) or water on a micrometer-scale line-patterned surface with a poly(3-sulfopropyl methacrylate) brush and a hydrophobic perfluoroalkyl monolayer was precisely measured by direct observation using optical microscopy and a selective dyeing method over a long period (178 days). When a liquid droplet was placed on the dyed line-patterned brush surface, the liquid penetrated and spread into the polymer brush layer, forming a precursor thin film that extended beyond the macroscopic contact line. The elongation proceeded in two stages by an adiabatic process followed by a diffusive process. The elongation distance X increased with time in proportion to t2.6 for water and t0.81 for EMI-TFSI during the adiabatic process. In a diffusive process, the advancing velocity of the precursor film was markedly reduced to be expressed as X ∝ t0.66 for water and X ∝ t0.21 for EMI-TFSI, indicating that the diffusive process was affected by the energy dissipation of the wetting system. The high viscosity and the strong molecular interaction of EMI-TFSI with the polymer brush gave a large entropy change during the wetting process to result in a slower spreading velocity.

A supersonic laser ablation beam source with narrow velocity spreads.

A supersonic beam source for SrF and BaF molecules is constructed by combining the expansion of carrier gas (a mixture of 2% SF6 and 98% argon) from an Even-Lavie valve with laser ablation of a barium/strontium metal target at a repetition rate of 10 Hz. Molecular beams with a narrow translational velocity spread are produced at relative values of Δv/v = 0.053(11) and 0.054(9) for SrF and BaF, respectively. The relative velocity spread of the beams produced in our source is lower in comparison with the results from other metal fluoride beams produced in supersonic laser ablation sources. The rotational temperature of BaF is measured to be 3.5 K. The source produces 6 × 108 and 107 molecules per steradian per pulse in the X2Σ+ (ν = 0, N = 1) state of BaF and SrF molecules, respectively, a state amenable to Stark deceleration and laser cooling.

Generation of ultrashort keV Ar+ ion pulses via femtosecond laser photoionization

Ion beams with energies in the keV regime are widely utilized in solid-state physics, but the ultrafast dynamics triggered by an ion impact onto a solid surface is to date exclusively accessible via simulations based on many untested assumptions and model parameters. A possible experimental access rests on the availability of a laser-synchronized ion source delivering sufficiently short ion pulses for time resolved pump–probe experiments. Here, we demonstrate a new miniaturized ion optical bunching setup for the creation of rare gas ion pulses using strong-field femtosecond laser photoionization. Neutral Ar gas atoms at room temperature are intercepted by a 50 fs, 800 nm laser pulse focused to ∼10 μm spot size. We demonstrate the generation of monoenergetic 2 keV Ar+ ion pulses with 180 ps duration (FWHM) at laser peak intensities around 1014 W cm−2 and of multiply charged Ar q+ ions (q = 1–5) at higher laser intensities. The results are in good agreement with detailed ion trajectory simulations, which show that the temporal resolution is essentially limited by the initial (thermal) velocity spread of the generated photo-ions, indicating the possibility to achieve even better time resolution by cooling the gas prior to ionization.

Marangoni fireworks: Atomization dynamics of binary droplets on an oil pool

Surface tension gradient due to concentration and temperature differences induces Marangoni forces. The Marangoni effect has been extensively studied to understand its fundamental underlying physics and its industrial applications. This paper describes the spreading and atomization dynamics of an aqueous 2-propanol (IPA) solution on sunflower oil. The spreading and self-atomization of droplets by the Marangoni effect with the evaporation of volatile components are herein observed. With 40 wt. % IPA solution droplets on sunflower oil, firework-like behavior was demonstrated with the Marangoni effect. To better understand the interplay between the evaporation and spreading/atomization characteristics, the temperature field on the oil pool was visualized and quantified using an infrared camera. The Marangoni flow driven by the temperature gradient near the spreading front was estimated and compared with the experimental spreading velocity. The experimental spreading velocity of the liquid film was found to roughly agree with the model prediction. By the atomization of the spreading IPA solution, thousands of atomized daughter droplets were counted, and the size distribution was determined. Additionally, fingering instability at the interface of the IPA solution and sunflower oil was quantitatively discussed, and the resulting wavelength on its interface was compared with the capillary model. We hope that our demonstration stimulates further studies that will yield deeper insights into the spreading and atomization dynamics of volatile binary droplets on a liquid pool.

Numerical and theoretical study on the spreading characteristics of droplet impact on a horizontal flowing liquid film

Droplet impacts on moving films involve many industrial applications. However, limited numerical and theoretical work has been performed to study droplet impact on flowing films. In this paper, the influence of the thickness of the moving liquid film on dynamic characteristics of a single droplet impact was studied by the CLSVOF method. An analysis of the velocity field, pressure field and vorticity cloud diagram inside the liquid showed that as the liquid film thickness increased, the migration distance of the asymmetric crown, the amount of liquid flowing into the crown wall and the angle between the crown wall and the liquid film all increased. Moreover, based on the conservation of mass and momentum, the mathematical model of droplet spreading was extended to a moving liquid film for the first time, which has a wider range of applications, and the mathematical model was also verified by combining the numerical and experimental results. The spreading velocity of the droplet on the moving liquid film is exactly equal to the average of the upstream and downstream spreading velocities; the relationship between the spreading diameter and contact time is a power-dependent trend with a power index of approximately 1/2, and with increasing liquid film thickness, the power index increases slightly. This study not only extends the kinetic theory of droplets impinging on a liquid film but also provides guidance for practical processes such as spray cooling and impinging stream chemical synthesis.

Methods of calculating the soil response spectrum for designing in seismically active area

The article is a continuation of the research of the effect of combining seismological and engineering-seismological methods based on real geological and geophysical data. A probabilistic seismic hazard analysis and numerical modeling of the soil response were performed for the site. Using the Monte Carlo method, 100 realizations of each of the 106 seismic models of the site were prepared to take into account the uncertainty and spread in shear wave velocities of the geological layers. Comparing and analyzing the results obtained, it was found that the generalized response spectrum of the soil relative to the surface, taking into account the parameters of a 30-meter thickness, provides for most of the considered spectral periods higher estimates of spectral accelerations than numerical modeling and can be used to preliminary estimate the amplitudes of the response spectrum of the soil profile before performing seismic microzoning.

Effect of wettability on droplet impact: Spreading and splashing

This study experimentally investigates the dynamic behaviors of water droplets over wide ranges of diameters and velocities (200 μm < D0 < 2600 μm, 50 < We < 3000) impacting on surfaces with various wettability (ranging from hydrophilic to superhydrophobic), and focuses on the effect of surface wettability on water droplet spreading and splashing. The experimental results show that the surface wettability contributes to the lift of the lamella, related to the cosine of the advancing angle (cosθa), that significantly influence the spreading and splashing. The lifting lamella promotes the spreading speed by reducing the surface friction and the viscous dissipation in the boundary layer. The mean spreading speed is linear to the impact velocity on each surface type, and the contact angle slightly affects the mean spreading velocity on the hydrophilic or hydrophobic surface. While, the mean spreading velocity on the superhydrophobic surface is much larger than the other surfaces due to the fewer viscous dissipation. The splashing behavior is dominated by the force from the air surrounding the lamella. The experimental results demonstrate that the splashing threshold K = OhRe1.25 is independent of the advancing angle for the case of θa < 90°, but is proportional to cosθa for the case θa > 90°. Furthermore, we put forward a prediction model of droplet splashing threshold, considering the effect of surface wettability, as K/K0 = 1 + αcosθa, which could quantitatively express the effect of surface wettability on water droplet splashing.

The Effect of Initial Conditions on the Laminar Flame Front Velocity in Gas Mixtures

In this paper, we consider the scatter of the laminar flame front velocities caused both by an error in the composition of the combustible mixture and by artificial initial disturbances. It is shown how the configuration of the initial perturbations of an initially flat front of a laminar flame in a gas mixture with the constant composition affects the velocity spread of expanding spherical flames. The effect of the error in the composition of the combustible mixture on the parameters that determine the flame front velocity was analyzed from the literature data on the spread of the flame propagation velocity. These parameters were recalculated for the possible variation in the mixture composition obtained based on the data on the accuracy of the equipment used in the experiments.