Radial Momentum Research Articles

Construction of Kinetic Model of Vacuum Arc Magnetic Fluid Under External Magnetic Field

The external magnetic field is an important control method of vacuum arc at present. Because of its high efficiency and economy, it has become an important method to study the kinetic model of vacuum arc magnetic fluid. Therefore, the kinetic model of vacuum arc magnetic fluid under the external magnetic field is constructed. Firstly, the mass conservation equation, radial momentum conservation equation, axial momentum conservation equation, energy conservation equation, electromagnetic equations are analyzed. The boundary conditions of the dynamic model are cathode boundary, anode boundary and fluid wall boundary. The fluid dynamics calculation software FLUENT is used as the calculation platform, and its secondary development is carried out. Combining VC++self-programming and self-defined macro functions, according to the boundary conditions, the kinetic model of vacuum arc magnetic fluid under external magnetic field is solved, and the kinetic model analysis of vacuum arc magnetic fluid under external magnetic field is realized. Experimental analysis shows under the action of longitudinal magnetic field, the intensity of longitudinal magnetic field increases, the maximum point of ion rotation speed in the magnetic fluid of vacuum arc moves outward along the edge of vacuum arc, the number density of electrons gradually moves from the central axis to the radial direction, and the high density area increases first and then decreases; Under the action of transverse magnetic field, the intensity of transverse magnetic field increases, the ion pressure in vacuum arc magnetic fluid gradually shifts and increases, and the ion number density also increases.

Particle deposition on the convergent nozzle walls of thermal-spray guns

The present study investigates particle depositions on convergent nozzle walls of (HVOF) spray guns, a process that reduces nozzle lifetime and degrades operation stability and reliability. We perform three-dimensional, two-way coupled simulations of reactive particle-laden compressible flow through a typical thermal spray gun nozzle. We determine critical radial particle velocities at which particles deposit on the wall. We characterize critical velocities concerning particle trajectories, particle diameter, and downstream location of particles deviating from the nozzle center. The critical velocity decreases with particle diameter and increases with the downstream location at which particles deviate from the nozzle center. We exclude the possibility that single particle–particle collision events create sufficient radial particle momentum for deposition. We use the gained insights to propose mitigating measures against particle depositions.

Generation of the radial higher-order orbital angular momentum mode in helically twisted elliptic fiber.

The generation mechanism and regulation method of the radial higher-order orbital angular momentum (OAM) mode based on helically twisted elliptic fiber (HTEF) are investigated. This work aims to reveal the quantitative relation between the twisted rate (α) and the target radial higher-order OAM mode. From theoretical simulations, it is concluded that α is the key to regulating the radial higher-order OAM mode. In the experiment, OAM2n modes with radial higher-order n of 2, 3, 4, and 5 are successfully generated at 1550 nm by regulating α of the HTEF. Finally, the transmission spectrum and purity measurements are performed for the OAM24 mode, and it can be concluded that the fabricated experimental samples have more than 90% conversion efficiency and close to 94% purity under the appropriate α and twisted length.

Conservative scattering of Reissner-Nordström black holes at third post-Minkowskian order

Using a recently developed effective field theory formalism for extreme mass ratios arXiv:2308.14832, we present a calculation of charged black hole scattering at third post-Minkowskian order. The charges and masses are kept arbitrary, and the result interpolates from the scattering of Schwarzschild to extremal charged black holes, and beyond to charged particles in electrodynamics — agreeing with previously reported results in all such limits. The computation of the radial action is neatly organized in powers of the mass ratio. The probe (0SF) contributions are readily computed by direct integration of the radial momentum, and we use the effective field theory to compute the subleading (1SF) contributions via background-field Feynman rules supplemented by an operator encoding recoil of the background. Together these contributions completely determine the conservative physics at order OG3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O}\\left({G}^3\\right) $$\\end{document}.

Inertial coalescence of drops with some viscosity

When two fluid drops touch, they coalesce due to surface tension. At early times, there is only a relatively small fluid bridge joining the drops. An asymptotic solution is presented for an inertial regime of early-time coalescence, in which inertial forces balance surface tension at leading order. It is demonstrated that viscosity nevertheless has a leading-order effect. Radial momentum is created at the tightly curved edge of the fluid bridge by the net force $2\gamma$ (per unit length) due to surface tension. This momentum is left behind the radially expanding bridge edge in a thin viscous wake. The divergent volume flux in the wake entrains fluid from above and below the bridge, and drives an inviscid irrotational flow in the drops on the scale of the bridge radius. This flow widens the gap between the drops ahead of the bridge, and the larger gap width results in a lower rate of coalescence. Including viscosity in this way improves the agreement between theory and the available experimental and numerical data.

On the rising dynamics of a Taylor bubble in sudden/gradual expansion tubes filled with viscoelastic liquids

In this paper, the rise behaviors of Taylor bubbles are investigated in sudden/gradual tubes filled with viscoelastic media via grid adaptive direct numerical simulations (DNS). The exponential Phan–Thien–Tanner (PTT) constitutive model is used to describe the viscoelastic rheological characteristics, and the phase interface is captured via the volume of fluid (VOF) method. The effects of tube structure (diameter ratio and structural angle) and fluid elasticity (expressed by the Weissenberg number Wi) on bubble dynamics have been studied. Our results indicate that bubbles are prone to rupture in the expansion tubes, mainly due to the dual effects of the wall and the elastic relaxation. The fluid elasticity suppresses the jet effect in a sudden expansion tube. Meantime, as the structural angle or the diameter ratio increases, the wall effect is weakened on axial or radial scales, inhibiting the bubble rupture. A large structure angle attenuates the wall effect, while changes in the diameter ratio slow down the radial momentum transfer near the wall region, both of which favor bubble integrity. We also obtain an exponential relationship between the critical rupture time and the structure angle. The dynamical Taylor bubbles can be operated by the structure of the tube and surrounding fluid viscoelasticity, which is of great significance in chemical engineering applications involving complex non-Newtonian fluids.

Thermal Analysis of MHD Hybrid Nanofluid on Stretching/Shrinking Non-Parallel Walls with Uncertain Volume Fractions

This study examines the heat transmission of magneto-hydrodynamic (MHD) hybrid nanofluid (Ag–[Formula: see text]O) flows between two stretchable or shrinkable walls. The nanoparticle volume fraction is considered an uncertain parameter and is represented using triangular and Gaussian fuzzy numbers. The governing radial momentum and energy equations are transformed into nonlinear ordinary differential equations using a similarity transformation and with the required boundary conditions. A generalized homotopy approach is used to analyze the impacts of embedding parameters, including the Reynolds number, stretching/shrinking parameter, opening angle, radiation parameter, Magnetic parameter, and solid volume fraction on velocity and thermal profiles in crisp and uncertain cases. The findings demonstrate that fluid velocity and temperature distribution increase as the stretching parameter increases. The correctness of the suggested technique is validated by comparing the results with the available numerical solution. It has been observed that there is a great agreement between the present results and the numerical ones.

Tracing the Layers of Photodissociated Gas in the Trifid Nebula

Photodissociated gas bears the signature of the dynamical evolution of the ambient interstellar medium impacted by the mechanical and radiative feedback from an expanding H ii region. Here we present an analysis of the kinematics of the young Trifid Nebula, based on velocity-resolved observations of the far-infrared fine structure lines of [C ii] at 158 μm and [O i] at 63 μm. The distribution of the photodissociated regions (PDRs) surrounding the nebula is consistent with a shell-like structure created by the H ii region expanding at a velocity of 5 km s−1. Comparison of ratios of [C ii] and [O i]63 μm intensities for identical velocity components with PDR models indicate a density of 104 cm−3. The redshifted and blueshifted PDR shells with a combined mass of 516 M ⊙ have a kinetic energy of ∼1047 erg. This is consistent with the thermal energy of the H ii region as well as with the energy deposited by the stellar wind luminosity from HD 169442A, an O7 V star, over the 0.5 Myr lifetime of the star. The observed momentum of the PDR shell is lower than what theoretical calculations predict for the radial momentum due to the shell being swept up by an expanding H ii region, which suggests that significant mass loss has occurred in M20 due to the dispersal of the surrounding gas by the advancing ionization front.

Analyzing the early impact dynamics of single droplets impacting onto wetted surfaces

Single droplet impacts onto thin wall-films are a common phenomenon in many applications. For sufficiently high impact velocities, the droplet impact process consists of three phases, i.e., initial contact stage, droplet deformation with radial momentum transfer inducing an upward rising lamella, and crown propagation. Here, we present the results of a combined numerical and experimental study focusing on the early dynamics of the impact process. Specifically, the effects of the initial droplet shape, wall-film thickness, and contact line motion are analyzed. Prior to impact, an oblate spheroidal droplet shape was observed. Using direct numerical simulation, we show that the droplet shape affects the impact dynamics only during the first two phases, as it is one of the key parameter influencing the correct prediction of the impact zone. The contact line propagation is described by a square-root-time dependence R¯CL=ατ for both, dry and wetted surfaces. On dry surfaces, the advancement of the contact line is determined by the rolling motion of the truncated droplet. On wetted surfaces, the value of the α-parameter is controlled by two concurrent effects, namely, rolling motion and wall-film inertia. For impact onto thin films, the rolling motion prevails. With increasing wall-film height, the droplet penetrates into the soft substrates and wall-film inertia becomes the controlling factor. These insights into the early impact dynamics on wetted surface are important for the formulation of a unified modeling approach.

CFD study of gas-powder injection characteristics in a novel lance with supersonic shrouding jet

Supersonic gas–solid jets are widely applied in the spraying coating processes, drug delivery, and metallurgical reactors, due to exceptional penetrating capabilities. In this study, a novel lance is designed to increase the penetration depth of gas–solid flow by utilizing a supersonic shrouding jet (SSJ). Subsequently, the numerical model based on the Eulerian-Lagrangian framework is applied to analyze the supersonic gas-powder flow within the lance. After validating the reliability of numerical model with experimental data, the gas-powder flow characteristics are discussed. The results indicate that the presence of SSJ enhances the powder velocity and penetration depth significantly by blending supersonic gas. Moreover, the particle-gas momentum interaction primarily occurs inside the central pipe and downstream of the lance exit. Along the radial direction, the particle dispersion is the result of the combined interplay between lift force and particle–particle collisions. The lift force enhances particle collision frequency, and these collisions, in turn, facilitate the conversion of axial momentum into radial momentum, resulting in substantial radial acceleration. When SSJ exits, the particle radial dispersion is constrained, primarily due to the reduced particle residence time. Additionally, the introduction of low-temperature oxygen from SSJ results in a reduction in both the carrier gas and particle temperatures. The results provide meaningful insights into understanding the supersonic gas–solid flow mechanisms of the SSJ lance.

Multi-step-index fiber with a large number of weakly coupled OAM mode groups for IM/DD systems in data centers: design, fabrication, and characterization

Mode division multiplexing (MDM) technique based on weakly coupled few-mode fibers (FMF) is promising to enhance the capacity of short-reach transmission. We design and fabricate a multi-step-index FMF (MSIF), which supports weakly coupled first-order radial orbital angular momentum mode group (OAMl,1 MG) for MDM transmission. We use three layers of core to regulate the minimum effective refractive index difference (min|Δn eff |) between OAMl,1 MG and the adjacent MGs. In experiments, we demonstrate that the fabricated MSIF can support up to OAM6,1 with the interferometric method, and the loss measured by an optical time-domain reflectometer (OTDR) can achieve <0.5 dB/km for the OAMl,1 with an order from |l| = 0 to |l| = 6. The inter-mode-group cross talk (XT) is tested by the power measurement, and the system-level XT after 20 km fiber transmission in the worst case is about −11.1 dB.

Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations

ABSTRACT Large spectroscopic surveys plus Gaia astrometry have shown us that the inner stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage (GES). With the richness of data at hand, there are a myriad of ways these accreted stars have been selected. We investigate these GES selections and their effects on the inferred progenitor properties using data constructed from APOGEE and Gaia. We explore selections made in eccentricity, energy-angular momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and [Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1279 GES stars with varying contamination from in-situ and other accreted stars. We also use the Auriga cosmological hydrodynamic simulations to benchmark the different GES dynamical selections. Applying the same observational GES cuts to nine Auriga galaxies with a GES, we find that the Jr-Lz method is best for sample purity and the eccentricity method for completeness. Given the average metallicity of GES (−1.28 < [Fe/H] < −1.18), we use the z = 0 mass–metallicity relationship to find an average $\rm M_{\star }$of ∼4 × 108 M⊙. We adopt a similar procedure and derive $\rm M_{\star }$ for the GES-like systems in Auriga and find that the eccentricity method overestimates the true $\rm M_{\star }$ by ∼2.6 × while E-Lz underestimates by ∼0.7 ×. Lastly, we estimate the total mass of GES to be $\rm 10^{10.5 - 11.1}~{\rm M}_{\odot }$ using the relationship between the metallicity gradient and the GES-to-in-situ energy ratio. In the end, we cannot just ‘pick and choose’ how we select GES stars, and instead should be motivated by the science question.

Experimental and theoretical study on cavitation of concrete targets penetrated by hypervelocity long rod projectiles

The cavitation phenomenon can seriously decrease the strike performance of military weapons. As military terminal-ballistic activities have turned to hypervelocity attacks, the cavitation effect existing in the hypervelocity penetration deserves more attention. In terms of long-rod hypervelocity penetration into concretes, this study presents an analytical cavity model for the prediction of the cavity diameter based on the two-stage cavitation mechanism and the u-v relationship. The two-stage mechanism involving mushrooming and radial momentum expansion is used to describe the cavitation modes and the penetration parameters for the projectiles used in the analytical model are determined by numerical simulations empirically. Besides, the three modes including rigidity, deformation and erosion are introduced into the cavity model to elucidate the projectile state. Moreover, the hypervelocity impact experiments on the long rods into semi-infinite concrete targets at velocities of 2117 ∼ 3086 m/s are carried out to observe the cavitation effect of concrete targets and for further validating the model through comparing with the experimental cavity profiles, where the cavity profile is quantified with silica gel, and the cavity diameter is found to be more than five times the projectile diameter. The cavitation effect is observed significantly and quantified from the experiment results. The cavity model is compared with the experimental cavity profiles, showing that the model has good applicability.

Stability of Schwarzshild black holes in quadratic gravitywith Weyl curvature domination

We study the linear stability of static and spherically symmetric (SSS)black holes (BHs) in the presence of a Weyl-squared curvaturebesides an Einstein-Hilbert term in the action.In this theory, there is always an exact Schwarzschild BHirrespective of the Weyl coupling constant α,with the appearance of a non-Schwarzschild solutionfor a particular range of the coupling oforder |α| ≈ r h 2 (where r h isthe horizon radius).On the SSS background, we show that the propagating degreesof freedom (DOFs) are three in the odd-parity sector and fourin the even-parity sector. Since the number of total seven DOFs coincides with those on the Minkowski and isotropic cosmological backgrounds, the Weyl gravity does not pose a strong coupling problem associated with the vanishing kinetic term of dynamical perturbations.The odd-parity perturbations possess at least one ghost mode,but the propagation speeds of all three dynamical modesare luminal.In the even-parity sector, our analysis, based on the WKB approximation, shows that, besides the appearance of at least one ghost mode, the Schwarzschild solution is prone to both radial and angular Laplacian instabilities of several dynamical perturbations for the Weyl coupling in the range |α| ≫ r h 2.For large radial and angular momentum modes, the time scales of such instabilities are much shorter than the horizon distance r h divided by the speed of light.In the coupling regime |α|≲ r h 2, the WKB approximation does not hold any longer, and a different analysis should be performedif one wants to state the stability of both the Schwarzschild andnon-Schwarzschild BH solutions in this range of model parameters.

Characterization of Sprays Generated by the Expansion of Emulsions with Liquid Carbon Dioxide

AbstractExpanding emulsions with liquid CO2 facilitates the creation of aerosols with an average droplet diameter in the low micrometer size range, which is challenging with conventional atomizers. The droplet formation process of the expansion of high‐pressure emulsions was investigated using a plain‐orifice atomizer and different swirl nozzles. The local droplet size and droplet velocities were measured and used to estimate the local Weber number and thus infer the droplet size reduction. Measurements of the local mass concentration in the aerosol showed that, for the swirl nozzle, the highest concentration was found outside of the central axis, indicating radial momentum generated by the swirl nozzle. Furthermore, it was shown that the type of expansion nozzle used has an influence on the resulting median droplet size in the aerosol. For a water mass load of 0.01, the median droplet diameter was reduced from 8 to 3 μm by increasing the swirl number from 0.01 to 0.1.

Experimental Investigation on the Influence of Swirl Ratio on Tornado-like Flow Fields by Varying Updraft Radius and Inflow Angle

The swirl ratio is the most critical parameter for determining the intensity and structure of tornado-like vortex, defined as the ratio of angular momentum to radial momentum. The angle of entry flow and the updraft radius are two key parameters affecting the swirl ratio. Many laboratory simulators have studied the effect of swirl ratio by changing the angle of entry flow, but there is a lack of research on the updraft radius. Therefore, for a deep sight of the impact of the updraft radius on the swirl ratio and tornado-like vortex, a laboratory tornado simulator capable of adjusting the updraft radius was designed, built, and tested. And, the effects of various swirl ratios caused by the updraft radius and the angle of entry flow on the tornado-like vortices were investigated, in terms of the dual-celled vortex transformation and vortex wandering. It was found that the effects of the updraft radius and the angle of turning vanes on the tornado-like vortices are quite different, and the formation of the dual-celled vortex is more sensitive to the updraft radius, because a larger angular momentum and axial pressure gradient can be provided. In addition, increasing the updraft radius has a greater inhibitory effect on the vortex wandering phenomenon compared to the angle of the turning vanes due to the flow fluctuations induced by turbulence.

Simple Convective Accretion Flows (SCAFs): Explaining the ≈ −1 Density Scaling of Hot Accretion Flows around Compact Accretors

Recent simulations find that hot gas accretion onto compact accretors is often highly turbulent and diskless, and shows a power-law density profile with slope α ρ ≈ −1. These results are consistent with observational constraints, but do not match existing self-similar solutions of radiatively inefficient accretion flows. We develop a theory for this new class of accretion flows, which we dub simple convective accretion flows (SCAFs). We use a set of hydrodynamic simulations to provide a minimalistic example of SCAFs, and develop an analytic theory to explain and predict key flow properties. We demonstrate that the turbulence in the flow is driven locally by convection, and argue that radial momentum balance, together with an approximate up–down symmetry of convective turbulence, yields α ρ = −1 ± few 0.1. Empirically, for an adiabatic hydrodynamic flow with γ ≈ 5/3, we get α ρ ≈ −0.8; the resulting accretion rate (relative to the Bondi accretion rate), , agrees very well with the observed accretion rates in Sgr A*, M87*, and a number of wind-fed supergiant X-ray binaries. We also argue that the properties of SCAFs are relatively insensitive to additional physical ingredients such as cooling and magnetic field; this explains their common appearance across simulations of different astrophysical systems.

Flexible generation of high-performance superposed perfect vortex beams with metasurfaces

Vortex beams have been adopted as optical communications tools to improve the information capacity of free-space communication. Topological charge (TC) is a key parameter for analyzing vortex beams, specifically when defining wavelength division multiplexing and information coding capabilities. TC is typically proportional to the radial intensity profile and orbital angular momentum. The radial intensity profile of perfect vortex (PV) beams is TC-independent, thereby improving the efficiency of image processing and optical fiber transmission; however, PV beams generated in traditional ways are restricted in their degree of freedom for multiplexing and it can be difficult to distinguish PV beams with different TCs. Herein, we numerically simulate a single all-dielectric geometric metasurface to generate PV beams with intensity patterns corresponding to the TC. We increase the types of intensity modes by superposing specific polarization states of Laguerre–Gaussian beams, creating many options for optical communication transmission purposes.

Topological effects on generalized Duffin-Kemmer-Petiau oscillator under Aharonov–Bohm flux field and Coulomb potential

In this paper, we study the relativistic quantum motions of the oscillator field of the wave equation under the influence of the Aharonov–Bohm (AB) flux field with a Coulomb vector potential in the background of the topological defects produced by a cosmic string and global monopole space-time. We derive the radial equation of the generalized Duffin-Kemmer-Petiau (DKP) oscillator in a static cosmic string space-time and solve it through the Heun function equation. Afterwards, we derive the radial equation of the same generalized DKP oscillator in a point-like global monopole background and obtain the eigenvalue solutions using the same procedure. The generalized oscillator field is studied by substituting the radial momentum operator ∂ r → (∂ r + i M ω η 0 f(r)), where f(r) is an arbitrary function other than linear and introduces a vector potential of Coulomb-types through a minimal substitution via ∂ μ → (∂ μ − i q A μ ) in the relativistic wave equation. It is shown that the eigenvalue solutions of the oscillator field are influenced by the topological defects of the cosmic string and point-like global monopole space-times and get them modified. Furthermore, we see that the eigenvalue solutions depend on the geometric quantum phase, and hence, shifted them more in addition to the topological defects that show the gravitational analogue to the Aharonov–Bohm effect for the bound-states.

A Simple Family of Tropical Cyclone Models

This review discusses a simple family of models capable of simulating tropical cyclone life cycles, including intensification, the formation of the axisymmetric version of boundary layer shocks, and the development of an eyewall. Four models are discussed, all of which are axisymmetric, f-plane, three-layer models. All four models have the same parameterizations of convective mass flux and air–sea interaction, but differ in their formulations of the radial and tangential equations of motion, i.e., they have different dry dynamical cores. The most complete model is the primitive equation (PE) model, which uses the unapproximated momentum equations for each of the three layers. The simplest is the gradient balanced (GB) model, which replaces the three radial momentum equations with gradient balance relations and replaces the boundary layer tangential wind equation with a diagnostic equation that is essentially a high Rossby number version of the local Ekman balance. Numerical integrations of the boundary layer equations confirm that the PE model can produce boundary layer shocks, while the GB model cannot. To better understand these differences in GB and PE dynamics, we also consider two hybrid balanced models (HB1 and HB2), which differ from GB only in their treatment of the boundary layer momentum equations. Because their boundary layer dynamics is more accurate than GB, both HB1 and HB2 can produce results more similar to the PE model, if they are solved in an appropriate manner.