Gas Surface Research Articles

Sound wave propagation in rarefied molecular gases

Sound wave propagation in rarefied flows of molecular gases confined in micro-channels is investigated numerically. We first validate the employed kinetic model against the experimental results and then systematically study the gas damping and surface force on the transducer as well as the resonance/anti-resonance in confined space. To quantify the impact of the finite relaxation rates of the translational and internal energies on wave propagation, we examine the roles of bulk viscosity and thermal conductivity in depth over a wide range of rarefactions and oscillation frequencies. It is found that the bulk viscosity only exerts influence on the pressure amplitude and its resonance frequency in the slip regime in high oscillations. In addition, the internal degree of freedom is frozen when the bulk viscosity of a molecular gas is large, resulting in the pressure amplitude of sound waves in the molecular gas being the same as in a monatomic gas. Meanwhile, the thermal conductivity has a limited influence on the pressure amplitude in all the simulated flows. In the case of the thermoacoustic wave, we prove that the Onsager–Casimir reciprocal relation also holds for molecular gases, i.e. the pressure deviation induced by the temperature variation is equal to the heat flux induced by the plate oscillation. Our findings enable an enhanced understanding of sound wave propagation in molecular gases, which may facilitate the design of nano-/micro-scale devices.

Spectral insight into the interface evolution of a carbon/carbon composite under high-enthalpy, nonequilibrium flow

Gas–surface interactions between thermal protection materials and high-enthalpy nonequilibrium flow are one of the greatest challenges in accurately predicting aerodynamic heating during supersonic flights. Finer microscopic details of flow properties are required for elaborate simulation of these interactions. Spectral insight, with quantum-state-specific characteristics, is provided in this work to investigate the physico-chemical processes in high temperature interface of a carbon/carbon (C/C) composite. The nonequilibrium air flow is produced by a 1.2 MW inductively coupled plasma wind tunnel at an enthalpy of 20.08 MJ/kg. The duration of each test is up to 100 s, and quartz is also tested for comparison. Spectral insights into the reaction mechanisms of the gas–surface interactions are acquired by the optical emission spectroscopy and laser absorption spectroscopy. Dynamic evolution of the chemical reaction pathways and thermal nonequilibrium are discussed based on the results of optical emission spectroscopy. Temporally and spatially resolved results of the translational temperature and number density of atomic oxygen are quantified by laser absorption spectroscopy. Controlling mechanisms in the surface chemistry are further analyzed in conjunction with the surface temperature, scanning electron microscopy, and energy dispersive spectroscopy. Reaction mechanisms on the C/C composite surface sequentially experience an oxidation-dominant, an intense competitive, a nitridation-dominant, and a recession dominant period. Distributions in the axial direction and dynamic characteristics of the translational temperature and number density of atomic oxygen are found closely related with surface swelling, recession, and chemical reactions. The results herein are consistent with each other and are instructive to further investigate the interface evolution on C/C composites.

VERTICO. VII. Environmental Quenching Caused by the Suppression of Molecular Gas Content and Star Formation Efficiency in Virgo Cluster Galaxies

We study how environment regulates the star formation cycle of 33 Virgo Cluster satellite galaxies on 720 pc scales. We present the resolved star-forming main sequence for cluster galaxies, dividing the sample based on their global H i properties and comparing to a control sample of field galaxies. H i–poor cluster galaxies have reduced star formation rate (SFR) surface densities with respect to both H i–normal cluster and field galaxies (∼0.5 dex), suggesting that mechanisms regulating the global H i content are responsible for quenching local star formation. We demonstrate that the observed quenching in H i–poor galaxies is caused by environmental processes such as ram pressure stripping (RPS), simultaneously reducing the molecular gas surface density and star formation efficiency (SFE) compared to regions in H i–normal systems (by 0.38 and 0.22 dex, respectively). We observe systematically elevated SFRs that are driven by increased molecular gas surface densities at fixed stellar mass surface density in the outskirts of early stage RPS galaxies, while SFE remains unchanged with respect to the field sample. We quantify how RPS and starvation affect the star formation cycle of inner and outer galaxy disks as they are processed by the cluster. We show both are effective quenching mechanisms, with the key difference being that RPS acts upon the galaxy outskirts while starvation regulates the star formation cycle throughout disk, including within the truncation radius. For both processes, the quenching is caused by a simultaneous reduction in the molecular gas surface densities and SFE at fixed stellar mass surface density.

Prediction of interfacial shear stress and pressure drop in vertical two-phase annular flow

The interfacial shear stress and pressure drop of an upward vertical annular flow of nitrogen–water, HFC134a–water, and nitrogen–95 % ethanol solution were comprehensively investigated considering the effect of the liquid–gas density ratio and surface tension. A direct link between the disturbance wave height and equivalent sand-grain roughness was noted through the analogy with the famous Moody chart for single-phase turbulent flows. A predictive model of the interfacial friction factor was developed based on this finding. To predict the pressure drop of the annular flow, a new model with good predictive performance for annular flows of various working fluids including steam–water under boiling water reactor operating condition (286 °C and 7 MPa) was proposed.

Laser fusion cutting: The missing link between gas dynamics and cut edge topography

In laser cutting, the fundamental role of the gas flow for melt removal and kerf formation is generally accepted. Beyond this vague understanding, however, the underlying physical mechanisms are not yet fully understood. In particular, detailed data concerning the momentum and heat transfer between the gas and melt have seldom been reported. This study addresses the local interactions between the cutting gas and kerf surface (melt film surface) in a fundamental way based on a combined experimental, theoretical, and numerical approach. Typical solid-state laser cut edges are analyzed considering the characteristic surface structures and the basic influences of the gas flow on the global and local melt movement. Here, apparent structures in the micrometer range indicate the effect of vortical gas structures close to the wall. Theoretical investigation of the gas boundary layer is conducted by semiempirical equations and the transfer of basic results from the boundary layer theory. It is shown that the boundary layer is in transition between the laminar and turbulent flow, and local flow separations and shock-boundary layer interactions primarily induce spatially periodic and quasistationary instability modes. An improved numerical model of the cutting gas flow confirms the theoretical results and exhibits good agreement with experimental cut edges, reproducing relevant instability modes and quantifying the local momentum and heat transfer distributions between the gas and melt. With the knowledge gained about the underlying physical mechanisms, promising approaches for improvements of the fusion cutting performance are proposed.

Radio-continuum decrements associated to shadowing from the central warp in transition disc DoAr 44

ABSTRACT Warps have often been used to explain disc properties, but well-characterized examples are important due to their role in disc evolution. Scattered light images of discs with central gaps have revealed sharp warps, such that the outer rings are shadowed by tilted inner discs. The near-infrared intensity drops along the ring around T-Tauri star DoAr 44 have been interpreted in terms of a central warp. We report new ALMA observations of DoAr 44 in the continuum at 230 and 350 GHz (at ∼10 au), along with a new epoch of Spectro Polarimetric High-contrast Exoplanet REsearch (SPHERE)/Infrared Dual-band Imager and Spectrograph differential polarized imaging taken during excellent weather conditions. The ALMA observations resolve the ring and confirm the decrements proposed from deconvolution of coarse 336 GHz data. The scattered light image constrains the dips, which correspond to a misaligned inner disc with a relative inclination ξ = 21.4 $^{+6.7}_{-8.3}$ deg. The SPHERE intensity profile shows a morphological change compared to a previous epoch that may be interpreted as a variable orientation of the inner disc, from ξ ∼ 30 deg to ξ ∼ 20 deg. The intensity dips probably correspond to temperature decrements, as their mm-spectral index, $\alpha ^{230 \textrm {GHz}}_{350 \textrm {GHz}} \sim$ 2.0 ± 0.1, is indicative of optically thick emission. The azimuth of the two temperature decrements are leading clockwise relative to the infrared-dips, by η = 14.95 deg and η = 7.92 deg. For a retrograde disc, such shifts are expected from a thermal lag and imply gas surface densities of Σg = 117 ± 10 g cm−2 and Σg = 48 ± 10 g cm−2. A lopsided disc, with contrast ratio fr = 2.4 ± 0.5, is also consistent with the large continuum crescent.

Metal-THINGS: a panchromatic analysis of the local scaling relationships of the dwarf irregular galaxy NGC 1569

ABSTRACT We investigate several panchromatic scaling relations (SRs) for the dwarf irregular galaxy NGC 1569 using Integral Field Unit (IFU) data from the Metal-THINGS Survey. Among the spatially resolved properties analysed, we explore SRs between the stellar mass, SFR, molecular gas, total gas, baryonic mass, gas metallicity, gas fraction, SFE, and effective oxygen yields. Such multiwavelength SRs are analysed at a spatial resolution of 180 pc, by combining our IFU observations with data from the surveys THINGS, CARMA, and archival data from DustPedia. Although we recover several known relations, our slopes are different to previously reported ones. Our star formation main sequence, Kennicutt–Schmidt (KS), and molecular KS relations show higher SFRs, lower scatter, and higher correlations, with steeper (1.21), and flatter slopes (0.96, 0.58), respectively. The shape of the SRs including metallicity, stellar mass, and gas fraction are flat, with an average value of 12 + log(O/H) ∼ 8.12 dex. The baryonic mass versus effective oxygen yields, and the stellar, gas and baryonic mass versus SFE show higher dispersions and lower correlations. Since we use the dust mass as a tracer of gas mass, we derive the dust-to-gas ratio and the CO luminosity-to-molecular gas mass conversion factors, showing differences of 0.16 and 0.95 dex for the total and molecular gas surface density, respectively, in comparison to previously reported values. We use a self-regulated feedback model to conclude that stellar feedback plays an important role generating outflows in NGC 1569.

The inefficiency of stellar feedback in driving galactic outflows in massive galaxies at high redshift

ABSTRACT Recent observations indicate that galactic outflows are ubiquitous in high-redshift (high-z) galaxies, including normal star-forming galaxies, quasar hosts, and dusty star-forming galaxies (DSFGs). However, the impact of outflows on the evolution of their hosts is still an open question. Here, we analyse the star-formation histories and galactic outflow properties of galaxies in massive haloes ($10^{12}\, {\rm M}_{\odot }\ \lt\ M_{\rm vir}\ \lt\ 5\times 10^{12}\, {\rm M}_{\odot }$) at z ≳ 5.5 in three zoom-in cosmological simulations from the MassiveFIRE suite, as part of the Feedback In Realistic Environments (FIRE) project. The simulations were run with the FIRE-2 model, which does not include feedback from active galactic nuclei. The simulated galaxies resemble z > 4 DSFGs, with star-formation rates of $\sim\!{1000}\ {\rm M}_{\odot }\, \rm yr^{-1}$ and molecular gas masses of Mmol ∼ 1010 M⊙. However, the simulated galaxies are characterized by higher circular velocities than those observed in high-z DSFGs. The mass loading factors from stellar feedback are of the order of ∼0.1, implying that stellar feedback is inefficient in driving galactic outflows and gas is consumed by star formation on much shorter time-scales than it is expelled from the interstellar medium. We also find that stellar feedback is highly inefficient in self-regulating star formation in this regime, with an average integrated star formation efficiency (SFE) per dynamical time of 30 per cent. Finally, compared with FIRE-2 galaxies hosted in similarly massive haloes at lower redshift, we find lower mass loading factors and higher SFEs in the high-z sample. We argue that both effects originate from the higher total and gas surface densities that characterize high-z massive systems.

Volume and surface effects in Cs-free regimes in NIO1

NIO1 (Negative Ion Optimization phase 1) is a compact multiaperture radiofrequency H- ion source whose design was optimized for sustainable prolonged beam on target operation; for installation economy, its size was drastically reduced with respect to fusion device D- sources, down to a plasma volume of 1.6 × 10-3 m-3. Even if Cesium improves H- production as well known, also Cs-free regimes (and intermediate regimes) well deserve some development effort, in view of avoiding long term contamination of the accelerator and for use as cleaning procedure. Data collected by NIO1 in a true Cs-free regime (before 2020) are thus very important, and need a thorough statistical analysis, with special attention to the technique of gas conditioning that was discovered in NIO1 and to the issues concerned with long term operation; N2, Ar, and Xe comparison to oxygen is here shown; all these gases proved useful for conditioning, to different extents. Gas conditioning macroscopically proves the importance of surface effects, even when the final production of H- happens in the source volume. As regards to the Electron Cyclotron Resonance Ion Sources (ECRIS), exchange of ideas and concepts, such aselectron economyand biased disks, liners and wall coatings, is considered. Gas mixing, conditioning and surface material perspectives are envisioned.

Influence of Effective Surface Area on Gas Sensing Properties and Surface Morphology of Ag Doped Cu2O Thin Films by Cost Effective Method of M-SILAR Technique

Influence of Effective Surface Area on Gas Sensing Properties and Surface Morphology of Ag Doped Cu2O Thin Films by Cost Effective Method of M-SILAR Technique

Neutral Atomic and Molecular Clouds and Star Formation in the Outer Carina Arm

We present a comprehensive investigation of H i (super)clouds, molecular clouds (MCs), and star formation in the Carina spiral arm of the outer Galaxy. Utilizing HI4PI and CfA CO survey data, we identify H i clouds and MCs based on the (l, v LSR) locations of the Carina arm. We analyzed 26 H i clouds and 48 MCs. Most of the identified H i clouds are superclouds, with masses exceeding 106 M ⊙. We find that 15 of these superclouds have associated MC(s) with M H I ≳ 106 M ⊙ and 50 M ⊙ pc−2. Our virial equilibrium analysis suggests that these CO-bright H i clouds are gravitationally bound or marginally bound. We report an anticorrelation between molecular mass fractions and Galactocentric distances, and a correlation with total gas surface densities. Nine CO-bright H i superclouds are associated with H ii regions, indicating ongoing star formation. We confirm the regular spacing of H i superclouds along the spiral arm, which is likely due to some underlying physical process, such as gravitational instabilities. We observe a strong spatial correlation between H ii regions and MCs, with some offsets between MCs and local H i column density peaks. Our study reveals that, in the context of H i superclouds, the star formation rate surface density is independent of H i and total gas surface densities but positively correlates with molecular gas surface density. This finding is consistent with both extragalactic studies of the resolved Kennicutt–Schmidt relation and local giant molecular clouds study of Lada et al. (2013), emphasizing the crucial role of molecular gas in regulating star formation processes.

Numerical investigation on characteristics of interfacial wave of liquid film in gas–liquid two-phase flow using OpenFOAM

Liquid film cooling as an advanced cooling technology is widely used in space vehicles. Stable operation of liquid film along the rocket combustion inner wall is crucial for thermal protection of rocket engines. The stability of liquid film is mainly determined by the characteristics of interfacial wave, which is rarely investigated right now. How to improve the stability of thin film has become a hot spot. In view of this, an advanced model based on the conventional Volume of Fluid (VOF) model is adopted to investigate the characteristics of interfacial wave in gas–liquid flow by using OpenFOAM, and the mechanism of formation and development of wave is revealed intuitively through numerical study. The effects from gas velocity, surface tension and dynamic viscosity of liquid (three factors) on the wave are studied respectively. It can be found that the gas velocity is critical to the formation and development of wave, and four modes of droplets generation are illustrated in this paper. Besides, a gas vortex near the gas–liquid interface can induce formation of wave easily, so changing the gas vortex state can regulate formation and development of wave. What’s more, the change rules of three factors influencing on the interfacial wave are obtained, and the surface tension has a negative effect on the formation and development of wave only when the surface tension coefficient is above the critical value, whereas the dynamic viscosity has a positive effect in this process. Lastly, the maximum height and maximum slope angle of wave will level off as the gas velocity increases. Meanwhile, the maximum slope angle of wave is usually no more than 38°, no matter what happens to the three factors.

Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies

ABSTRACT The cold dark matter model predicts galaxies have 100 times more dark matter mass than stars. Nevertheless, recent observations report the existence of dark-matter-deficient galaxies with less dark matter than expected. To solve this problem, we investigate the physical processes of galaxy formation in head-on collisions between gas-containing dark matter subhaloes (DMSHs). Analytical estimation of the collision frequency between DMSHs associated with a massive host halo indicates that collisions frequently occur within one-tenth of the virial radius of the host halo, with a collision time-scale of about $10\, \mathrm{Myr}$, and the most frequent relative velocity increases with increasing radius. Using analytical models and numerical simulations, we show the bifurcation channel of the formation of dark-matter-dominated and dark-matter-deficient galaxies. In the case of low-velocity collisions, a dark-matter-dominated galaxy is formed by the merging of two DMSHs. In the case of moderate-velocity collisions, the two DMSHs penetrate each other. However, the gas medium collides, and star formation begins as the gas density increases, forming a dwarf galaxy without dark matter at the collision surface. In the case of high-velocity collisions, shock-breakout occurs due to the shock waves generated at the collision surface reaching the gas surface, and no galaxy forms. For example, the simulation demonstrates that a pair of DMSHs with a mass of $10^9\, \mathrm{M_\odot }$ containing gas of 0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar mass of $10^7\, \mathrm{M_\odot }$ for a relative velocity of $200\, \mathrm{km\, s^{-1}}$.

Mixing Characteristics and Parameter Effects on the Mixing Efficiency of High-Viscosity Solid–Liquid Mixtures under High-Intensity Acoustic Vibration

High-intensity acoustic vibration is a new technology for solving the problem of uniform dispersion of highly viscous materials. In this study, we investigate the mixing characteristics of high-viscosity solid–liquid phases under high-intensity acoustic vibration and explore the effect of vibration parameters on the mixing efficiency. A numerical simulation model of solid–liquid–gas multiphase flow, employing the volume of fluid (VOF) and discrete phase model (DPM), was developed and subsequently validated through experimental verification. The results show that the movement and deformation of the gas–liquid surface over the entire field are critical for achieving rapid and uniform mixing of the solid–liquid phases under acoustic vibration. Increasing the amplitude or frequency of vibration can intensify the movement and deformation of the free surface of gas and liquid, improve the mixing efficiency, and shorten the mixing time. Under the condition of constant acceleration, the mixing efficiency of materials is higher at low frequency and high amplitude. Further, we define a relationship that predicts desirable mixing conditions as a function of amplitude and frequency. This serves as a valuable reference guide for evaluating the minimum requirements when selecting operating parameters.

Corrosion Mechanism of Typical Shale Gas Gathering and Transportation Pipelines in Southwest China

The shale gas surface gathering and transportation pipeline in southwest China has experienced serious internal corrosion, which affects the production of shale gas wells and has great potential safety hazards. To find out the corrosion type and corrosion mechanism, the corrosion analysis and mechanism research of gathering and transportation pipelines are carried out, including corrosion morphology observation, corrosion product analysis, fluid mechanics calculation, etc. The results show that the corrosion products of the gathering and transportation pipeline are mainly FeCO3 with high S content, and the corrosion type is sulfate-reducing bacteria (SRB) and CO2 corrosion. At the same time, the corrosion pits are mainly distributed on the approach stream side, which is closely related to the scouring effect of the medium flow in the vertical branch pipe on the wall surface of the tee. The corrosion weight loss test shows that the corrosion rate of sterile solution is significantly lower than that of bacterium solution, indicating that the presence of SRB significantly expedites the corrosion of materials.

Forming rocky exoplanets around K-dwarf stars

Context. New space telescopes, such as the upcoming PLATO mission, aim to detect and study thousands of exoplanets, especially terrestrial planets around main-sequence stars. This motivates us to study how these planets formed. How multiple close-in super-Earths form around stars with masses lower than that of the Sun is still an open issue. Several recent modeling studies have focused on planet formation around M-dwarf stars, but so far no studies have focused specifically on K dwarfs, which are of particular interest in the search for extraterrestrial life. Aims. We aim to reproduce the currently known population of close-in super-Earths observed around K-dwarf stars and their system characteristics. Additionally, we investigate whether the planetary systems that we form allow us to decide which initial conditions are the most favorable. Methods. We performed 48 high-resolution N-body simulations of planet formation via planetesimal accretion using the existing GENGA software running on GPUs. In the simulations we varied the initial protoplanetary disk mass and the solid and gas surface density profiles. Each simulation began with 12 000 bodies with radii of between 200 and 2000 km around two different stars, with masses of 0.6 and 0.8 M⊙. Most simulations ran for 20 Myr, with several simulations extended to 40 or 100 Myr. Results. The mass distributions for the planets with masses between 2 and 12 M⊕ show a strong preference for planets with masses Mp < 6 M⊕ and a lesser preference for planets with larger masses, whereas the mass distribution for the observed sample increases almost linearly. However, we managed to reproduce the main characteristics and architectures of the known planetary systems and produce mostly long-term angular-momentum-deficit-stable, nonresonant systems, but we require an initial disk mass of 15 M⊕ or higher and a gas surface density value at 1 AU of 1500 g cm−2 or higher. Our simulations also produce many low-mass planets with M < 2 M⊕, which are not yet found in the observed population, probably due to the observational biases. Earth-mass planets form quickly (usually within a few million years), mostly before the gas disk dispersal. The final systems contain only a small number of planets with masses Mp > 10 M⊕, which could possibly accrete substantial amounts of gas, and these formed after the gas had mostly dissipated. Conclusions. We mostly manage to reproduce observed K-dwarf exoplanetary systems from our GPU simulations.

Wide-field CO isotopologue emission and the CO-to-H2 factor across the nearby spiral galaxy M101

Carbon monoxide (CO) emission constitutes the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H2 conversion factor, α12CO(1−0), links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of α12CO(1−0) across all environments within and across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using spectral line observations of CO and its isotopologues mapped across a nearby galaxy, we can constrain the molecular gas conditions and link them to a variation in α12CO(1−0). Here, we present new, wide-field (10 × 10 arcmin2) IRAM 30-m telescope 1 mm and 3 mm line observations of 12CO, 13CO, and C18O across the nearby, grand-design, spiral galaxy M101. From the CO isotopologue line ratio analysis alone, we find that selective nucleosynthesis and changes in the opacity are the main drivers of the variation in the line emission across the galaxy. In a further analysis step, we estimated α12CO(1−0) using different approaches, including (i) via the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) local thermal equilibrium (LTE) based measurements using the optically thin 13CO(1–0) intensity. We find an average value of ⟨α12CO(1 − 0)⟩ = 4.4 ± 0.9 M⊙ pc−2 (K km s−1)−1 across the disk of the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based α12CO(1−0) values are lower by a factor of 2–3 across the disk relative to the dust-based result. Accounting for α12CO(1−0) variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy’s center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if α12CO(1−0) variations are not accounted for.

Distributions of the Density and Kinetic Temperature of the Molecular Gas in the Central Region of NGC 613 Using Hierarchical Bayesian Inference

We present position–position–velocity (PPV) cubes of the physical and chemical properties of the molecular medium in the central 1.2 kpc region of the active galaxy NGC 613 at a PPV resolution of 0.″8 × 0.″8 × 10 km s−1 (0.″8 = ∼68 pc). We used eight molecular lines (13CO(1–0), C18O(1–0), HCN(1–0), HCO+(1–0), CS(2–1), HCN(4–3), HCO+(4–3), and CS(7–6)) obtained with the Atacama Large Millimeter/submillimeter Array. Non-LTE calculation with hierarchical Bayesian inference was used to construct PPV cubes of the gas kinetic temperature (T kin), molecular hydrogen volume density (), column densities (), and fractional abundances of four molecules (12C18O, HCN, HCO+, and CS). The derived , , and T kin ranged from 103.21−3.85 cm−3, 1020.8−22.1 cm−2, and 102.33−2.64 K, respectively. Our first application of the non-LTE method with the hierarchical Bayesian inference to external galaxies yielded compatible results compared with the previous studies of this galaxy, demonstrating the efficacy of this method for application to other galaxies. We examined the correlation between gas surface density (converted from ) and the star formation rate ΣSFR obtained from the 110 GHz continuum flux map and found two distinct sequences in the –ΣSFR diagram; the southwestern subregion of the star-forming ring exhibited a ∼0.5 dex higher star formation efficiency (SFE; ΣSFR/) than the eastern subregion. However, they exhibited no systematic difference in , which is often argued as a driver of SFE variation. We suggest that the deficiency of molecular gas in the southwestern subregion, where no significant gas supply is evident along the offset ridges in the bar, is responsible for the elevated SFE.

Drag on a sphere in a slow flow of a binary mixture of rarefied gases

The drag force on a sphere in an isothermal binary mixture of rarefied gases is calculated on the basis of the McCormack model for the linearized Boltzmann equation. The diffuse scattering of the gas–surface interaction law is assumed as the boundary condition. The rigid-spheres model of interatomic interaction potential is used as well as ab initio potentials for helium, argon, and krypton gases. The force is obtained in a wide range of the gas rarefaction, which covers the free molecular, transition, and slip regimes of the gas flow. In the free molecular and slip flow regimes, the problem is solved analytically, while in the transition regime, the system of kinetic equations is solved numerically via the discrete velocity method optimized to overcome the problem of discontinuity of the distribution function of molecular velocities on the convex surface. The calculations are carried out for the mixtures helium–argon and helium–krypton at 300 K. In the slip flow regime, the data available in the literature for the viscous slip coefficient of the helium–argon mixture are used, while for the mixture helium–krypton, it is calculated. The influence of the interatomic interaction potential, molar fraction, and ratio of atomic mass of species in the mixture on the drag force is analyzed.

Rarefied gas–surface interaction under multi-incidence parameter coupling conditions: A molecular dynamics simulation

An effective gas–surface interaction model is crucial for accurately predicting the aerodynamic thermal properties of hypersonic vehicles. However, the applicability of classical interface models has been severely challenged under hypersonic conditions since the boundary properties predicted by different gas-surface interaction models vary greatly, which seriously threatens the flight safety. Therefore, in order to accurately describe gas–surface interaction under hypersonic conditions, we analyzed the reflection behavior of the gas on a silicon-based surface through molecular dynamics simulations and presented a modified Maxwell model. Furthermore, the dependence of model parameters on incident energy, angle, and gas type was also obtained. By analyzing gas–surface interaction time and potential well depth, the endogenous mechanism of the accommodation coefficient variation under multi-incidence parameter coupling conditions is revealed. These results not only elucidate the microscopic mechanism of gas–surface interaction but also provide an important basis for establishing accurate boundary conditions for larger-scale simulations.