Low-density Regions Research Articles

Magnetic Fields of the Starless Core L 1512

We present JCMT POL-2 850 μm dust polarization observations and Mimir H-band stellar polarization observations toward the starless core L 1512. We detect the highly ordered core-scale magnetic field traced by the POL-2 data, of which the field orientation is consistent with the parsec-scale magnetic fields traced by Planck data, suggesting the large-scale fields thread from the low-density region to the dense core region in this cloud. The surrounding magnetic field traced by the Mimir data shows a wider variation in the field orientation, suggesting there could be a transition of magnetic field morphology at the envelope-scale. L 1512 was suggested to be presumably older than 1.4 Myr in a previous study via time-dependent chemical analysis, hinting that the magnetic field could be strong enough to slow the collapse of L 1512. In this study, we use the Davis–Chandrasekhar–Fermi method to derive a plane-of-sky magnetic field strength (B pos) of 18 ± 7 μG and an observed mass-to-flux ratio (λ obs) of 3.5 ± 2.4, suggesting that L 1512 is magnetically supercritical. However, the absence of significant infall motion and the presence of an oscillating envelope are inconsistent with the magnetically supercritical condition. Using a virial analysis, we suggest the presence of a hitherto hidden line-of-sight magnetic field strength of ∼27 μG with a mass-to-flux ratio (λ tot) of ∼1.6, in which case both magnetic and kinetic pressures are important in supporting the L 1512 core. On the other hand, L 1512 may have just reached supercriticality and will collapse at any time.

Towards Low Iridium Loading in Proton-Exchange Membrane Water Electrolysis: Full-Cell Performance of IrOx@TiO2 Core-Shell Particles in Anode Catalyst Layers

Hydrogen as a promising energy carrier to bridge the gap between supply and demand of renewable energies can be produced by proton-exchange membrane water electrolysis (PEMWE). However, the scarcity of iridium limits the widespread and large-scale implementation of PEMWE. Thus, the state-of-the-art iridium loading for the oxygen evolution reaction (OER) of ~1-2 mgIr·cm-2 must be distinctly reduced.1, 2 A decrease in loading involves a reduction of the catalyst layer (CL) thickness. The reduction is, however, only possible down to a certain threshold (≤ 0.5 mgIr·cm-2), below which the CL shows cracks leading to incomplete connection of the catalyst particles.1, 3 To overcome this issue, we developed a novel catalyst structure with reduced iridium content (< 50 wt% Ir). The synthesized catalyst exhibits a core-shell structure consisting of an insulating TiO2 core (d ≈ 1 µm) coated with iridium oxide (IrOx@TiO2). This optimized structure enables a reduction of the loading while maintaining the thickness and, thus, the electrical connection within the CL.In two related contributions, we demonstrate the suitability of the catalyst by a comprehensive analysis: The first contribution mainly focuses on structural optimization, activity, and stability investigations of the catalyst powder. In this contribution, we optimize the catalyst layer aiming towards high catalyst utilization and in-plane conductivity at iridium loadings below 0.5 mgIr·cm-2. PEMWE full-cell performance is investigated in catalyst-coated membrane (CCM) configuration and the structure and thickness of the CL are characterized by SEM cross-sections. The results of the full-cell tests with our catalyst in comparison to a state-of-the-art IrO2/TiO2 catalyst are shown in Fig. 1. The polarization curve of our catalyst exhibits high OER activity and performance at low loadings (Graph A: 1.76 V @ 1.5 A·cm-2). In comparison to the state-of-the-art catalyst (Graph B), our catalyst outperforms the commercial catalyst in terms of activity. This becomes evident when comparing the HFR-free polarization curves in the low current density region. It could be assigned to a higher catalyst utilization due to the maintained CL thickness or, more likely, to a higher activity of the catalyst itself.1 The performance difference between our and the state-of-the-art catalyst in higher current density regions mainly stems from ohmic losses in the electrode as can be seen from the high frequency resistance (HFR) in Fig. 1. These ohmic losses, however, can supposedly be assigned to the material itself and not to a decreased in-plane conductivity due to a disconnection of parts of the CL.In conclusion, we successfully synthesized an IrOx@TiO2 catalyst with a core-shell structure. This catalyst enables the fabrication of catalyst layers with decreased loadings below 0.5 mgIr·cm-2 while maintaining the catalyst utilization demonstrated in full-cell testing. Therefore, our newly developed catalyst contributes to a scale-up of the PEMWE technology by drastically reducing the demand for the scarce element iridium.Figure 1: Polarization curves, HFR-free polarization curves and HFR values at 80°C (ambient pressure, 100 mlH2O·min-1 on anode) of CCMs fabricated with different anode catalyst types and loading. A: in-house core-shell catalyst with 0.2 mgIr·cm-2 and B: commercial catalyst with 0.3 mgIr·cm-2 (IrO2/TiO2 with 75 wt% Ir, Umicore).References M. Möckl, M. F. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva and H. A. Gasteiger, J. Electrochem. Soc., 169(6), 64505 (2022).C. Minke, M. Suermann, B. Bensmann and R. Hanke-Rauschenbach, Int. J. of Hydrog. Energy, 46(46), 23581–23590 (2021).M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 165(5), F305-F314 (2018). Figure 1

Single Cell Performance and Durability of Pt and Pt-Based Catalysts Supported on Mesoporous Carbon

Introduction In Japan, an extremely high cell performance, i.e., 0.84 V at 0.2 A/cm2 and 0.70 V at 3.0 A/cm2, is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. In addition to high oxygen reduction reaction (ORR) activity of the Pt and the Pt-based cathode catalysts for increasing cell voltage in the low current density (LCD) region, high oxygen diffusivity is crucial for achieving the cell voltage in the high current density (HCD) region [2]. Therefore, it is considered that carbon supports for the catalyst NPs would play an important role for realizing the high cell performance. In this study, a mesoporous carbon (MPC) was selected as the support for Pt and Pd@Pt core-shell structured catalyst NPs and cell performance of the catalysts were compared with that of the catalyst supported on a conventional Ketjen Black (KB). Furthermore, durability of the MPC support against a high potential range was investigated. Experimental KB-600JD (SBET: 1,335 m2/g, LION) and MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO [3]) were used as carbon supports. The Pt/MPC catalyst, Pt diameter of 2.5 nm and metal loading of 50 wt.%, was synthesized through impregnation of Pt(NO2)2(NH3)2 followed by thermal reduction in N2 at 400oC. Pt/Pd/KB and Pt/Pd/MPC core-shell catalysts were synthesized by a direct displacement reaction (DDR) method [4]. Diameter of Pd@Pt NPs was 5.8 nm and Pt and Pd metal loading were 20 wt.% and 30 wt.%, respectively. Cyclic voltammogram was recorded in Ar-saturated 0.1 M HClO4 at 25oC with a scan rate of 50 mV/s and ORR activity was evaluated in O2-saturated 0.1 M HClO4 at 25oC with a positive potential scan rate of 10 mV/s at 1,600 rpm. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2. Nafion DE2020 was used as an ionomer for the catalyst ink preparation (I/C ratio was set to 0.83-1.2) and Nafion NRE 211 with 25 µm in thickness was used as a membrane. Pt loading in the cathode catalyst layer was set to 0.1 mg/cm2. H2 and air were supplied to the anode and the cathode by 418 NmL/min. and 988 NmL/min., respectively, and pressurized to 150 kPa at gas outlets. The cell temperature was set to 80oC and humidified from 35% to 95% relative humidity (RH). H2 utilization was 5% at a current density of 3.0 A/cm2. Durability of the carbon supports was evaluated by using the single cell. H2 and N2 were fed to anode and cathode, respectively, and cathode potential was swept from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mV/s for 1,000 cycles. Results and Discussion Pore size distribution of the carbon supports is shown in Figure 1. Both KB-600JD and MPC (CNovel MH-18) carbon supports have mesopores, and the MPC exhibits a narrower pore size distribution centered at 4 nm. Figure 2 demonstrates I-V performance of Pd@Pt core-shell catalysts, indicating that Pt/Pd/MPC catalyst shows a higher cell voltage in HCD region (> 2 A/cm2) than that of Pt/Pd/KB-600JD catalyst. Interconnectivity of the mesopores analyzed by 3D-TEM observation of the carbon supports is depicted in Figure 3. It is clear that density of interconnected mesopores is much higher in the MPC support, which is considered to contribute to the higher cell voltage in HCD region of the Pt/Pd/MPC catalyst. Figure 4 displays cross-sectional SEM image of the Pt/Pd/MPC catalyst. In addition to the well-interconnected mesopores (Figure 3), macropores coexist in the MPC support, which is also considered to contribute to the higher cell voltage in HCD region.A high potential durability test was performed by using potential cycling of 1.0-1.5 V vs. RHE for 1,000 cycles at 80oC. Thickness of Pt/MPC cathode catalyst layer was decreased by 36% after the durability test, while the decrease was mitigated to 14% with a heat treatment of the MPC support at 2,200oC in Ar. (Figures 5). As is well known, it was demonstrated that the high temperature heat treatment has a positive effect for durability improvement of the carbon support against the high potential range.This study was partly supported by NEDO, Japan.

Surface Forces Characterization of Concentrated PMMA Brush Layers under Applied Load and Shear.

Concentrated polymer brushes (CPBs) are known to exhibit excellent lubrication properties. However, the frictional behaviors of CPBs vary, depending on their preparation and operating conditions. In order to understand such complicated properties, it is necessary to determine their structures and correlate them with their properties, during shear motion. In this study, we employed surface forces and resonance shear measurement (RSM) as well as refractive index measurement using fringes of equal chromatic order (FECO) for studying the structure of the CPBs of poly(methyl methacrylate) (PMMA) in toluene. The obtained elastic (ks) and viscous (bs) parameters based on the RSM for the PMMA-PMMA were higher than those obtained for PMMA-silica over the entire distance range. With the increasing shear amplitude on the PMMA-PMMA under an applied load, the bs value first increased and then decreased while the ks value monotonically decreased. These behaviors were consistent with those of the thicker CPBs reported in a previous paper (Soft Matter, 2019). Thus, the dynamics of the CPBs under the applied load and shear were not dependent on the thickness of the polymer brushes in this case. The density distribution of the swollen PMMA brushes along the distance in the thickness direction of the brush layer was estimated by using the measured refractive index values, showing that the fraction of the PMMA brushes in the outer region from the surface (20% in the thickness) was ca. 10%. This lower density region near the surface of the swollen CPBs enabled them to interpenetrate with each other. Changes in the refractive index value under shear were observed, indicating that the interpenetrated PMMA chains were pulled out with increasing shear amplitude. These results demonstrated that broader applications of CPBs are possible by regulating the friction between them under different operating conditions, even for usually lubricious CPBs.

Experimental investigation on a novel external-mixing prefilming atomizer for advanced aircraft engines

As a crucial component of aircraft engine combustors, the external-mixing prefilming atomizer exhibits significant advantages in enhancing combustion efficiency and reducing pollutant emissions. However, the intricate relationship between spray characteristics, droplet breakup mechanisms, flow fields, and geometric parameters of the external-mixing prefilming atomizer remains poorly understood. This paper presents a novel external-mixing prefilming atomizer, aiming to address this knowledge gap. Additionally, particle image velocimetry (PIV), high-speed Schlieren photography, and particle image analysis (PIA) measuring systems were employed to gain insights into the spray characteristics and droplet breakup mechanism of the external-mixing prefilming atomizer under various swirl flow fields. The obtained results reveal the formation of two distinct regions, namely a low-density area and a high-density area. Notably, the number of droplets in the low-density area, located within 0–2 cm from the central axis, is significantly lower compared to the high-density area situated further away from the central axis. The breakup of droplets in the low-density region is primarily attributed to the inner swirl airflow, while the outer swirl airflow induces droplet breakup in the high-density region. As the Reynolds number of the incoming flow increases, the air-liquid ratio (ALR) and Weber number also increase, thereby enhancing the fuel breakup effect. The Schlieren results reveal a two-stage spray development process. The initial stage is characterized by the dominant momentum of the dense spray, with the spray tip penetration (STP) exhibiting a relatively slow increase. The aerodynamic force of airflow plays a crucial role in governing the breakup and transport of droplets, leading to a linear variation of STP over time. As the swirl number increases, there is an enhanced momentum exchange between droplets and airflow, resulting in accelerated spray penetration and droplet breakup.

Revealing quasi-excitations in the low-density homogeneous electron gas with model exchange–correlation kernels

Time-dependent density functional theory within the linear response regime provides a solid mathematical framework to capture excitations. The accuracy of the theory, however, largely depends on the approximations for the exchange–correlation (xc) kernels. Away from the long-wavelength (or q = 0 short wave-vector) and zero-frequency (ω = 0) limit, the correlation contribution to the kernel becomes more relevant and dominant over exchange. The dielectric function, in principle, can encompass xc effects relevant to describe low-density physics. Furthermore, besides collective plasmon excitations, the dielectric function can reveal collective electron–hole excitations, often dubbed “ghost excitons.” Besides collective excitons, the physics of the low-density regime is rich, as exemplified by a static charge-density wave that was recently found for rs &amp;gt; 69, and was shown to be associated with softening of the plasmon mode. These excitations are seen to be present in much higher density 2D homogeneous electron gases of rs ≳ 4. In this work, we perform a thorough analysis with xc model kernels for excitations of various nature. The uniform electron gas, as a useful model of real metallic systems, is used as a platform for our analysis. We highlight the relevance of exact constraints as we display and explain screening and excitations in the low-density region.

Studying the Effects of Annealing on the Mechanical Properties and Microstructure of Ultrasonic Vibration Welds

The impact of heat treatment on the mechanical characteristics and microstructure of an ultrasonic vibration metal inert gas (MIG) weld is examined in this study. The findings show that the heat treatment method has a significant influence on the mechanical characteristics and structure of the welding specimens. The annealing will increase the grains’ size, thereby reducing the hardness and increasing the ductility of the joint. Due to the impact of ultrasonic during welding, the microstructure in fusion zone of both annealed and non-annealed specimens showed small and fine grain structure (with the grain size being measured at less than 40µm). The result also demonstrates that during annealing, the carbon tended to diffuse from the area of high density to the region of low density, resulting in a reduction in hardness compared to the initial sample. Also, it was discovered that elements that lower weld strength (such as the irregularity of the grain structure and the dendritic structure produced by metal crystallization) significantly decreased.

Grain boundary migration behavior and microstructure evolution during multilayer additive hot-compression bonding

Multilayer additive hot-compression bonding (MAHCB) was a new technology to prepare large-size homogeneous materials, the strain hardening at the bonding interface was an urgent issue for its application. In this paper, the effect of grain boundary migration on strain hardening was studied by the MAHCB process of pure copper. The results show that prolonging the holding time can significantly promote both interfacial grain boundary migration (IGBM) and matrix grain boundary migration (MGBM). The bonding interface became tortuous due to the promotion of IGBM, accompanied by the reduction in dislocation density at the interface and the gradual elimination of interfacial defects. It occurred mainly by the evolution of the triple junction grain boundary from T-type to equal type, which was driven by the decrease in the interface energy. On the other hand, the growth of dynamic recrystallization (DRX) grains was improved with MGBM, and enlarged the low dislocation density region. With the combined effect of both, strain hardening was completely eliminated, when the strain was 0.15 and the holding time was 3 h. Then the optimal bonding properties were obtained, the average tensile-shear strength and elongation were 191.2 MPa and 212 %, and the recovery rates were 99.7 % and 98.9 %, respectively, compared to the as-cast pure Cu samples.

Exploring the natural ventilation potential for supertall buildings considering vertical meteorology: A case study in Harbin, China

The natural ventilation strategy is gaining increasing attention from architects due to its advantages in reducing energy consumption and maintaining a healthy indoor environment. For supertall buildings, the vertical variation in outdoor meteorology has inspired us to explore the energy-saving potential by adopting natural ventilation strategy. This study investigates the impact of vertical variation in urban meteorology on the natural ventilation potential (NVP) of supertall buildings based on numerical simulation and field observation. Firstly, the Weather Research and Forecasting (WRF) model is used to simulate the urban meteorology of the study region by integrating the local climate zone (LCZ) map. Next, meteorological data from sounding experiments by unmanned aerial vehicle (UAV) and monitoring stations are used to validate the simulated result, and thereby to explore the vertical meteorology in urban area. Finally, the vertical NVP in supertall buildings is quantitatively assessed considering the impact of LCZ in urban areas based on thermal comfort models. The findings reveal that atmospheric temperature decreases approximately linearly with increasing altitude, with a maximum temperature gradient of up to −0.82 °C/100 m. Static models underestimated the annual natural ventilation hours (NVHs) by at least 348 h compared to the adaptive thermal comfort model. Compared to low-density building regions, the higher NVP is indicated in high-density building regions. This study aims to illustrate the importance of considering the vertical variations of urban meteorology, and is expected to assist architects and policy makers in quantifying the energy saving potential of natural ventilation in supertalls.

A numerical model for simulation of two-phase flows interaction with flexible slender bodies

This paper introduces a fluid–structure interaction (FSI) model for simulation of the coupled dynamics between two-phase flows and elastic slender structures. This model is extended from the early FSI model for single-phase flows by Wang et al. [“A coupled flow and beam model for fluid–slender body interaction,” J. Fluids Struct. 115, 103781 (2022)] to two-phase flows with interfaces of two liquids or free surfaces between gas and liquid. To better capture the interface movement of the two-phase flows and the interaction with the deformable structure, a consistent mass and momentum flux scheme is developed to reduce the spurious oscillation of fluid velocities near the interface, especially in the lower density (e.g., air) region. The proposed model is validated by a series of two-dimensional laboratory experiments of flow impact on a deformable plate, demonstrating that the model has good capabilities of conserving mass and momentum during the process of plate deformation by impulsive flow forces. The model is also applied to the investigation of three-dimensional dam break flow impact on a column of elastic plates. The complex interaction between the plates and the flow is discussed based on the simulation results.

Trajectories of square lattice staircase polygons

The distribution of monomers in a coating of grafted and adsorbing polymers is modelled using a grafted staircase polygon in the square lattice. The adsorbing staircase polygon consists of a bottom and a top lattice path (branches) and the asymptotic probability density that vertices in the lattice are occupied by these paths is determined. This is a model consisting of two linear polymers grafted to a hard wall in a coating. The probability that either the bottom path, or the top path, of a staircase polygon passes through a lattice site with coordinates , for 0 < ϵ < 1 and δ ≥ 0, is determined asymptotically as n → ∞ . This gives the probability density of vertices in the staircase polygon in the scaling limit (as n → ∞ ): = The densities of other cases, grafted and adsorbed, and at the adsorption critical point (or special point), are also determined. In these cases the most likely and mean trajectories of the paths are determined. The results show that low density regions in the distribution induce entropic forces on test particles which confine them next to the hard wall, or between branches of the staircase polygon. These results give qualitative mechanisms for the stabilisation of a drug particle confined to a polymer coating in a drug delivery system such as a drug-eluding stent covered by a grafted polymer.

Thermal conductivity analysis of natural fiber-derived porous thermal insulation materials

Natural fibers, derived from plants such as wood, hemp, straw and cotton, have been explored for the fabrication of porous structures for thermal insulation applications due to their widespread availability, sustainability, and cost-effectiveness. Understanding the fundamental heat transfer mechanisms within natural fiber-derived porous structures is crucial for both optimized geometric design and real-world insulation applications. Herein, we developed a theoretical framework considering geometric parameters, including pore size, fiber diameter and porosity (i.e., density), to examine the contribution of various heat transfer modes (i.e., conduction, convection, and radiation) on the effective thermal conductivity of porous structures derived from natural fibers. Our results indicate that thermal radiation is largely responsible for the rapid increase in effective thermal conductivity of the natural fiber-derived porous insulations in low-density regions (< 50 kg/m3) and that natural convection rarely occurs within these materials. The insulation materials derived from natural fibers with diameters in the micron range (5–50 μm) can achieve their minimum thermal conductivity at an optimal density of 50–90 kg/m³. Effective strategies to lower the effective thermal conductivity of natural fiber-derived porous materials include increasing porosity to curtail solid conduction, incorporating nanoscale pores by using nanosize fibers to diminish gaseous thermal conductivity. This research offers valuable insights into the heat transfer mechanisms in natural fiber-derived materials and should guide the structural design and optimization process toward developing super-thermal insulation materials derived from natural fibers.

Eulerian investigation of the mechanism of Ultra-low NOx emissions from CFB boilers with finer bed material

Industrial practice has confirmed that decreasing the bed material size substantially diminishes nitrogen oxide (NOx) emissions in coal-fired circulating fluidized bed (CFB) boilers. This reduction allows for compliance with the stringent ultra-low emissions standard (NOx < 50 mg/Nm3@6%O₂, dry) proposed by the Chinese government, all without the need for supplementary denitrification devices. Consequently, it becomes essential to delve into the mechanisms through which particle size, as a gas–solid flow characteristic, directly affects the chemical reaction dynamics within the coal combustion process. Due to the difficulty of field tests, this paper employs the Eulerian-Eulerian numerical method suitable for wide particle size distributions to investigate the impact of bed material particle size on gas–solid flow and NOx concentration. Furthermore, the generalized QC-EMMS drag model is utilized to predict the heterogeneous flow across varying particle size conditions. Industrial test data validated the simulation accuracy, with prediction errors of bed pressure drop, temperature, and NOx emission concentration ranging from 5 % to 8.1 %. The analysis revealed that the particle concentration distribution heterogeneity decreases as the particle size diminishes. The variation trends of NOx with particle size exhibit distinct differences between the lower dense phase region and the upper dilute phase region within the bed. For finer bed material particles, the high NOx concentration generated in the dense phase region experiences a more thorough reduction in the dilute phase region. Through the analysis of material concentration and reaction rate, it is believed that this phenomenon is related to the increased catalytic effect of fine ash particles in the dilute phase region, that is, the reduction in particle size leads to an increase in catalytic specific surface area, enhancing NOx reduction, thus achieving ultra-low emissions at the outlet. These research results provide theoretical support for further development of ultra-low emission coal-fired CFB boilers.

Density-graded Voronoi honeycombs – A local transversely isotropic description

Density-graded cellular materials, in which their relative density varies with location, are amenable to design for multi-functions, and have been used in mechanical buffers and protective structures. To estimate the force–deformation responses of density-graded cellular materials efficiently without resorting to modelling individual cells, a constitutive model based on local transverse isotropy is adopted. Metallic Voronoi honeycomb-like specimens with cell sizes that vary with position, are fabricated via 3D printing, to represent a density-graded cellular material with local transverse isotropy that varies with position. From quasi-static compression tests and cell-based finite element (FE) simulations, the influence of relative density on the continuum constitutive model is investigated and demonstrated to be significant. The model is then used to predict the global stress–strain response and deformation of cellular materials with positive and negative density-gradients, defined along the loading direction, subjected to uniaxial quasi-static compression, quasi-static indentation, and dynamic compression. The results show that for quasi-static compression, severe plastic deformation progresses from the low relative density region to the high relative density region; however, impact loading and indentation of negative density-gradient Voronoi honeycombs generates two zones of gross deformation – in the vicinity of the applied load and the opposite lower density end. Predictions based on the proposed constitutive model display good correlation with experimental and cell-based FE results.

HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30 m maps of the Orion B GMC

Context. Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (n(H2) &gt; several 104 cm−3 or AV &gt; 8 mag) and linking their line luminosity with the star formation rate is of critical importance. Aims. Our aim is to determine the origin and physical conditions of the HCN-emitting gas and study their relation to those of other molecules. Methods. In the context of the IRAM 30m ORION-B large program, we present 5 deg2 (~250 pc2) HCN, HNC, HCO+, and CO J =1–0 maps of the Orion B GMC, complemented with existing wide-field [C I] 492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. Furthermore, we study the HCN/HNC isomeric abundance ratio with updated photochemical models. Results. We spectroscopically resolve the HCN J = 1–0 hyperfine structure (HFS) components (and partially resolved J = 2−1 and 3−2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN J = 1−0 luminosity, L′(HCN J = 1−0) = 110 K km s−1 pc−2, arises from AV &lt; 8 mag. The HCN/CO J = 1−0 line intensity ratio, widely used as a tracer of the dense gas fraction, shows a bimodal behavior with an inflection point at AV &lt; 3 mag typical of translucent gas and illuminated cloud edges. We find that most of the HCN J = 1−0 emission arises from extended gas with n(H2) &lt; 104 cm−3, and even lower density gas if the ionization fraction is χe ≥ 10−5 and electron excitation dominates. This result contrasts with the prevailing view of HCN J = 1−0 emission as a tracer of dense gas and explains the low-AV branch of the HCN/CO J = 1−0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~ 0.1) at AV &lt; 3 mag correspond to regions of high [C I] 492 GHz/CO J = 1−0 intensity ratios (&gt;1) characteristic of low-density photodissociation regions. The low surface brightness (≲ 1 K km s−1) and extended HCN and HCO+ J = 1−0 emission scale with IFIR – a proxy of the stellar far-ultraviolet (FUV) radiation field – in a similar way. Together with CO J = 1−0, these lines respond to increasing IFIR up to G0 ≃ 20. On the other hand, the bright HCN J = 1−0 emission (&gt; 6 K km s−1) from dense gas in star-forming clumps weakly responds to IFIR once the FUV field becomes too intense (G0 &gt; 1500). In contrast, HNC J = 1−0 and [C I] 492 GHz lines weakly respond to IFIR for all G0. The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages. Conclusions. Given the widespread and extended nature of the [C I] 492 GHz emission, as well as its spatial correlation with that of HCO+, HCN, and 13CO J = 1−0 lines (in this order), we argue that the edges of GMCs are porous to FUV radiation from nearby massive stars. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and it leads to a relatively low value of the dense gas mass to total luminosity ratio, α (HCN) = 29 M⊙/(K km s−1pc2) in Orion B. As a corollary for extragalactic studies, we conclude that high HCN/CO J = 1−0 line intensity ratios do not always imply the presence of dense gas, which may be better traced by HNC than by HCN.

Large eddy simulation of shock-combustion-film interaction at hydrogen injection in a model scramjet combustor

Supersonic film cooling, which utilizes fuel onboard, is a promising method for protecting the wall of a scramjet combustor from intense heat and friction loads during high-speed flight. Large eddy simulation (LES) is conducted to study the coupled interactions among shock, combustion, and film. First, reference solutions for individual effects of shock and combustion on film performance are computed. Then the coupled effects are analyzed based on the reactive hydrogen film impinged by an oblique shock at a deflection angle of 2° The results show that the incident shock will enhance the transport processes after the impingement point by an increased turbulence production in the separation bubble. On the contrary, boundary layer combustion has the effect of constraining the transport processes in the mixing layer due to the low-density region caused by thermal expansion. This leads to a decrease in the mixing rate and wall friction. Although the heat flux transported to the film is also reduced by combustion, the wall temperature still increases due to the excessive compensation of heat from intense endothermic chemistry. Besides, the boundary layer combustion of hydrogen film exhibits self-limited characteristic since the mixing process is further delayed under combustion circumstance. Therefore, an incident shock will break the mixing bottleneck of the boundary layer combustion, leading to a more intense reaction with a more wrinkled flame front. The current findings indicate that the additional gaining of cooling and friction reduction performance is exclusive, but also point out the direction for simultaneous improvement of the two performances.

Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

ABSTRACT Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, hth/r = 0.3, 0.1, and 0.03. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with hth/r = 0.03. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of z/r = ±0.03 and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGNs). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass-loss rates in good agreement with the properties of ultrafast outflows observed in AGN.

The Applications and Pitfalls of Cone-Beam Computed Tomography-Based Synthetic Computed Tomography for Adaptive Evaluation in Pencil-Beam Scanning Proton Therapy.

The study evaluates the efficacy of cone-beam computed tomography (CBCT)-based synthetic CTs (sCT) as a potential alternative to verification CT (vCT) for enhanced treatment monitoring and early adaptation in proton therapy. Seven common treatment sites were studied. Two sets of sCT per case were generated: direct-deformed (DD) sCT and image-correction (IC) sCT. The image qualities and dosimetric impact of the sCT were compared to the same-day vCT. The sCT agreed with vCT in regions of homogeneous tissues such as the brain and breast; however, notable discrepancies were observed in the thorax and abdomen. The sCT outliers existed for DD sCT when there was an anatomy change and for IC sCT in low-density regions. The target coverage exhibited less than a 5% variance in most DD and IC sCT cases when compared to vCT. The Dmax of serial organ-at-risk (OAR) in sCT plans shows greater deviation from vCT than small-volume dose metrics (D0.1cc). The parallel OAR volumetric and mean doses remained consistent, with average deviations below 1.5%. The use of sCT enables precise treatment and prompt early adaptation for proton therapy. The quality assurance of sCT is mandatory in the early stage of clinical implementation.

Energy absorption and impact resistance of hybrid triply periodic minimal surface (TPMS) sheet-based structures

Triply periodic minimal surface (TPMS)-sheet cellular structures have attracted considerable attention owing to their light weight and superior mechanical properties. In this study, the compressive performance and energy absorption of a sheet-based structure comprising of Schwarz primitives (P) and cubic cellular components were investigated. The hybrid cellular structure synthesises stretching-dominated and bending-dominated deformation mechanisms, indicating a trade-off between the energy absorption modes of types-I and -II. Furthermore, numerical simulations demonstrated that the hybrid structure outperforms the independent Schwarz P and cubic structures in terms of impact energy dissipation capability. Additionally, the study identified that the compressive performance of novel cellular structures is significantly affected by the cell size and scale factor. Reducing the size and increasing the scale factor can improve the specific energy absorption of the structure. The Schwarz P+cubic hybrid structures exhibit specific energy absorption of 18.76–30.16 kJ/kg and a relative density of 6.8 %, positioning them in the low-density and high-performance region of the Ashby chart. Overall, the Schwarz P + cubic hybrid cellular structures possess excellent crashworthiness and show great potential for protective and impact engineering applications.

Molecular dynamics simulation of deposition of amorphous carbon films on sapphire surfaces

The growth of amorphous carbon films on a sapphire surface was investigated using classical molecular dynamics simulation. The kinetic energy of carbon particles was set as 10 eV and ReaxFF potential was used to express the interaction between different kinds of particles. The results of the temperature distribution in both deposition time and deposition space are reported. Simulation results reveal that the grown amorphous carbon film consists of four regions, namely interlayer, low density, stable growth, and surface regions. In the interlayer region, the interlayer between substrate and pure carbon film is formed. In the low density region, a pure carbon film is grown while the film density decreases initially and then increases. In the stable growth region, the film density remains almost constant. The film density decreases rapidly in the surface region. The radial distribution function (RDF) analysis suggests that a structure similar to that of diamond exists in the stable growth region of the film. The lower film density in the low density and surface regions was interpreted to indicate the existence of abundant sp1 chain structures, which is supported by the depth profile of the sp fractions. The present results are in good agreement with previous experimental and simulation results and demonstrate the suitability of the ReaxFF potential in the simulation of amorphous carbon growth on sapphire substrate. Our study provides a good starting point for the simulation study of amorphous carbon films on sapphire substrates.