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•A mechanistic model of wire arc additive manufacturing is validated experimentally.•3D transient model considers mass addition, heat transfer, fluid flow and free surface.•New ...results on cooling rates, solidification parameters, droplet impact & finger penetration.•Effects of power, welding speed, wire diameter and feed rate are evaluated.
Structure, properties and serviceability of components made by wire arc additive manufacturing (WAAM) depend on the process parameters such as arc power, travel speed, wire diameter and wire feed rate. However, the selection of appropriate processing conditions to fabricate defect free and structurally sound components by trial and error is expensive and time consuming. Here we develop, test and utilize a three-dimensional heat transfer and fluid flow model of WAAM to calculate temperature and velocity fields, deposit shape and size, cooling rates and solidification parameters. The calculated fusion zone geometries and cooling rates for various arc power and travel speed and thermal cycles considering convective flow of molten metal agreed well with the corresponding experimental data for H13 tool steel deposits. It was found that convection is the main mechanism of heat transfer inside the molten pool. Faster travel speed enhanced the cooling rate but reduced the ratio of temperature gradient to solidification growth rate indicating increased instability of plane front solidification of components. Higher deposition rates could be achieved by increasing the heat input, using thicker wires and rapid wire feeding.
We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and ...coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around 10^{-5} mbar, minimal temperatures along the cavity axis in the millikelvin regime are observed. Simultaneously, the center-of-mass (c.m.) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred millikelvin. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behavior of the c.m. temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined.
Global urbanization drastically reshapes the land–atmosphere interactions, biogeochemical cycles, and ecosystems in the integrated earth system. Urban trees are used extensively for providing ...desirable ecosystem services, especially to mitigate elevated thermal stress in cities. Urban trees differ markedly from their natural counterparts in the physiology due to their close interactions with anthropogenic stressors. Here we present the cooling capacity of urban trees in response to thermal extremes in cities of the contiguous United States. The cooling capacity is quantified as the surface cooling rate, which is expressed as the negative ratio of land surface temperature (LST) changes to fractional tree cover (FTC) changes, i.e., –ΔLST/ΔFTC. Results show that the surface cooling rate is dominated by plant transpiration up to 1.336 °C per percentage of FTC in heat waves; its temperature dependence remarkably resembles the thermodynamic liquid-water-vapor equilibrium. Urban trees also exert pseudo cooling effect in cold waves in contrast to the anthropogenic heat emission. The average surface cooling rate in cold waves is 0.022 °C per percentage of FTC, which is much smaller than that in heat waves (0.202 °C per percentage of FTC). The enhanced cooling capacity of urban trees will enable their provision of better ecosystem services to the urban environment for projected future increase of extreme heat.
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•The surface cooling rate of urban trees during heat and cold waves is quantified.•Temperature dependence of tree cooling resembles the phase equilibrium of water.•The cooling of urban trees is dominated by plant transpiration in heat waves.•The synthetic cooling of cities is regulated by anthropogenic heat in cold waves.•Urban trees are ahead of the global change responses than their rural counterparts.
The high temperature and electron degeneracy attained during a supernova allow for the formation of a large muon abundance within the core of the resulting protoneutron star. If new pseudoscalar ...degrees of freedom have large couplings to the muon, they can be produced by this muon abundance and contribute to the cooling of the star. By generating the largest collection of supernova simulations with muons to date, we show that observations of the cooling rate of SN 1987A place strong constraints on the coupling of axionlike particles to muons, limiting the coupling to g_{aμ}<10^{-8.1} GeV^{-1}.
Medium- to long-chain alkanes can form upon cooling intermediate phases between isotropic liquid and solid crystalline, called rotator phases, where relative freedom of the molecules to rotate about ...their long axis is combined with long range translational order. Rotator phases are well documented experimentally but the mechanism of their formation at the molecular level is still not fully explained. In a previous work, we have shown that molecular dynamics simulations can produce rotator phases upon cooling of hexadecane S. Iliev et al., J. Col. Int. Sci., 2023, 638, 743. The aim of the current work is to develop a procedure to identify the specific ordered phase obtained in the simulations. The influence of the cooling rate on the freezing process of hexadecane (bulk and surfactant-interfaced to water) is tested as well. Several parameters are combined to quantify the degree of ordering and the type of phase in the studied systems. These are the tilt angle of the molecules with respect to the crystallite plane, the radial distribution function of the centre of mass of the molecules in the crystallite, the percentage of the gauche torsion angles in the molecules, the angle of the second principal axis of each molecule with respect to the x axis of the coordinate system, and estimates from Voronoi analysis. The results show that the systems form a rotator phase, which transitions gradually towards the thermodynamically most stable triclinic crystal, and the transformation progresses to different extent depending on the system. The influence of the cooling rate is related only to the size of the largest crystallite formed, the other parameters of the freezing process remain unaffected. The work also presents a robust procedure for obtaining and identifying different types of ordered phases in alkane-containing systems with thoroughly tested computational protocol and a comprehensive set of structural analyses. Several key characteristics are advanced, compared to previous research Ryckaert et al., Mol. Phys., 1989, 67, 957; Wentzel et al., J. Chem. Phys. 2011, 134, 224504, namely, a new methodology is proposed to compute the unit cell deformation parameter and azimuthal angle from MD simulation trajectories of the freezing process in alkane-containing systems. The suggested structural analysis, which is independent of the coordinate system, is applicable to any linear-chain system with polycrystalline structure.
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•The cooling rate effect on rotator phases formation in hexadecane is traced by MD.•Hexadecane exhibits multiple rotator phases upon freezing.•Slower cooling yields larger main crystallites in bulk or at an interface.•Thermodynamic and structural parameters quantify the degree of ordering.•The phase state of the systems is identified with a set of structural descriptors.
ABSTRACT
We analyse the average annual and seasonal air temperature conditions in the ‘local climate zones’ (LCZs) of Szeged, Hungary. The basis of our analysis is a 1‐year dataset from 2014 to 2015 ...for a 20‐station urban meteorological network. The network and its corresponding LCZ classes put temperature studies in Szeged into a new spatial framework to assess local climate and urban heat island (UHI) conditions. The stations were installed at locally representative sites using a Geographic Information System (GIS) method based on the standard surface parameters of the LCZ classification. The network was purposely designed to monitor thermal differences among LCZ classes in Szeged. We provide detailed site metadata for each of the monitoring stations used in the analysis. Our results show that the densely built‐up LCZ classes have higher annual and monthly mean and minimum air temperatures than structurally open and more vegetated classes, with nocturnal differences of >4 °C observed under calm, clear skies. Among select temperature indices measured in the urban LCZ classes, frost days, cooling degree‐days, and tropical nights differ markedly from the background rural LCZ classes. This difference suggests that local climatologies exist within Szeged, and that these have implications for thermal comfort, urban energy use, and urban agriculture. Finally, the evaluation of heating and cooling rates in Szeged shows an important role for LCZs in UHI analysis.
We report three-dimensional (3D) cooling of a levitated nanoparticle inside an optical cavity. The cooling mechanism is provided by cavity-enhanced coherent scattering off an optical tweezer. The ...observed 3D dynamics and cooling rates are as theoretically expected from the presence of both linear and quadratic terms in the interaction between the particle motion and the cavity field. By achieving nanometer-level control over the particle location we optimize the position-dependent coupling and demonstrate axial cooling by two orders of magnitude at background pressures of 6×10^{-2} mbar. We also estimate a significant (>40 dB) suppression of laser phase noise heating, which is a specific feature of the coherent scattering scheme. The observed performance implies that quantum ground state cavity cooling of levitated nanoparticles can be achieved for background pressures below 1×10^{-7} mbar.
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