Sharp Temperature Gradient Research Articles

Circumpolar and Regional Seascape Drivers of Genomic Variation in a Southern Ocean Octopus.

Understanding how ecological, environmental and geographic features influence population genetic patterns provides crucial insights into a species' evolutionary history, as well as their vulnerability or resilience under climate change. In the Southern Ocean, population genetic variation is influenced across multiple spatial scales ranging from circum-Antarctic, which encompasses the entire continent, to regional, with varying levels of geographic separation. However, comprehensive analyses testing the relative importance of different environmental and geographic variables on genomic variation across these scales are generally lacking in the Southern Ocean. Here, we examine genome-wide single nucleotide polymorphisms of the Southern Ocean octopus Pareledone turqueti across the Scotia Sea and the Antarctic continental shelf, at depths between 102 and 1342 m, throughout most of this species' range. The circumpolar distribution of P. turqueti is biogeographically structured with a clear signature of isolation-by-geographical distance, but with long-distance genetic connectivity also detected between East and West Antarctica. Genomic variation of P. turqueti was also associated with bottom water temperature at a circumpolar scale, driven by a genotype-temperature association with the warmer sub-Antarctic Shag Rocks and South Georgia. Within the Scotia Sea, geographic distance, oxygen and fine-scale isolation-by-water depth were apparent drivers of genomic variation at regional scales. Putative positive selection of haemocyanin (oxygen transport protein), calcium ion transport and genes linked to RNA modification, detected within the Scotia Sea, suggest physiological adaptation to the regional sharp temperature gradient (~0-+2°C). Overall, we identified seascape drivers of genomic variation in the Southern Ocean at circumpolar and regional scales in P. turqueti and contextualised the role of environmental adaptations in the Southern Ocean.

Vapor flux induced by temperature gradient is responsible for providing liquid water to hypoliths

Commonly comprised of cyanobacteria, algae, bacteria and fungi, hypolithic communities inhabit the underside of cobblestones and pebbles in diverse desert biomes. Notwithstanding their abundance and widespread geographic distribution and their growth in the driest regions on Earth, the source of water supporting these communities remains puzzling. Adding to the puzzle is the presence of cyanobacteria that require liquid water for net photosynthesis. Here we report results from six-year monitoring in the Negev Desert (with average annual precipitation of ~ 90 mm) during which periodical measurements of the water content of cobblestone undersides were carried out. We show that while no effective wetting took place following direct rain, dew or fog, high vapor flux, induced by a sharp temperature gradient, took place from the wet subsurface soil after rain, resulting in wet-dry cycles and wetting of the cobblestone undersides. Up to 12 wet-dry cycles were recorded following a single rain event, which resulted in vapor condensation on the undersides of the cobblestones, with the daily wet phase lasting for several hours during daylight. This ‘concealed mechanism’ expands the distribution of photoautotrophic organisms into hostile regions where the abiotic conditions limit their growth, and provides the driving force for important evolutionary processes not yet fully explored.

Influence of Solid-Phase and Melt-Quenching Na3Fe2(PO4)3 Polycrystal Production Technology on Their Structure and Ionic Conductivity

This article studies the influence of solid-phase (type 1 samples) and melt-quenching (type 2 samples) technological modes of obtaining Na3Fe2(PO4)3 polycrystals on their structures and ion-conducting properties. α-Na3Fe2(PO4)3 polycrystals of the 1st type are formed predominantly under an isothermal firing regime, and the synthesis of the 2nd type is carried out under sharp temperature gradient conditions, contributing to the formation of glassy precursors possessing a reactive and deformed structure, in which the crystallization of crystallites occurs faster than in precursors obtained under isothermal firing. The elemental composition of α-Na3Fe2(PO4)3 type 2 polycrystals is maintained within the normal range despite the sharp non-equilibrium thermodynamic conditions of synthesis. The microstructure of the type 1 Na3Fe2(PO4)3 polycrystals is dominated by chaotically arranged crystallites of medium (7–10 μm) and large (15–35 μm) sizes, while the polycrystals of type 2 are characterized by the preferential formation of small (3–4 μm) and medium (7–10 μm) crystallites, causing uniaxial deformations in their structure, which contribute to a partial increase in their symmetry. The advantage of type 2 polycrystals is that they have higher density and conductivity and are synthesized faster than type 1 samples by a factor of 4. The article also considers the issues of crystallization in a solid-phase precursor from the classical point of view, i.e., the process of the formation of small solid-phase nuclei in the metastable phase and their growth to large particles due to association with small crystallites using phase transitions. Possible variants and models of crystallite growth in Na3Fe2(PO4)3 polycrystals, as well as distinctive features of crystallization between two types of samples, are discussed.

Thermal Imprints of Mesoscale Eddies in the Global Ocean

Abstract Mesoscale eddies have been widely documented for their significant role in regulating oceanic heat absorption and redistribution. However, our understanding of their thermal impacts on a global scale has been hampered by the scarcity of eddy-targeted observations. Here, we perform a comprehensive global analysis of over 2 million historical hydrographic profile measurements collocated with satellite-based eddy observations between 1993 and 2019, revealing rich geographical variability in the intensity, vertical extent, and asymmetry of the temperature anomalies induced by eddies. In tropical and subtropical oceans, temperature anomalies within eddies are dominated by eddy pumping and heavily influenced by near-surface vertical stratification, resulting in a relatively shallow extent of eddy effects (around 500 m) with subsurface maximum temperature anomalies occurring near 100 m. In midlatitude main current systems with sharp horizontal temperature gradients, the impact of eddy trapping becomes evident, leading to temperature anomalies extending to depths of 1000 m, with vertical peak values exceeding 4°C between 300 and 700 m, equivalent to perturbations in the local mean-state temperature of nearly 30%. This process also results in substantial temperature anomalies on isopycnal surfaces, indicating notable cross-frontal transport of heat and water mass with varying properties. The estimated meridional heat transport by eddy movement is on the order of O(1) MW, generally one order of magnitude smaller than previous estimates of the time-varying meridional heat transport (). However, this does not imply that they are unimportant, as this long-range cross-frontal heat transport may have significant consequences on regional temperature extremes and biogeochemical environments. Significance Statement Mesoscale eddies with a horizontal scale of O(100) km play a crucial role in modulating oceanic heat absorption and redistribution. However, their activities vary significantly across the global ocean, and our understanding of their thermal impacts remains incomplete. In this study, we conduct a global analysis of the influence of eddies on ocean temperature using over 2 million historical in situ temperature and salinity profile measurements combined with satellite-based eddy observations. Our analysis reveals substantial regional variability in the intensity, vertical extent, and asymmetry of temperature anomalies within eddies of different polarities and discusses the underlying mechanisms. Additionally, we estimate the meridional heat transport caused by eddy movement within each 2° × 2° grid, providing insights into the efficient pathways for eddy heat transport in western boundary currents and their extensions. These results enhance our understanding of the global-scale thermal impact of eddies and provide valuable references for assessing their biogeochemical and ecological consequences.

Guldborgsund Arson House Fire Experiment and Numerical Investigation

AbstractThis paper describes the Guldborgsund arson house fire experiment performed in Denmark and the subsequent numerical investigation. Gas temperatures were measured with four thermocouple trees, and smoke detector activation times were recorded in all rooms. A two-step approach was used to perform the numerical modelling for reproduction of the fire scene. The measured temperatures in the room of fire origin were used as an input for back calculating the Heat Release Rate (HRR) with the two-zone model Argos. As a next step, this HRR was used in the Fire Dynamic Simulator (FDS) to predict the temperatures and the smoke detectors’ activation times in other rooms. The FDS model was partly build using output files from the laser scanning. A sensitivity analysis is presented, where the effect of nine input parameters was investigated, including HRR, the material properties, the height of the fuel bed, the fire area, level of geometrical detail of the first item ignited etc. This study showed that the measured soot deposition heights on the walls differed from the heights of measured sharp temperature gradients used to indicate the hot smoke layer. The numerical simulations resulted in less than 50% error for most of the temperature measurement points during the fuel-controlled stage of the fire and results were the most sensitive to the input HRR. Material properties in FDS had a significant influence on the computed upper-layer gas temperatures at late stages of the fire.

A Mechanism-Based Reaction-Diffusion Model for Accelerated Discovery of Thermoset Resins Frontally Polymerized by Olefin Metathesis.

Frontal ring-opening metathesis polymerization (FROMP) involves a self-perpetuating exothermic reaction, which enables the rapid and energy-efficient manufacturing of thermoset polymers and composites. Current state-of-the-art reaction-diffusion FROMP models rely on a phenomenological description of the olefin metathesis kinetics, limiting their ability to model the governing thermo-chemical FROMP processes. Furthermore, the existing models are unable to predict the variations in FROMP kinetics with changes in the resin composition and as a result are of limited utility toward accelerated discovery of new resin formulations. In this work, we formulate a chemically meaningful model grounded in the established mechanism of ring-opening metathesis polymerization (ROMP). Our study aims to validate the hypothesis that the ROMP mechanism, applicable to monomer-initiator solutions below 100 °C, remains valid under the nonideal conditions encountered in FROMP, including ambient to >200 °C temperatures, sharp temperature gradients, and neat monomer environments. Through extensive simulations, we demonstrate that our mechanism-based model accurately predicts the FROMP behavior across various resin compositions, including polymerization front velocities and thermal characteristics (e.g., Tmax). Additionally, we introduce a semi-inverse workflow that predicts FROMP behavior from a single experimental data point. Notably, the physiochemical parameters utilized in our model can be obtained through DFT calculations and minimal experiments, highlighting the model's potential for rapid screening of new FROMP chemistries in pursuit of thermoset polymers with superior thermo-chemo-mechanical properties.

Rethinking the Roughness Height: An Improved Description of Temperature Profiles over Short Vegetation

In this study, we present an extension to the Monin–Obukov similarity theory (MOST) for the roughness sublayer (RSL) over short vegetation. We test our theory using temperature measurements from fiber optic cables in an array-shaped set-up. This provides a high vertical measurement resolution that enables us to measure the sharp temperature gradients near the surface. It is well-known that MOST is invalid in the RSL as the flow is distorted by roughness elements. However, to derive the surface temperature, it is common practice to extrapolate the logarithmic profiles down to the surface through the RSL. Instead of logarithmic behaviour defined by MOST near the surface, our observations show near-linear temperature profiles. This log-to-linear transition is described over an aerodynamically smooth surface by the Van Driest equation in classical turbulence literature. Here we propose that the Van Driest equation can also be used to describe this transition over a rough surface, by replacing the viscous length scale with a surface length scale Ls\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L_s$$\\end{document} that represents the size of the smallest eddies near the grass structures. We show that Ls\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L_s$$\\end{document} scales with the geometry of the vegetation and that the model shows the potential to be scaled up to tall canopies. The adapted Van Driest model outperforms the roughness length concept in describing the temperature profiles near the surface and predicting the surface temperature.

Transient simulation of conjugate heat transfer of kerosene with thermal oxidative and surface coking reactions at a supercritical pressure

The regenerative cooling technology in aerospace applications often involves supercritical-pressure heat transfer of a hydrocarbon fuel, in which thermal oxidative reactions of the fuel would lead to surface coke accumulation on the cooling channel wall. In this paper, the long-term transient conjugate heat transfer of kerosene has been numerically investigated in a circular mini tube at a supercritical pressure of 3 MPa. The modified chemical mechanism and kinetics of thermal oxidative and surface coking reactions of kerosene are proposed, which, with an additional surface reaction, are capable of treating large coke deposition in regions with sharp temperature gradients. Transient surface coke accumulation and its effect on heat transfer are handled by a dynamic mesh technique. A series of numerical studies are conducted to elucidate the influences of surface heat flux, dissolved oxygen mass fraction, inlet fuel mass flow rate, and coke layer thermal conductivity on the supercritical-pressure heat transfer and thermal oxidative reactions. The effects of different reaction pathways on surface coke accumulation are analyzed comprehensively, and it is revealed that the oxygen-consuming surface coke deposition is initiated only at the high fuel temperature at around 590 K. The present numerical results would enhance fundamental understanding of the long-term transient heat transfer process of a hydrocarbon fuel at supercritical pressures in the regenerative cooling applications.

A new conceptual model of the Travale (Italy) enhanced geothermal system: Insights from electromagnetic geothermometry

The paper presents an electromagnetic geothermometer-based two-dimensional temperature model for the Travale (Italy) enhanced geothermal system derived from magnetotelluric and temperature data. Joint analysis of the temperature and electrical resistivity models made feasible a new self-consistent conceptual model of this site, which agrees with available geological data and previous geophysical results. It explains the nature of seismic reflection horizons detected in 1978 and unusual seismicity distribution in depth which could not be properly addressed earlier without temperature predicting beyond boreholes. It is concluded that the heat source feeding this system may be a granitic intrusion located at a depth below 4 km, which corresponds to the isotherm of 600 °C. A sharp temperature gradient above this intrusion forms a contact thermo-metamorphic halo in the depth range of 3–4 km. Assessment of the released fluid volume fraction from the electrical resistivity model indicates that the general amount of thermo-metamorphic and magmatic fluids in the rock-dominated Travale geothermal system is negligible. It is concluded that meteoric waters are the main source of fluids penetrating along normal faults up to 3 km. Despite some supercritical fluids of thermo-metamorphic origin could circulate in the system below this depth, their amount is hardly sufficient for economically justified exploration drilling. The results of the study indicate that building the temperature model of the geothermal area is crucially important for the exploration of enhanced geothermal systems.

Simulating the Geometric Distortion of Remanufactured Parts Using the Inherent Strain Method

Directed energy deposition (DED) is an additive manufacturing technique that is very well adapted for remanufacturing metallic components. The process works by depositing material onto either a substrate or, in the case of remanufacturing, existing component using a metal wire or powder that is fed into an energy source, usually either a laser or electron beam. DED naturally introduces a large amount of heat into the component in a manner that generally produces both large and sharp temperature gradients. These gradients generate residual stresses in the material which cause the component to warp in ways that are often difficult to accurately predict. To use this method for remanufacturing components with specific geometrical demands, such as turbine blades, any potential warping must be accounted for and minimized. A simulation methodology based on inherent strain (IS) is proposed as a high-throughput high accuracy method of evaluating the warping based on the geometry and parameters of an individual component. The study compares the results of this high throughput IS-based simulation tool with more traditional thermomechanical simulations with respect to both accuracy and time efficiency when applied to an industrial case.

Interfacial Microstructure Produced during Dissimilar AA6013/Ti-6Al-4V Friction Stir Lap Welding under Zero-Penetration Condition

The purpose of this study was to investigate the interfacial microstructure that was produced during dissimilar friction stir lap welding (FSW) of 6013 aluminum alloy and Ti-6Al-4V. FSW was conducted under a zero-penetration condition, i.e., the welding tool was plunged exclusively into the upper (aluminum) plate of the dissimilar lap joint. To facilitate the interpretation of microstructural processes, finite element modeling (FEM) was applied to evaluate the temperature field within the weld zone. The FEM simulation revealed a very sharp temperature gradient across the dissimilar interface. This effect was attributed to the generation of FSW heat exclusively within the aluminum part and a relatively low thermal conductivity of titanium. The abrupt temperature drop on the titanium side imposed a strict limitation on the diffusion penetration of aluminum and thus resulted in a relatively thin (~0.5 μm) and discontinuous intermetallic compound. Due to the complex chemical composition of the FSWed aluminum alloy, the diffusion processes also involved alloying elements. Consequently, the evolved intermetallic compound had a complicated chemical composition, with the principal elements being aluminum, titanium, silicon, manganese, and magnesium.

Phase-field modeling of thermal cracking in hardening mass concrete

The manuscript is concerned with the phase-field modeling of thermal cracking in massive hardening concrete structures. Owing to the exothermic reactions, known as hydration, a considerable amount of heat is generated during the setting and hardening of concrete. Since concrete has relatively low thermal conductivity and fairly high heat capacity, sharp temperature gradients may occur between the interior and exterior parts of massive concrete structures. This uneven temperature distribution results in a non-uniform thermal expansion that, in turn, may lead to thermal cracking in conjunction with restraining external boundary conditions. Hence, it is of key importance to conduct the crack risk assessment of massive concrete structures at various stages of their construction and thereafter where real-size experiments are often unrealizable. To this end, we develop a multi-field computational approach to simulate thermal cracking in hardening concrete. In particular, we propose a novel chemo-thermo-mechanical model developed within the framework of the theory of reactive porous media coupled with a quasi-brittle phase-field regularized cohesive zone model to explicitly account for the initiation and propagation of thermal cracks. In the model, chemical, thermal, mechanical, and phase-field fracture problems are coupled through the constitutive equations. To our best knowledge, this is the first study where a quasi-brittle phase-field fracture model is used to model thermal cracking in hardening massive concrete through a novel incremental formulation. The potential of the proposed approach is demonstrated through the benchmark thermomechanical problems of a normal concrete member and a roller-compacted concrete dam. In the latter, original multi-field interface elements are devised between the lifts.

Thermo-hydraulic characteristics of thermal interface induced by natural convection in water tank with internal heat source

Large-scale water tank or pool with internal heat source is common structures in solar energy and nuclear power. In this paper, a visible thermal interface is captured in the experiments in a square water tank with a horizontal heating rod, which is an interesting phenomenon hardly reported before. The direct shadowgraphy method is employed to record gray graphs of the forming process and flow regimes of the thermal interface. Corresponding transient numerical simulations with the large eddy simulation (LES) method are conducted to display detailed thermo-hydraulic characteristics of the thermal interface. Based on the temperature distribution and flow field, a natural convection circulation driven by buoyancy is formed above a stagnant zone, which is regarded as a two-layer stratified shear flow. The horizontal shear flow above the stagnant zone induces the visible thermal interface. Because the heat transfer mode switches from convection to conduction when crossing the interface, the sharp temperature gradient and density variation occur at the interface. The effect of the heat flux of the heating rod is investigated, and three different flow regimes of the thermal interface are observed under different heat loads. The stability of the flow regimes is analyzed by the gradient Richardson number, which is the ratio of the buoyancy to the turbulent shear. At low heat load, the thermal interface is destabilized because the turbulent shear overcomes the density barrier. As the heat loading increases, the buoyancy increasing rate exceeds the shear increasing rate, and the stronger potential barrier dominates the flow structure, which is beneficial to the interface stability.

Frontally polymerized foams: thermodynamic and kinetical aspects of front hindrance by particles.

Frontal polymerization (FP) is a solvent-free, energy-efficient process where a self-propagating polymerization reaction with a characteristic sharp temperature gradient at the front head propagates through the resin to provide the curing conditions. It relies on the enthalpic balance, which spreads the reaction to unreacted resin in the neighborhood. Therefore, the FP is sensitive to the presence of non-reactive volumes, such as boundaries, fillers, or other additives, that retain heat from the front but produce no enthalpy in return. On the other hand, the front's high temperature could be used to initiate other processes, such as foaming, incorporating them into a simple single-step fabrication procedure. This study used silica particles of two different sizes (14 nm and 200-300 nm) in an epoxy-based FP foam as a representative filler to probe the constraints imposed by non-reactive additives. The presence of particles visibly hindered the front propagation, increased the foam density and even corrupted the frontal regime in some cases. We show that preheating or chemical composition changes are viable approaches to address the fillers' adverse effects. Furthermore, we present evidence that the reduced reaction enthalpy caused by silica nanoparticles, was balanced by the lower heat capacity of our model system. At the same time, the front hindrance was attributed to changes in reaction kinetics and the heat distribution around the front. These results set up essential narratives for the design and practical applications of frontally polymerized foams with non-reactive fillers.

Anisotropic response in mechanical behavior of additively manufactured Al–Mn-Sc alloys by in-situ EBSD tensile tests

The anisotropic of microstructure for additively manufactured Al alloys is attributed to sharp temperature gradient during laser melting, resulting in anisotropy of mechanical properties. The correlation of grain orientation, size and aspect ratio on mechanical properties were investigated using in-situ EBSD tensile method on selective laser melting Al–Mn-Sc alloys in longitudinal section and cross section. The lattice rotation behavior and the effects of grain orientation, grain size and aspect ratio on mechanical anisotropy were explored. The predominant <001> orientation of the columnar crystals in the longitudinal section and the <102 > and <101> orientation that dominates the equiaxed crystals in the cross-section had a certain influence on the anisotropy of mechanical properties. The grains with different shapes and sizes exhibited different rotation behavior, and lattice rotation of equiaxed crystals with smaller grain sizes was more complex. By introducing anisotropy coefficients (Ka), the anisotropy sensitivity to grain orientations was revealed. Based on the anisotropy coefficient, grain size and aspect ratio, the mechanical anisotropy in both directions was accurately predicted.

Resonant transformation of sonic-Alfven modes in solar spicules

The photospheric 5 min oscillations of the Sun can convert to Alfven waves by some conditions such as considering of a region and of a damping mechanism. Actually, 5 min oscillations appear in the photosphere and it does not seem to be able to pierce into the solar upper atmosphere because of the acoustic cut off of stratified medium and the sharp temperature gradient in the transition region. By perturbing the equilibrium state of the magnetized plasma, we showed that the acoustic and Alfven waves can interact through the nonlinear terms of the ideal MHD equations. We found that Alfven waves with twice the period of acoustic waves can propagate into upper layers when the speeds of sound and Alfven waves are equal. This condition obtain when ϖ=1/2ϖ .

Geochemical and microbiological profiles in hydrothermal extreme acidic environments (Pisciarelli Spring, Campi Flegrei, Italy).

Although terrestrial hydrothermal systems are considered among the most fascinating environments, how their unique and extreme conditions can affect microorganisms selection and the role in biogeochemical cycles has not yet been well elucidated. A combined geochemical and microbiological exploration in waters and sediments from 10 sampling points along a sharp temperature gradient (15-90°C) within an extremely acidic hydrothermal system (Pisciarelli Spring, Campi Flegrei area, southern Italy) displayed how hydrothermal fluids influence the microbial dynamics. This area was characterized by high levels of reduced gaseous species (e.g. H2S, H2, CH4, CO) and very low pH values (<2.3). Thermodynamic calculations revealed a high microbial catabolic potential in oxidation/reduction reactions of N-, S- and Fe-bearing species. Overall, an increase of the archaeal/bacterial abundance ratio was observed by decreasing temperature and pH values. In particular, Archaea and Bacteria were present in almost equal cell abundance (up to 1.1 × 109 and 9.3 × 108 cell/g, respectively) in the <70°C sampling points (average pH = 2.09); on the contrary, the highest temperature waters (85-90°C; average pH = 2.26) were characterized by a low abundance of archaeal cells. The high-throughput sequencing of the 16S rRNA genes indicated strong differences in archaeal and bacterial communities composition along the temperature gradient. However, the microbiome in this extreme environment was mainly constituted by chemoautotrophic microorganisms that were likely involved in N-, S- and Fe-bearing species transformations (e.g. Acidianus infernus, Ferroplasma acidarmanus, Acidithiobacillus,Sulfobacillus,Thaumarchaeota), in agreement with thermodynamic calculations.

Coaxial-Injector Surrogate Modeling Based on Reynolds-Averaged Navier–Stokes Simulations Using Deep Learning

Facing the need to increase the accuracy of rocket engine design tools, the present work introduces an innovative methodology for the design and optimization of rocket engine combustion chambers using numerical simulations and deep learning. An experimental test case of a single coaxial injector is taken as a reference point, and a design of experiments is generated by varying nine parameters (geometrical and operative conditions). Reynolds-averaged Navier–Stokes simulations are carried out to generate the data set. The data are used to train surrogate models of different fidelities, from low-dimensional outputs (zero-dimensional and one-dimensional) toward the two-dimensional temperature field. Attention is given on the selection of the proper machine learning technique. For low-dimensional outputs, results show that deep neural networks outperform other standard machine learning tools, namely, radial basis function and kriging. Regarding high-dimensional outputs, convolutional neural networks with gradient-based loss functions are found effective to capture the large and smooth temperature variations, as well as the thin and sharp temperature gradients at the flame front. Eventually, the models are used in the framework of an optimization problem. Results highlight the benefits of new design and optimization tools based on deep learning, capable of real-time predictions of complex flowfields.

Acoustic Speed Measurement Platform for Monitoring Highly Concentrated Gas Temperature Distribution

Acoustic thermography reconstructs 2-D gas temperature distributions by using sound speeds from multiple paths across a plane. However, a reliable platform that can provide accurate sound speeds in a highly concentrated temperature distribution environment has not been fully discussed. Most of the existing temperature reconstruction algorithms are developed using simulated data and validated with customized systems. The results from the reported systems are difficult to reproduce due to the lack of critical information, such as the methods for determining the time of flight (TOF). Moreover, previous attempts have not investigated the effect of a sharp temperature gradient, in which the difference of acoustic impedance between high- and low-temperature regions cannot be neglected. In response, this letter demonstrates a reliable acoustic platform designed specifically for measuring the acoustic speeds in an environment with a critical temperature distribution. The main components of the proposed platform are discussed in detail, including the TOF calculation technique, the hardware architecture, and the working principle. The performance of the platform has been evaluated by comparing the captured acoustic velocities and the reconstructed temperature map with results from a thermocouple array. The average acoustic velocity measured by the proposed platform for 100 times is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$341 \pm \ 3$</tex-math></inline-formula> m/s at room temperature (25 °C) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$354 \pm \ 5\ $</tex-math></inline-formula> m/s after heater turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> (38.27 °C). Moreover, the 2-D temperature map reconstructed by acquired acoustic velocities also matches the results from the thermocouple array. Therefore, the proposed platform can be considered as an accurate and reliable acoustic speed measurement system for monitoring the 2-D temperature distribution.

A Comparison Between Hardness-Scaling and Ball-Indentation Techniques on Predicting Stress/Strain Distribution and Failure Behavior of Resistance Spot Welded Advanced High Strength Steel

Abstract To accurately model the resistance spot welding (RSW) joint in finite element analysis (FEA), the constitutive behavior of materials in various weld regions such as heat-affected zone (HAZ) should be measured. Due to the sharp temperature gradient through RSW specimens, microstructural and corresponding mechanical properties of weld regions are different. Additionally, the size of RSW is small; hence, it is challenging to directly measure the stress–strain curve of materials. In this regard, hardness-scaling and ball-indentation techniques are among the popular methods to in-directly measure the stress–strain curve of these materials. However, the effectiveness of these two techniques on predicting the stress/strain distribution and failure behavior of resistance spot welded advanced high strength steels (AHSS) is not clear. In the present work, the stress–strain curves obtained through hardness-scaling and ball-indentation techniques have been compared. The stress/strain distribution and failure behavior of the resistance spot welded AHSS specimen have been simulated by the stress–strain data obtained using the two methods. The simulation results have been compared with experimental analysis. The results showed that both methods can accurately predict the failure location. With the comparison of FEA results with experiment analysis, it was shown that the ball-indentation method provides slightly better predictions of failure behavior compared to the hardness-scaling method. However, the harness scaling method is a simple and convenient technique, which can be implemented as a qualitative analysis for the failure behavior of RSW joints.