292 research outputs found
Nondestructive testing of fiber array with multiple missing fibers
National audienceOur goal is to detect defects in composite materialscomposed by multilayer planar plates with a periodicset of circular cylindrical fibers embedded in each layer. As astarter, the work presented is electromagnetic (EM) modelingand imaging of missing fibers within a fiber array standingin air. The multiple scattering method is utilized to analyzethe electromagnetic behavior, and the corresponding imagingmodel is established directly from Lippman-Schwinger integralformulation. Standard MUltiple SIgnal Classification (MUSIC)and the proposed joint sparsity which borrows the idea ofsparse theory are applied to retrieve the locations of missingfibers. Numerical results are provided to confirm availabilityand accuracy of EM modeling and defect imaging
Evidence for elevated emissions from high-latitude wetlands contributing to high atmospheric CH4 concentration in the early Holocene
The major increase in atmospheric methane (CH4) concentration during the last glacial-interglacial transition provides a useful example for understanding the interactions and feedbacks among Earth\u27s climate, biosphere carbon cycling, and atmospheric chemistry. However, the causes of CH4 doubling during the last deglaciation are still uncertain and debated. Although the ice-core data consistently suggest a dominant contribution from northern high-latitude wetlands in the early Holocene, identifying the actual sources from the ground-based data has been elusive. Here we present data syntheses and a case study from Alaska to demonstrate the importance of northern wetlands in contributing to high atmospheric CH4concentration in the early Holocene. Our data indicate that new peatland formation as well as peat accumulation in northern high-latitude regions increased more than threefold in the early Holocene in response to climate warming and the availability of new habitat as a result of deglaciation. Furthermore, we show that marshes and wet fens that represent early stages of wetland succession were likely more widespread in the early Holocene. These wetlands are associated with high CH4 emissions due to high primary productivity and the presence of emergent plant species that facilitate CH4 transport to the atmosphere. We argue that early wetland succession and rapid peat accumulation and expansion (not simply initiation) contributed to high CH4 emissions from northern regions, potentially contributing to the sharp rise in atmospheric CH4 at the onset of the Holocene
Quantifying Peat Carbon Accumulation in Alaska Using a Process-Based Biogeochemistry Model
This study uses an integrated modeling framework that couples the dynamics of hydrology, soil thermal regime, and ecosystem carbon and nitrogen to quantify the long-term peat carbon accumulation in Alaska during the Holocene. Modeled hydrology, soil thermal regime, carbon pools and fluxes, and methane emissions are evaluated using observation data at several peatland sites in Minnesota, Alaska, and Canada. The model is then applied for a 10,000 year (15 ka to 5 ka; 1 ka = 1000 cal years before present) simulation at four peatland sites. We find that model simulations match the observed carbon accumulation rates at fen sites during the Holocene (R2 = 0.88, 0.87, 0.38, and -0.05 using comparisons in 500 year bins). The simulated (2.04 m) and observed peat depths (on average 1.98 m) were also compared well (R2 = 0.91). The early Holocene carbon accumulation rates, especially during the Holocene thermal maximum (HTM) (35.9 g Cm-2 yr-1), are estimated up to 6 times higher than the rest of the Holocene (6.5 g Cm-2 yr-1). Our analysis suggests that high summer temperature and the lengthened growing season resulted from the elevated insolation seasonality, along with wetter-than-before conditions might be major factors causing the rapid carbon accumulation in Alaska during the HTM. Our sensitivity tests indicate that, apart from climate, initial water table depth and vegetation canopy are major drivers to the estimated peat carbon accumulation. When the modeling framework is evaluated for various peatland types in the Arctic, it can quantify peatland carbon accumulation at regional scales
MM-Vet: Evaluating Large Multimodal Models for Integrated Capabilities
We propose MM-Vet, an evaluation benchmark that examines large multimodal
models (LMMs) on complicated multimodal tasks. Recent LMMs have shown various
intriguing abilities, such as solving math problems written on the blackboard,
reasoning about events and celebrities in news images, and explaining visual
jokes. Rapid model advancements pose challenges to evaluation benchmark
development. Problems include: (1) How to systematically structure and evaluate
the complicated multimodal tasks; (2) How to design evaluation metrics that
work well across question and answer types; and (3) How to give model insights
beyond a simple performance ranking. To this end, we present MM-Vet, designed
based on the insight that the intriguing ability to solve complicated tasks is
often achieved by a generalist model being able to integrate different core
vision-language (VL) capabilities. MM-Vet defines 6 core VL capabilities and
examines the 16 integrations of interest derived from the capability
combination. For evaluation metrics, we propose an LLM-based evaluator for
open-ended outputs. The evaluator enables the evaluation across different
question types and answer styles, resulting in a unified scoring metric. We
evaluate representative LMMs on MM-Vet, providing insights into the
capabilities of different LMM system paradigms and models. Code and data are
available at https://github.com/yuweihao/MM-Vet.Comment: Code and data: https://github.com/yuweihao/MM-Ve
Phase-Modulated Elastic Properties of Two-Dimensional Magnetic FeTe: Hexagonal and Tetragonal Polymorphs
Two-dimensional (2D) layered magnets, such as iron chalcogenides, have
emerged these years as a new family of unconventional superconductor and
provided the key insights to understand the phonon-electron interaction and
pairing mechanism. Their mechanical properties are of strategic importance for
the potential applications in spintronics and optoelectronics. However, there
is still lack of efficient approach to tune the elastic modulus despite the
extensive studies. Herein, we report the modulated elastic modulus of 2D
magnetic FeTe and its thickness-dependence via phase engineering. The grown 2D
FeTe by chemical vapor deposition can present various polymorphs, i.e.
tetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe,
ferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation
method showed an obvious thickness-dependence, from 290.9+-9.2 to 113.0+-8.7
GPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In
comparison, the elastic modulus of h-FeTe remains unchanged. Our results could
shed light on the efficient modulation of mechanical properties of 2D magnetic
materials and pave the avenues for their practical applications in nanodevices.Comment: 19 pages, 4 figure
Modeling Holocene Peatland Carbon Accumulation in North America
Peatlands are a large carbon reservoir. Yet the quantification of their carbon stock still has a large uncertainty due to lacking observational data and wellâtested peatland biogeochemistry models. Here, a processâbased peatland model was calibrated using longâterm peat carbon accumulation data at multiple sites in North America. The model was then applied to quantify the peat carbon accumulation rates and stocks within North America over the last 12,000 years. We estimated that 85â174 Pg carbon was accumulated in North American peatlands over the study period including 0.37â0.76 Pg carbon in subtropical peatlands. During the period from 10,000 to 8,000 years ago, the warmer and wetter conditions might have played an important role in stimulating peat carbon accumulation by enhancing plant photosynthesis. Enhanced peat decomposition due to warming slowed the carbon accumulation through the rest of the Holocene. While recent modeling studies indicate that the northern peatlands will continue to act as a carbon sink in this century, our studies suggest that future enhanced peat decomposition accompanied by peatland areal changes induced by permafrost degradation and other disturbances shall confound the sink and source analysis
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