Articles de revues : « Reconstruction and flux simulations »

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Zerrik, E., et H. Bourray. « Flux reconstruction : sensors and simulations ». Sensors and Actuators A : Physical 109, n^o 1-2 (décembre 2003) : 34–46. http://dx.doi.org/10.1016/s0924-4247(03)00358-3.

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Khazari, Adil, et Ali Boutoulout. « Flux reconstruction for hyperbolic systems : Sensors and simulations ». Evolution Equations & ; Control Theory 4, n^o 2 (2015) : 177–92. http://dx.doi.org/10.3934/eect.2015.4.177.

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Cruz, Fernando, José P. Faria, Miguel Rocha, Isabel Rocha et Oscar Dias. « A review of methods for the reconstruction and analysis of integrated genome-scale models of metabolism and regulation ». Biochemical Society Transactions 48, n^o 5 (17 septembre 2020) : 1889–903. http://dx.doi.org/10.1042/bst20190840.

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The current survey aims to describe the main methodologies for extending the reconstruction and analysis of genome-scale metabolic models and phenotype simulation with Flux Balance Analysis mathematical frameworks, via the integration of Transcriptional Regulatory Networks and/or gene expression data. Although the surveyed methods are aimed at improving phenotype simulations obtained from these models, the perspective of reconstructing integrated genome-scale models of metabolism and gene expression for diverse prokaryotes is still an open challenge.

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Zunz, V., et H. Goosse. « Influence of freshwater input on the skill of decadal forecast of sea ice in the Southern Ocean ». Cryosphere 9, n^o 2 (17 mars 2015) : 541–56. http://dx.doi.org/10.5194/tc-9-541-2015.

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Abstract. Recent studies have investigated the potential link between the freshwater input derived from the melting of the Antarctic ice sheet and the observed recent increase in sea ice extent in the Southern Ocean. In this study, we assess the impact of an additional freshwater flux on the trend in sea ice extent and concentration in simulations with data assimilation, spanning the period 1850–2009, as well as in retrospective forecasts (hindcasts) initialised in 1980. In the simulations with data assimilation, the inclusion of an additional freshwater flux that follows an autoregressive process improves the reconstruction of the trend in ice extent and concentration between 1980 and 2009. This is linked to a better efficiency of the data assimilation procedure but can also be due to a better representation of the freshwater cycle in the Southern Ocean. The results of the hindcast simulations show that an adequate initial state, reconstructed thanks to the data assimilation procedure including an additional freshwater flux, can lead to an increase in the sea ice extent spanning several decades that is in agreement with satellite observations. In our hindcast simulations, an increase in sea ice extent is obtained even in the absence of any major change in the freshwater input over the last decades. Therefore, while the additional freshwater flux appears to play a key role in the reconstruction of the evolution of the sea ice in the simulation with data assimilation, it does not seem to be required in the hindcast simulations. The present work thus provides encouraging results for sea ice predictions in the Southern Ocean, as in our simulation the positive trend in ice extent over the last 30 years is largely determined by the state of the system in the late 1970s.

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Trojak, W., N. R. Vadlamani, J. Tyacke, F. D. Witherden et A. Jameson. « Artificial compressibility approaches in flux reconstruction for incompressible viscous flow simulations ». Computers & ; Fluids 247 (octobre 2022) : 105634. http://dx.doi.org/10.1016/j.compfluid.2022.105634.

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Wilson, David J., Allison Youngblood, Odette Toloza, Jeremy J. Drake, Kevin France, Cynthia S. Froning, Boris T. Gänsicke, Seth Redfield et Brian E. Wood. « Testing Lyα Emission-line Reconstruction Routines at Multiple Velocities in One System ». Astrophysical Journal 936, n^o 2 (1 septembre 2022) : 189. http://dx.doi.org/10.3847/1538-4357/ac87a8.

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Abstract The 1215.67 Å H i Lyα emission line dominates the ultraviolet flux of low-mass stars, including the majority of known exoplanet hosts. Unfortunately, strong attenuation by the interstellar medium (ISM) obscures the line core in most stars, requiring the intrinsic Lyα flux to be reconstructed based on fits to the line wings. We present a test of the widely used Lyα emission-line reconstruction code lyapy using phase-resolved, medium-resolution STIS G140M observations of the close white dwarf–M dwarf binary EG UMa. The Doppler shifts induced by the binary orbital motion move the Lyα emission line in and out of the region of strong ISM attenuation. Reconstructions of each spectrum should produce the same Lyα profile regardless of phase, under the well-justified assumption that there is no intrinsic line variability between observations. Instead, we find that the reconstructions underestimate the Lyα flux by almost a factor of 2 for the lowest velocity, most attenuated spectrum, due to a degeneracy between the intrinsic Lyα and ISM profiles. Our results imply that many stellar Lyα fluxes derived from G140M spectra reported in the literature may be underestimated, with potential consequences for, for example, estimates of extreme-ultraviolet stellar spectra and ultraviolet inputs into simulations of exoplanet atmospheres.

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Huang, Lawrence, Rupert A. C. Croft et Hitesh Arora. « Deep forest : Neural network reconstruction of the Lyman-α forest ». Monthly Notices of the Royal Astronomical Society 506, n^o 4 (19 juillet 2021) : 5212–22. http://dx.doi.org/10.1093/mnras/stab2041.

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ABSTRACT We explore the use of Deep Learning to infer physical quantities from the observable transmitted flux in the Ly α forest. We train a Neural Network using redshift z = 3 outputs from cosmological hydrodynamic simulations and mock data sets constructed from them. We evaluate how well the trained network is able to reconstruct the optical depth for Ly α forest absorption from noisy and often saturated transmitted flux data. The Neural Network outperforms an alternative reconstruction method involving log inversion and spline interpolation by approximately a factor of 2 in the optical depth root mean square error. We find no significant dependence in the improvement on input data signal to noise, although the gain is greatest in high optical depth regions. The Ly α forest optical depth studied here serves as a simple, one dimensional, example but the use of Deep Learning and simulations to approach the inverse problem in cosmology could be extended to other physical quantities and higher dimensional data.

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Zhou, Bowen, et Fotini Katopodes Chow. « Large-Eddy Simulation of the Stable Boundary Layer with Explicit Filtering and Reconstruction Turbulence Modeling ». Journal of the Atmospheric Sciences 68, n^o 9 (1 septembre 2011) : 2142–55. http://dx.doi.org/10.1175/2011jas3693.1.

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Abstract Large-eddy simulation (LES) of the stably stratified atmospheric boundary layer is performed using an explicit filtering and reconstruction approach with a finite difference method. Turbulent stresses are split into the resolvable subfilter-scale and subgrid-scale stresses. The former are recovered from a reconstruction approach, and the latter are represented by a dynamic eddy-viscosity model. The resulting dynamic reconstruction model (DRM) can sustain resolved turbulence with less stringent resolution requirements than conventional closure models, even under strong atmospheric stability. This is achieved by proper representation of subfilter-scale (SFS) backscatter of turbulent kinetic energy (TKE). The flow structure and turbulence statistics for the moderately stable boundary layer (SBL) are analyzed with high-resolution simulations. The DRM simulations show good agreement with established empirical formulations such as flux and gradient-based surface similarity, even at relatively coarse resolution. Similar results can be obtained with traditional closure models at the cost of higher resolution. SBL turbulence under strong stability is also explored. Simulations show an intermittent presence of elevated TKE below the low-level jet. Overall, the explicit filtering and reconstruction approach is advantageous for simulations of the SBL. At coarse resolution, it can extend the working range of LES to stronger stability, while maintaining agreement to similarity theory; at fine resolution, good agreement with theoretical formulations provides confidence in the results and allows for detailed investigation of the flow structure under moderate to strong stability conditions.

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Zorrilla, Francisco, Filip Buric, Kiran R. Patil et Aleksej Zelezniak. « metaGEM : reconstruction of genome scale metabolic models directly from metagenomes ». Nucleic Acids Research 49, n^o 21 (6 octobre 2021) : e126-e126. http://dx.doi.org/10.1093/nar/gkab815.

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Abstract Metagenomic analyses of microbial communities have revealed a large degree of interspecies and intraspecies genetic diversity through the reconstruction of metagenome assembled genomes (MAGs). Yet, metabolic modeling efforts mainly rely on reference genomes as the starting point for reconstruction and simulation of genome scale metabolic models (GEMs), neglecting the immense intra- and inter-species diversity present in microbial communities. Here, we present metaGEM (https://github.com/franciscozorrilla/metaGEM), an end-to-end pipeline enabling metabolic modeling of multi-species communities directly from metagenomes. The pipeline automates all steps from the extraction of context-specific prokaryotic GEMs from MAGs to community level flux balance analysis (FBA) simulations. To demonstrate the capabilities of metaGEM, we analyzed 483 samples spanning lab culture, human gut, plant-associated, soil, and ocean metagenomes, reconstructing over 14,000 GEMs. We show that GEMs reconstructed from metagenomes have fully represented metabolism comparable to isolated genomes. We demonstrate that metagenomic GEMs capture intraspecies metabolic diversity and identify potential differences in the progression of type 2 diabetes at the level of gut bacterial metabolic exchanges. Overall, metaGEM enables FBA-ready metabolic model reconstruction directly from metagenomes, provides a resource of metabolic models, and showcases community-level modeling of microbiomes associated with disease conditions allowing generation of mechanistic hypotheses.

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Liu, Chunlei, et Richard P. Allan. « Unrealistic Increases in Wind Speed Explain Reduced Eastern Pacific Heat Flux in Reanalyses ». Journal of Climate 31, n^o 8 (20 mars 2018) : 2981–93. http://dx.doi.org/10.1175/jcli-d-17-0642.1.

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Abstract Tropical eastern Pacific sea surface temperature plays a pivotal role in mechanisms that determine global mean surface temperature variability. In this study, the surface flux contribution to recent cooling of the tropical eastern Pacific is investigated using data from three atmospheric reanalyses with full assimilation of observations, an observation-based net surface energy flux reconstruction, and 15 atmosphere-only climate model simulations. For ERA-Interim, 78% of the decrease in net surface flux (−0.65 W m−2 yr−1 over 1988–2008) is explained by the latent heat flux variability. Latent heat flux variability differs between datasets, and this is investigated using a bulk formula. It is found that discrepancies in wind speed change explain contrasting latent heat flux trends across datasets. The significant increase in wind speed of 0.26 m s−1 decade−1 over the tropical eastern Pacific in ERA-Interim is not reproduced by satellite or buoy observations or atmosphere-only climate model simulations, casting questions on the reliability of reanalysis-based surface fluxes over the tropical eastern Pacific.

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Virtanen, I. O. I., A. A. Pevtsov, I. I. Virtanen et K. Mursula. « Reconstructing solar magnetic fields from historical observations ». Astronomy & ; Astrophysics 652 (août 2021) : A79. http://dx.doi.org/10.1051/0004-6361/202140656.

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Context. The evolution of the photospheric magnetic field can be simulated with surface flux transport (SFT) simulations, which allow for the study of the evolution of the entire field, including polar fields, solely using observations of the active regions. However, because only one side of the Sun is visible at a time, active regions that emerge and decay on the far-side are not observed and not included in the simulations. As a result, some flux is missed. Aims. We construct additional active regions and apply them to the far-side of the Sun in an SFT simulation to assess the possible effects and the magnitude of error that the missing far-side flux causes. We estimate how taking the missing far-side flux into account affects long-term SFT simulations. Methods. We identified active regions from synoptic maps of the photospheric magnetic field between 1975 and 2019. We divided them into solar cycle wings and determined their lifetimes. Using the properties of observed active regions with sufficiently short lifetimes, we constructed additional active regions and inserted them into an SFT simulation. Results. We find that adding active regions with short lifetimes to the far-side of the Sun results in significantly stronger polar fields in minimum times and slightly delayed polarity reversals. These results partly remedy the earlier results, which show overly weak polar fields and polarity reversals that are slightly too early when far-side emergence is not taken into account. The far-side active regions do not significantly affect poleward flux surges, which are mostly caused by larger long-living active regions. The far-side emergence leads to a weak continuous flow of flux, which affects polar fields over long periods of time.

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Spring, Aaron, István Dunkl, Hongmei Li, Victor Brovkin et Tatiana Ilyina. « Trivial improvements in predictive skill due to direct reconstruction of the global carbon cycle ». Earth System Dynamics 12, n^o 4 (15 novembre 2021) : 1139–67. http://dx.doi.org/10.5194/esd-12-1139-2021.

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Abstract. State-of-the art climate prediction systems have recently included a carbon component. While physical-state variables are assimilated in reconstruction simulations, land and ocean biogeochemical state variables adjust to the state acquired through this assimilation indirectly instead of being assimilated themselves. In the absence of comprehensive biogeochemical reanalysis products, such an approach is pragmatic. Here we evaluate a potential advantage of having perfect carbon cycle observational products to be used for direct carbon cycle reconstruction. Within an idealized perfect-model framework, we reconstruct a 50-year target period from a control simulation. We nudge variables from this target onto arbitrary initial conditions, mimicking an assimilation simulation generating initial conditions for hindcast experiments of prediction systems. Interested in the ability to reconstruct global atmospheric CO2, we focus on the global carbon cycle reconstruction performance and predictive skill. We find that indirect carbon cycle reconstruction through physical fields reproduces the target variations. While reproducing the large-scale variations, nudging introduces systematic regional biases in the physical-state variables to which biogeochemical cycles react very sensitively. Initial conditions in the oceanic carbon cycle are sufficiently well reconstructed indirectly. Direct reconstruction slightly improves initial conditions. Indirect reconstruction of global terrestrial carbon cycle initial conditions are also sufficiently well reconstructed by the physics reconstruction alone. Direct reconstruction negligibly improves air–land CO2 flux. Atmospheric CO2 is indirectly very well reconstructed. Direct reconstruction of the marine and terrestrial carbon cycles slightly improves reconstruction while establishing persistent biases. We find improvements in global carbon cycle predictive skill from direct reconstruction compared to indirect reconstruction. After correcting for mean bias, indirect and direct reconstruction both predict the target similarly well and only moderately worse than perfect initialization after the first lead year. Our perfect-model study shows that indirect carbon cycle reconstruction yields satisfying initial conditions for global CO2 flux and atmospheric CO2. Direct carbon cycle reconstruction adds little improvement to the global carbon cycle because imperfect reconstruction of the physical climate state impedes better biogeochemical reconstruction. These minor improvements in initial conditions yield little improvement in initialized perfect-model predictive skill. We label these minor improvements due to direct carbon cycle reconstruction “trivial”, as mean bias reduction yields similar improvements. As reconstruction biases in real-world prediction systems are likely stronger, our results add confidence to the current practice of indirect reconstruction in carbon cycle prediction systems.

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Luo, Jun, Lijun Xuan et Kun Xu. « Comparison of Fifth-Order WENO Scheme and Finite Volume WENO-Gas-Kinetic Scheme for Inviscid and Viscous Flow Simulation ». Communications in Computational Physics 14, n^o 3 (septembre 2013) : 599–620. http://dx.doi.org/10.4208/cicp.110212.021112a.

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AbstractThe development of high-order schemes has been mostly concentrated on the limiters and high-order reconstruction techniques. In this paper, the effect of the flux functions on the performance of high-order schemes will be studied. Based on the same WENO reconstruction, two schemes with different flux functions, i.e., the fifth-order WENO method and the WENO-Gas-kinetic scheme (WENO-GKS), will be compared. The fifth-order finite difference WENO-SW scheme is a characteristic variable reconstruction based method which uses the Steger-Warming flux splitting for inviscid terms, the sixth-order central difference for viscous terms, and three stages Runge-Kutta time stepping for the time integration. On the other hand, the finite volume WENO-GKS is a conservative variable reconstruction based method with the same WENO reconstruction. But, it evaluates a time dependent gas distribution function along a cell interface, and updates the flow variables inside each control volume by integrating the flux function along the boundary of the control volume in both space and time. In order to validate the robustness and accuracy of the schemes, both methods are tested under a wide range of flow conditions: vortex propagation, Mach 3 step problem, and the cavity flow at Reynolds number 3200. Our study shows that both WENO-SW and WENO-GKS yield quantitatively similar results and agree with each other very well provided a sufficient grid resolution is used. With the reduction of mesh points, the WENO-GKS behaves to have less numerical dissipation and present more accurate solutions than those from the WENO-SW in all test cases. For the Navier-Stokes equations, since the WENO-GKS couples inviscid and viscous terms in a single flux evaluation, and the WENO-SW uses an operator splitting technique, it appears that the WENO-SW is more sensitive to the WENO reconstruction and boundary treatment. In terms of efficiency, the finite volume WENO-GKS is about 4 times slower than the finite differenceWENO-SW in two dimensional simulations. The current study clearly shows that besides high-order reconstruction, an accurate gas evolution model or flux function in a high-order scheme is also important in the capturing of physical solutions. In a physical flow, the transport, stress deformation, heat conduction, and viscous heating are all coupled in a single gas evolution process. Therefore, it is preferred to develop such a scheme with multi-dimensionality, and unified treatment of inviscid and dissipative terms. A high-order scheme does prefer a high-order gas evolution model. Even with the rapid advances of high-order reconstruction techniques, the first-order dynamics of the Riemann solution becomes the bottleneck for the further development of high-order schemes. In order to avoid the weakness of the low order flux function, the development of high-order schemes relies heavily on the weak solution of the original governing equations for the update of additional degree of freedom, such as the non-conservative gradients of flow variables, which cannot be physically valid in discontinuous regions.

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Mihalache, Ovidiu, et Toshihiko Yamaguchi. « Fast reconstruction of the magnetization of a Halbach magnet in EMAT using experimental measurements ». International Journal of Applied Electromagnetics and Mechanics 64, n^o 1-4 (10 décembre 2020) : 905–12. http://dx.doi.org/10.3233/jae-209404.

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The paper presents a fast and accurate algorithm to reconstruct the magnetization model of parallelepiped magnet (parts of a Halbach magnet array) or the Halbach magnet array used in an electromagnetic acoustic (EMAT) transducer. The model accuracy is validated against measurements of the magnetic flux distributions above surface of block magnets or Halbach magnet array using 2D/3D theoretical formula to compute magnetic flux distribution, validated by 2D/3D FEM simulations, and based on a non-uniform distribution of reconstructed magnetization model (using only main magnetization component). The illness of inverse reconstruction model is controlled through very small enough increments steps in an adaptive iterative algorithm but large enough increments to assure faster reconstruction, and converging to the same magnetization model of magnet blocks or Halbach array magnet, independent of the initial magnetization values.

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Sajib, Saurav Z. K., Munish Chauhan, Oh In Kwon et Rosalind J. Sadleir. « Magnetic-resonance-based measurement of electromagnetic fields and conductivity in vivo using single current administration—A machine learning approach ». PLOS ONE 16, n^o 7 (22 juillet 2021) : e0254690. http://dx.doi.org/10.1371/journal.pone.0254690.

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Diffusion tensor magnetic resonance electrical impedance tomography (DT-MREIT) is a newly developed technique that combines MR-based measurements of magnetic flux density with diffusion tensor MRI (DT-MRI) data to reconstruct electrical conductivity tensor distributions. DT-MREIT techniques normally require injection of two independent current patterns for unique reconstruction of conductivity characteristics. In this paper, we demonstrate an algorithm that can be used to reconstruct the position dependent scale factor relating conductivity and diffusion tensors, using flux density data measured from only one current injection. We demonstrate how these images can also be used to reconstruct electric field and current density distributions. Reconstructions were performed using a mimetic algorithm and simulations of magnetic flux density from complementary electrode montages, combined with a small-scale machine learning approach. In a biological tissue phantom, we found that the method reduced relative errors between single-current and two-current DT-MREIT results to around 10%. For in vivo human experimental data the error was about 15%. These results suggest that incorporation of machine learning may make it easier to recover electrical conductivity tensors and electric field images during neuromodulation therapy without the need for multiple current administrations.

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Sahade, Abril, Angelos Vourlidas, Laura A. Balmaceda et Mariana Cécere. « Understanding the Deflection of the “Cartwheel CME” : Data Analysis and Modeling ». Astrophysical Journal 953, n^o 2 (1 août 2023) : 150. http://dx.doi.org/10.3847/1538-4357/ace420.

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Abstract We study the low corona evolution of the “Cartwheel” coronal mass ejection (CME; 2008 April 9) by reconstructing its three-dimensional path and modeling it with magnetohydrodynamic simulations. This event exhibited a double deflection that has been reported and analyzed in previous works but whose underlying cause remained unclear. The Cartwheel CME traveled toward a coronal hole (CH) and against the magnetic gradients. Using a high-cadence, full-trajectory reconstruction, we accurately determine the location of the magnetic flux rope (MFR) and, consequently, the magnetic environment in which it is immersed. We find a pseudostreamer (PS) structure whose null point may be responsible for the complex evolution of the MFR at the initial phase. From the preeruptive magnetic field reconstruction, we estimate the dynamic forces acting on the MFR and provide a new physical insight into the motion exhibited by the 2008 April 9 event. By setting up a similar magnetic configuration in a 2.5D numerical simulation we are able to reproduce the observed behavior, confirming the importance of the PS null point. We find that the magnetic forces directed toward the null point cause the first deflection, directing the MFR toward the CH. Later, the magnetic pressure gradient of the CH produces the reversal motion of the MFR.

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Dzanic, T., S. S. Girimaji et F. D. Witherden. « Partially-averaged Navier–Stokes simulations of turbulence within a high-order flux reconstruction framework ». Journal of Computational Physics 456 (mai 2022) : 110992. http://dx.doi.org/10.1016/j.jcp.2022.110992.

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Le Bras, Sophie, Hugues Deniau et Christophe Bogey. « A technique of flux reconstruction at the interfaces of nonconforming grids for aeroacoustic simulations ». International Journal for Numerical Methods in Fluids 91, n^o 12 (23 octobre 2019) : 587–614. http://dx.doi.org/10.1002/fld.4767.

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Heng, Yi, Lars Hoffmann, Sabine Griessbach, Thomas Rößler et Olaf Stein. « Inverse transport modeling of volcanic sulfur dioxide emissions using large-scale simulations ». Geoscientific Model Development 9, n^o 4 (2 mai 2016) : 1627–45. http://dx.doi.org/10.5194/gmd-9-1627-2016.

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Abstract. An inverse transport modeling approach based on the concepts of sequential importance resampling and parallel computing is presented to reconstruct altitude-resolved time series of volcanic emissions, which often cannot be obtained directly with current measurement techniques. A new inverse modeling and simulation system, which implements the inversion approach with the Lagrangian transport model Massive-Parallel Trajectory Calculations (MPTRAC) is developed to provide reliable transport simulations of volcanic sulfur dioxide (SO2). In the inverse modeling system MPTRAC is used to perform two types of simulations, i.e., unit simulations for the reconstruction of volcanic emissions and final forward simulations. Both types of transport simulations are based on wind fields of the ERA-Interim meteorological reanalysis of the European Centre for Medium Range Weather Forecasts. The reconstruction of altitude-dependent SO2 emission time series is also based on Atmospheric InfraRed Sounder (AIRS) satellite observations. A case study for the eruption of the Nabro volcano, Eritrea, in June 2011, with complex emission patterns, is considered for method validation. Meteosat Visible and InfraRed Imager (MVIRI) near-real-time imagery data are used to validate the temporal development of the reconstructed emissions. Furthermore, the altitude distributions of the emission time series are compared with top and bottom altitude measurements of aerosol layers obtained by the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite instruments. The final forward simulations provide detailed spatial and temporal information on the SO2 distributions of the Nabro eruption. By using the critical success index (CSI), the simulation results are evaluated with the AIRS observations. Compared to the results with an assumption of a constant flux of SO2 emissions, our inversion approach leads to an improvement of the mean CSI value from 8.1 to 21.4 % and the maximum CSI value from 32.3 to 52.4 %. The simulation results are also compared with those reported in other studies and good agreement is observed. Our new inverse modeling and simulation system is expected to become a useful tool to also study other volcanic eruption events.

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Jacobus, Cooper, Peter Harrington et Zarija Lukić. « Reconstructing Lyα Fields from Low-resolution Hydrodynamical Simulations with Deep Learning ». Astrophysical Journal 958, n^o 1 (1 novembre 2023) : 21. http://dx.doi.org/10.3847/1538-4357/acfcb5.

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Abstract Hydrodynamical cosmological simulations are a powerful tool for accurately predicting the properties of the intergalactic medium (IGM) and for producing mock skies that can be compared against observational data. However, the need to resolve density fluctuation in the IGM puts a stringent requirement on the resolution of such simulations, which in turn limits the volumes that can be modeled, even on the most powerful supercomputers. In this work, we present a novel modeling method that combines physics-driven simulations with data-driven generative neural networks to produce outputs that are qualitatively and statistically close to the outputs of hydrodynamical simulations employing eight times higher resolution. We show that the Lyα flux field, as well as the underlying hydrodynamic fields, have greatly improved statistical fidelity over a low-resolution simulation. Importantly, the design of our neural network allows for sampling multiple realizations from a given input, enabling us to quantify the model uncertainty. Using test data, we demonstrate that this model uncertainty correlates well with the true error of the Lyα flux prediction. Ultimately, our approach allows for training on small simulation volumes and applying it to much larger ones, opening the door to producing accurate Lyα mock skies in volumes of Hubble size, as will be probed with DESI and future spectroscopic sky surveys.

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Gazith, Dotan, et Barak Zackay. « Precision Speckle Pattern Reconstruction for High-contrast Imaging ». Astronomical Journal 167, n^o 2 (10 janvier 2024) : 53. http://dx.doi.org/10.3847/1538-3881/ad0b6f.

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Abstract In high-contrast imaging, a large instrumental, technological, and algorithmic effort is made to reduce residual speckle noise and improve the detection capabilities. In this work, we explore the potential of using a precise physical description of speckle images, in conjunction with the optimal detection statistic to perform high-contrast imaging. Our method uses short-exposure speckle images, reconstructing the point-spread function (PSF) of each image with phase retrieval algorithms. Using the reconstructed PSFs, we calculate the optimal detection statistic for all images. We analyze the arising bias due to the use of a reconstructed PSF and correct for it completely up to its accumulation over 104 images. We measure in simulations the method’s sensitivity loss due to overfitting in the reconstruction process and get to an estimated 5σ detection limit of 5 × 10−7 flux ratio at angular separations of 0.″1–0.″5 for a 1h observation of Sirius A with a 2 m telescope.

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Vásquez Anacona, Hugo, Cristian Mattar, Nicolás G. Alonso-de-Linaje, Héctor H. Sepúlveda et Jessica Crisóstomo. « Wind Simulations over Western Patagonia Using the Weather Research and Forecasting model and Reanalysis ». Atmosphere 14, n^o 7 (23 juin 2023) : 1062. http://dx.doi.org/10.3390/atmos14071062.

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The Chilean Western Patagonia has the highest wind potential resources in South America. Its complex terrain deserves a special attention for wind modeling and assessments. In this work, we have performed a comprehensive meso-scale climate simulation on Weather Research and Forecasting (WRF) in order to provide new insights into the wind climatology in Western Patagonia. Simulations were carried out from 1989 to 2020, and we considered a previous sensitivity analysis for their configuration. In situ data from a wind mast, meteorological information and data from eddy flux stations were used to evaluate the results. Reanalysis data from ERA-5, MERRA-2 and RECON80-17 were also used to perform a comparison of the obtained results with the WRF simulation. The results show that the WRF simulation using ERA-5 presented in this work is slightly different to a mathematical reconstruction using MERRA-2 (RECON80-17), which is widely accepted in Chile for wind resource assessments, presenting a statistical difference of about EMD = 0.8 [m s−1] and RMSE = 0.5. Non-significative differences were found between the WRF simulation and MERRA-2 reanalysis, while ERA-5 with MERRA-2 presented a remarkable statistical difference of about EMD = 1.64 [m s−1] and RMSE = 1.8. In relation to flux comparison, reanalysis and WRF in contrast with in situ observations presented a good performance during the summer season, although a spatial resolution bias was noticed. These results can be used as an input for further research related to WRF simulations in Western Patagonia to provide reliable information on wind energy exploration and extreme climatological phenomena such as heat waves.

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Dash, Soumyaranjan, Marc L. DeRosa, Mausumi Dikpati, Xudong 旭东 Sun 孙, Sushant S. Mahajan, Yang 扬. Liu 刘 et J. Todd Hoeksema. « Ensemble Kalman Filter Data Assimilation into the Surface Flux Transport Model to Infer Surface Flows : An Observing System Simulation Experiment ». Astrophysical Journal 975, n^o 2 (1 novembre 2024) : 288. http://dx.doi.org/10.3847/1538-4357/ad7eac.

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Abstract Knowledge of the global magnetic field distribution and its evolution on the Sun’s surface is crucial for modeling the coronal magnetic field, understanding the solar wind dynamics, computing the heliospheric open flux distribution, and predicting the solar cycle strength. As the far side of the Sun cannot be observed directly and high-latitude observations always suffer from projection effects, we often rely on surface flux transport (SFT) simulations to model the long-term global magnetic field distribution. Meridional circulation, the large-scale north–south component of the surface flow profile, is one of the key components of the SFT simulation that requires further constraints near high latitudes. Prediction of the photospheric magnetic field distribution requires knowledge of the flow profile in the future, which demands reconstruction of that same flow at the current time so that it can be estimated at a later time. By performing Observing System Simulation Experiments, we demonstrate how the ensemble Kalman filter technique, when used with an SFT model, can be utilized to make “posterior” estimates of flow profiles into the future that can be used to drive the model forward to forecast the photospheric magnetic field distribution.

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Teh, Wai-Leong. « Multiple Flux Rope Dynamics : MMS Observations and Reconstruction Results ». Astrophysical Journal 969, n^o 2 (1 juillet 2024) : 96. http://dx.doi.org/10.3847/1538-4357/ad47f9.

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Abstract A series of six ion-scale magnetic flux ropes (FR1–6) and a thin current sheet, encountered by Magnetospheric Multiscale spacecraft in the magnetosheath side of the dayside magnetopause boundary layer, are studied for multiple flux rope dynamics in terms of energy conversion and two-dimensional geometry structure. The thin current sheet is identified in between FR1 and FR2. The energy exchange between electromagnetic fields and plasmas is dynamic in FR1–5 and also surrounding the flux ropes, while a low-energy conversion rate is seen in FR6. Different energy conversions exist in the flux ropes: energy dynamo is predominant in FR1 and FR5, while energy dissipation is dominated in FR2–4. Both the energy dynamo and dissipation primarily result from j ∥ E ∥. Strong dissipation, surrounded by an energy dynamo, happens at the thin current sheet and is accompanied by ion agyrotropic behavior. From the reconstructed magnetic field maps, the estimated aspect ratios of the six flux ropes are 0.78, 0.16, 0.66, 0.11, 0.40, and 0.46 in order, and the thickness of the thin current sheet is ∼63 km (∼1.8 ion inertial length). Evidently, the magnetic field map shows that FR4 is a coalescing flux rope where a pronounced dissipation is present. The overall finding agrees with the previous simulation studies of multiple island coalescence.

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Shi, Jingchang, et Hong Yan. « Turbulence amplification in the shock wave/turbulent boundary layer interaction over compression ramp by the flux reconstruction method ». Physics of Fluids 35, n^o 1 (janvier 2023) : 016122. http://dx.doi.org/10.1063/5.0134222.

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Wall-resolved large eddy simulation on a supersonic turbulent boundary layer over a [Formula: see text] compression ramp is performed under the framework of high order discontinuous methods for a free-stream Mach number [Formula: see text] and Reynolds number [Formula: see text]. The turbulent flow is resolved by the high order flux reconstruction method, and the shock is captured by a high-resolution, but stable weighted essentially non-oscillation limiter. The solver used in this paper is validated by the double Mach reflection case and the Taylor–Green vortex case. The results of shock wave/turbulent boundary layer interaction at the ramp corner are validated by the numerical simulations and the experimental data in the literature. The analysis of the physics behind the turbulence amplification at around the ramp corner is presented. The shear effects and the flow deceleration/acceleration are the main reasons of the turbulence amplification.

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Kang, Myeongseok, et Donghyun You. « A Low Dissipative and Stable Cell-Centered Finite Volume Method with the Simultaneous Approximation Term for Compressible Turbulent Flows ». Mathematics 9, n^o 11 (26 mai 2021) : 1206. http://dx.doi.org/10.3390/math9111206.

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A simultaneous-approximation term is a non-reflecting boundary condition that is usually accompanied by summation-by-parts schemes for provable time stability. While a high-order convective flux based on reconstruction is often employed in a finite-volume method for compressible turbulent flow, finite-volume methods with the summation-by-parts property involve either equally weighted averaging or the second-order central flux for convective fluxes. In the present study, a cell-centered finite-volume method for compressible Naiver–Stokes equations was developed by combining a simultaneous-approximation term based on extrapolation and a low-dissipative discretization method without the summation-by-parts property. Direct numerical simulations and a large eddy simulation show that the resultant combination leads to comparable non-reflecting performance to that of the summation-by-parts scheme combined with the simultaneous-approximation term reported in the literature. Furthermore, a characteristic boundary condition was implemented for the present method, and its performance was compared with that of the simultaneous-approximation term for a direct numerical simulation and a large eddy simulation to show that the simultaneous-approximation term better maintained the average target pressure at the compressible flow outlet, which is useful for turbomachinery and aerodynamic applications, while the characteristic boundary condition better preserved the flow field near the outlet.

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Brochard, G., J. Bao, C. Liu, N. Gorelenkov, G. Choi, G. Dong, P. Liu et al. « Verification and validation of linear gyrokinetic and kinetic-MHD simulations for internal kink instability in DIII-D tokamak ». Nuclear Fusion 62, n^o 3 (28 janvier 2022) : 036021. http://dx.doi.org/10.1088/1741-4326/ac48a6.

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Abstract Verification and linear validation of the internal kink instability in tokamak have been performed for both gyrokinetic (GTC) and kinetic-MHD codes (GAM-solver, M3D-C1-K, NOVA, XTOR-K). Using realistic magnetic geometry and plasma profiles from the same equilibrium reconstruction of the DIII-D shot #141216, these codes exhibit excellent agreement for the growth rate and mode structure of the internal kink mode when all kinetic effects are suppressed. The simulated radial mode structures, obtained from linear simulations, are in reasonable agreement with the normalised electron cyclotron emission measurement after adjusting, within the experimental uncertainty, the safety factor q = 1 flux-surface location in the equilibrium reconstruction. Compressible magnetic perturbations strongly destabilize the kink, while poloidal variations of the equilibrium current density reduce the growth rate of the kink. Furthermore, kinetic effects of thermal ions are found to decrease the kink growth rate in kinetic-MHD simulations, but increase the kink growth rate in gyrokinetic simulations, due to the additional drive of the ion temperature gradient and parallel electric field. Kinetic thermal electrons are found to have negligible effects on the internal kink instability.

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Ni, Zining, Jinsen Xie, Muhammad Abdul Wasaye, Erpin Zhang et Tao Yu. « Effect of Higher-Order Harmonics on the Steady-State Neutron Flux in Accelerator-Driven Subcritical Reactors ». International Journal of Energy Research 2023 (5 avril 2023) : 1–14. http://dx.doi.org/10.1155/2023/4114886.

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Higher-order neutron fluxes (i.e., higher-order harmonics) are widely applied in perturbation theory and modal kinetics, and they are important for research on the physical characteristics of accelerator-driven subcritical reactors (ADSRs). This paper presents a computational scheme for reconstructing the neutron flux in the steady state according to the biorthogonal properties of the forward and adjoint neutron fluxes, which can be used to analyze how higher-order harmonics affect the steady-state neutron flux under λ - and prompt α -modes. Simulation results indicated that a modal synthesis method based on λ - and prompt α -modes can effectively reconstruct the steady-state neutron flux and core power in an ADSR with a power reconstruction accuracy of within 5%. The higher-order harmonics can be classified into three types according to their physical characteristics: the first type contributes significantly to the steady-state neutron flux, the second type contributes almost nothing to the steady-state neutron flux, and the third type contributes nothing to the steady-state neutron flux. The external neutron source contributes only to specific harmonic expansion orders, which are characterized by significant axial and radial symmetry for both the λ - and prompt α -modes.

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MIYAJI, Koji, et Rei NAGASAWA. « Extension of the Flux Reconstruction Method to High-Reynolds Number RANS Simulations around High-Lift Devices ». TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES 60, n^o 1 (2017) : 18–26. http://dx.doi.org/10.2322/tjsass.60.18.

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Agoshkov, Valery I., Eugene I. Parmuzin, Natalia B. Zakharova et Victor P. Shutyaev. « Variational assimilation with covariance matrices of observation data errors for the model of the Baltic Sea dynamics ». Russian Journal of Numerical Analysis and Mathematical Modelling 33, n^o 3 (26 juin 2018) : 149–60. http://dx.doi.org/10.1515/rnam-2018-0013.

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Abstract The mathematical model of the Baltic Sea dynamics developed at the Institute of Numerical Mathematics of RAS is considered. The problem of variational assimilation of average daily data for the sea surface temperature (SST) is formulated and studied with the use of covariance matrices of observation data errors. Based on variational assimilation of satellite observation data, we propose an algorithm for solving the inverse problem of the heat flux reconstruction on the sea surface. The results of numerical experiments on reconstruction of the heat flux function are presented for the problem of variational assimilation of observation SST data.

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Pereira, Carlos A., et Brian C. Vermeire. « Performance and accuracy of hybridized flux reconstruction schemes ». Journal of Computational Physics 457 (mai 2022) : 111039. http://dx.doi.org/10.1016/j.jcp.2022.111039.

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Ma, Libin, Chao Yan et Jian Yu. « A Modal-Decay-Based Shock-Capturing Approach for High-Order Flux Reconstruction Method ». Aerospace 10, n^o 1 (24 décembre 2022) : 14. http://dx.doi.org/10.3390/aerospace10010014.

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The increasing demand for high-fidelity simulations of compressible turbulence on complex geometries poses a number of challenges for numerical schemes, and plenty of high-order methods have been developed. The high-order methods may encounter spurious oscillations or even blow up for strongly compressible flows, and a number of approaches have been developed, such as slope limiters and artificial viscosity models. In the family of artificial viscosity, which measures smoothness using the modal coefficients, the averaged modal decay (MDA) model employs all of the modes instead of only the highest mode as in the highest modal decay (MDH) model, which tends to underestimate the smoothness. However, the MDA approach requires high-order accuracy (usually P≥4) to deliver a reliable estimation of smoothness. In this work, an approach used to extend the MDA model to lower orders, such as P2 and P3, referred to as MDAEX, was proposed, where neighboring elements were incorporated to involve more information in the estimation process. A further controlling of the value of artificial viscosity was also introduced. The proposed model was applied to several typical benchmark cases and compared with other typical models. The results show that the MDAEX model recovers the expected accuracy better than the MDA model for P2 and P3 and captures flow structures well for shock-dominated flows.

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Kovbasa, S., S. Dymko et M. Zhelinskyi. « SPEED OBSERVER FOR SENSORLESS ENERGY GENERATION SYSTEMS BASED ON FIELD ORIENTED INDUCTION GENERATOR ». ELECTRICAL AND COMPUTER SYSTEMS 33, n^o 108 (30 novembre 2020) : 9–15. http://dx.doi.org/10.15276/eltecs.32.108.2020.1.

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The paper presents a novel adaptive speed – rotor flux linkage observer for sensorless field oriented control of induction generators. The proposed solution is based on Matsuse observer structure to- gether with specially designed correction terms and utilize current measurement and calculated stator volt- ages for real-time flux and speed reconstruction. A special coordinate transformation is used to avoid non- linear parametrization in the right side of flux linkages differential equations. Adaptive observer is designed in two steps: at the first step nonadaptive flux linkage observer is designed under condition of speed meas- urement; at the second step adaptive to rotor speed version of the flux linkage observer is designed. Infor- mation about rotor speed for adaptive flux observer is calculated by developed speed observer. A second Lyapunov’s method together with adaptive control theory are utilized for observer’s correction terms synthe- sis and stability proof. Designed adaptive observer under persistence of excitation conditions guarantees local exponential estimation of constant rotor speed and flux linkage vector components of induction ma- chine. From the practical point of view persistency excitation conditions are met if rotor flux linkages are nonzero. Proposed solution investigated by simulations. It is shown, that developed adaptive speed observer provides asymptotic estimation of induction motor currents, speed and flux linkage components under con- stant speed conditions. For varying speed proposed observer provides estimation of required variables with a small dynamic error. Proposed observer can be used in energy generation systems based on induction generators as well as in sensorless induction motor-based drive systems with constant or slowly varying ro- tor speed.

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Zhu, Hui, Song Fu, Lei Shi et Z. J. Wang. « Implicit Large-Eddy Simulation for the High-Order Flux Reconstruction Method ». AIAA Journal 54, n^o 9 (septembre 2016) : 2721–33. http://dx.doi.org/10.2514/1.j054826.

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Virtanen, I. O. I., I. I. Virtanen, A. A. Pevtsov, L. Bertello, A. Yeates et K. Mursula. « Reconstructing solar magnetic fields from historical observations ». Astronomy & ; Astrophysics 627 (25 juin 2019) : A11. http://dx.doi.org/10.1051/0004-6361/201935606.

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Aims. The evolution of the photospheric magnetic field has only been regularly observed since the 1970s. The absence of earlier observations severely limits our ability to understand the long-term evolution of solar magnetic fields, especially the polar fields that are important drivers of space weather. Here, we test the possibility to reconstruct the large-scale solar magnetic fields from Ca II K line observations and sunspot magnetic field observations, and to create synoptic maps of the photospheric magnetic field for times before modern-time magnetographic observations. Methods. We reconstructed active regions from Ca II K line synoptic maps and assigned them magnetic polarities using sunspot magnetic field observations. We used the reconstructed active regions as input in a surface flux transport simulation to produce synoptic maps of the photospheric magnetic field. We compared the simulated field with the observed field in 1975−1985 in order to test and validate our method. Results. The reconstruction very accurately reproduces the long-term evolution of the large-scale field, including the poleward flux surges and the strength of polar fields. The reconstruction has slightly less emerging flux because a few weak active regions are missing, but it includes the large active regions that are the most important for the large-scale evolution of the field. Although our reconstruction method is very robust, individual reconstructed active regions may be slightly inaccurate in terms of area, total flux, or polarity, which leads to some uncertainty in the simulation. However, due to the randomness of these inaccuracies and the lack of long-term memory in the simulation, these problems do not significantly affect the long-term evolution of the large-scale field.

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Frasson, Thomas, Stéphane Labrosse, Henri-Claude Nataf, Nicolas Coltice et Nicolas Flament. « On the impact of true polar wander on heat flux patterns at the core–mantle boundary ». Solid Earth 15, n^o 5 (14 mai 2024) : 617–37. http://dx.doi.org/10.5194/se-15-617-2024.

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Abstract. The heat flux across the core–mantle boundary (CMB) is a fundamental variable for Earth evolution and internal dynamics. Seismic tomography provides access to seismic heterogeneities in the lower mantle, which can be related to present-day thermal heterogeneities. Alternatively, mantle convection models can be used to either infer past CMB heat flux or to produce statistically realistic CMB heat flux patterns in self-consistent models. Mantle dynamics modifies the inertia tensor of the Earth, which implies a rotation of the Earth with respect to its spin axis, a phenomenon called true polar wander (TPW). This rotation must be taken into account to link the dynamics of the mantle to the dynamics of the core. In this study, we explore the impact of TPW on the CMB heat flux over long timescales (∼1 Gyr) using two recently published mantle convection models: one model driven by a plate reconstruction and a second that self-consistently produces a plate-like behaviour. We compute the geoid in both models to correct for TPW. In the plate-driven model, we compute a total geoid and a geoid in which lateral variations of viscosity and density are suppressed above 350 km depth. An alternative to TPW correction is used for the plate-driven model by simply repositioning the model in the original paleomagnetic reference frame of the plate reconstruction. The average TPW rates range between 0.4 and 1.8° Myr−1, but peaks up to 10° Myr−1 are observed. We find that in the plate-driven mantle convection model used in this study, the maximum inertia axis produced by the model does not show a long-term consistency with the position of the magnetic dipole inferred from paleomagnetism. TPW plays an important role in redistributing the CMB heat flux, notably at short timescales (≤10 Myr). Those rapid variations modify the latitudinal distribution of the CMB heat flux, which is known to affect the stability of the magnetic dipole in geodynamo simulations. A principal component analysis (PCA) is computed to obtain the dominant CMB heat flux pattern in the different cases. These heat flux patterns are representative of the mantle convection cases studied here and can be used as boundary conditions for geodynamo models.

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TAN, X., J. X. ZHONG et G. W. YANG. « GROWTH MECHANISM OF RING SHAPED NANOSTRUCTURES SELF-ASSEMBLY UPON DROPLET EPITAXY ». Surface Review and Letters 19, n^o 03 (juin 2012) : 1250029. http://dx.doi.org/10.1142/s0218625x12500291.

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A quantitatively kinetic model has been established to address the self-assembly of the ring shaped nanostructures upon the droplet epitaxy via kinetic Monte Carlo simulations. The theoretical predictions about the temperature and As flux dependences of the self-assembly of the ring shaped nanostructures were in well agreement with recent experiments. It was found that the morphological evolution of the ring shaped nanostructures was attributed to the cooperation of the enhanced diffusion barriers of free Ga atoms in the inner ring region and the effects of the surface reconstruction around the Ga droplets during the arsenization step.

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Watanabe, Michio, Hiroaki Tatebe, Hiroshi Koyama, Tomohiro Hajima, Masahiro Watanabe et Michio Kawamiya. « Importance of El Niño reproducibility for reconstructing historical CO<sub>2</sub> ; flux variations in the equatorial Pacific ». Ocean Science 16, n^o 6 (18 novembre 2020) : 1431–42. http://dx.doi.org/10.5194/os-16-1431-2020.

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Abstract. Based on a set of climate simulations utilizing two kinds of Earth system models (ESMs) in which observed ocean hydrographic data are assimilated using exactly the same data assimilation procedure, we have clarified that the successful simulation of the observed air–sea CO2 flux variations in the equatorial Pacific is tightly linked to the reproducibility of coupled physical air–sea processes. When an ESM with a weaker ENSO (El Niño–Southern Oscillations) amplitude than that of the observations was used for historical simulations with ocean data assimilation, the observed equatorial anticorrelated relationship between the sea surface temperature (SST) and the air–sea CO2 flux on interannual to decadal timescales could not be represented. The simulated CO2 flux anomalies were upward (downward) during El Niño (La Niña) periods in the equatorial Pacific. The reason for this was that the non-negligible correction term in the governing equation of ocean temperature, which was added via the ocean data assimilation procedure, caused an anomalous, spurious equatorial upwelling (downwelling) during El Niño (La Niña) periods, which brought more (less) subsurface layer water rich in dissolved inorganic carbon (DIC) to the surface layer. On the other hand, in the historical simulations where the observational data were assimilated into the other ESM with a more realistic ENSO representation, the correction term associated with the assimilation procedure remained small enough so as not to disturb an anomalous advection–diffusion balance for the equatorial ocean temperature. Consequently, spurious vertical transport of DIC and the resultant positively correlated SST and air–sea CO2 flux variations did not occur. Thus, the reproducibility of the tropical air–sea CO2 flux variability with data assimilation can be significantly attributed to the reproducibility of ENSO in an ESM. Our results suggest that, when using data assimilation to initialize ESMs for carbon cycle predictions, the reproducibility of the internal climate variations in the model itself is of great importance.

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Todarello, Elisa, Andre Scaffidi, Marco Regis et Marco Taoso. « Constraining below-threshold radio source counts with machine learning ». Journal of Cosmology and Astroparticle Physics 2024, n^o 01 (1 janvier 2024) : 062. http://dx.doi.org/10.1088/1475-7516/2024/01/062.

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Abstract We propose a machine-learning-based technique to determine the number density of radio sources as a function of their flux density, for use in next-generation radio surveys. The method uses a convolutional neural network trained on simulations of the radio sky to predict the number of sources in several flux bins. To train the network, we adopt a supervised approach wherein we simulate training data stemming from a large domain of possible number count models going down to fluxes a factor of 100 below the threshold for source detection. We test the model reconstruction capabilities as well as benchmark the expected uncertainties in the model predictions, observing good performance for fluxes down to a factor of ten below the threshold. This work demonstrates that the capabilities of simple deep learning models for radio astronomy can be useful tools for future surveys.

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Kahnt, Maik, Konstantin Klementiev, Vahid Haghighat, Clemens Weninger, Tomás S. Plivelic, Ann E. Terry et Alexander Björling. « Measurement of the coherent beam properties at the CoSAXS beamline ». Journal of Synchrotron Radiation 28, n^o 6 (5 octobre 2021) : 1948–53. http://dx.doi.org/10.1107/s1600577521009140.

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The CoSAXS beamline at the MAX IV Laboratory is a modern multi-purpose (coherent) small-angle X-ray scattering (CoSAXS) instrument, designed to provide intense and optionally coherent illumination at the sample position, enabling coherent imaging and speckle contrast techniques. X-ray tracing simulations used to design the beamline optics have predicted a total photon flux of 1012–1013 photons s−1 and a degree of coherence of up to 10% at 7.1 keV. The normalized degree of coherence and the coherent flux of this instrument were experimentally determined using the separability of a ptychographic reconstruction into multiple mutually incoherent modes and thus the Coherence in the name CoSAXS was verified. How the beamline can be used both for coherent imaging and XPCS measurements, which both heavily rely on the degree of coherence of the beam, was demonstrated. These results are the first experimental quantification of coherence properties in a SAXS instrument at a fourth-generation synchrotron light source.

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Kahnt, Maik, Konstantin Klementiev, Vahid Haghighat, Clemens Weninger, Tomás S. Plivelic, Ann E. Terry et Alexander Björling. « Measurement of the coherent beam properties at the CoSAXS beamline ». Journal of Synchrotron Radiation 28, n^o 6 (5 octobre 2021) : 1948–53. http://dx.doi.org/10.1107/s1600577521009140.

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The CoSAXS beamline at the MAX IV Laboratory is a modern multi-purpose (coherent) small-angle X-ray scattering (CoSAXS) instrument, designed to provide intense and optionally coherent illumination at the sample position, enabling coherent imaging and speckle contrast techniques. X-ray tracing simulations used to design the beamline optics have predicted a total photon flux of 1012–1013 photons s−1 and a degree of coherence of up to 10% at 7.1 keV. The normalized degree of coherence and the coherent flux of this instrument were experimentally determined using the separability of a ptychographic reconstruction into multiple mutually incoherent modes and thus the Coherence in the name CoSAXS was verified. How the beamline can be used both for coherent imaging and XPCS measurements, which both heavily rely on the degree of coherence of the beam, was demonstrated. These results are the first experimental quantification of coherence properties in a SAXS instrument at a fourth-generation synchrotron light source.

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Xiao, Tianbai. « A flux reconstruction kinetic scheme for the Boltzmann equation ». Journal of Computational Physics 447 (décembre 2021) : 110689. http://dx.doi.org/10.1016/j.jcp.2021.110689.

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Lu, Jinhua, Haiyan Lei, Chuanshan Dai, Liming Yang et Chang Shu. « Analyses and reconstruction of the lattice Boltzmann flux solver ». Journal of Computational Physics 453 (mars 2022) : 110923. http://dx.doi.org/10.1016/j.jcp.2021.110923.

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Hussey, Daniel Seth, Michael Cyrus Daugherty, Youngju Kim, David Jacobson et Jacob Michael LaManna. « Simulations of Wolter Optics Tomography of a Fuel Cell ». ECS Meeting Abstracts MA2023-02, n^o 37 (22 décembre 2023) : 1735. http://dx.doi.org/10.1149/ma2023-02371735mtgabs.

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Wolter optics present the opportunity to realize a practical neutron lens, which would convert a neutron imaging instrument from a pinhole camera to a microscope. This conversion would result in significant increases in neutron flux (up to 10,000 gain over the pinhole geometry) while maintaining high spatial resolution (about 3 μm). The Wolter optic system is currently being fabricated, and the first mirrors are expected to arrive in SEP 2023. As such, we focus this presentation on image simulation efforts. A ray tracing code has been developed that is able to simulate full three-dimensional imaging of extended objects placed in the Wolter optics neutron microscope. To simulate a fuel cell, we employ tomography measurements based on X-ray tomography and CAD-model derived volumes. The reason the Wolter optics system realizes such a large gain in flux is by focusing much of the flux from an extended neutron guide onto a small spot. Thus, conventional radiography of an extended object will not be possible, as the beam is similar to a cone beam which emanates from a small spot. We derive a tomography algorithm for this spot and will show its effectiveness on reconstructing simulated images. We also report on expected performance of the fabricated optical system in the current instrument configuration. Figure 1

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Montiel, Miguel, et Roque Corral. « Time-Inclined Method for High-Fidelity Rotor/Stator Simulations ». Aerospace 10, n^o 5 (18 mai 2023) : 475. http://dx.doi.org/10.3390/aerospace10050475.

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The application of the time-inclined method in a fourth-order unstructured flux-reconstruction code for turbomachinery is demonstrated. Inviscid and viscous unsteady results due to the interaction of an incoming gust of total pressure with a linear cascade of flat plates and a linear cascade of T106A low-pressure turbine airfoils are reported. The agreement between the time-inclined method and the equivalent full-annulus multipassage solution is very high for both cases. Viscous solutions at Reynolds numbers of 104 and 105 were conducted. A high degree of matching was obtained between the time-inclined and the whole annulus approaches. The limitations of the method are explored and discussed. While the evolution of the unsteady boundary layers created by the interaction with the incoming wakes was very well captured, the mixing associated with the trailing edge vortex shedding was less accurate. The critical parameter controlling the method’s accuracy is the local Strouhal number. It was demonstrated that the benefit of retaining the exact blade count in the simulations overcomes the slight differences in the mixing due to the limitation of the time-inclined method to model viscous effects accurately in all situations.

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Shi, Jingchang, Hong Yan et Z. J. Wang. « Flux Reconstruction Implementation of an Algebraic Wall Model for Large-Eddy Simulation ». AIAA Journal 58, n^o 7 (juillet 2020) : 3051–62. http://dx.doi.org/10.2514/1.j058957.

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Bull, J. R., et A. Jameson. « Simulation of the Taylor–Green Vortex Using High-Order Flux Reconstruction Schemes ». AIAA Journal 53, n^o 9 (septembre 2015) : 2750–61. http://dx.doi.org/10.2514/1.j053766.

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Skarolek, Vilem, et Koji Miyaji. « Numerical Simulation of Transitional Flow Past a Wing Using Flux Reconstruction Method ». AIAA Journal 54, n^o 4 (avril 2016) : 1424–37. http://dx.doi.org/10.2514/1.j054324.

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Yang, L. M., C. Shu et J. Wu. « A Hybrid Lattice Boltzmann Flux Solver for Simulation of Viscous Compressible Flows ». Advances in Applied Mathematics and Mechanics 8, n^o 6 (19 septembre 2016) : 887–910. http://dx.doi.org/10.4208/aamm.2015.m1172.

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AbstractIn this paper, a hybrid lattice Boltzmann flux solver (LBFS) is proposed for simulation of viscous compressible flows. In the solver, the finite volume method is applied to solve the Navier-Stokes equations. Different from conventional Navier-Stokes solvers, in this work, the inviscid flux across the cell interface is evaluated by local reconstruction of solution using one-dimensional lattice Boltzmann model, while the viscous flux is still approximated by conventional smooth function approximation. The present work overcomes the two major drawbacks of existing LBFS [28–31], which is used for simulation of inviscid flows. The first one is its ability to simulate viscous flows by including evaluation of viscous flux. The second one is its ability to effectively capture both strong shock waves and thin boundary layers through introduction of a switch function for evaluation of inviscid flux, which takes a value close to zero in the boundary layer and one around the strong shock wave. Numerical experiments demonstrate that the present solver can accurately and effectively simulate hypersonic viscous flows.

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Yu, Hengyong, Changguo Ji et Ge Wang. « SART-Type Image Reconstruction from Overlapped Projections ». International Journal of Biomedical Imaging 2011 (2011) : 1–7. http://dx.doi.org/10.1155/2011/549537.

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To maximize the time-integrated X-ray flux from multiple X-ray sources and shorten the data acquisition process, a promising way is to allow overlapped projections from multiple sources being simultaneously on without involving the source multiplexing technology. The most challenging task in this configuration is to perform image reconstruction effectively and efficiently from overlapped projections. Inspired by the single-source simultaneous algebraic reconstruction technique (SART), we hereby develop a multisource SART-type reconstruction algorithm regularized by a sparsity-oriented constraint in the soft-threshold filtering framework to reconstruct images from overlapped projections. Our numerical simulation results verify the correctness of the proposed algorithm and demonstrate the advantage of image reconstruction from overlapped projections.

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