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Matringe, Sebastien F., Ruben Juanes e Hamdi A. Tchelepi. "Tracing Streamlines on Unstructured Grids From Finite Volume Discretizations". SPE Journal 13, n. 04 (1 dicembre 2008): 423–31. http://dx.doi.org/10.2118/103295-pa.
Testo completoAbstract (sommario):
Summary The accuracy of streamline reservoir simulations depends strongly on the quality of the velocity field and the accuracy of the streamline tracing method. For problems described on complex grids (e.g., corner-point geometry or fully unstructured grids) with full-tensor permeabilities, advanced discretization methods, such as the family of multipoint flux approximation (MPFA) schemes, are necessary to obtain an accurate representation of the fluxes across control volume faces. These fluxes are then interpolated to define the velocity field within each control volume, which is then used to trace the streamlines. Existing methods for the interpolation of the velocity field and integration of the streamlines do not preserve the accuracy of the fluxes computed by MPFA discretizations. Here we propose a method for the reconstruction of the velocity field with high-order accuracy from the fluxes provided by MPFA discretization schemes. This reconstruction relies on a correspondence between the MPFA fluxes and the degrees of freedom of a mixed finite-element method (MFEM) based on the first-order Brezzi-Douglas-Marini space. This link between the finite-volume and finite-element methods allows the use of flux reconstruction and streamline tracing techniques developed previously by the authors for mixed finite elements. After a detailed description of our streamline tracing method, we study its accuracy and efficiency using challenging test cases. Introduction The next-generation reservoir simulators will be unstructured. Several research groups throughout the industry are now developing a new breed of reservoir simulators to replace the current industry standards. One of the main advances offered by these next generation simulators is their ability to support unstructured or, at least, strongly distorted grids populated with full-tensor permeabilities. The constant evolution of reservoir modeling techniques provides an increasingly realistic description of the geological features of petroleum reservoirs. To discretize the complex geometries of geocellular models, unstructured grids seem to be a natural choice. Their inherent flexibility permits the precise description of faults, flow barriers, trapping structures, etc. Obtaining a similar accuracy with more traditional structured grids, if at all possible, would require an overwhelming number of gridblocks. However, the added flexibility of unstructured grids comes with a cost. To accurately resolve the full-tensor permeabilities or the grid distortion, a two-point flux approximation (TPFA) approach, such as that of classical finite difference (FD) methods is not sufficient. The size of the discretization stencil needs to be increased to include more pressure points in the computation of the fluxes through control volume edges. To this end, multipoint flux approximation (MPFA) methods have been developed and applied quite successfully (Aavatsmark et al. 1996; Verma and Aziz 1997; Edwards and Rogers 1998; Aavatsmark et al. 1998b; Aavatsmark et al. 1998c; Aavatsmark et al. 1998a; Edwards 2002; Lee et al. 2002a; Lee et al. 2002b). In this paper, we interpret finite volume discretizations as MFEM for which streamline tracing methods have already been developed (Matringe et al. 2006; Matringe et al. 2007b; Juanes and Matringe In Press). This approach provides a natural way of reconstructing velocity fields from TPFA or MPFA fluxes. For finite difference or TPFA discretizations, the proposed interpretation provides mathematical justification for Pollock's method (Pollock 1988) and some of its extensions to distorted grids (Cordes and Kinzelbach 1992; Prévost et al. 2002; Hægland et al. 2007; Jimenez et al. 2007). For MPFA, our approach provides a high-order streamline tracing algorithm that takes full advantage of the flux information from the MPFA discretization.
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Zerrik, E., e H. Bourray. "Flux reconstruction: sensors and simulations". Sensors and Actuators A: Physical 109, n. 1-2 (dicembre 2003): 34–46. http://dx.doi.org/10.1016/s0924-4247(03)00358-3.
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Khazari, Adil, e Ali Boutoulout. "Flux reconstruction for hyperbolic systems: Sensors and simulations". Evolution Equations & Control Theory 4, n. 2 (2015): 177–92. http://dx.doi.org/10.3934/eect.2015.4.177.
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Jessen, Kristian, Margot G. Gerritsen e Bradley T. Mallison. "High-Resolution Prediction of Enhanced Condensate Recovery Processes". SPE Journal 13, n. 02 (1 giugno 2008): 257–66. http://dx.doi.org/10.2118/99619-pa.
Testo completoAbstract (sommario):
Summary This paper investigates the accuracy of first- and high-order numerical methods in simulating enhanced condensate processes in 1D, 2D, and 3D. We compare the predictions of a standard single point upwind (SPU) scheme with a third-order accurate finite difference (FD) simulator based on a third-order essentially nonoscillatory (ENO) flux reconstruction with matching temporal accuracy. We include physical dispersion in the mathematical model of these multiphase, multicomponent systems. The comparisons demonstrate that SPU schemes may fail to predict the formation of the mobile liquid bank at the leading edge of the displacement unless an impractical number of gridblocks is used in the simulations. In contrast, the high-order FD simulator is demonstrated to accurately predict the liquid bank at much lower grid resolution, providing for a more efficient simulation approach. In 2D displacement calculations with gravity included, the CPU requirement of the SPU scheme was found to be more than 50 times larger than for the ENO scheme for a given level of accuracy. In 2D vertical cross-sections, the predicted component recovery is demonstrated to vary upward of 8% depending on the selected numerical scheme for a given grid resolution and dispersivity. In these settings, the SPU solutions converge to the ENO results upon significant grid refinement. In 3D displacement calculations, the magnitude of the predicted condensate bank is also found to be very different depending on the selected numerical scheme. Relative to the 2D displacement calculations, condensate banking and gravity segregation is observed to have less impact on the process performance prediction because of the permeability configuration in the 3D model used here, but it could have a high impact in different settings. We include an explicit representation of longitudinal and transverse dispersion in the porous medium to demonstrate the grid resolution required to resolve physical dispersion at a given simulation length scale, and to show that condensate banks can also form in more realistic dispersive systems. Grid-refinement studies in 1D and 2D demonstrate, again, that the ENO scheme outperforms the SPU scheme for a given Peclet (Pe) number. Converged solutions are obtained with the ENO scheme using a relatively small number of grid cells. In addition, we show the behavior of the two schemes for varying Peclet numbers on a fixed simulation grid. For this grid, the ENO scheme is shown to be sensitive to the Peclet number, signifying that physical dispersion is not overwhelmed by numerical diffusion. For the SPU scheme, however, the solutions are almost independent of the Peclet number, which indicates that numerical diffusion dominates. Introduction Significant portions of the current hydrocarbon reserves are found in gas-condensate-carrying formations. Production of hydrocarbons from these reserves is expected to increase upward of 100% by 2015 (Cambridge Energy Research Associates 2005). Primary production of these reserves will result in significant loss of the heavy ends because of liquid dropout once the reservoir pressure reaches the dew point pressure. Enhanced condensate recovery by gas cycling/injection schemes are often applied to extend the lifetime of condensate reservoirs. These processes are inherently compositional, as the component transfer between an immobile liquid phase and a mobile gas phase is the key mechanism for enhancing recovery. Numerical simulation of such processes is very challenging because the prediction of the local displacement efficiency and the global sweep can be very sensitive to numerical diffusion. Various authors have shown that numerical artifacts can alter the displacement characteristics and lead to significant underprediction of the local displacement efficiency (Stalkup et al. 1990; Lim et al. 1997; Johns et al. 2002; Jessen et al. 2004). In their numerical studies of gas injection in depleted condensate reservoirs, Høier and Whitson (2001) demonstrated that near-miscible gas injection may, in some cases, lead to the formation of a condensate bank at the leading edge of the displacement. In addition, for some injection settings, the liquid bank was shown to exceed the critical liquid saturation and hence become mobile. Their analysis was based on 1D displacement calculations. The work of Jessen and Orr (2004) demonstrated that the prediction of condensate banks that exceed the critical condensate saturation by numerical calculations requires a firm control of numerical diffusion. They used analytical solutions based on the method of characteristics (MOC) (Johns et al. 1993) as well as a high-resolution FD simulator developed by Mallison et al. (2005) to investigate the complex interplay of flow and phase behavior in enhanced condensate recovery processes in 1D. In this work, we extend this investigation of enhanced condensate recovery processes to 2D and 3D. We include gravity to study the impact of a mobile liquid bank on the overall efficiency of the enhanced condensate recovery (ECR) process. We investigate the grid resolutions needed for both numerical schemes to resolve the condensate banks, and the impact of numerical errors on the predicted recovery in the presence of gravity. We also study the importance of physical dispersion in ECR processes. In particular, we are interested in understanding the grid resolution that is required to resolve the physical dispersion terms by controlling the level of numerical diffusion. We note that physical dispersion/diffusion is required to obtain a converged solution in 2D and 3D for this type of displacement problem. In the following section, we introduce the mathematical model for multicomponent multiphase flow in porous media, including an explicit representation of dispersive terms. We then describe the implementation in our compositional simulator. Next, we discuss the condensate system investigated in this work and present simulation results for enhanced condensate recovery in 1D, 2D, and 3D. Finally, we draw conclusions from the presented material.
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Trojak, W., N. R. Vadlamani, J. Tyacke, F. D. Witherden e A. Jameson. "Artificial compressibility approaches in flux reconstruction for incompressible viscous flow simulations". Computers & Fluids 247 (ottobre 2022): 105634. http://dx.doi.org/10.1016/j.compfluid.2022.105634.
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Török, T., M. G. Linton, J. E. Leake, Z. Mikić, R. Lionello, V. S. Titov e C. Downs. "Solar Eruptions Triggered by Flux Emergence below or near a Coronal Flux Rope". Astrophysical Journal 962, n. 2 (1 febbraio 2024): 149. http://dx.doi.org/10.3847/1538-4357/ad1826.
Testo completoAbstract (sommario):
Abstract Observations have shown a clear association of filament/prominence eruptions with the emergence of magnetic flux in or near filament channels. Magnetohydrodynamic (MHD) simulations have been employed to systematically study the conditions under which such eruptions occur. These simulations to date have modeled filament channels as 2D flux ropes or 3D uniformly sheared arcades. Here we present MHD simulations of flux emergence into a more realistic configuration consisting of a bipolar active region containing a line-tied 3D flux rope. We use the coronal flux-rope model of Titov et al. as the initial condition and drive our simulations by imposing boundary conditions extracted from a flux emergence simulation by Leake et al. We identify three mechanisms that determine the evolution of the system: (i) reconnection displacing footpoints of field lines overlying the coronal flux rope, (ii) changes of the ambient field due to the intrusion of new flux at the boundary, and (iii) interaction of the (axial) electric currents in the preexisting and newly emerging flux systems. The relative contributions and effects of these mechanisms depend on the properties of the preexisting and emerging flux systems. Here we focus on the location and orientation of the emerging flux relative to the coronal flux rope. Varying these parameters, we investigate under which conditions an eruption of the latter is triggered.
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Ding, Yan, Sung-Chan Kim, Richard B. Styles e Rusty L. Permenter. "SIMULATION OF LONG-TERM SHORELINE CHANGE DRIVEN BY LONGSHORE AND CROSS-SHORE SEDIMENT TRANSPORT". Coastal Engineering Proceedings, n. 36v (28 dicembre 2020): 31. http://dx.doi.org/10.9753/icce.v36v.sediment.31.
Testo completoAbstract (sommario):
Driven by wave and current, sediment transport alongshore and cross-shore induces shoreline changes in coasts. Estimated by breaking wave energy flux, longshore sediment transport in littoral zone has been studied for decades. Cross-shore sediment transport can be significant in a gentle-slope beach and a barred coast due to bar migration. Short-term beach profile evolution (typically for a few days or weeks) has been successfully simulated by reconstructing nonlinear wave shape in nearshore zone (e.g. Hsu et al 2006, Fernandez-Mora et al. 2015). However, it is still lack of knowledge on the relationship between cross-shore sediment transport and long-term shoreline evolution. Based on the methodology of beach profile evolution modeling, a semi-empirical closure model is developed for estimating phase-average net cross-shore sediment transport rate induced by waves, currents, and gravity. This model has been implemented into GenCade, the USACE shoreline evolution model.
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Huang, Lawrence, Rupert A. C. Croft e Hitesh Arora. "Deep forest: Neural network reconstruction of the Lyman-α forest". Monthly Notices of the Royal Astronomical Society 506, n. 4 (19 luglio 2021): 5212–22. http://dx.doi.org/10.1093/mnras/stab2041.
Testo completoAbstract (sommario):
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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Zunz, V., e H. Goosse. "Influence of freshwater input on the skill of decadal forecast of sea ice in the Southern Ocean". Cryosphere 9, n. 2 (17 marzo 2015): 541–56. http://dx.doi.org/10.5194/tc-9-541-2015.
Testo completoAbstract (sommario):
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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Liu, Chunlei, e Richard P. Allan. "Unrealistic Increases in Wind Speed Explain Reduced Eastern Pacific Heat Flux in Reanalyses". Journal of Climate 31, n. 8 (20 marzo 2018): 2981–93. http://dx.doi.org/10.1175/jcli-d-17-0642.1.
Testo completoAbstract (sommario):
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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Oudrane, Abdellatif, Benaoumeur Aour, Belkacem Zeghmati, Xavier Chesneau e Messaoud Hamouda. "Etude et calcul de bilan de la densité du flux solaire pour un simple habitat dans la région d’Adrar". Journal of Renewable Energies 20, n. 1 (12 ottobre 2023): 51–60. http://dx.doi.org/10.54966/jreen.v20i1.608.
Testo completoAbstract (sommario):
L’objectif principal de ce travail est d’étudier l’effet des conditions climatiques externes, sur l’évolution de la densité du flux solaire journalier incident sur les parois verticales d’un bâtiment situé dans la région d’Adrar au Sud de l’Algérie avec diffèrent orientation. Ce bâtiment est conçu pour des a1pplications de type chauffage ou climatisation. Les simulations numériques abordées dans ce travail ont permis de comparer la variation de la densité du flux solaire incident durant une journée complète sur les parois Sud, Est, Nord et Ouest du bâtiment aux valeurs du flux solaire global sur une paroi horizontale (le plafond externe). Le flux solaire global horizontal est calculé à l’aide d’une fonction Gaussienne sinusoïdale. Les simulations ont été effectuées dans le cas d’un bâtiment situé dans une zone désertique. Les résultats de la simulation numérique, ont montré l’effet de l’orientation du bâtiment sur l’évolution du flux solaire journalier incident.
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Kruegler, James, Jesus Gomez-Velez, Laura K. Lautz e Theodore A. Endreny. "Dynamic Evapotranspiration Alters Hyporheic Flow and Residence Times in the Intrameander Zone". Water 12, n. 2 (5 febbraio 2020): 424. http://dx.doi.org/10.3390/w12020424.
Testo completoAbstract (sommario):
Hyporheic zones (HZs) influence biogeochemistry at the local reach scale with potential implication for water quality at the large catchment scale. The characteristics of the HZs (e.g., area, flux rates, and residence times) change in response to channel and aquifer physical properties, as well as to transient perturbations in the stream–aquifer system such as floods and groundwater withdraws due to evapotranspiration (ET) and pumping. In this study, we use a numerical model to evaluate the effects of transient near-stream evapotranspiration (ET) on the area, exchange flux, and residence time (RT) of sinuosity-induced HZs modulated by regional groundwater flow (RGF). We found that the ET fluxes (up to 80 mm/day) consistently increased HZ area and exchange flux, and only increased RTs when the intensity of regional groundwater flow was low. Relative to simulations without ET, scenarios with active ET had more than double HZ area and exchange flux and about 20% longer residence times (as measured by the median of the residence time distribution). Our model simulations show that the drawdown induced by riparian ET increases the net flux of water from the stream to the nearby aquifer, consistent with field observations. The results also suggest that, along with ET intensity, the magnitude of the HZ response is influenced by the modulating effect of both gaining and losing RGF and the sensitivity of the aquifer to daily cycles of ET withdrawal. This work highlights the importance of representing near-stream ET when modeling sinuosity-induced hyporheic zones, as well as the importance of including riparian vegetation in efforts to restore the ecosystem functions of streams.
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Dzanic, T., S. S. Girimaji e F. D. Witherden. "Partially-averaged Navier–Stokes simulations of turbulence within a high-order flux reconstruction framework". Journal of Computational Physics 456 (maggio 2022): 110992. http://dx.doi.org/10.1016/j.jcp.2022.110992.
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Le Bras, Sophie, Hugues Deniau e 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. 12 (23 ottobre 2019): 587–614. http://dx.doi.org/10.1002/fld.4767.
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Mihalache, Ovidiu, e 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. 1-4 (10 dicembre 2020): 905–12. http://dx.doi.org/10.3233/jae-209404.
Testo completoAbstract (sommario):
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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Halloran, P. R. "Does atmospheric CO<sub>2</sub> seasonality play an important role in governing the air-sea flux of CO<sub>2</sub>?" Biogeosciences 9, n. 6 (28 giugno 2012): 2311–23. http://dx.doi.org/10.5194/bg-9-2311-2012.
Testo completoAbstract (sommario):
Abstract. The amplitude, phase, and form of the seasonal cycle of atmospheric CO2 concentrations varies on many time and space scales (Peters et al., 2007). Intra-annual CO2 variation is primarily driven by seasonal uptake and release of CO2 by the terrestrial biosphere (Machta et al., 1977; Buchwitz et al., 2007), with a small (Cadule et al., 2010; Heimann et al., 1998), but potentially changing (Gorgues et al., 2010) contribution from the ocean. Variability in the magnitude, spatial distribution, and seasonal drivers of terrestrial net primary productivity (NPP) will be induced by, amongst other factors, anthropogenic CO2 release (Keeling et al., 1996), land-use change (Zimov et al., 1999) and planetary orbital variability, and will lead to changes in CO2atm seasonality. Despite CO2atm seasonality being a dynamic and prominent feature of the Earth System, its potential to drive changes in the air-sea flux of CO2 has not previously (to the best of my knowledge) been explored. It is important that we investigate the impact of CO2atm seasonality change, and the potential for carbon-cycle feedbacks to operate through the modification of the CO2atm seasonal cycle, because the decision had been made to prescribe CO2atm concentrations (rather than emissions) within model simulations for the fifth IPCC climate assessment (Taylor et al., 2009). In this study I undertake ocean-model simulations within which different magnitude CO2atm seasonal cycles are prescribed. These simulations allow me to examine the effect of a change in CO2atm seasonal cycle magnitude on the air-sea CO2 flux. I then use an off-line model to isolate the drivers of the identified air-sea CO2 flux change, and propose mechanisms by which this change may come about. Three mechanisms are identified by which co-variability of the seasonal cycles in atmospheric CO2 concentration, and seasonality in sea-ice extent, wind-speed and ocean temperature, could potentially lead to changes in the air-sea flux of CO2 at mid-to-high latitudes. The sea-ice driven mechanism responds to an increase in CO2atm seasonality by pumping CO2 into the ocean, the wind-speed and solubility-driven mechanisms, by releasing CO2 from the ocean (in a relative sense). The relative importance of the mechanisms will be determined by, amongst other variables, the seasonal extent of sea-ice. To capture the described feedbacks within earth system models, CO2atm concentrations must be allowed to evolve freely, forced only by anthropogenic emissions rather than prescribed CO2atm concentrations; however, time-integrated ocean simulations imply that the cumulative net air-sea flux could be at most equivalent to a few ppm CO2atm. The findings presented here suggest that, at least under pre-industrial conditions, the prescription of CO2atm concentrations rather than emissions within simulations will have little impact on the marine anthropogenic CO2 sink.
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Lionello, Roberto, Cooper Downs, Emily I. Mason, Jon A. Linker, Ronald M. Caplan, Pete Riley, Viacheslav S. Titov e Marc L. DeRosa. "Global MHD Simulations of the Time-dependent Corona". Astrophysical Journal 959, n. 2 (1 dicembre 2023): 77. http://dx.doi.org/10.3847/1538-4357/ad00be.
Testo completoAbstract (sommario):
Abstract We describe, test, and apply a technique to incorporate full-Sun, surface flux evolution into an MHD model of the global solar corona. Requiring only maps of the evolving surface flux, our method is similar to that of Lionello et al., but we introduce two ways to correct the electric field at the lower boundary to mitigate spurious currents. We verify the accuracy of our procedures by comparing to a reference simulation, driven with known flows and electric fields. We then present a thermodynamic MHD calculation lasting one solar rotation driven by maps from the magnetic flux evolution model of Schrijver & DeRosa. The dynamic, time-dependent nature of the model corona is illustrated by examining the evolution of the open flux boundaries and forward-modeled EUV emission, which evolve in response to surface flows and the emergence and cancellation flux. Although our main goal is to present the method, we briefly investigate the relevance of this evolution to properties of the slow solar wind, examining the mapping of dipped field lines to the topological signatures of the “S-Web” and comparing charge state ratios computed in the time-dependently driven run to a steady-state equivalent. Interestingly, we find that driving on its own does not significantly improve the charge state ratios, at least in this modest resolution run that injects minimal helicity. Still, many aspects of the time-dependently driven model cannot be captured with traditional steady-state methods, and such a technique may be particularly relevant for the next generation of solar wind and coronal mass ejection models.
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Zheng, Jianqiu, Qiong Zhang, Qiang Li, Qiang Zhang e Ming Cai. "Contribution of sea ice albedo and insulation effects to Arctic amplification in the EC-Earth Pliocene simulation". Climate of the Past 15, n. 1 (13 febbraio 2019): 291–305. http://dx.doi.org/10.5194/cp-15-291-2019.
Testo completoAbstract (sommario):
Abstract. In the present work, we simulate the Pliocene climate with the EC-Earth climate model as an equilibrium state for the current warming climate induced by rising CO2 in the atmosphere. The simulated Pliocene climate shows a strong Arctic amplification featuring pronounced warming sea surface temperature (SST) over the North Atlantic, in particular over the Greenland Sea and Baffin Bay, which is comparable to geological SST reconstructions from the Pliocene Research, Interpretation and Synoptic Mapping group (PRISM; Dowsett et al., 2016). To understand the underlying physical processes, the air–sea heat flux variation in response to Arctic sea ice change is quantitatively assessed by a climate feedback and response analysis method (CFRAM) and an approach similar to equilibrium feedback assessment. Given the fact that the maximum SST warming occurs in summer while the maximum surface air temperature warming happens during winter, our analyses show that a dominant ice-albedo effect is the main reason for summer SST warming, and a 1 % loss in sea ice concentration could lead to an approximate 1.8 W m−2 increase in shortwave solar radiation into open sea surface. During the winter months, the insulation effect induces enhanced turbulent heat flux out of the sea surface due to sea ice melting in previous summer months. This leads to more heat released from the ocean to the atmosphere, thus explaining why surface air temperature warming amplification is stronger in winter than in summer.
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Zhou, Bowen, e 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. 9 (1 settembre 2011): 2142–55. http://dx.doi.org/10.1175/2011jas3693.1.
Testo completoAbstract (sommario):
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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Wilson, David J., Allison Youngblood, Odette Toloza, Jeremy J. Drake, Kevin France, Cynthia S. Froning, Boris T. Gänsicke, Seth Redfield e Brian E. Wood. "Testing Lyα Emission-line Reconstruction Routines at Multiple Velocities in One System". Astrophysical Journal 936, n. 2 (1 settembre 2022): 189. http://dx.doi.org/10.3847/1538-4357/ac87a8.
Testo completoAbstract (sommario):
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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Su, Yingna, Bernhard Kliem, Adriaan van Ballegooijen e Edward Deluca. "Numerical simulations of the CME on 2010 April 8". Proceedings of the International Astronomical Union 8, S294 (agosto 2012): 575–76. http://dx.doi.org/10.1017/s174392131300327x.
Testo completoAbstract (sommario):
AbstractWe present 3D zero-beta ideal MHD simulations of the solar flare/CME event that occurred in Active Region 11060 on 2010 April 8. The initial magnetic configurations of the two simulations are stable nonlinear force-free field and unstable magnetic field models constructed by Su et al. (2011) using the flux rope insertion method. The MHD simulations confirm that the stable model relaxes to a stable equilibrium, while the unstable model erupts as a CME. Comparisons between observations and MHD simulations of the CME are also presented.
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Sullivan, Ryan C., V. Rao Kotamarthi e Yan Feng. "Recovering Evapotranspiration Trends from Biased CMIP5 Simulations and Sensitivity to Changing Climate over North America". Journal of Hydrometeorology 20, n. 8 (31 luglio 2019): 1619–33. http://dx.doi.org/10.1175/jhm-d-18-0259.1.
Testo completoAbstract (sommario):
Abstract Future projections of evapotranspiration (ET) are of critical importance for agricultural and freshwater management and for predicting land–atmosphere feedbacks on the climate system. However, ET from phase 5 of the Coupled Model Intercomparison Project (CMIP5) simulations exhibits substantial biases, bolstering little confidence in future ET projections. Despite poor predictive skill and large bias of ET from the global climate models, the information content necessary to calculate ET offline is available in the models’ archived outputs: temperature T, water vapor pressure e, atmospheric pressure P, and surface net radiation R. A relatively simple three-source energy balance model [Penman–Monteith (PM)], along with the mean annual cycle of remotely sensed vegetation properties, can then be used to reconstruct ET with a substantially reduced bias relative to in situ turbulent heat flux measurements. This methodology is used here to reconstruct ET projections from 2006 through 2100 over North America using output from selected CMIP5 models and to attribute projected ET trends to specific atmospheric controls. CMIP5 ET exhibits substantial bias in annual ET relative to in situ flux measurements across North America (38%–73%; 2006–15), but ET reconstructed from the CMIP5 meteorology with the PM method greatly reduces this bias (−8% to +14%). Present-day North American ET is more sensitive to changes in atmospheric demand for ET (temperature and water vapor pressure) than energy limitation (net radiation), and to a lesser extent vegetation properties (leaf area index). Accordingly, ET is projected to increase 0.26–0.87 mm yr−1 yr−1 over North America through 2100 driven primarily by trends in temperature.
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Luo, Jun, Lijun Xuan e 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. 3 (settembre 2013): 599–620. http://dx.doi.org/10.4208/cicp.110212.021112a.
Testo completoAbstract (sommario):
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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Cruz, Fernando, José P. Faria, Miguel Rocha, Isabel Rocha e 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. 5 (17 settembre 2020): 1889–903. http://dx.doi.org/10.1042/bst20190840.
Testo completoAbstract (sommario):
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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Bisbas, T. G., A. P. Whitworth, R. Wünsch, D. A. Hubber e S. Walch. "Radiation Driven Implosion and Triggered Star Formation". Proceedings of the International Astronomical Union 6, S270 (maggio 2010): 263–66. http://dx.doi.org/10.1017/s1743921311000482.
Testo completoAbstract (sommario):
AbstractWe present simulations of stable isothermal clouds exposed to ionizing radiation from a discrete external source, and identify the conditions that lead to Radiatively Driven Implosion and Star Formation. We use the Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We find that the incident ionizing flux is the critical parameter determining the evolution; high fluxes disperse the cloud, whereas low fluxes trigger star formation. We find a clear connection between the intensity of the incident flux and the parameters of star formation.
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MIYAJI, Koji, e 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. 1 (2017): 18–26. http://dx.doi.org/10.2322/tjsass.60.18.
Testo completo
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Gray, M. "Reconstruction of Spectra obtained with PYTHEAS through Photometric Methods". International Astronomical Union Colloquium 149 (1995): 311–13. http://dx.doi.org/10.1017/s0252921100023228.
Testo completoAbstract (sommario):
PYTHEAS is an imaging spectrophotometer giving high spectral and spatial resolutions in joining together several optical concepts (Le Coarer et al 1992, and Georgelin et al. in these Proceedings). It will be shown in this paper that, contrary to other spectrographs using somewhat complex methods of data reduction, the PYTHEAS imaging spectrometer requires simple photometric methods in order to reconstruct the spectra of astrophysical objects.The CCD image of a continuous spectral energy distribution for a permanent gap between the layers of the Fabry-Perot consists in a series of channelled spectra: each elementary beam, spectrally filtered by the FP interferometer and sampled by a microlens in the frame, gives light to the whole surface of the grism which separates the various F-P orders by displaying them in a line on the detector. Each line or channelled spectrum consists of a series of Fabry pupils (spectral elements), each of them containing the luminous flux emitted on a certain wavelength by the object under investigation. After scanning the FP interferometer across its free spectral range, we obtain a series of shifted channelled spectra whose set provides us with a chart showing the photometric values of flux according to the wavelength. Consequently, some forms of calibration (continuum lamp, spectral lamp) allow the reconstruction of the spectra of the astrophysical object through simple photometric measures.
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Guennou, Chloé, Etienne Pariat, James E. Leake e Nicole Vilmer. "Testing predictors of eruptivity using parametric flux emergence simulations". Journal of Space Weather and Space Climate 7 (2017): A17. http://dx.doi.org/10.1051/swsc/2017015.
Testo completoAbstract (sommario):
Solar flares and coronal mass ejections (CMEs) are among the most energetic events in the solar system, impacting the near-Earth environment. Flare productivity is empirically known to be correlated with the size and complexity of active regions. Several indicators, based on magnetic field data from active regions, have been tested for flare forecasting in recent years. None of these indicators, or combinations thereof, have yet demonstrated an unambiguous eruption or flare criterion. Furthermore, numerical simulations have been only barely used to test the predictability of these parameters. In this context, we used the 3D parametric magnetohydrodynamic (MHD) numerical simulations of the self-consistent formation of the flux emergence of a twisted flux tube, inducing the formation of stable and unstable magnetic flux ropes of Leake et al. (2013, 2014). We use these numerical simulations to investigate the eruptive signatures observable in various magnetic scalar parameters and provide highlights on data analysis processing. Time series of 2D photospheric-like magnetograms are used from parametric simulations of stable and unstable flux emergence, to compute a list of about 100 different indicators. This list includes parameters previously used for operational forecasting, physical parameters used for the first time, as well as new quantities specifically developed for this purpose. Our results indicate that only parameters measuring the total non-potentiality of active regions associated with magnetic inversion line properties, such as the Falconer parameters Lss, WLss, Lsg, and WLsg, as well as the new current integral WLsc and length Lsc parameters, present a significant ability to distinguish the eruptive cases of the model from the non-eruptive cases, possibly indicating that they are promising flare and eruption predictors. A preliminary study about the effect of noise on the detection of the eruptive signatures is also proposed.
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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. 3 (28 gennaio 2022): 036021. http://dx.doi.org/10.1088/1741-4326/ac48a6.
Testo completoAbstract (sommario):
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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Gazith, Dotan, e Barak Zackay. "Precision Speckle Pattern Reconstruction for High-contrast Imaging". Astronomical Journal 167, n. 2 (10 gennaio 2024): 53. http://dx.doi.org/10.3847/1538-3881/ad0b6f.
Testo completoAbstract (sommario):
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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Nagy, Melinda, Alexandre Lemerle e Paul Charbonneau. "Impact of nonlinear surface inflows into activity belts on the solar dynamo". Journal of Space Weather and Space Climate 10 (2020): 62. http://dx.doi.org/10.1051/swsc/2020064.
Testo completoAbstract (sommario):
We examine the impact of surface inflows into activity belts on the operation of solar cycle models based on the Babcock–Leighton mechanism of poloidal field regeneration. Towards this end we introduce in the solar cycle model of Lemerle & Charbonneau (2017. ApJ 834: 133) a magnetic flux-dependent variation of the surface meridional flow based on the axisymmetric inflow parameterization developped by Jiang et al. (2010. ApJ 717: 597). The inflow dependence on emerging magnetic flux thus introduces a bona fide nonlinear backreaction mechanism in the dynamo loop. For solar-like inflow speeds, our simulation results indicate a decrease of 10–20% in the strength of the global dipole building up at the end of an activity cycle, in agreement with earlier simulations based on linear surface flux transport models. Our simulations also indicate a significant stabilizing effect on cycle characteristics, in that individual cycle amplitudes in simulations including inflows show less scatter about their mean than in the absence of inflows. Our simulations also demonstrate an enhancement of cross-hemispheric coupling, leading to a significant decrease in hemispheric cycle amplitude asymmetries and temporal lag in hemispheric cycle onset. Analysis of temporally extended simulations also indicate that the presence of inflows increases the probability of cycle shutdown following an unfavorable sequence of emergence events. This results ultimately from the lower threshold nonlinearity built into our solar cycle model, and presumably operating in the sun as well.
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Ma, Libin, Chao Yan e Jian Yu. "A Modal-Decay-Based Shock-Capturing Approach for High-Order Flux Reconstruction Method". Aerospace 10, n. 1 (24 dicembre 2022): 14. http://dx.doi.org/10.3390/aerospace10010014.
Testo completoAbstract (sommario):
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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Ekama, G. A., e P. Marais. "Comparison of the 1D flux theory with a 2D hydrodynamic secondary settling tank model". Water Science and Technology 50, n. 7 (1 ottobre 2004): 195–204. http://dx.doi.org/10.2166/wst.2004.0454.
Testo completoAbstract (sommario):
The applicability of the 1D idealized flux theory (1DFT) for design of secondary settling tanks (SSTs) is evaluated by comparing its predicted maximum surface overflow (SOR) and solids loading (SLR) rates with that calculated from the 2D hydrodynamic model SettlerCAD using as a basis 35 full scale SST tress tests conducted on different SSTs with diameters from 30 to 45m and 2.25 to 4.1m side water depth, with and without Stamford baffles. From the simulations, a relatively consistent pattern appeared, i.e. that the 1DFT can be used for design but its predicted maximum SLR needs to be reduced by an appropriate flux rating, the magnitude of which depends mainly on SST depth and hydraulic loading rate (HLR). Simulations of the sloping bottom shallow (1.5-2.5 m SWD) Dutch SSTs tested by STOWa and the Watts et al. SST, all with doubled SWDs, and the Darvill new (4.1 m) and old (2.5 m) SSTs with interchanged depths, were run to confirm the sensitivity of the flux rating to depth and HLR. Simulations with and without a Stamford baffle were also done. While the design of the internal features of the SST, such as baffling, have a marked influence on the effluent SS concentration for underloaded SSTs, these features appeared to have only a small influence on the flux rating, i.e. capacity, of the SST. In the meantime until more information is obtained, it would appear that from the simulations so far that the flux rating of 0.80 of the 1DFT maximum SLR recommended by Ekama and Marais remains a reasonable value to apply in the design of full scale SSTs – for deep SSTs (4 m SWD) the flux rating could be increased to 0.85 and for shallow SSTs (2.5 m SWD) decreased to 0.75. It is recommended that (i) while the apparent interrelationship between SST flux rating and depth suggests some optimization of the volume of the SST, that this be avoided and that (ii) the depth of the SST be designed independently of the surface area as is usually the practice and once selected, the appropriate flux rating is applied to the 1DFT estimate of the surface area.
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Cornardeau, Baptiste. "Joseph Margolis et la fragilité du monde humain". Archives de Philosophie Tome 87, n. 2 (9 aprile 2024): 103–19. http://dx.doi.org/10.3917/aphi.872.0101.
Testo completoAbstract (sommario):
Hybride et émergent, artéfactuel et culturel sans cesser d’être naturel, le monde humain tel que l’entend Joseph Margolis est aussi, en cela même, dans ses différentes dimensions, un monde fragile, instable et en perpétuelle reconstruction. Les enquêtes scientifiques et philosophiques ne peuvent en effet s’affranchir d’une condition d’impermanence où le flux l’emporte sur la fixité, ni s’émanciper d’un horizon de discours symbiotique ancré dans le langage ordinaire. Métisses et solidaires d’un monde médian conversationnel, les langues humaines répondent également, dans leur indétermination même, aux exigences de formes de vie partagées et changeantes. Les personnes humaines enfin, enculturées et enlangagées, restent inscrites dans un monde Intentionnel et historique, qu’elles s’incorporent et transforment.
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Qin, G., F. J. Kong e S. S. Wu. "The Efficiency of Electron Acceleration by ICME-driven Shocks". Astrophysical Journal 942, n. 2 (1 gennaio 2023): 63. http://dx.doi.org/10.3847/1538-4357/aca60e.
Testo completoAbstract (sommario):
Abstract We present a study of the efficiency of the acceleration of suprathermal electrons at collisionless shock waves driven by interplanetary coronal mass ejections (ICMEs), with the data analysis from both the spacecraft observations and test-particle simulations. The observations are from the 3DP/EESA instrument on board Wind during the 74 shock events listed in Yang et al., and the test-particle simulations are carried out through 315 cases with different shock parameters. A total of seven energy channels ranging from 0.428–4.161 keV are selected. In the simulations, using a backward-in-time method, we calculate the average downstream flux in the 90° pitch angle. On the other hand, the average downstream and upstream fluxes in the 90° pitch angle can also be directly obtained from the 74 observational shock events. In addition, the variation in the event number ratio with the downstream to upstream flux ratio above a threshold value in terms of the shock angle (the angle between the shock normal and upstream magnetic field), upstream Alfvén Mach number, and shock compression ratio is statistically obtained. It is shown from both the observations and simulations that a large shock angle, upstream Alfvén Mach number, and shock compression ratio can enhance the efficiency of the shock acceleration. Our results suggest that shock drift acceleration is more efficient in the electron acceleration by ICME-driven shocks, which confirms the findings of Yang et al.
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Kovbasa, S., S. Dymko e M. Zhelinskyi. "SPEED OBSERVER FOR SENSORLESS ENERGY GENERATION SYSTEMS BASED ON FIELD ORIENTED INDUCTION GENERATOR". ELECTRICAL AND COMPUTER SYSTEMS 33, n. 108 (30 novembre 2020): 9–15. http://dx.doi.org/10.15276/eltecs.32.108.2020.1.
Testo completoAbstract (sommario):
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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Dupuy, A., O. Banton e M. Razack. "Contamination nitratée des eaux souterraines d'un bassin versant agricole hétérogène: 1. Évaluation des apports à la nappe (modèle Agriflux)". Revue des sciences de l'eau 10, n. 1 (12 aprile 2005): 23–40. http://dx.doi.org/10.7202/705268ar.
Testo completoAbstract (sommario):
Au cours des dernières décennies, la hausse de la productivité agricole s'est accompagnée d'une forte augmentation des fertilisations azotées qui a entraîné l'augmentation des concentrations en nitrates dans les eaux souterraines. Récemment, les études sur la gestion des pollutions diffuses agricoles ont intégré l'échelle du bassin versant hydrologique. Dans cet article, une approche basée sur un découpage en secteurs pédologiquement et agronomiquement homogènes a été élaborée pour permettre l'utilisation d'un modèle d'évaluation (AgriFlux), et obtenir les flux d'eau et de nitrates sortant de la zone racinaire sur l'ensemble d'un bassin. La modélisation du bassin de La Jannerie a porté sur une période de quatre ans impliquant 19 zones de simulation. L'influence du cycle végétatif des cultures et leur nature sur l'évolution des flux de nitrates a été mise en évidence par des simulations préliminaires, de même que l'influence du type de sol. L'évolution des concentrations moyennes saisonnières en nitrates sortant de la zone racinaire montre que les fertilisations minérales ne sont pas les seules sources importantes de nitrates dans les sols. Les pratiques culturales, comme le retournement des prairies, l'enfouissement des résidus de récoltes ou l'assolement, ont une forte influence sur la dynamique spatiale et temporelle des flux de nitrates percolant vers la nappe.
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Nameche, T., e J. L. Vassel. "Bilan thermique sous climat tempéré des lagunes aérées et naturelles". Revue des sciences de l'eau 12, n. 1 (12 aprile 2005): 65–91. http://dx.doi.org/10.7202/705344ar.
Testo completoAbstract (sommario):
Un modèle décrivant la température d'équilibre des lagunes a été développé, tenant compte des différents flux de chaleur que celles-ci échangent avec l'air et le sol environnant. Six composantes différentes ont été inclues dans le calcul de ce bilan thermique: radiation solaire, évaporation, convection, rayonnement atmosphérique, rayonnement de la surface du plan d'eau, échange via les parois en contact avec le sol. Le modèle ainsi obtenu a été testé avec efficacité sur deux lagunes aérées et une lagune naturelle situées sous climat tempéré ; sa précision sur l'estimation des températures d'équilibre étant de l'ordre de 0.7 °C. Des simulations en continu ont également pu être effectuées au moyen d'une variante dynamique, tenant compte de l'inertie thermique qu'entraîne le volume des bassins. Quelle que soit la saison envisagée, la principale forme d'apport de chaleur est représentée par la radiation solaire tandis que la dissipation d'énergie se partage entre les flux d'évaporation et la balance des deux flux de rayonnement. Les bassins échangeraient en moyenne plus de 250 W/m2 ; le maximum de transfert de chaleur correspondant au printemps et à la période estivale. Enfin, l'analyse de sensibilité du modèle nous a permis de mettre en évidence la contribution de chacun des termes intervenant dans le calcul de ce bilan thermique et de révéler sa dépendance vis-à-vis principalement de la température d'entrée, du rayonnement solaire et de la température de l'air.
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Becquart, C. S., P. C. Clapp e J. A. Rifkin. "Molecular dynamics simulations of surface reconstruction at the edges of a crack in ruthenium aluminum". Journal of Materials Research 9, n. 3 (marzo 1994): 548–52. http://dx.doi.org/10.1557/jmr.1994.0548.
Testo completoAbstract (sommario):
Using molecular dynamics computer simulations and interatomic potentials derived partly by Voter and Chen1 and Rifkin et al.,2 we studied the surface reconstruction taking place on free surfaces of arrays of RuAl. Surface reconstruction appears to be very important on {111} and {110} types of planes and almost nonexistent on {100} type of planes. Cracks oriented so that their crack planes were either {111} types or {110} types exhibit on their internal free surface important surface reconstruction. It is believed that this effect may have some contribution in the brittle versus ductile behavior of the crack.
40
Jung, Chunyong, e Gary M. Lackmann. "The Response of Extratropical Transition of Tropical Cyclones to Climate Change: Quasi-Idealized Numerical Experiments". Journal of Climate 34, n. 11 (giugno 2021): 4361–81. http://dx.doi.org/10.1175/jcli-d-20-0543.1.
Testo completoAbstract (sommario):
AbstractThis study uses small ensembles of convection-allowing, quasi-idealized simulations to examine the response of North Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET) to climate change. Using HURDAT2 and ERA5 data over a 40-yr period from 1979 to 2018, we developed storm-relative composite fields for past North Atlantic recurving, oceanic ET events. The quasi-idealized present-day simulations are initialized from these composites and run in an aquaplanet domain. A pseudo–global warming approach is used for future simulations: Thermodynamic changes between late twenty-first century and twentieth century, derived from an ensemble of 20 CMIP5 GCMs under the RCP8.5 scenario, are added to the present-day initial and lateral boundary conditions. The composite-initialized present-day simulations exhibit realistic ET characteristics. Future simulations show greater intensity, heavier precipitation, and stronger downstream midlatitude wave train development relative to the present-day case. Specifically, the future ET event is substantially stronger before ET completion, though the system undergoes less reintensification after ET completion. Reductions in lower-tropospheric baroclinicity associated with Arctic amplification could contribute to this result. The future simulation exhibits 3-hourly ensemble-mean precipitation rate increases ranging from ~23% to ~50%, depending on ET phase and averaging radius. In addition, larger eddy kinetic energy accompanies the future storm, partly created by increased baroclinic conversion, resulting in stronger amplification of downstream energy maxima via intensified ageostrophic geopotential flux convergence and divergence. These results suggest that future TCs undergoing ET could have greater potential to cause high-impact weather in western Europe through both direct and remote processes.
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Spring, Aaron, István Dunkl, Hongmei Li, Victor Brovkin e Tatiana Ilyina. "Trivial improvements in predictive skill due to direct reconstruction of the global carbon cycle". Earth System Dynamics 12, n. 4 (15 novembre 2021): 1139–67. http://dx.doi.org/10.5194/esd-12-1139-2021.
Testo completoAbstract (sommario):
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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Shukhtina, M. A., e E. Gordeev. "In situ magnetotail magnetic flux calculation". Annales Geophysicae 33, n. 6 (18 giugno 2015): 769–81. http://dx.doi.org/10.5194/angeo-33-769-2015.
Testo completoAbstract (sommario):
Abstract. We explore two new modifications of the magnetotail magnetic flux (F) calculation algorithm based on the Petrinec and Russell (1996) (PR96) approach of the tail radius determination. Unlike in the PR96 model, the tail radius value is calculated at each time step based on simultaneous magnetotail and solar wind observations. Our former algorithm, described in Shukhtina et al. (2009), required that the "tail approximation" requirement were fulfilled, i.e., it could be applied only tailward x ∼ −15 RE. The new modifications take into account the approximate uniformity of the magnetic field of external sources in the near and middle tail. Tests, based on magnetohydrodynamics (MHD) simulations, show that this approach may be applied at smaller distances, up to x ∼ −3 RE. The tests also show that the algorithm fails during long periods of strong positive interplanetary magnetic field (IMF) Bz. A new empirical formula has also been obtained for the tail radius at the terminator (at x = 0) which improves the calculations.
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Čemeljić, Miljenko, Hai Yang, Feng Yuan e Hsien Shang. "Formation of Episodic Jets and Associated Flares from Black Hole Accretion Systems". Astrophysical Journal 933, n. 1 (1 luglio 2022): 55. http://dx.doi.org/10.3847/1538-4357/ac70cc.
Testo completoAbstract (sommario):
Abstract Episodic ejections of blobs (episodic jets) are widely observed in black hole sources and usually associated with flares. In this paper, by performing and analyzing three-dimensional general relativity magnetohydrodynamical numerical simulations of accretion flows, we investigate their physical mechanisms. We find that magnetic reconnection occurs in the accretion flow, likely due to the turbulent motion and differential rotation of the accretion flow, resulting in flares and formation of flux ropes. Flux ropes formed inside of 10–15 gravitational radii are found to mainly stay within the accretion flow, while flux ropes formed beyond this radius are ejected outward by magnetic forces and form the episodic jets. These results confirm the basic scenario proposed in Yuan et al. Moreover, our simulations find that the predicted velocity of the ejected blobs is in good consistency with observations of Sgr A*, M81, and M87. All of the processes were found to occur quasiperiodically, with the period being the orbital time at the radius where the flux rope is formed. The predicted period of the flares and ejections is consistent with those found from the light curves or image of Sgr A*, M87, and PKS 1510–089. The possible applications to protostellar accretion systems are discussed.
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Zhao, Yiqing, Ke Lan, Peng Song, Wudi Zheng e Xin Li. "A method to determine the flux limiter via the motion of the M-band emission region in Au hohlraum". Laser and Particle Beams 30, n. 3 (25 maggio 2012): 387–96. http://dx.doi.org/10.1017/s0263034612000225.
Testo completoAbstract (sommario):
AbstractIn this paper, the influence of the electron flux limiter fe on hohlraum plasmas is studied by using the two-dimensional code LARED-H, and a method to experimentally determine fe via the motion of the M-band emission region in Au hohlraum is proposed. From our simulations, the limited free streaming flux may dominates the heat conduction in the regions with steep temperature gradient, including the laser deposition region, the flux-heated overcritical region, and the laser channel boundary between the hot laser plasmas and the surrounding radiation ablated plasmas, while these are important X-ray emission regions. Hence, the choosing of fe may influence the wall plasma expansion and the laser spot motion, and further influence the motion of the emission regions. From our study, the motion of the M-band (>1.5 keV) emission region is sensitive to fe when the limited free streaming flux dominates the heat conduction of the wall plasma expansion region, and so it is possible to determine fe via the motion of the M-band emission region. In this work, the model used in our simulations is taken from the wall and laser spot motion experiments done by Huser et al. (2009).
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TAN, X., J. X. ZHONG e G. W. YANG. "GROWTH MECHANISM OF RING SHAPED NANOSTRUCTURES SELF-ASSEMBLY UPON DROPLET EPITAXY". Surface Review and Letters 19, n. 03 (giugno 2012): 1250029. http://dx.doi.org/10.1142/s0218625x12500291.
Testo completoAbstract (sommario):
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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Frasson, Thomas, Stéphane Labrosse, Henri-Claude Nataf, Nicolas Coltice e Nicolas Flament. "On the impact of true polar wander on heat flux patterns at the core–mantle boundary". Solid Earth 15, n. 5 (14 maggio 2024): 617–37. http://dx.doi.org/10.5194/se-15-617-2024.
Testo completoAbstract (sommario):
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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Jung, Chunyong, e Gary M. Lackmann. "Extratropical Transition of Hurricane Irene (2011) in a Changing Climate". Journal of Climate 32, n. 15 (11 luglio 2019): 4847–71. http://dx.doi.org/10.1175/jcli-d-18-0558.1.
Testo completoAbstract (sommario):
Abstract Tropical cyclones (TCs) undergoing strong extratropical transition (ET) can produce adverse societal impacts in areas that rarely experience direct TC impacts. This, in conjunction with projected environmental changes in climatological ET regions, motivates the investigation of possible future changes in ET characteristics. We utilize a small ensemble of numerical model simulations to examine how warming affects the ET of Hurricane Irene. To assess the effects of climate change, we use the pseudo-global warming method in which thermodynamic changes, derived from an ensemble of 20 CMIP5 GCMs, are applied to analyzed initial and lateral boundary conditions of model simulations. We find increased storm intensity in the future simulations, both in reduced minimum sea level pressure and strengthened 10-m wind speed. Storm-centered composites indicate a strengthening of tropospheric potential vorticity near the center of Irene, consistent with enhanced latent heat release. The results also demonstrate that Irene’s precipitation in the warmed simulations increases at a rate that exceeds Clausius–Clapeyron scaling, owing to enhanced moisture flux convergence and an additional contribution from increased surface evaporation. The duration of the transition process increased in the warmed simulations due to a weakened midtropospheric trough and reduced vertical wind shear and meridional SST gradient with a slower northward translation. These results suggest that transitioning storms may exhibit an increased ability to extend TC-like conditions poleward, and motivates additional research.
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Todarello, Elisa, Andre Scaffidi, Marco Regis e Marco Taoso. "Constraining below-threshold radio source counts with machine learning". Journal of Cosmology and Astroparticle Physics 2024, n. 01 (1 gennaio 2024): 062. http://dx.doi.org/10.1088/1475-7516/2024/01/062.
Testo completoAbstract (sommario):
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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Montiel, Miguel, e Roque Corral. "Time-Inclined Method for High-Fidelity Rotor/Stator Simulations". Aerospace 10, n. 5 (18 maggio 2023): 475. http://dx.doi.org/10.3390/aerospace10050475.
Testo completoAbstract (sommario):
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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Kahnt, Maik, Konstantin Klementiev, Vahid Haghighat, Clemens Weninger, Tomás S. Plivelic, Ann E. Terry e Alexander Björling. "Measurement of the coherent beam properties at the CoSAXS beamline". Journal of Synchrotron Radiation 28, n. 6 (5 ottobre 2021): 1948–53. http://dx.doi.org/10.1107/s1600577521009140.
Testo completoAbstract (sommario):
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.