The Experts below are selected from a list of 10155 Experts worldwide ranked by ideXlab platform
François Primeau - One of the best experts on this subject based on the ideXlab platform.
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An offline Implicit Solver for simulating prebomb radiocarbon
Ocean Modelling, 2014Co-Authors: Ann Bardin, François Primeau, Keith LindsayAbstract:Abstract It takes several thousand years for the deep-ocean concentration of natural radiocarbon to come to equilibrium with surface fluxes, making it computationally too expensive to routinely simulate it with moderate- to high-resolution ocean models. We present an Implicit Solver for computing prebomb Δ 14 C that requires the equivalent of only a few tens of model years to reach equilibrium. The Solver uses a Newton–Krylov algorithm with a preconditioner based on a coarse-grained annually-averaged tracer-transport operator. Coarse-graining provides a general approach for developing preconditioners for models of increasing resolution. We implemented and tested the Solver for the ocean component of the Community Earth System Model (CESM) with a nominal horizontal resolution of 1 ° × 1 ° and with 60 vertical levels. Simulated Δ 14 C values are in good agreement with observations at the surface and in the North Atlantic, but the deep North Pacific simulated values show a substantial bias, with prebomb radiocarbon Δ 14 C values translating to ages that are twice the observationally based estimate. This bias is substantially larger than published simulations obtained with coarser resolution models, suggesting that increasing model resolution does not automatically improve the fidelity of the deep ocean ventilation processes. We therefore recommend that natural Δ 14 C be used as a deep-ocean ventilation metric for critically evaluating deep ocean circulation.
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optimization and sensitivity study of a biogeochemistry ocean model using an Implicit Solver and in situ phosphate data
Global Biogeochemical Cycles, 2006Co-Authors: Eun Young Kwon, François PrimeauAbstract:[1] A new Implicit method for obtaining equilibrium solutions and their sensitivity to changes in parameters is described and applied to an OCMIP-2 type ocean-biogeochemistry model. The method is used to optimize model parameters by minimizing the difference between the observed and simulated PO4 distribution. The optimized parameters include (1) the exponent α in the power law vertical profile for particulate organic matter (POM) fluxes, (2) the fraction σ of biological production allocated to dissolved organic matter (DOM) and (3) the rate constant κ for the remineralization of DOM. Global PO4 observations constrain σ and κ but not independently because their sensitivity patterns are highly correlated. In contrast, the sensitivity pattern for α is uncorrelated to those of the other parameters, allowing it to be independently constrained. We show that export production from POC is well constrained by the distribution of PO4 in an OCMIP-2 type model, but that new production and export production from DOC are not well constrained. With the optimal parameter set (α = −1.0, σ = 0.74, and κ = 1.0 yrs−1) the fraction of the spatial PO4 variance captured by our model increases from 60% with the reference OCMIP-2 parameters to 70%. Combined changes in σ and κ account for most of the improvements by reducing but not completely eliminating the nutrient trapping effect in the Eastern Equatorial Pacific and northern Indian Ocean that causes the model to over-predict PO4 concentrations. Important remaining model-data misfits in the deep North Atlantic where PO4 is over predicted and in the North Pacific where the model does not produce the observed sharp nutricline are likely attributable to deficiencies in ocean transport. The fact that the fraction of unexplained variance is large at the optimal parameter values highlights the importance of properly simulating physical transport for ocean biogeochemical modeling.
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sensitivity and optimization study of a biogeochemistry ocean model using an Implicit Solver and in situ phosphate data
Global Biogeochemical Cycles, 2006Co-Authors: François Primeau, Ey KwonAbstract:JOURNAL OF GEOPHYSICAL RESEARCH: EARTH SURFACE, VOL. 118, 1746–1753, doi:10.1002/jgrf.20125, 2013 Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher-order model M. Morlighem, 1 H. Seroussi, 2 E. Larour, 2 and E. Rignot 1,2 Received 7 March 2013; revised 24 July 2013; accepted 25 July 2013; published 9 September 2013. [ 1 ] Basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics. Here we use a complete map of surface velocity of the Antarctic Ice Sheet to infer the basal friction over the entire continent by combining these observations with a three-dimensional, thermomechanical, higher-order ice sheet numerical model from the Ice Sheet System Model open source software. We demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization, at a spatial resolution as high as 3 km along the coastline. We compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation. The results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80% of the ice sheet. The basal friction coefficient, which relates basal friction to basal velocity, is, however, significantly heterogeneous: it is low on fast moving ice and high near topographic divides. Areas with low values extend far out into the interior, along glacier and ice stream tributaries, almost to the flanks of topographic divides, suggesting that basal sliding is widespread beneath the Antarctic Ice Sheet. Citation: Morlighem, M., H. Seroussi, E. Larour, and E. Rignot (2013), Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher-order model, J. Geophys. Res. Earth Surf., 118, 1746–1753, doi:10.1002/jgrf.20125. 1. Introduction [ 2 ] Realistic modeling of the Antarctic Ice Sheet is essential to improve projections of its past, present, and future contributions to sea level rise in a warming climate [IPCC-AR4, 2007]. Boundary conditions are required inputs for ice sheet numerical models. Among these boundary conditions, basal friction is one of the main controls of ice sheet mechanics and it is also one of the most poorly known variables because it cannot be observed directly. Inverse methods that combine ice sheet modeling and surface observations provide a viable alternative to constrain basal conditions. This approach has been applied to simplified two-dimensional ice sheet models [MacAyeal, 1992] and extended to higher-order and full-Stokes models [Morlighem et al., 2010; Seroussi et al., 2011; Jay-Allemand et al., 2011]. Larour et al. [2012] and Gillet-Chaulet et al. [2012] applied this approach to the Greenland Ice Sheet using different ice flow models, but inversion of basal friction has never been Department of Earth System Science, University of California, Irvine, California, USA. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. Corresponding author: M. Morlighem, Department of Earth System Science, University of California, Croul Hall, Irvine, CA 92697-3100, USA. (Mathieu.Morlighem@uci.edu) ©2013. American Geophysical Union. All Rights Reserved. 2169-9003/13/10.1002/jgrf.20125 attempted at the scale of the Antarctic continent, which is 7 times larger than Greenland. Pollard and DeConto [2012] recently used a simplified approach to infer basal friction beneath the Antarctic Ice Sheet at a resolution of 40 km by tuning basal friction to best match observations of ice sheet surface elevation. [ 3 ] Here, we present and apply an inverse method to the entire Antarctic Ice Sheet using a three-dimensional, thermomechanical, higher-order, ice flow model combined with high-resolution (300 m) ice motion data. To apply this method to the entire continent, the approach needs to be scal- able and the cost function must accommodate flow regimes spanning from near stagnant ice in the interior (cm/yr) to fast-flowing ice along the periphery (km/yr), almost 6 orders of magnitude difference in speed. [ 4 ] Inverting for basal friction requires the construction of an adjoint model. A common approximation is to neglect the nonlinearity of ice viscosity (e.g., MacAyeal [1992]). The impact of this incomplete adjoint approximation on the performance of the inversion has not been fully established. Goldberg and Sergienko [2011] showed that for a hybrid model [Schoof and Hindmarsh, 2010; Goldberg, 2011], the exact adjoint may be advantageous in some cases to minimize the cost function. Here, we address this issue by deriving the exact solution of the adjoint model and by comparing the results to those obtained with the incomplete adjoint. We also compare the performance of two descent algorithms. Finally, we analyze and discuss the inferred pattern of basal friction in Antarctica and the implications of the results for ice sheet modeling.
E. Codina - One of the best experts on this subject based on the ideXlab platform.
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Numerical simulation of a supersonic ejector for vacuum generation with explicit and Implicit Solver in openfoam
Energies, 2019Co-Authors: Ll Macia, R. Castilla, P. J. Gamez-montero, S. Camacho, E. CodinaAbstract:Supersonic ejectors are used extensively in all kind of applications: compression of refrigerants in cooling systems, pumping of volatile fluids or in vacuum generation. In vacuum generation, also known as zero-secondary flow, the ejector has a transient behaviour. In this paper, a numerical and experimental research of a supersonic compressible air nozzle is performed in order to investigate and to simulate its behaviour. The CFD toolbox OpenFOAM 6 was used, with two density-based Solvers: explicit Solver rhoCentralFoam, which implements Kurganov Central-upwind schemes, and Implicit Solver HiSA, which implements the AUSM+up upwind scheme. The behaviour of the transient evacuation ranges between adiabatic polytropic exponent at the beginning of the process and isothermal at the end. A model for the computation of the transient polytropic exponent is proposed. During the evacuation, two regimes are encountered in the second nozzle. In the supercritic regime, the secondary is choked and sonic flow is reached. In the subcritic regime, the secondary flow is subsonic. The final agreement is good with the two different Solvers, although simulation tends to slightly overestimate flow rate for large values region.
Kengo Nakajima - One of the best experts on this subject based on the ideXlab platform.
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a fast scalable Implicit Solver with concentrated computation for nonlinear time evolution problems on low order unstructured finite elements
International Parallel and Distributed Processing Symposium, 2018Co-Authors: Tsuyoshi Ichimura, Kohei Fujita, Masashi Horikoshi, Larry Meadows, Kengo Nakajima, Takuma Yamaguchi, Kentaro Koyama, Hikaru Inoue, Akira Naruse, Keisuke KatsushimaAbstract:Many supercomputers are shifting to architectures with low B (byte/s; memory transfer capability) per F (FLOPS capability) ratios. However, utilizing increased F is difficult for applications that inherently require large B. Targeting an Implicit unstructured low-order finite-element analysis Solver, which typically requires large B, we have developed a concentrated computation algorithm that yields significant performance improvements on low B/F supercomputers. 35.7% peak performance was achieved for a sparse matrix-vector multiplication kernel, and 15.6% peak performance was achieved for the whole Solver on the second generation Xeon Phi-based Oakforest-PACS. This is 5.02 times faster than (and 6.90 times the peak performance of) the state-of-the-art Solver (the SC14 Gordon Bell finalist Solver). On Oakforest-PACS, the proposed Solver was approximately 2.42 times faster than the state-of-the-art Solver running on the K computer. The proposed approach has implications for systems and applications and is expected to have significant impact on various fields that use finite-element methods for nonlinear time evolution problems.
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SC - A fast scalable Implicit Solver for nonlinear time-evolution earthquake city problem on low-ordered unstructured finite elements with artificial intelligence and transprecision computing
SC18: International Conference for High Performance Computing Networking Storage and Analysis, 2018Co-Authors: Tsuyoshi Ichimura, Kohei Fujita, Takuma Yamaguchi, Akira Naruse, Jack C. Wells, Thomas C. Schulthess, Tjerk Straatsma, Christopher Zimmer, Maxime Martinasso, Kengo NakajimaAbstract:To address problems that occur due to earthquake in urban areas, we propose a method that utilizes artificial intelligence (AI) and transprecision computing to accelerate a nonlinear dynamic low-order unstructured finite-element Solver. The AI is used to improve the convergence of iterative Solver leading to 5.56-fold reduction in arithmetic count from a standard Solver, and FP16-FP21-FP32-FP64 computing is used to accelerate the sparse matrix-vector product kernel, which demonstrated 71.4% peak FP64 performance on Summit. This is 25.3 times faster than a standard Solver and 3.99 times faster than the state-of-the-art SC14 Gordon Bell Finalist Solver. Furthermore, the proposed Solver demonstrated high scalability (88.8% on the K computer and 89.5% on Piz Daint), leading to 14.7% peak FP64 performance on 4096 nodes of Summit. The proposed approach utilizing AI and FP16 arithmetic has implications for accelerating other Implicit Solvers used for earthquake city simulations as well as various fields.
T L Lin - One of the best experts on this subject based on the ideXlab platform.
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use of a coupled explicit Implicit Solver for calculating spring back in automotive body panels
Journal of Materials Processing Technology, 1995Co-Authors: M Finn, P C Galbraith, J O Hallquist, L Lum, T L LinAbstract:Abstract LS-DYNA3D, an explicit code, and LS-NIKE3D, an Implicit code, have been coupled to facilitate the finite element (FE) modelling of sheet metal forming. The explicit FE code is used to model the forming process, in which the deformable blank contacts rigid tools. The Implicit FE code is used to model the subsequent spring-back which occurs after the tooling is removed. In this way, the explicit code with its robust handling of contact during forming is combined with the Implicit code and its large time steps during spring-back. The result is an efficient method for solving even very large (>20 000 deformable elements) sheet forming models. Three examples of the application of this method are given.
Ll Macia - One of the best experts on this subject based on the ideXlab platform.
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Numerical simulation of a supersonic ejector for vacuum generation with explicit and Implicit Solver in openfoam
Energies, 2019Co-Authors: Ll Macia, R. Castilla, P. J. Gamez-montero, S. Camacho, E. CodinaAbstract:Supersonic ejectors are used extensively in all kind of applications: compression of refrigerants in cooling systems, pumping of volatile fluids or in vacuum generation. In vacuum generation, also known as zero-secondary flow, the ejector has a transient behaviour. In this paper, a numerical and experimental research of a supersonic compressible air nozzle is performed in order to investigate and to simulate its behaviour. The CFD toolbox OpenFOAM 6 was used, with two density-based Solvers: explicit Solver rhoCentralFoam, which implements Kurganov Central-upwind schemes, and Implicit Solver HiSA, which implements the AUSM+up upwind scheme. The behaviour of the transient evacuation ranges between adiabatic polytropic exponent at the beginning of the process and isothermal at the end. A model for the computation of the transient polytropic exponent is proposed. During the evacuation, two regimes are encountered in the second nozzle. In the supercritic regime, the secondary is choked and sonic flow is reached. In the subcritic regime, the secondary flow is subsonic. The final agreement is good with the two different Solvers, although simulation tends to slightly overestimate flow rate for large values region.
