Velocity and depth distributions in stream reaches: testing European models in Ecuador

We tested how European statistical hydraulic models developed in France and Germany predicted the frequency distributions of water depth and point-velocity measured in 14 reaches in Ecuador during 25 surveys. We first fitted the observed frequency distributions to parametric functions defined in Europe and predicted the parameters from the average characteristics of reaches (e.g. discharge rate, mean depth and width) using European regressions. When explaining the frequency of three classes of velocity and three classes of depth among reach surveys, the fitted and predicted distributions had a low absolute bias (< 3%). The residual variance of fits relative to the mean class variance was < 18%. The residual variance of predicted frequencies was 30-61% for velocity classes and 20-36% for depth classes. Overall, the European models appeared appropriate for Ecuadorian stream reaches but could be improved. Our study demonstrates the transferability of statistical hydraulic models between widely-separated geographic regions

Physics informed Neural Networks applied to the description of wave-particle resonance in kinetic simulations of fusion plasmas

The Vlasov-Poisson system is employed in its reduced form version (1D1V) as a test bed for the applicability of Physics Informed Neural Network (PINN) to the wave-particle resonance. Two examples are explored: the Landau damping and the bump-on-tail instability. PINN is first tested as a compression method for the solution of the Vlasov-Poisson system and compared to the standard neural networks. Second, the application of PINN to solving the Vlasov-Poisson system is also presented with the special emphasis on the integral part, which motivates the implementation of a PINN variant, called Integrable PINN (I-PINN), based on the automatic-differentiation to solve the partial differential equation and on the automatic-integration to solve the integral equation

Parallel bottleneck in the Quasineutrality solver embedded in GYSELA

This report shows some performance results of the Quasineutraliy Poisson solver used in the GYSELA code. The numerical schemes of this Poisson solver is explained, and the computation and communication steps on a parallel machine are described. Benchmarks shows several time measurement from 32 cores to 4096 cores. Present bottlenecks and problems of the parallel algorithm are discussed. Some possible solutions are foreseen.Ce rapport présente des prises de performances du solveur Poisson Quasi-neutre utilisé dans le code GYSELA. Le schéma numérique de ce solveur Poisson est décrit, ainsi que les différentes étapes de calculs et de communication sur machine parallèle. Une série de benchmarks on été effectués de 32 à 4096 coeurs, cela donne un aperçu des performances de ce solveur parallèle. Les goulots d'étranglement et les limitations de l'algorithme parallèle utilisé sont explicités. Enfin, des solutions possibles sont envisagées

Gyroaverage operator for a polar mesh

International audienceIn this work, we are concerned with numerical approximation of the gyroaverage operators arising in plasma physics to take into account the effects of the finite Larmor radius corrections. The work initiated in [5] is extended here to polar geometries. A direct method is proposed in the space configuration which consists in integrating on the gyrocircles using interpolation operator (Hermite or cubic splines). Numerical comparisons with a standard method based on a Padé approximation are performed: (i) with analytical solutions, (ii) considering the 4D drift-kinetic model with one Larmor radius and (iii) on the classical linear DIII-D benchmark case [6]. In particular, we show that in the context of a drift-kinetic simulation, the proposed method has similar computational cost as the standard method and its precision is independent of the radius. PACS. PACS-key discribing text of that key – PACS-key discribing text of that ke

Some numerical aspects of the conservative PSM scheme in a 4D drift-kinetic code.

The purpose of this work is simulation of magnetised plasmas in the ITER project framework. In this context, kinetic Vlasov-Poisson like models are used to simulate core turbulence in the tokamak in a toroidal geometry. This leads to heavy simulations because a 6D dimensional problem has to be solved, even if reduced to a 5D in so called gyrokinetic models. Accurate schemes, parallel algorithms need to be designed to bear these simulations. This paper describes the numerical studies to improve robustness of the conservative PSM scheme in the context of its development in the GYSELA code. In this paper, we only consider the 4D drift-kinetic model which is the backbone of the 5D gyrokinetic models and relevant to build a robust and accurate numerical method

Non regression testing for the JOREK code

Non Regression Testing (NRT) aims to check if software modifications result in undesired behaviour. Suppose the behaviour of the application previously known, this kind of test makes it possible to identify an eventual regression, a bug. Improving and tuning a parallel code can be a time-consuming and difficult task, especially whenever people from different scientific fields interact closely. The JOREK code aims at investing Magnetohydrodynamic (MHD) instabilities in a Tokamak plasma. This paper describes the NRT procedure that has been tuned for this simulation code. Automation of the NRT is one keypoint to keeping the code healthy in a source code repository.Comment: No. RR-8134 (2012

Scaling and optimizing the Gysela code on a cluster of many-core processors

International audienceThe current generation of the Xeon Phi Knights Landing (KNL) processor provides a highly multi-threaded environment on which regular programming models such as MPI/OpenMP can be used. This specific hardware offers both large memory bandwidth and large computing resources and is currently available on computing facilities. Many factors impact the performance achieved by applications, one of the key points is the efficient exploitation of SIMD vector units, another one is the memory access pattern. Thus, vectorization and optimization works have been conducted on a plasma turbulence application, namely Gysela. A set of different techniques have been used: loop splitting, inlining, grouping a set of LU solve operations, removing conditionals and some loop nests, auto-tuning of one computation kernel, changing a key numerical scheme – Lagrange interpolation instead of cubic splines. As a result, KNL execution times have been reduced by up to a factor 3 in some configurations. This effort has also permitted to gain a speedup of 2x on Broadwell architecture and 3x on Skylake. Nice scalability curves up to a few thousands cores have been obtained on a strong scaling experiment. Incremental work for vectorizing the Gysela code meant a large payoff without resorting to writing assembly code or using low-level intrinsics

Implantation d'un raymarching adaptatif pour la visualisation de données volumiques issues de simulations gyrocin étiques

Ce rapport technique décrit la mise en oeuvre d'une méthode de rendu volumique efficace adaptée aux données volumiques issues d'une simulation gyrocin étique. Cette méthode est basée sur un lancer de rayon adaptatif implanté en utilisant la programmation des cartes graphiques. La géométrie spécifique d'un tokamak (dispositif physique dédié aux plasmas) est exploitée dans le but d'obtenir un rendu interactif. Une bonne approximation de l'intégration du rayon continu est obtenue en introduisant une table de pré-intégration échantillonnée. Une comparaison avec une implentation récente de visualisation de simulations gyrocinétiques est également faite et démontre l'efficacité de notre approche en terme de qualité et de fréquence d'affichage

Adaptive Raymarching and Multi-Sampled Pre-integration on Graphics Hardware: Application to the Visualization of Gyrokinetic Plasmas Simulations

The study of plasmas is a rising research topic in the field of physics. It implies the need for specific algorithms allowing physicists to easily visualize and thus correctly interpret related complex datasets. This paper describes an innovative and highly efficient volume rendering method designed for gyrokinetic simulations. Based on a Raymarching approach that benefits from the latest functionalities offered by current graphics programmable hardware, we exploit the geometrical properties of the tokamak (a physical device used to create plasmas) in order to achieve a rendering at interactive framerates. We therefore introduce a novel and general method for Raymarching volumes with an adaptive step. A good approximation of the rays' continuous integration is obtained by introducing a Multi-Sampled Pre-integrated table. A comparison with a recent gyrokinetic simulations visualization technique demonstrates the improved efficiency of our approach in terms of framerate and quality