Unified transport scaling laws for plasma blobs and depletions

Wiesenberger, Matthias; Held, Markus; Kube, Ralph; Garcia, Odd Erik

doi:10.1063/1.4985318

Cited by 13 publications

(22 citation statements)

References 35 publications

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“…5 Applications Programs that execute two-and threedimensional simulations: read in input file(s), simulate, and either write results to disc or directly visualize them on screen. Some examples led to journal publications in the past [63,32,43,62].…”

Section: Overviewmentioning

confidence: 99%

“…To conclude the introduction let us here briefly mention the capabilities and background of our code. Feltor is a modular and free software package that we have developed particularly for the use in full-F (no Oberbeck-Boussinesq approximation) drift-and gyro-fluid models [32,62,41,31]. We use discontinuous Galerkin methods [12,6] to spatially discretize model equations.…”

Section: Introductionmentioning

confidence: 99%

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Reproducibility, accuracy and performance of the Feltor code and library on parallel computer architectures

Wiesenberger

Einkemmer

Held

et al. 2019

Computer Physics Communications

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Feltor is a modular and free scientific software package. It allows developing platform independent code that runs on a variety of parallel computer architectures ranging from laptop CPUs to multi-GPU distributed memory systems. Feltor consists of both a numerical library and a collection of application codes built on top of the library. Its main target are two-and three-dimensional drift-and gyro-fluid simulations with discontinuous Galerkin methods as the main numerical discretization technique.We observe that numerical simulations of a recently developed gyro-fluid model produce non-deterministic results in parallel computations. First, we show how we restore accuracy and bitwise reproducibility algorithmically and programmatically. In particular, we adopt an implementation of the exactly rounded dot product based on long accumulators, which avoids accuracy losses especially in parallel applications. However, reproducibility and accuracy alone fail to indicate correct simulation behaviour. In fact, in the physical model slightly different initial conditions lead to vastly different end states. This behaviour translates to its numerical representation. Pointwise convergence, even in principle, becomes impossible for long simulation times. We briefly discuss alternative methods to ensure the correctness of results like the convergence of reduced physical quantities of interest, ensemble simulations, invariants or reduced simulation times.In a second part, we explore important performance tuning considerations. We identify latency and memory bandwidth as the main performance indicators of our routines. Based on these, we propose a parallel performance model that predicts the execution time of algorithms implemented in Feltor and test our model on a selection of parallel hardware architectures. We are able to predict the execution time with a relative error of less than 25% for problem sizes between 10 −1 and 10 3 MB. Finally, we find that the product of latency and bandwidth gives a minimum array size per compute node to achieve a scaling efficiency above 50% (both strong and weak).

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Section: Overviewmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Reproducibility, accuracy and performance of the Feltor code and library on parallel computer architectures

Wiesenberger

Einkemmer

Held

et al. 2019

Computer Physics Communications

Self Cite

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“…( 10) at the cost of introducing an auxiliary field (see e.g. [11] for the zeroth order). The presented set of equations ( 8)- (10) have the advantages that they have a straight-forward generalization to systems with multiple participating species.…”

Section: Perturbative Setupmentioning

confidence: 99%

Consistency in drift-ordered fluid equations

Gath

Wiesenberger

2019

Physics of Plasmas

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We address several concerns related to the derivation of drift-ordered fluid equations. Starting from a fully Galilean invariant fluid system, we show how consistent sets of perturbative drift-fluid equations in the case of a isothermal collisionless fluid can be obtained. Treating all the dynamical fields on equal footing in the singular-drift expansion, we show under what conditions a set of perturbative equations can have a non-trivial quasi-neutral limit. We give a suitable perturbative setup where we provide the full set of perturbative equations for obtaining the first-order corrected fields and show that all the constants of motion are preserved at each order. With the dynamical field variables under perturbative control, we subsequently provide a quantitative analysis by means of numerical simulations. With direct access to first-order corrections the convergence properties are addressed for different regimes of parameter space and the validity of the first-order approximation is discussed in the three settings: cold ions, hot ions and finite charge density.

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“…Since detailed physical models increase the analytical complexity significantly, the scientific community relies on numerical simulations of isolated blobs and fully turbulent simulations of the scrape off layer. Numerical simulations in two dimensions [26][27][28][29][30][31][32] and three dimensions [33][34][35][36][37][38][39][40][41] have enhanced the understanding of the underlying mechanisms of blob and filament propagation in the scrape off layer. Most of these numerical simulations investigate idealized isolated blobs modeled as positive symmetrical Gaussian perturbations on a constant plasma background.…”

Section: Introductionmentioning

confidence: 99%

Blob interactions in 2D scrape-off layer simulations

et al. 2020

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Interaction of coherent structures known as blobs in the scrape-off layer of magnetic confinement fusion devices is investigated. Isolated and interacting seeded blobs as well as full plasma turbulence are studied with a two dimensional fluid code. The features of the blobs (size, amplitude, position) are determined with a blob tracking algorithm, which identifies them as coherent structures above a chosen density threshold and compared to a conventional center of mass approach. The agreement of these two methods is shown to be affected by the parameters of the blob tracking algorithm. The benchmarked approach is then extended to a population of interacting plasma blobs with statistically distributed amplitudes, sizes and initial positions for different levels of intermittency. As expected, for decreasing intermittency, we observe an increasing number of blobs deviating from size-velocity scaling laws of perfectly isolated blobs. This is found to be caused by the interaction of blobs with the electrostatic potential of one another, leading to higher average blob velocities. The degree of variation from the picture of perfectly isolated blobs is quantified as a function of the average waiting time of the seeded blobs.

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Unified transport scaling laws for plasma blobs and depletions

Cited by 13 publications

References 35 publications

Reproducibility, accuracy and performance of the Feltor code and library on parallel computer architectures

Reproducibility, accuracy and performance of the Feltor code and library on parallel computer architectures

Consistency in drift-ordered fluid equations

Blob interactions in 2D scrape-off layer simulations

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