23 research outputs found
Three discontinuous Galerkin schemes for the anisotropic heat conduction equation on non-aligned grids
We present and discuss three discontinuous Galerkin (dG) discretizations for
the anisotropic heat conduction equation on non-aligned cylindrical grids. Our
most favourable scheme relies on a self-adjoint local dG (LDG) discretization
of the elliptic operator. It conserves the energy exactly and converges with
arbitrary order. The pollution by numerical perpendicular heat fluxes degrades
with superconvergence rates. We compare this scheme with aligned schemes that
are based on the flux-coordinate independent approach for the discretization of
parallel derivatives. Here, the dG method provides the necessary interpolation.
The first aligned discretization can be used in an explicit time-integrator.
However, the scheme violates conservation of energy and shows up stagnating
convergence rates for very high resolutions. We overcome this partly by using
the adjoint of the parallel derivative operator to construct a second
self-adjoint aligned scheme. This scheme preserves energy, but reveals
unphysical oscillations in the numerical tests, which result in a decreased
order of convergence. Both aligned schemes exhibit low numerical heat fluxes
into the perpendicular direction. We build our argumentation on various
numerical experiments on all three schemes for a general axisymmetric magnetic
field, which is closed by a comparison to the aligned finite difference (FD)
schemes of References [1,2
The collisional drift wave instability in steep density gradient regimes
The collisional drift wave instability in a straight magnetic field
configuration is studied within a full-F gyro-fluid model, which relaxes the
Oberbeck-Boussinesq (OB) approximation. Accordingly, we focus our study on
steep background density gradients. In this regime we report on corrections by
factors of order one to the eigenvalue analysis of former OB approximated
approaches as well as on spatially localised eigenfunctions, that contrast
strongly with their OB approximated equivalent. Remarkably, non-modal phenomena
arise for large density inhomogeneities and for all collisionalities. As a
result, we find initial decay and non-modal growth of the free energy and
radially localised and sheared growth patterns. The latter non-modal effect
sustains even in the nonlinear regime in the form of radially localised
turbulence or zonal flow amplitudes.Comment: accepted at Nuclear Fusio
Beyond the Oberbeck-Boussinesq and long wavelength approximation
We present the first simulations of a reduced magnetized plasma model that
incorporates both arbitrary wavelength polarization and non-Oberbeck-Boussinesq
effects. Significant influence of these two effects on the density, electric
potential and ExB vorticity and non-linear dynamics of blobs are reported.
Arbitrary wavelength polarization implicates so-called gyro-amplification that
compared to a long wavelength approximation leads to highly amplified
small-scale ExB vorticity fluctuations. These strongly increase the coherence
and lifetime of blobs and alter the motion of the blobs through a faster
blob-disintegration. Non-Oberbeck-Boussinesq effects incorporate plasma
inertia, which substantially decreases the growth rate and linear acceleration
of high amplitude blobs, while the maximum blob velocity is not affected.
Finally, we generalize and numerically verify unified scaling laws for blob
velocity, acceleration and growth rate that include both ion temperature and
arbitrary blob amplitude dependence
Non-Oberbeck-Boussinesq zonal flow generation
Novel mechanisms for zonal flow (ZF) generation for both large relative
density fluctuations and background density gradients are presented. In this
non-Oberbeck-Boussinesq (NOB) regime ZFs are driven by the Favre stress, the
large fluctuation extension of the Reynolds stress, and by background density
gradient and radial particle flux dominated terms. Simulations of a nonlinear
full-F gyro-fluid model confirm the predicted mechanism for radial ZF
propagation and show the significance of the NOB ZF terms for either large
relative density fluctuation levels or steep background density gradients
Consistency in Drift-ordered Fluid Equations
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.Comment: 22 page
Unified transport scaling laws for plasma blobs and depletions
We study the dynamics of seeded plasma blobs and depletions in an (effective)
gravitational field. For incompressible flows the radial center of mass
velocity of blobs and depletions is proportional to the square root of their
initial cross-field size and amplitude. If the flows are compressible, this
scaling holds only for ratios of amplitude to size larger than a critical
value. Otherwise, the maximum blob and depletion velocity depends linearly on
the initial amplitude and is independent of size. In both cases the
acceleration of blobs and depletions depends on their initial amplitude
relative to the background plasma density, is proportional to gravity and
independent of their cross-field size. Due to their reduced inertia plasma
depletions accelerate more quickly than the corresponding blobs. These scaling
laws are derived from the invariants of the governing drift-fluid equations and
agree excellently with numerical simulations over five orders of magnitude. We
suggest an empirical model that unifies and correctly captures the radial
acceleration and maximum velocities of both blobs and depletions
Pad\'e-based arbitrary wavelength polarization closures for full-F gyro-kinetic and -fluid models
We propose a solution to the long-standing short wavelength polarization
closure shortfall of full-F gyro-fluid models. This is achieved by first
finding an appropriate quadratic form of the gyro-fluid moment over the
polarization part of the gyro-center Hamiltonian. Secondly, we deduce
Pad\'e-based approximations to the latter expression that produce a
polarization charge density with the desired order of accuracy and retain
linear polarization effects for arbitrary wavelengths. The proposed closures
feature proper energy conservation and the anticipated Oberbeck-Boussinesq and
long wavelength limits
Reproducibility, accuracy and performance of the Feltor code and library on parallel computer architectures
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.
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
0.1 and 1000 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)
Reproducibility Strategies for Parallel Preconditioned Conjugate Gradient
The Preconditioned Conjugate Gradient method is often used in numerical simulations. While being widely used, the solver is also known for its lack of accuracy while computing the residual. In this article, we aim at a twofold goal: enhance the accuracy of the solver but also ensure its reproducibility in a message-passing implementation. We design and employ various strategies starting from the ExBLAS approach (through preserving every bit of information until final rounding) to its more lightweight performance-oriented variant (through expanding the intermediate precision). These algorithmic strategies are reinforced with programmability suggestions to assure deterministic executions. Finally, we verify these strategies on modern HPC systems: both versions deliver reproducible number of iterations, residuals, direct errors, and vector-solutions for the overhead of only 29 % (ExBLAS) and 4 % (lightweight) on 768 processes