325 research outputs found
On the Heat Transfer in Rayleigh-Benard systems
In this paper we discuss some theoretical aspects concerning the scaling laws
of the Nusselt number versus the Rayleigh number in a Rayleigh-Benard cell. We
present a new set of numerical simulations and compare our findings against the
predictions of existing models. We then propose a new theory which relies on
the hypothesis of Bolgiano scaling. Our approach generalizes the one proposed
by Kadanoff, Libchaber and coworkers and solves some of the inconsistencies
raised in the recent literature.Comment: 10 pages, 5 figure
Towards a unified lattice kinetic scheme for relativistic hydrodynamics
We present a systematic derivation of relativistic lattice kinetic equations
for finite-mass particles, reaching close to the zero-mass ultra-relativistic
regime treated in the previous literature. Starting from an expansion of the
Maxwell-Juettner distribution on orthogonal polynomials, we perform a
Gauss-type quadrature procedure and discretize the relativistic Boltzmann
equation on space-filling Cartesian lattices. The model is validated through
numerical comparison with standard benchmark tests and solvers in relativistic
fluid dynamics such as Boltzmann approach multiparton scattering (BAMPS) and
previous relativistic lattice Boltzmann models. This work provides a
significant step towards the formulation of a unified relativistic lattice
kinetic scheme, covering both massive and near-massless particles regimes
Universality of anisotropic fluctuations from numerical simulations of turbulent flows
We present new results from a direct numerical simulation of a three
dimensional homogeneous Rayleigh-Benard system (HRB), i.e. a convective cell
with an imposed linear mean temperature profile along the vertical direction.
We measure the SO(3)-decomposition of both velocity structure functions and
buoyancy terms. We give a dimensional prediction for the values of the
anisotropic scaling exponents in this Rayleigh-Benard systems. Measured scaling
does not follow dimensional estimate, while a better agreement can be found
with the anisotropic scaling of a different system, the random-Kolmogorov-flow
(RKF). Our findings support the conclusion that scaling properties of
anisotropic fluctuations are universal, i.e. independent of the forcing
mechanism sustaining the turbulent flow.Comment: 4 pages, 3 figure
Kinetic approach to relativistic dissipation
Despite a long record of intense efforts, the basic mechanisms by which
dissipation emerges from the microscopic dynamics of a relativistic fluid still
elude a complete understanding. In particular, no unique pathway from kinetic
theory to hydrodynamics has been identified as yet, with different approaches
leading to different values of the transport coefficients. In this Letter, we
approach the problem by matching data from lattice kinetic simulations with
analytical predictions. Our numerical results provide neat evidence in favour
of the Chapman-Enskog procedure, as suggested by recently theoretical analyses,
along with qualitative hints at the basic reasons why the Chapman-Enskog
expansion might be better suited than Grad's method to capture the emergence of
dissipative effects in relativistic fluids
Evolution of a double-front Rayleigh-Taylor system using a GPU-based high resolution thermal Lattice-Boltzmann model
We study the turbulent evolution originated from a system subjected to a
Rayleigh-Taylor instability with a double density at high resolution in a 2
dimensional geometry using a highly optimized thermal Lattice Boltzmann code
for GPUs. The novelty of our investigation stems from the initial condition,
given by the superposition of three layers with three different densities,
leading to the development of two Rayleigh-Taylor fronts that expand upward and
downward and collide in the middle of the cell. By using high resolution
numerical data we highlight the effects induced by the collision of the two
turbulent fronts in the long time asymptotic regime. We also provide details on
the optimized Lattice-Boltzmann code that we have run on a cluster of GPU
Matched filters for coalescing binaries detection on massively parallel computers
We discuss some computational problems associated to matched filtering of
experimental signals from gravitational wave interferometric detectors in a
parallel-processing environment. We then specialize our discussion to the use
of the APEmille and apeNEXT processors for this task. Finally, we accurately
estimate the performance of an APEmille system on a computational load
appropriate for the LIGO and VIRGO experiments, and extrapolate our results to
apeNEXT.Comment: 19 pages, 6 figure
Energy-efficiency evaluation of Intel KNL for HPC workloads
Energy consumption is increasingly becoming a limiting factor to the design
of faster large-scale parallel systems, and development of energy-efficient and
energy-aware applications is today a relevant issue for HPC code-developer
communities. In this work we focus on energy performance of the Knights Landing
(KNL) Xeon Phi, the latest many-core architecture processor introduced by Intel
into the HPC market. We take into account the 64-core Xeon Phi 7230, and
analyze its energy performance using both the on-chip MCDRAM and the regular
DDR4 system memory as main storage for the application data-domain. As a
benchmark application we use a Lattice Boltzmann code heavily optimized for
this architecture and implemented using different memory data layouts to store
its lattice. We assessthen the energy consumption using different memory
data-layouts, kind of memory (DDR4 or MCDRAM) and number of threads per core
Using accelerators to speed up scientific and engineering codes: perspectives and problems
Accelerators are quickly emerging as the leading technology to further boost
computing performances; their main feature is a massively parallel on-chip architecture. NVIDIA
and AMD GPUs and the Intel Xeon-Phi are examples of accelerators available today. Accelerators
are power-efficient and deliver up to one order of magnitude more peak performance than
traditional CPUs. However, existing codes for traditional CPUs require substantial changes to
run efficiently on accelerators, including rewriting with specific programming languages.
In this contribution we present our experience in porting large codes to NVIDIA GPU and Intel
Xeon-Phi accelerators. Our reference application is a CFD code based on the Lattice
Boltzmann (LB) method. The regular structure of LB algorithms makes them suitable for
processor architectures with a large degree of parallelism. However, the challenge of
exploiting a large fraction of the theoretically available performance is not easy to
met. We consider a state-of-the-art two-dimensional LB model based on 37 populations (a
D2Q37 model), that accurately reproduces the thermo-hydrodynamics of a 2D-fluid obeying the
equation-of-state of a perfect gas.
We describe in details how we implement and optimize our LB code for Xeon-Phi and
GPUs, and then analyze performances on single- and multi-accelerator systems. We
finally compare results with those available on recent traditional multi-core CPUs
Numerical evidence of electron hydrodynamic whirlpools in graphene samples
We present an extension of recent relativistic Lattice Boltzmann methods
based on Gaussian quadratures for the study of fluids in (2+1) dimensions. The
new method is applied to the analysis of electron flow in graphene samples
subject to electrostatic drive; we show that the flow displays hydro-electronic
whirlpools in accordance with recent analytical calculations as well as
experimental results
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