27 research outputs found
Revisiting Kinematic Fast Dynamo in 3-dimensional magnetohydrodynamic plasmas: Dynamo transition from non-Helical to Helical flows
Dynamos wherein magnetic field is produced from velocity fluctuations are
fundamental to our understanding of several astrophysical and/or laboratory
phenomena. Though fluid helicity is known to play a key role in the onset of
dynamo action, its effect is yet to be fully understood. In this work, a fluid
flow proposed recently [Yoshida et al. Phys. Rev. Lett. 119, 244501 (2017)] is
invoked such that one may inject zero or finite fluid helicity using a control
parameter, at the beginning of the simulation. Using a simple kinematic fast
dynamo model, we demonstrate unambiguously the strong dependency of short scale
dynamo on fluid helicity. In contrast to conventional understanding, it is
shown that fluid helicity does strongly influence the physics of short scale
dynamo. To corroborate our findings, late time magnetic field spectra for
various values of injected fluid helicity is presented along with rigorous
``geometric'' signatures of the 3D magnetic field surfaces, which shows a
transition from ``untwisted'' to ``twisted'' sheet to ``cigar'' like
configurations. It is also shown that one of the most studied ABC dynamo model
is not the ``fastest'' dynamo model for problems with lower magnetic Reynolds
number. This work brings out, for the first time, the role of fluid helicity in
moving from ``non-dynamo'' to ``dynamo'' regime systematically
Three Dimensional Pseudo-Spectral Compressible Magnetohydrodynamic GPU Code for Astrophysical Plasma Simulation
This paper presents the benchmarking and scaling studies of a GPU accelerated
three dimensional compressible magnetohydrodynamic code. The code is developed
keeping an eye to explain the large and intermediate scale magnetic field
generation is cosmos as well as in nuclear fusion reactors in the light of the
theory given by Eugene Newman Parker. The spatial derivatives of the code are
pseudo-spectral method based and the time solvers are explicit. GPU
acceleration is achieved with minimal code changes through OpenACC
parallelization and use of NVIDIA CUDA Fast Fourier Transform library (cuFFT).
NVIDIAs unified memory is leveraged to enable over-subscription of the GPU
device memory for seamless out-of-core processing of large grids. Our
experimental results indicate that the GPU accelerated code is able to achieve
upto two orders of magnitude speedup over a corresponding OpenMP parallel, FFTW
library based code, on a NVIDIA Tesla P100 GPU. For large grids that require
out-of-core processing on the GPU, we see a 7x speedup over the OpenMP, FFTW
based code, on the Tesla P100 GPU. We also present performance analysis of the
GPU accelerated code on different GPU architectures - Kepler, Pascal and Volta
Long time fate of two-dimensional incompressible high Reynolds number Navier-Stokes turbulence: A quantitative comparison between theory and simulation
Predicting the long time or late time states of two-dimensional
incompressible, high Reynolds number, slowly decaying turbulence has been one
of the long-standing problems. Using ``point vortices'' as ``inviscid''
building blocks, which do not respect incompressibility, statistical mechanical
models conserving only total energy and zero total circulation result in the
well-known sinh-Poisson relation between vorticity and stream function. On the
other hand, statistical mechanics of ``inviscid patch'' vortices, which
respects incompressibility by conserving regions of zero and nonzero vorticity,
predicts a generalized relaxed state, which has never been systematically
compared with direct numerical simulations (DNS). In this study, starting from
highly packed regions of nonzero initial vorticity, we demonstrate using high
resolution, high Reynolds number DNS that the late time states agree with
predictions from patch vortex models. As total circulation is reduced or
diluted, we show that late time states of our DNS systematically and
unambiguously lead to the sinh-Poisson relationship between vorticity and
stream function. We believe that our quantitative findings solve one of the
long-standing problems in two-dimensional turbulence