arXiv subjects

Rupak Mukherjee

Publications and source records attributed to Rupak Mukherjee.

28 records · Page 2Linked to original sources

arXiv2019

Study of Dynamo Action in Three Dimensional Magnetohydrodynamic Plasma with Arnold-Beltrami-Childress Flow

For a three dimensional magnetohydrodynamic (MHD) plasma the dynamo action with ABC flow as initial condition has been studied. The study delineates crucial parameter that gives a transition from coherent nonlinear oscillation to dynamo. Further, for both kinematic and dynamic models at magnetic Prandtl number equal to unity the dynamo action is studied for driven ABC flows. The magnetic resistivity has been chosen at a value where the fast dynamo occurs and the growth rate shows no further variation with the change of magnetic Reynold's number. The exponent of growth of magnetic energy increases, indicating a faster dynamo, if a higher wave number is excited compared to the one with a lower wave number. The result has been found to hold good for both kinematic and externally forced dynamic dynamos where the backreaction of magnetic field on the velocity field is no more negligible. In case of an externally forced dynamic dynamo, the super Alfvenic flows have been found to excite strong dynamos giving rise to the growth of magnetic energy of seven orders of magnitude. The back-reaction of magnetic field on the velocity field through Lorentz force term has been found to affect the dynamics of the velocity field and in turn the dynamics of magnetic field, leading to a saturation, when the dynamo action is very prominent.

physics.plasm-ph↗

arXiv2019

Viscoelastic effects on asymmetric two-dimensional vortex patterns in a strongly coupled dusty plasma

Strongly coupled dusty plasma medium is often described as a viscoelastic fluid that retains its memory. In a flowing dusty plasma medium, vortices of different sizes appear when the flow does not remain laminar. The vortices also merge to transfer energy between different scales. In the present work, we study the effect of viscoelasticity and compressibility over a localized vortex structure and multiple rotational vortexes in a strongly coupled viscoelastic dusty plasma medium. In case of single rotating vortex flow, a transverse wave is generated from the localized vortex source and the evolution time of generated waves is found to be reduced due to finite viscoelasticity and compressibility of the medium. It is found that the viscoelasticity suppresses the dispersion of vorticity. In the presence of multiple vortices, we find, the vortex mergers get highly affected in the presence of memory effect of the fluid, and thus the dynamics of the medium gets completely altered compared to a non-viscoelastic fluid. For a compressible fluid, viscoelasticity damps out the energy in the sonic waves generated in the medium. Thus a highly viscoelastic and compressible fluid, in some cases, behaves similarly to an incompressible viscoelastic fluid. The wave-front like rings propagate in elliptical orbits keeping the footprint of the earlier position of the point-vortex. The rings collide with each other even within the patch vortex region forming regions of high vorticity at the point of intersection and pass through each other.

physics.plasm-ph↗

arXiv2019

Recurrence in three dimensional magnetohydrodynamic plasma

We report on a numerical observation of recurrence phenomenon in a three dimensional magneto-hydrodynamic (MHD) plasma for certain classes of initial flow profiles. Our simulations discover such a behaviour for an initial Taylor-Green (TG) type of flow whereas under identical conditions an initial Arnold-Beltrami-Childress (ABC) flow fails to show recurrent behaviour. This difference in the dynamical behaviours is traced to the nature of the Rayleigh quotient in each case. The Rayleigh quotient which is an approximate measure of the number of active degrees of freedom in a dynamical system is found to remain small and bounded in time for the TG flow but grows indefinitely in time for the ABC flow. An estimate of the effective degrees of freedom for the TG flow case is provided and its practical implications for the nonlinear behaviour of the continuum nonlinear MHD plasma system is discussed.

physics.plasm-ph↗

arXiv2018

Numerical relaxation of a 3D MHD Taylor - Woltjer state subject to abrupt expansion

Since the advent of Taylor-Woltjer theory [J B Taylor, PRL, 33, 1139 (1974), L Woltjer, PNAS, 44, 489 (1958)], it has been widely believed that situations with perfectly conducting boundaries and near ideal conditions, the final state of MHD system would be force-free Taylor-Woltjer states defined as $\vec{\nabla} \times \vec{B} = α\vec{B}$ with $α$ as a constant and $\vec{B}$ is the magnetic field defined over a volume $V$. These states are of fundamental importance in fusion plasmas [J B Taylor, RMP 58, 741 (1986)]. More recently, several new MHD models have been proposed - for example Reduced Multi-region relaxed MHD [S R Hudson {\it et al}, Phys. Plasmas, 19, 112502 (2012)] and arbitrary scale relaxation model to Taylor-Woltjer state [H Qin {\it et al}, PRL, 109, 235001 (2012)] to mention a few. In the present work, we use a 3D compressible MHD solver in cartesian geometry which can handle conducting or periodic as well has mixed boundary conditions to investigate numerically the arbitrary scale relaxation model proposed by Qin et al [H Qin {\it et al}, PRL, 109, 235001 (2012)]. For this purpose, we consider two volumes $V_{init}$ and $V_{final}$. We load the 3D MHD solver in the limit of zero compressibility with a Taylor-Woltjer state $B_{init}(x,y,z,t=0)$ and let it again a numerical evolve with conducting boundaries at $V_{init}$ to make sure that we have obtained a numerically steady Taylor - Woltjer state for volume $V_{init}$. Followed by this procedure, we "suddenly" relax the boundaries to a new volume $V_{final}$, such that $V_{init}$ $<$ $V_{final}$ and evaluate whether or not the system attains quasi-steady state. Details of the numerical method used, the protocol followed, the expansion technique and the novelty of this numerical experiment and details of our results have been presented in this paper.

physics.plasm-ph↗

arXiv2018

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.

physics.comp-ph↗

arXiv2018

Correlation between two non-linear events in a complex dusty plasma system

A phenomenological model using fluid theory is developed to show that the decay of two seemingly independent nonlinear structures namely the dust void and dust soliton strongly depends on the plasma ionization parameter in an unmagnetized complex plasma system. Numerical solution of model equations has shown that the evolution of dust voids and their subsequent decay in a time frame is intimately related with the ionization parameter. A similar result also holds good in case of dust solitons where stability of soliton is found to depend critically upon ionization parameter. Most importantly, it is observed that time of the collapse of a dust soliton precedes the onset time of a dust void decay and therefore soliton decay acts as a precursor for void decay to occur in a given dusty plasma system.

physics.plasm-ph↗

arXiv2018

Amplitude Mediated Chimera States with Active and Inactive Oscillators

The emergence and nature of amplitude mediated chimera states, spatio-temporal patterns of co-existing coherent and incoherent regions, are investigated for a globally coupled system of active and inactive Ginzburg-Landau oscillators. The existence domain of such states is found to shrink and shift in parametric space as the fraction of inactive oscillators is increased. The role of inactive oscillators is found to be two fold - they get activated to form a separate region of coherent oscillations and in addition decrease the common collective frequency of the coherent regions by their presence. The dynamical origin of these effects is delineated through a detailed bifurcation analysis of a reduced model equation that is based on a mean field approximation. Our results may have practical implications for the robustness of such states in biological or physical systems where age related deterioration in the functionality of components can occur.

nlin.CD↗

arXiv2018

Compressibility effects on quasistationary vortex and transient hole patterns through vortex merger

The effect of compressibility in hydrodynamic vortex merging has been discussed. In the past, in incompressible limit it has been observed that the merging of a collection of intense point-like vortices arranged uniformly outside a circular vortex, can lead to quasistationary vortex patch and transient hole pattern inside the patch via nonlinear merger process. These patterns are akin to \textquoteleft vortex crystals\textquoteright. Compressibility can introduce a natural acoustic scale to the problem. We find that the natural mode is independent of the number of point-like vortices and the amplitude scales linearly with compressibility. Further it has been identified that after merging, the system exhibits oscillation at a natural frequency together with its harmonics and beats with its own harmonics. The power of the frequency is found to scale as $M^{-2}$, where $M$ is the Mach number. Also the vortex crystals formed out of the merging process are found to melt faster as compressibility is increased.

physics.flu-dyn↗

arXiv2017

Single Particle Closed Orbit in Yukawa Potential

We study the orbit of a single particle moving under the Yukawa potential and observe the precessing ellipse type orbits. The amount of precession can be tuned through the coupling parameter $α$. With a suitable choice of the coupling parameter; we can get a closed bound orbit. In some cases we have observed some petals which can also have a closed bound nature with an appropriate choice of the coupling constant. A threshold energy has also been calculated for the boundness of the orbits.

physics.plasm-ph↗

arXiv2017

Constraining scalar-Gauss-Bonnet Inflation by Reheating, Unitarity and PLANCK

We revisit the inflationary dynamics in detail for theories with Gauss-Bonnet gravity coupled to scalar functions, in light of the Planck data. Considering the chaotic inflationary scenario, we constrain the parameters of two models involving inflaton-Gauss-Bonnet coupling by current Planck data. For non zero inflaton-Gauss-Bonnet coupling $β$, an inflationary analysis provides us a big cosmologically viable region in the space of $(m,β)$, where $m$ is the mass of inflaton. However, we study further on constraining $β$ arising from reheating considerations and unitarity of tree-level amplitude involving we have studied the constraints on $β$ arising from reheating considerations and unitarity of tree level amplitude involving $2$ graviton $\rightarrow 2$ graviton ($h h\rightarrow hh$) scattering. Our analysis, particularly on reheating significantly reduces the parameter space of $(m,β)$ for all models. he quadratic Gauss-Bonnet coupling parameter turns out to be more strongly constrained than that of the linear coupling. For the linear Gauss-Bonnet coupling function, we obtain $β\lesssim 10^3$, with the condition $β(m/M_P)^2 \simeq 10^{-4}$. However, study of the Higgs inflation scenario in the presence of Gauss-Bonnet term turned out to be strongly disfavored.

gr-qc↗