Research

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Research Interests

  • Computational Fluid Dynamics : Hydrodynamics, Magneto-hydrodynamics & Radiative transfer.
  • Theoretical star formation : Accretion disk physics, Jets and Outflow formation.
  • Particle Acceleration in AGN jets : Modeling Non-Thermal spectral signatures
  • Inter-Stellar Medium : Shock-Cloud Interaction, Collapse of molecular cores, Shock induced chemistry.
  • Astrophysical Code Development : MPI Parallel Programming, Visualization software tools, best practices of coding with C and Python.

List of Recent Selected Published Works

1. Simulation of MHD modes in Active Region of Sun.

There is considerable observational evidence of implosion of magnetic loop systems inside solar coronal active regions following high-energy events like solar flares. In this work, we propose that such collapse can be modeled in three dimensions quite accurately within the framework of ideal magnetohydrodynamics. We furthermore argue that the dynamics of loop implosion is only sensitive to the transmitted disturbance of one or more of the system variables, e.g., velocity generated at the event site. This indicates that to understand loop implosion, it is sensible to leave the event site out of the simulated active region. Toward our goal, a velocity pulse is introduced to model the transmitted disturbance generated at the event site. Magnetic field lines inside our simulated active region are traced in real time, and it is demonstrated that the subsequent dynamics of the simulated loops closely resemble observed imploding loops. Our work highlights the role of plasma β in regards to the rigidity of the loop systems and how that might affect the imploding loops’ dynamics. Compressible magnetohydrodynamic modes such as kink and sausage are also shown to be generated during such processes, in accordance with observations.

Density evolution along a field line showing indicatication of the standing sausage wave oscillation.

2. Runge-Kutta Legendre Method for Parabolic PDEs in PLUTO code : Application to Thermal Conduction Problems.

Effect of Anisotropic Thermal conduction in the problem of 2D blast wave.

An important ingredient in numerical modelling of high temperature magnetized astrophysical plasmas is the anisotropic transport of heat along magnetic field lines from higher to lower temperatures. Magnetohydrodynamics typically involves solving the hyperbolic set of conservation equations along with the induction equation. Incorporating anisotropic thermal conduction requires to also treat parabolic terms arising from the diffusion operator. An explicit treatment of parabolic terms will considerably reduce the simulation time step due to its dependence on the square of the grid resolution ( x) for stability. Although an implicit scheme relaxes the constraint on stability, it is difficult to distribute efficiently on a parallel architecture.

Strong scaling test for the newly implemented Runge-Kutta Legendre Method in PLUTO code.

Treating parabolic terms with accelerated super-time- stepping (STS) methods has been discussed in literature, but these methods suffer from poor accuracy (first order in time) and also have difficult-to- choose tuneable stability parameters. In this work, we highlight a second-order (in time) Runge–Kutta–Legendre (RKL) scheme (first described by Meyer, Balsara & Aslam 2012) that is robust, fast and accurate in treating parabolic terms alongside the hyperbolic conversation laws. We demonstrate its superiority over the first-order STS schemes with standard tests and astrophysical applications. We also show that explicit conduction is particularly robust in handling saturated thermal conduction. Parallel scaling of explicit conduction using RKL scheme is demonstrated up to more than 104 processors.

3. Interaction of Hydrodynamic Shock with Self-gravitating cloud.

Time evolution of density (X-Y plane) of interaction of 3D self-gravitating cloud with weak shock

We describe the results of 3D simulations of the interaction of hydrodynamic shocks with Bonnor-Ebert spheres performed with an adaptive mesh refinement code. The calculations are isothermal and the clouds are embedded in a medium in which the sound speed is either 4 or 10 times that in the cloud. The strengths of the shocks are such that they induce gravitational collapse in some cases and not in others, and we derive a simple estimate for the shock strength required for this to occur. These results are relevant to dense cores and Bok globules in star-forming regions subjected to shocks produced by stellar feedback.

4. Interplay Magnetic reconnection and non-axisymmetric instabilities in Jets.

Formation of short wavelength pressure driven instabilities in density of jet column with toroidal magnetic field lines.

Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as S 1/2 (where S is the Lundquist number) and yields time-scales several order of magnitude larger than the observed ones. Earlier two- dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of S (`fast reconnection' regime) due to the presence of the secondary tearing instability that takes place for S ≳ 1 × 10 4 . We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at S ≃ 1 × 10 3 .

The dominant kink mode for current driven instability seen in the density along turbulent current sheets.

5. Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index

Comparison of Sod Shock tube solution for Ideal EoS and EoS which depends on H/He composition and has non-constant adiabatic index.

An equation of state (EoS) is a relation between thermodynamic state variables and it is essential for closing the set of equations describing a fluid system. Although an ideal EoS with a constant adiabatic index Γ is the preferred choice owing to its simplistic implementation, many astrophysical fluid simulations may benefit from a more sophisticated treatment that can account for diverse chemical processes. In the present work we first review the basic thermodynamic principles of a gas mixture in terms of its thermal and caloric EoS by including effects like ionization, dissociation, and temperature dependent degrees of freedom such as molecular vibrations and rotations. The formulation is revisited in the context of plasmas that are either in equilibrium conditions (local thermodynamic- or collisional excitation-equilibria) or described by non- equilibrium chemistry coupled to optically thin radiative cooling.

Effect of H/He EoS in the solution of 2D axi-symmetric MHD jet in comparison to ideal EoS.

We then present a numerical implementation of thermally ideal gases obeying a more general caloric EoS with non-constant adiabatic index in Godunov-type numerical schemes. We discuss the necessary modifications to the Riemann solver and to the conversion between total energy and pressure (or vice versa) routinely invoked in Godunov-type schemes. We then present two different approaches for computing the EoS. The first employs root-finder methods and it is best suited for EoS in analytical form. The second is based on lookup tables and interpolation and results in a more computationally efficient approach, although care must be taken to ensure thermodynamic consistency. A number of selected benchmarks demonstrate that the employment of a non-ideal EoS can lead to important differences in the solution when the temperature range is 500- 10 4K where dissociation and ionization occur. The implementation of selected EoS introduces additional computational costs although the employment of lookup table methods (when possible) can significantly reduce the overhead by a factor of ~ 3-4.