arXiv subjects

Souvik Naskar

Publications and source records attributed to Souvik Naskar.

11 recordsLinked to original sources

arXiv2026

Solid and Quasi-Solid Electrolytes for Zinc Batteries: Balancing Water Activity, Ion Transport, and Interfaces

Zinc batteries offer a compelling route to safe and low-cost energy storage, yet their reliance on aqueous electrolytes promotes hydrogen evolution, corrosion, cathode dissolution, and non-uniform zinc deposition. Replacing the liquid with a solid or quasi-solid electrolyte can suppress these processes, but it also removes the medium that enables rapid Zn2+ transport and conformal electrode contact. This tension, the price of removing water, has been obscured by inconsistent use of the term solid state and by comparisons based largely on bulk ionic conductivity. Here we critically examine zinc electrolytes across a continuum from water-rich hydrogels to dry polymers, solvated crystals, and inorganic conductors. We distinguish water content from thermodynamic water activity and classify these materials according to phase state, mobile-solvent fraction, and dominant transport mechanism. We show that neither high conductivity nor nominally water-free composition reliably predicts cell performance: electrolyte thickness, Zn2+ transference, interfacial resistance, and evolving contact often determine the practical outcome. Controlled-solvation and hybrid electrolytes therefore provide the most credible near-term path, whereas genuinely solvent-free Zn2+ conductors remain a longer-term scientific target. Progress will require transparent reporting of solvent state and validation using thin electrolytes, realistic electrode loadings, limited zinc excess, and calendar-life testing.

cond-mat.mtrl-sci↗

arXiv2026

Paleomagnetic signatures of core-mantle interactions inferred from top-heavy thermochemical geodynamo simulations

The time-averaged geomagnetic field provides crucial insights into deep Earth dynamics and thermal core-mantle interactions. Paleomagnetic observations and numerical dynamo simulations are equivocal regarding the longitudinal structure of the time-averaged field, though the latter have often considered a generic buoyancy source, which may obscure distinct signatures of thermal and chemical buoyancy that arise near the equator and poles, respectively. In this study, we present a new suite of top-heavy geodynamo simulations, varying the relative strengths of thermal and chemical driving and comparing the resultant magnetic signatures to observational field models spanning centuries to tens of thousands of years. None of the spatially-averaged measures of field morphology and variability we tested could robustly distinguish between different levels of chemical driving or the presence of heterogeneous outer boundary heat flux. On the other hand, observational constraints requiring longitudinal variations in time-averaged inclination anomaly are readily matched by simulations with heterogeneous outer boundary thermal forcing, in contrast to those with homogeneous mantle heat flux. Longitudinal field structures are reduced, but not erased, by elevated chemical driving, which also promotes the formation and deepening of polar minima in the radial magnetic field. Our simulations indicate that both the strong heat flux heterogeneity and chemical driving in Earth's core are likely to result in small but persistent departures from the geocentric axial dipole approximation.

physics.geo-ph↗

arXiv2026

Geomagnetic signatures of the slurry F-layer inferred from dynamo simulations

Seismic observations indicate that the lowermost portion of Earth's liquid core is density stratified. The existence of this so-called F-layer challenges classical theories of core dynamics, where the geodynamo process that generates Earth's main magnetic field is assumed to be powered by heat and light element release at the inner core boundary. The seismically-inferred thickness, density, and velocity anomaly can be reproduced by a dynamical model that represents the F-layer as a two-phase two-component slurry on the liquidus, with a ``snow'' of solid iron particles falling through a quasi-static iron-oxygen liquid. Here, we present the first fluid dynamical simulations of thermochemically driven rotating convection and dynamo action that include a simple representation of the stratified slurry F-layer at the base of the spherical shell geometry. We show that the F-layer can create a barrier to columnar quasi-geostrophic flow, which is expressed near the core surface as a migration of peak radial and azimuthal flow speeds to lower latitudes as the thickness and stratification strength increase. In dynamo simulations, this effect induces polar minima in the radial magnetic field at the outer boundary ($B_r$) that strengthen and deepen with increasing stratification, and peaks in latitudinal profiles of $B_r$ moving to lower latitudes with reduced temporal variability. The geomagnetic signature of the F-layer is most prominent in time-averaged $B_r$, when resolved to at least spherical harmonic degree 5, and a trend of increasingly negative zonal degree 3 and 5 Gauss coefficients as the F-layer thickness and stratification strength increase. Our results suggest that an F-layer thickness of 600~km is incompatible with geomagnetic observations and favour weak stratification (normalised Brunt-V\"ais\"al\"a frequency $<1$) and a layer $<400$~km thick.

physics.geo-ph↗

arXiv2025

Thermochemical models of outer core convection with heterogeneous core-mantle boundary heat flux

Convection in Earth's outer core is driven by the release of heat and light elements at the inner core boundary. A key question is whether these buoyancy sources drive convection throughout the core, or whether a stable layer exists just below the core-mantle boundary (CMB). Recent simulations incorporating CMB heat flux heterogeneities propose locally stable ``regional inversion lenses'' (RILs) rather than a global layer, allowing stable and unstable regions to coexist. However, these simulations combine thermal and compositional anomalies, ignoring differences in diffusivities and boundary conditions. Here we simulate thermal, chemical, and thermochemical convection at Ekman number $E=10^{-5}$, with thermal and chemical flux Rayleigh numbers $\widetilde{Ra}_T=30-4000$ and $\widetilde{Ra}_\xi=30-100000$, and Prandtl numbers $Pr_T=1$ and $Pr_\xi=10$. Purely chemical simulations accumulate light elements below the CMB, forming locally stable regions near the poles or global layers, depending on $\widetilde{Ra}_\xi$. These chemically stratified regions persist in thermochemical simulations even when thermal forcing is destabilising. Introducing heterogeneous CMB heat flux produces thermally stratified RILs even with strongly destabilising compositional buoyancy. Our simulations reveal a diverse range of locations, properties, and morphologies of stable regions depending on $\widetilde{Ra}_T$ and $\widetilde{Ra}_\xi$, they can have a seismically detectable thickness and strength and might also have a signature in geomagnetic observations.

astro-ph.EP↗

arXiv2025

Accessing the dipole-multipole transition in rapidly rotating spherical shell dynamos

Polarity reversals are a key feature of Earth's magnetic field, yet the processes governing them are still poorly understood. Dipole reversals have been found in many numerical dynamo simulations and often occur close to the transition between dipolar and multipolar regimes. Simulated conditions are far from those in Earth's liquid iron core because of the long runtimes needed to capture polarity transitions. We develop a unidimensional path theory in an attempt to simplify the search for the dipole-multipole transition at increasingly realistic physical conditions. We build 3 paths, all based on a constant magnetic Reynolds number $Rm$; one aiming for Magnetic, Coriolis, and Archimedean (MAC), and 2 aiming for inertia-MAC force balance. We add inertia due to its role in simulated reversals. Results show reasonable agreement with predictions within the accessible parameter space, but deviate from predicted behaviour for certain quantities, e.g. magnetic field strength and magnetic/kinetic energy ratio. Further, simulations move into the dipolar non-reversing regime as they are advanced along the path. By increasing the buoyancy driving (via higher Rayleigh number) above the values predicted by the path theory, we are able to access the dipole-multipole transition down to an Ekman number $E\sim 10^{-6}$, comparable to the most extreme conditions reported to date. Results demonstrate that our approach is an efficient method for seeking the dipole-multipole transition at low $E$. However, the conditions under which we access the dipole-multipole transition become increasingly hard to access numerically and also increasingly unrealistic because $Rm$ rises beyond plausible bounds inferred from geophysical observations. Future work combining path theory with variations in the core buoyancy distribution, appears a promising approach to accessing the transition at extreme physical conditions.

physics.geo-ph↗

arXiv2023

A generalized curvilinear solver for spherical shell Rayleigh-B\'enard convection

A three-dimensional finite-difference solver has been developed and implemented for Boussinesq convection in a spherical shell. The solver transforms any complex curvilinear domain into an equivalent Cartesian domain using Jacobi transformation and solves the governing equations in the latter. This feature enables the solver to account for the effects of the non-spherical shape of the convective regions of planets and stars. Apart from parallelization using MPI, implicit treatment of the viscous terms using a pipeline alternating direction implicit scheme and HYPRE multigrid accelerator for pressure correction makes the solver efficient for high-fidelity direct numerical simulations. We have performed simulations of Rayleigh-B\'enard convection at three Rayleigh numbers $Ra=10^{5}, 10^{7}$ and $10^{8}$ while keeping the Prandtl number fixed at unity ($Pr=1$). The average radial temperature profile and the Nusselt number match very well, both qualitatively and quantitatively, with the existing literature. Closure of the turbulent kinetic energy budget, apart from the relative magnitude of the grid spacing compared to the local Kolmogorov scales, assures sufficient spatial resolution.

physics.comp-ph↗

arXiv2022

Energy pathways in large- and small-scale convection-driven dynamos

We investigate the energy pathways between the velocity and the magnetic fields in a rotating plane layer dynamo driven by Rayleigh-B\'enard convection using direct numerical simulations. The kinetic and magnetic energies are divided into mean and turbulent components to study the production, transport, and dissipation associated with large and small-scale dynamos. This energy balance-based characterization reveals distinct mechanisms for large- and small-scale magnetic field generation in dynamos, depending on the nature of the velocity field and the conditions imposed at the boundaries.

physics.flu-dyn↗

arXiv2022

Path integral approach to driven quantum harmonic oscillator using Markov chain Monte Carlo methods

We have simulated the ground states of quantum harmonic oscillators driven either by constant forces of different magnitudes or time-dependent driving forces. The expectation values of position for various combinations of mass, natural angular frequency, and the coupling constant $\lambda$ were calculated for both driving modes. For constant forcing, coherent states were obtained. The results for both forcing scenarios match the theoretically expected values almost exactly. For the simulations, the Metropolis algorithm was implemented on a discrete time lattice to evaluate the imaginary time path integral of the systems.

quant-ph↗

arXiv2022

Effects of kinematic and magnetic boundary conditions on the dynamics of convection-driven plane layer dynamos

Rapidly rotating convection-driven dynamos are investigated under different kinematic and magnetic boundary conditions using DNS. At a fixed rotation rate, represented by the Ekman number $E=5\times10^{-7}$, the thermal forcing is varied from 2 to 20 times its value at the onset of convection ($\mathcal{R}=Ra/Ra_c=2-20$), keeping the fluid properties constant ($Pr=Pr_m=1$). The statistical behavior, force balance and heat transport characteristics of the dynamos depend on boundary conditions that dictate both boundary layer and the interior dynamics. At a fixed thermal forcing ($\mathcal{R}=3$), the Ekman plumes in the presence of viscous boundary layers lead to energetic vortices that result in higher enstrophy and kinetic helicity with no-slip boundaries compared to free-slip boundaries. The structure and strength of the magnetic field are also dictated by the boundary conditions. Though the leading order force balance remains geostrophic, Lorentz force dominates inside the thermal boundary layer with no-slip, electrically conducting walls. Here, the Lorentz work term in the turbulent kinetic energy budget is found to have components that exchange energy from the velocity field to the magnetic field, and vice-versa. However, with no-slip, insulated walls, all Lorentz work components perform unidirectional energy transfer to produce magnetic energy from the kinetic energy of the fluid. The heat transfer enhancement in dynamos, compared to non-magnetic rotating convection, exhibits a peak in the range $\mathcal{R}=3-5$. For free-slip conditions, dynamo action may alter the heat transport by suppressing the formation of large-scale vortices. However, the highest heat transfer enhancement occurs when the boundaries are no-slip, electrically conducting walls.

physics.flu-dyn↗

arXiv2021

Direct numerical simulations of optimal thermal convection in rotating plane layer dynamos

The heat transfer behavior of convection-driven dynamos in a rotating plane layer between two parallel plates, heated from below and cooled from the top, is investigated. At a fixed rotation rate (Ekman Number, $E=10^{-6}$) and fluid properties (thermal and magnetic Prandtl numbers, $Pr=Pr_m=1$), both dynamo convection (DC) and non-magnetic rotating convection (RC) simulations are performed to demarcate the effect of magnetic field on heat transport at different thermal forcings (Rayleigh Number, $Ra=3.83\times10^{9}-3.83\times10^{10}$). In this range, our turbulence resolving simulations demonstrate the existence of an optimum thermal forcing, at which heat transfer between the plates in DC exhibits maximum enhancement, as compared to the heat transport in the RC simulations. Unlike any global force balance reported in the literature, the present simulations reveal an increase in the Lorentz force in the \textit{thermal boundary layer}, due to stretching of magnetic field line by the vortices near the walls with no-slip boundary condition. This increase in Lorentz force mitigates turbulence suppression owing to the Coriolis force, resulting in enhanced turbulence and heat transfer

physics.flu-dyn↗

arXiv2012

Application of Integral Value Transformation (IVT) in a Specialized Computer Network Design

Integral Value Transformation (IVT) is a family of transformations from N0kto N0. An algebraic result has been established in p-adic IVT systems and an application of the result is described in this paper. The result in this paper provides the rule to find the pth pre image of a natural number for the Collatz like bijective functions in p-adic IVT systems. Using this result a routing algorithm is proposed. This proposed routing algorithm reduces number of address calculation.

cs.DM↗