arXiv subjects

Naveen Jingade

Publications and source records attributed to Naveen Jingade.

6 recordsLinked to original sources

arXiv2024

Correlation times of velocity and kinetic helicity fluctuations in nonhelical hydrodynamic turbulence

Nonhelical turbulence within a linear shear flow has demonstrated efficient amplification of large-scale magnetic fields in numerical simulations, but its precise mechanism remains elusive. The incoherent $α$ mechanism proposes that a zero-mean fluctuating transport coefficient $α$ (linked to kinetic helicity) in the shear flow is a candidate driver. Previous renovating-flow models have proposed that the correlation time of helicity fluctuations must be sufficiently extended to overcome turbulent magnetic diffusivity, yet only empirical validation of this concept has been obtained. In this study, we conduct direct numerical simulations of weakly compressible nonhelical hydrodynamic turbulence. We scrutinize the correlation times of velocity and kinetic helicity fluctuations in distinct flow configurations, including rotation, shearing, and Keplerian flows, as well as the shearing burgulence counterpart. Our findings indicate that rotation contributes to a prolonged correlation time of helicity compared to velocity, particularly notable in auto-correlations of both volume-averaged quantities and individual Fourier modes due to the formation of large-scale vortices. In contrast, moderate shear strength does not exhibit significant scale separation, with shear flows elongating vortices in the shear direction. Shearing burgulence, characterized by shorter helicity correlation times, appears less conducive to hosting the incoherent $α$ effect. Notably, at modest shear rates, only Keplerian flows exhibit sufficiently coherent helicity fluctuations, in contrast to shearing flows. However, the relative strength of helicity fluctuations compared to turbulent diffusivity is significantly lower, raising doubts about the viability of the incoherent $α$ effect as a potential dynamo driver in the subsonic flows examined in this study.

physics.plasm-ph↗

arXiv2021

Mean field dynamo action in shearing flows. II: fluctuating kinetic helicity with zero mean

Here we explore the role of temporal fluctuations in kinetic helicity on the generation of large-scale magnetic fields in presence of a background linear shear flow. Key techniques involved here are same as in our earlier work \citep[][hereafter paper~I]{JS20}, where we have used the renovating flow based model with shearing waves. Both, the velocity and the helicity fields, are treated as stochastic variables with finite correlation times, $τ$ and $τ_h$, respectively. Growing solutions are obtained when $τ_h > τ$, even when this time-scale separation, characterised by $m=τ_h/τ$, remains below the threshold for causing the turbulent diffusion to turn negative. In regimes when turbulent diffusion remains positive, and $τ$ is on the order of eddy turnover time $T$, the axisymmetric modes display non-monotonic behaviour with shear rate $S$: both, the growth rate $γ$ and the wavenumber $k_\ast$ corresponding to the fastest growing mode, first increase, reach a maximum and then decrease with $|S|$, with $k_\ast$ being always smaller than eddy-wavenumber, thus boosting growth of magnetic fields at large length scales. The cycle period $P_{\rm cyc}$ of growing dynamo wave is inversely proportional to $|S|$ at small shear, exactly as in the fixed kinetic helicity case of paper~I. This dependence becomes shallower at larger shear. Interestingly enough, various curves corresponding to different choices of $m$ collapse on top of each other in a plot of $m P_{\rm cyc}$ with $|S|$.

physics.flu-dyn↗

arXiv2020

Mean field dynamo action in shear flows. I: fixed kinetic helicity

We study mean-field dynamo action in a background linear shear flow by employing pulsed renewing flows with fixed kinetic helicity and nonzero correlation time ($τ$). We use plane shearing waves in terms of time-dependent exact solutions to the Navier-Stokes equation as derived by Singh \& Sridhar (2017). This allows us to self-consistently include the anisotropic effects of shear on the stochastic flow. We determine the average response tensor governing the evolution of mean magnetic field, and study the properties of its eigenvalues which yield the growth rate ($γ$) and the cycle period ($P_{\rm cyc}$) of the mean magnetic field. Non-axisymmetric modes of the mean magnetic field decay as $t \to \infty$ and hence are deemed unimportant for mean-field dynamo. Both, $γ$ and the wavenumber corresponding to the fastest growing axisymmetric mode vary non-monotonically with shear rate $S$ when $τ$ is comparable to the eddy turnover time $T$, in which case, we also find quenching of dynamo when shear becomes too strong. When $τ/T\sim{\cal O}(1)$, the cycle period ($P_{\rm cyc}$) of growing dynamo wave scales with shear as $P_{\rm cyc} \propto |S|^{-1}$ at small shear, and it becomes nearly independent of shear as shear becomes too strong.This asymptotic behaviour at weak and strong shear has implications for magnetic activity cycles of stars in recent observations. Our study thus essentially generalizes the standard $αΩ$ (or $α^2Ω$) dynamo as also the $α$ effect is affected by shear and the modelled random flow has a finite memory.

astro-ph.SR↗

arXiv2018

Generation of large-scale magnetic fields due to fluctuating $α$ in shearing systems

We explore the growth of large-scale magnetic fields in a shear flow, due to helicity fluctuations with a finite correlation time, through a study of the Kraichnan-Moffatt model of zero-mean stochastic fluctuations of the $α$ parameter of dynamo theory. We derive a linear integro-differential equation for the evolution of large-scale magnetic field, using the first-order smoothing approximation and the Galilean invariance of the $α$-statistics. This enables construction of a model that is non-perturbative in the shearing rate $S$ and the $α$-correlation time $τ_α$. After a brief review of the salient features of the exactly solvable white-noise limit, we consider the case of small but non-zero $τ_α$. When the large-scale magnetic field varies slowly, the evolution is governed by a partial differential equation. We present modal solutions and conditions for the exponential growth rate of the large-scale magnetic field, whose drivers are the Kraichnan diffusivity, Moffatt drift, Shear and a non-zero correlation time. Of particular interest is dynamo action when the $α$-fluctuations are weak; i.e. when the Kraichnan diffusivity is positive. We show that in the absence of Moffatt drift, shear does not give rise to growing solutions. But shear and Moffatt drift acting together can drive large scale dynamo action with growth rate $γ\propto |S|$.

astro-ph.GA↗

arXiv2015

Adiabatic black hole growth in Sérsic models of elliptical galaxies

We have examined the effect of slow growth of a central black hole on spherical galaxies that obey Sérsic or $R^{1/m}$ surface-brightness profiles. During such growth the actions of each stellar orbit are conserved, which allows us to compute the final distribution function if we assume that the initial distribution function is isotropic. We find that black-hole growth leads to a central cusp or ``excess light', in which the surface brightness varies with radius as $R^{-1.3}$ (with a weak dependence on Sérsic index $m$), the line-of-sight velocity dispersion varies as $R^{-1/2}$, and the velocity anisotropy is $β\simeq -0.24$ to $-0.28$ depending on $m$. The excess stellar mass in the cusp scales approximately linearly with the black-hole mass, and is typically 0.5--0.85 times the black-hole mass. This process may strongly influence the structure of nuclear star clusters if they contain black holes.

astro-ph.GA↗

arXiv2015

Numerical studies of dynamo action in a turbulent shear flow - I

We perform numerical experiments to study the shear dynamo problem where we look for the growth of large--scale magnetic field due to non--helical stirring at small scales in a background linear shear flow, in previously unexplored parameter regimes. We demonstrate the large--scale dynamo action in the limit when the fluid Reynolds number (${\rm Re}$) is below unity whereas the magnetic Reynolds number (${\rm Rm}$) is above unity; the exponential growth rate scales linearly with shear, which is consistent with earlier numerical works. The limit of low ${\rm Re}$ is particularly interesting, as seeing the dynamo action in this limit would provide enough motivation for further theoretical investigations, which may focus the attention to this analytically more tractable limit of ${\rm Re} < 1$ as compared to more formidable limit of ${\rm Re} > 1$. We also perform simulations in the regimes when, (i) both (${\rm Re}$, ${\rm Rm}$) $< 1$; (ii) ${\rm Re} > 1$ & ${\rm Rm} < 1$, and compute all components of the turbulent transport coefficients ($α_{ij}$ and $η_{ij}$) using the test--field method. A reasonably good agreement is seen between our results and the results of earlier analytical works (Sridhar & Singh 2010; Singh & Sridhar 2011) in the similar parameter regimes.

astro-ph.GA↗