arXiv subjects

A. Mishra

Publications and source records attributed to A. Mishra.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Dynamo Driven by Inertial Waves

We demonstrate, by studying precession-driven flows, that inertial wave hydrodynamic turbulence can drive a robust magnetic dynamo action. Motivated by the stronger damping of large-scale geostrophic vortices in rapidly rotating planetary and stellar interiors, we introduce a controlled damping of the vortices, which usually accompany inertial wave turbulence and feed on wave energy. It is shown that even a small vortex damping results in a significant increase of the growth rate of the dynamo due to inertial waves in the kinematic regime, allowing it to persist for magnetic Prandtl numbers as low as $Pm \sim 10^{-3}$ and Poincar\'e numbers $Po\sim 0.025$. These critical values of $Po$ and $Pm$ for the dynamo onset decrease with increasing Reynolds number. The onset and growth of the dynamo appear to correlate with the coherent fluctuations of kinetic helicity. Spectral analysis shows that magnetic energy growth is primarily due to inertial-wave-induced induction over a broad range of scales. These results establish inertial waves as an efficient mechanism for magnetic field amplification in rapidly rotating low-$Pm$ flows relevant to planetary and stellar interiors.

physics.flu-dyn↗

arXiv2025

Isomer-selective dissociation dynamics of 1,2-dibromoethene ionized by femtosecond-laser radiation

We study the isomer-specific photoionization and photofragmentation of 1,2- dibromoethene (DBE) under strong-field fs-laser irradiation in the gas phase complementing previous studies utilising ns- and ps-laser excitation. Our findings are compatible with a dissociative multiphoton-ionization mechanism producing a variety of ionic photofragments. Using both Stark deflection and chemical separation of the two isomers, pronounced isomer-specific photofragmentation dynamics could be observed for different product channels. While for Br+ formation, the isomer specificity appears to originate from different photoexcitation efficiencies, for the C2H2Br+ channel it is more likely caused by differences in the coupling to the exit channel. By contrast, the formation of the C2H2+ photofragment does not seem to exhibit a pronounced isomeric dependence under the present conditions. The present work underlines the importance of isomeric effects in photochemistry even in small polyatomics like the present system as well as their pronounced dependence on the photoexcitation conditions.

physics.chem-ph↗

arXiv2025

The effect of split endcaps on the flow dynamics in a tall Taylor-Couette setup

The effects of axial boundaries, or endcaps are of fundamental interest in many Taylor-Couette (TC) flow experiments. A main challenge in those experiments has been to minimize these effects, which can substantially alter the flow structure compared to the axially unbounded idealized case. Therefore, understanding and disentangling the influence of endcaps on the TC flow dynamics is essential for the unambiguous interpretation of experimental results, particularly when other dynamical processes (instabilities) in TC flows are involved. In this paper, we study the hydrodynamic evolution of a quasi-Keplerian TC flow in the presence of split endcaps for high Reynolds numbers, $Re$, up to $2\times 10^5$, which are larger than those considered in related previous studies. At these $Re$, the flow deviates from the ideal TC flow profile without endcaps, resulting in about $15\%$ deviation in angular velocity at the mid-height of the cylinders. Aside from turbulent fluctuations caused by shearing instability near the endcaps, the bulk flow remains nearly axially independent and exhibits overall Rayleigh-stability. We characterize the scalings of the Ekman and Stewartson layer sizes with $Re$ as well as examine the effect of the ratio of the outer to inner cylinders' angular velocities on the flow. The implications of these findings for ongoing magnetorotational instability (MRI) experiments based on the similar axially bounded TC setup are also discussed. Specifically, it is shown that when imposing a constant axial magnetic field in all the considered configurations, the flow profile modified by the endcaps lowers the critical threshold for the onset of MRI that in turn can facilitate its emergence and detection in those experiments.

physics.flu-dyn↗

arXiv2025

Strong and weak dynamo regimes in Taylor-Couette flows

We reveal a nonlinear magnetic dynamo in a Taylor-Couette flow at small magnetic Prandtl numbers $Pm\leq 1$, which has been previously believed to exist only at higher $Pm\gtrsim 10$ in this flow. The amplitude of initial perturbations, $Pm$ and domain aspect ratio play a key role in the onset and evolution of the dynamo. It exists in two main states -- a weak state dominated by large-scale modes and a strong, turbulent state with higher amplitude dominated by small-scale modes. These findings can be important for dynamo processes in various astrophysical objects with small $Pm$.

physics.flu-dyn↗

arXiv2024

Adaptive Beam Broadening for DoA Estimation in Dynamic and Resource-Constrained DFRC Systems

Dual-function radar communication (DFRC) systems incorporate both radar and communication functions by sharing spectrum, hardware and radio frequency (RF) chains. In this work, we consider a conceptual DFRC scheduler model which shares RF chains between radar and communication functions. If such a scheduler is tuned for prioritizing communication performance, the RF chains and time allocated to radar are less and varying. We propose a practical, low-latency and resource-aware technique for sensing the entire field-of-view (FOV) and Direction-of-Arrival (DoA) estimation in such settings by leveraging time-sliced beam allocation along with adaptive windowing. This results in a balanced cumulative array factor over the FOV thereby ensuring better DoA estimation reliability. Extensive simulation studies show that the technique has consistent target detection and angle estimation performance in all directions and adapts to varying resource availability with time.

eess.SP↗

arXiv2024

Geometry-influenced cooling performance of lithium-ion battery

Battery geometry (shape and size) is one of the important parameters which governs the battery capacity and thermal behavior. In the dynamic conditions or during the operation, the performance of batteries become much more complex. Herein, the changes in thermal behavior of lithium-ion battery (LIB)by altering the geometry i.e., length to diameter ratio (l/d), is investigated. The geometries considered are named as large geometry (LG), datum geometry (DG) and small geometry (SG) with the l/d ratio of 5.25, 3.61, and 2.38, respectively. A three-dimensional (3D) multi-partition thermal model is adopted, and the numerical results are validated by the published experimental data. For three different cooling approaches such as radial, both-tab and mixed cooling, the average battery temperature and temperature heterogeneity are thoroughly examined considering the heat transfer coefficients (h) of50 and 100 W/m2K at discharge rates of 1, 2 and 3C. Amongst, the minimum average battery temperature is exhibited by DG, the minimum radial temperature heterogeneity is obtained from LG, and substantial outperformance in terms of faster cooling rate is identified for SG, irrespective of the cooling approach employed

physics.app-ph↗

arXiv2024

One-winged butterflies: mode selection for azimuthal magnetorotational instability by thermal convection

The effects of thermal convection on turbulence in accretion discs, and particularly its interplay with the magnetorotational instability (MRI), are of significant astrophysical interest. Despite extensive theoretical and numerical studies, such an interplay has not been explored experimentally. We conduct linear analysis of the azimuthal version of MRI (AMRI) in the presence of thermal convection and compare the results with our experimental data published before. We show that the critical Hartmann number ($Ha$) for the onset of AMRI is reduced by convection. Importantly, convection breaks symmetry between $m = \pm 1$ instability modes ($m$ is the azimuthal wavenumber). This preference for one mode over the other makes the AMRI-wave appear as a ``one-winged butterfly''.

astro-ph.SR↗

arXiv2024

Non-axisymmetric modes of magnetorotational and possible hydrodynamical instabilities in the upcoming DRESDYN-MRI experiments -- linear and nonlinear dynamics

The quest for an unambiguous detection of magnetorotational instability (MRI) in experiments is still ongoing despite recent promising results. To conclusively identify MRI in the laboratory, a large cylindrical Taylor-Couette experiment with liquid sodium is under construction within the DRESDYN project. In this paper, we investigate the linear and nonlinear dynamics of non-axisymmetric MRI in the magnetized Taylor-Couette flow of liquid sodium, which is a model flow in this experiment. We show that the achievable highest Lundquist $Lu = 10$ and magnetic Reynolds $Rm = 40$ numbers in this experiment are large enough for the linear instability of non-axisymmetric modes with azimuthal wavenumber $|m|=1$, although the corresponding critical values of these numbers are usually higher than those for the axisymmetric mode. The structure of the ensuing nonlinear saturated state and its scaling properties with respect to Reynolds number $Re$ are analyzed, which are important for the DRESDYN-MRI experiment having very high $Re \gtrsim 10^6$. It is shown that for $Re \lesssim 4\times 10^4$, the non-axisymmetric MRI modes eventually decay, since the modified shear profile of the mean azimuthal velocity due to the nonlinear axisymmetric MRI appears to be stable against non-axisymmetric instabilities. By contrast, for larger $Re \gtrsim 4\times 10^4$, a rapid growth and saturation of the non-axisymmetric modes of nonmagnetic origin occurs, which are radially localized near the inner cylinder wall, forming a turbulent boundary layer. However, for all the parameters considered, the saturation amplitude of these non-axisymmetric modes is always a few orders smaller than that of the axisymmetric MRI mode. Therefore, the results of our previous axisymmetric study on the scaling properties of nonlinear MRI states also hold when non-axisymmetric modes are included.

physics.flu-dyn↗

arXiv2024

Collisional alignment and molecular rotation control chemi-ionization of individual conformers

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Using a novel crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces crucially influences reaction pathways for distinct conformers which is, however, countered by molecular rotation. We demonstrate how the interplay between molecular geometry, chemical steering forces and rotational dynamics govern the outcome of reactions and illustrate the capability of advanced molecule-control techniques to unravel these effects.

physics.chem-ph↗

arXiv2023

Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: scaling properties and relation to upcoming DRESDYN-MRI experiment

Magnetorotational instability (MRI) is the most likely mechanism driving angular momentum transport in astrophysical disks. However, despite many efforts, a conclusive experimental evidence of MRI is still missing. Recently, performing 1D linear analysis of the standard MRI (SMRI) in a cylindrical Taylor-Couette (TC) flow with an axial magnetic field, we showed that SMRI can be detected in the upcoming DRESDYN-MRI experiment based on a magnetized TC flow of liquid sodium. In this study, also related to DRESDYN-MRI experiments, we focused on the nonlinear evolution and saturation properties of SMRI and analyzed its scaling behavior with respect to the main parameters of the TC flow. We did a detailed analysis over the extensive ranges of magnetic Reynolds number $Rm\in [8.5, 37.1]$, Lundquist number $Lu\in[1.5, 15.5]$ and Reynolds number, $Re\in[10^3, 10^5]$. We considered small magnetic Prandtl numbers, $Pm \ll 1$, down to $Pm\sim 10^{-4}$, aiming at values typical of liquid sodium in the experiments. In the saturated state, the magnetic energy of SMRI and torque due to perturbations on the cylinders, which characterizes angular momentum transport, both increase with $Rm$ for fixed $(Lu, Re)$, while for fixed $(Lu, Rm)$, the magnetic energy decreases and torque increases with increasing $Re$. We studied the scaling of the magnetic energy and torque in the saturated state as a function of $Re$ and find a power law dependence $Re^{-0.6...-0.5}$ for the magnetic energy and $Re^{0.4...0.5}$ for the torque at all $(Lu, Rm)$ and high $Re\geq 4000$. We also explored the dependence on Lundquist number and angular velocity of the cylinders. These scaling laws will be instrumental in the subsequent analysis of more realistic finite-length TC flows and comparison of numerical results with those obtained from the DRESDYN-MRI experiments to unambiguously identify SMRI in laboratory.

physics.flu-dyn↗

arXiv2022

Low-Energy Electron-Track Imaging for a Liquid Argon Time-Projection-Chamber Telescope Concept using Probabilistic Deep Learning

The GammaTPC is an MeV-scale single-phase liquid argon time-projection-chamber gamma-ray telescope concept with a novel dual-scale pixel-based charge-readout system. It promises to enable a significant improvement in sensitivity to MeV-scale gamma-rays over previous telescopes. The novel pixel-based charge readout allows for imaging of the tracks of electrons scattered by Compton interactions of incident gamma-rays. The two primary contributors to the accuracy of a Compton telescope in reconstructing an incident gamma-ray's original direction are its energy and position resolution. In this work, we focus on using deep learning to optimize the reconstruction of the initial position and direction of electrons scattered in Compton interactions, including using probabilistic models to estimate predictive uncertainty. We show that the deep learning models are able to predict locations of Compton scatters of MeV-scale gamma-rays from simulated pixel-based data to better than 0.6 mm RMS error, and are sensitive to the initial direction of the scattered electron. We compare and contrast different deep learning uncertainty estimation algorithms for reconstruction applications. Additionally, we show that event-by-event estimates of the uncertainty of the locations of the Compton scatters can be used to select those events that were reconstructed most accurately, leading to improvement in locating the origin of gamma-ray sources on the sky.

astro-ph.IM↗

arXiv2022

From helical to standard magnetorotational instability: predictions for upcoming liquid sodium experiments

We conduct a linear analysis of axisymmetric magnetorotational instability (MRI) in a magnetized cylindrical Taylor-Couette (TC) flow for its standard version (SMRI) with a purely axial background magnetic field and two further types -- helically modified SMRI (H-SMRI) and helical MRI (HMRI) -- in the presence of combined axial and azimuthal magnetic fields. This study is intended as preparatory for upcoming large-scale liquid sodium MRI experiments planned within the DRESDYN project at Helmholtz-Zentrum Dresden-Rossendorf, so we explore these instability types for typical values of the main parameters: the magnetic Reynolds number, the Lundquist number and the ratio of the angular velocities of the cylinders, which are attainable in these experiments. In contrast to previous attempts at detecting MRI in the lab, our results demonstrate that SMRI and its helically modified version can in principle be detected in the DRESDYN-TC device for the range of the above parameters, including the astrophysically most important Keplerian rotation, despite the extremely small magnetic Prandtl number of liquid sodium. Since in the experiments we plan to approach (H-)SMRI from the previously studied HMRI regime, we characterise the continuous and monotonous transition between the both regimes. We show that H-SMRI, like HMRI, represents an overstability (travelling wave) with non-zero frequency linearly increasing with azimuthal field. Because of its relevance to finite size flow systems in experiments, we also analyse the absolute form of H-SMRI and compare its growth rate and onset criterion with the convective one.

physics.flu-dyn↗

arXiv2021

Imaging the interface of a qubit and its quantum-many-body environment

Decoherence affects all quantum systems, natural or artificial, and is the primary obstacle impeding quantum technologies. We show theoretically that for a Rydberg qubit in a Bose condensed environment, experiments can image the system-environment interface that is central for decoherence. High precision absorption images of the condensed environment will be able to capture transient signals that show the real time build up of a mesoscopic entangled state in the environment. This is possible before decoherence sources other than the condensate itself can kick in, since qubit decoherence time-scales can be tuned from the order of nanoseconds to microseconds by choice of the excited Rydberg principal quantum number ν. Imaging the interface will allow detailed explorations of open quantum system concepts and may offer guidance for coherence protection in challenging scenarios with non-Markovian environments.

quant-ph↗

arXiv2021

Convective, absolute and global azimuthal magnetorotational instabilities

We study the convective and absolute forms of azimuthal magnetorotational instability (AMRI) in a Taylor-Couette (TC) flow with an imposed azimuthal magnetic field. We show that the domain of the convective AMRI is wider than that of the absolute AMRI. Actually, it is the absolute instability which is the most relevant and important for magnetic TC flow experiments. The absolute AMRI, unlike the convective one, stays in the device, displaying a sustained growth that can be experimentally detected. We also study the global AMRI in a TC flow of finite height using DNS and find that its emerging butterfly-type structure -- a spatio-temporal variation in the form of upward and downward traveling waves -- is in a very good agreement with the linear stability analysis, which indicates the presence of two dominant absolute AMRI modes in the flow giving rise to this global butterfly pattern.

physics.flu-dyn↗

arXiv2021

Effect of Passive flow Control of Bifurcation Phenomenon in Sudden Expansion channel

In this article we describe, a numerical simulation has been performed for a channel flow with sudden expansion. Different geometric expansion ratios are considered in the current study such as 1:2, 1:3, and 1:4. The bifurcation phenomenon has been investigated for the baseline case and the critical Reynolds number is controlled using a passive modification in the channel. The geometry of sudden expansion channel is modified by introducing a two-dimensional plate along the center line of the channel at the expansion plane. This is one of the passive control techniques, which has been explored in the present study. The computations indicate that the critical Reynolds number of the symmetry-breaking bifurcation increases as the length of the plate increases. An empirical relation has been developed amongst the critical Reynolds number, length of control plates and expansion ratio (h/H). The critical Reynolds number predicted by the empirical relation for different plate lengths are in good agreement with computational results.

physics.flu-dyn↗

arXiv2019

Examining the Presence of Gender Bias in Customer Reviews Using Word Embedding

Humans have entered the age of algorithms. Each minute, algorithms shape countless preferences from suggesting a product to a potential life partner. In the marketplace algorithms are trained to learn consumer preferences from customer reviews because user-generated reviews are considered the voice of customers and a valuable source of information to firms. Insights mined from reviews play an indispensable role in several business activities ranging from product recommendation, targeted advertising, promotions, segmentation etc. In this research, we question whether reviews might hold stereotypic gender bias that algorithms learn and propagate Utilizing data from millions of observations and a word embedding approach, GloVe, we show that algorithms designed to learn from human language output also learn gender bias. We also examine why such biases occur: whether the bias is caused because of a negative bias against females or a positive bias for males. We examine the impact of gender bias in reviews on choice and conclude with policy implications for female consumers, especially when they are unaware of the bias, and the ethical implications for firms.

cs.CL↗

arXiv2017

Multi-wavelength temporal and spectral variability of the blazar OJ 287 during and after the December 2015 flare: a major accretion disc contribution

We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $γ$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR--UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible--UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $\sim \rm 1.8 \times10^{10} M_{\odot}$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different $γ$-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak $γ$-ray emission that shows a shift in the SED peak is consistent with $γ$-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump.

astro-ph.HE↗

arXiv2016

Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016

We present recent optical photometric observations of the blazar OJ 287 taken during September 2015 -- May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 ground based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ~ 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour and spectral variability.

astro-ph.HE↗