arXiv subjects

S. Sarkar

Publications and source records attributed to S. Sarkar.

At least 19 recordsLinked to original sources

arXiv2026

Secondary Flows and Near-Wall Turbulence in Channel Flow with Longitudinal Ribs

This study employs the Large Eddy Simulation (LES) to investigate secondary flows and near-wall turbulence induced by two types of surface-mounted longitudinal ribs, namely rectangular and triangular, in a channel flow. The friction Reynolds number, based on friction velocity and channel height H, is set at 220. The rib aspect ratio W/h, where W and h represent the width and height of the rib, is 2, and the rib spacing, S is 0.6H. The results indicate formation of two counter-rotating vortices between the adjacent ribs for both the cases considered. The roughness function is higher with the rectangular rib as compared to that of the triangular rib. At the location of the mid-plane on the rectangular ribs, the wall shear stress is relatively lower as compared to that of the location of mid-plane between the ribs. Conversely, for the case of triangular rib, the opposite pattern is observed. Normal Reynolds stress exhibits strong anisotropic behaviour near the wall for both the cases, overlapping above 0.4H. Between 0.4H and 0.8H, the variation in normal Reynolds stresses is linear. Near the wall, higher production of turbulent kinetic energy (TKE) and normal Reynolds stresses are observed with the triangular rib as compared to those of the rectangular rib. The ratio of production to dissipation is unity in the log-law region.

physics.flu-dyn↗

arXiv2025

Measurement of ion acceleration and diffusion in a laser-driven magnetized plasma

Here we present results from an experiment performed at the GSI Helmholtz Centre for Heavy Ion Research. A mono-energetic beam of chromium ions with initial energies of $\sim 450$ MeV was fired through a magnetized interaction region formed by the collision of two counter-propagating laser-ablated plasma jets. While laser interferometry revealed the absence of strong fluid-scale turbulence, acceleration and diffusion of the beam ions was driven by wave-particle interactions. A possible mechanism is particle acceleration by electrostatic, short scale length kinetic turbulence, such as the lower-hybrid drift instability.

physics.plasm-ph↗

arXiv2025

Transitional and Near-Wall Turbulence Dynamics over Rib-Roughened Surfaces

This study utilizes Large Eddy Simulation (LES) to investigate the impact of longitudinal triangular riblets on the laminar-to-turbulent transition in boundary layer flow. Five cases are examined: one involving a flat plate and four with ribbed plates. Among the ribbed cases, three use a riblet aspect ratio of two, whereas one has an aspect ratio of one. Arrays of longitudinal triangular riblets are positioned on a flat plate, and the transition to turbulence is initiated by controlled excitation of a Tollmien-Schlichting (TS) wave imposed on a Blasius velocity profile in a stable region. The longitudinal triangular riblets attenuate the TS wave, leading to a lower growth rate of turbulence. For higher riblet height ($h$) and width ($w$), with inner-scaled dimensions $h^+ = 25$, $w^+ = S^+ = 50$ (where $S$ is the spacing between two riblets), an early transition is triggered by high-frequency disturbances generated at the leading edge of the roughness elements. However, increasing riblet spacing to $S^+ = 75$ delays the transition by 17.5 percent. Both cases exhibited increased drag compared to the flat plate. For $h^+ = 12.5$ and $w^+ = S^+ = 25$, transition was delayed by 37 percent, with a modest overall drag reduction of 8.8 percent. The most significant result from the considered cases, $h^+ = w^+ = S^+ = 12.5$, showed a 47 percent delay in transition and a 13.69 percent reduction in overall drag. Smaller riblets cause minimal disturbance at the leading edge of roughness, resulting in a transition mechanism similar to a flat plate, while also reducing pressure loss, secondary flows, and velocity fluctuations.

physics.flu-dyn↗

arXiv2025

QSHS: An Axion Dark Matter Resonant Search Apparatus

We describe a resonant cavity search apparatus for axion dark matter constructed by the Quantum Sensors for the Hidden Sector (QSHS) collaboration. The apparatus is configured to search for QCD axion dark matter, though also has the capability to detect axion-like particles (ALPs), dark photons, and some other forms of wave-like dark matter. Initially, a tuneable cylindrical oxygen-free copper cavity is read out using a low noise microwave amplifier feeding a heterodyne receiver. The cavity is housed in a dilution refrigerator and threaded by a solenoidal magnetic field, nominally 8T. The apparatus also houses a magnetic field shield for housing superconducting electronics, and several other fixed-frequency resonators for use in testing and commissioning various prototype quantum electronic devices sensitive at a range of axion masses in the range 2.0 to 40 micro-eV/c2. The apparatus as currently configured is intended as a test stand for electronics over the relatively wide frequency band attainable with the TM010 cavity mode used for axion searches. We present performance data for the resonator, dilution refrigerator, and magnet, and plans for the first science run.

physics.ins-det↗

arXiv2024

From Urban Clusters to Megaregions: Mapping Australia's Evolving Urban Regions

This study employs percolation theory to investigate the hierarchical organisation of Australian urban centres through the connectivity of their road networks. The analysis demonstrates how discrete urban clusters have developed into integrated regional entities, delineating the pivotal distance thresholds that regulate these urban transitions. The study reveals the interconnections between disparate urban clusters, shaped by their functional differentiation and historical development. Furthermore, the study identifies a dichotomy of urban agglomeration forces and a persistent spatial disconnection between Australia's wider urban landscape. This highlights the interplay between urban densification and peripheral growth. It suggests the need for new thinking on potential integrated governance structures that bridge urban development with broader social and economic policies across regional and national scales. Additionally, the study emphasises the growing importance of national coordination in Australian urban development planning to ensure regional consistency, equity, and productivity.

physics.soc-ph↗

arXiv2024

The study of $0\nu\beta\beta$ decay of $^{136}$Xe using nonclosure approach in nuclear shell model

In this investigation, we compute the nuclear matrix elements (NMEs) relevant to the light neutrino-exchange mechanism governing neutrinoless double beta ($0\nu\beta\beta$) decay in $^{136}$Xe. Our method is based on the nonclosure approach within the interacting nuclear shell model framework. This approach considers the genuine effects arising from the excitation energies of two hundred states for each spin-parity of the intermediary nucleus $^{136}$Cs. All computations are performed using the effective shell model Hamiltonian GCN5082. To understand the impact of nuclear structure on $0\nu\beta\beta$ decay, we explore the dependence of the NME on various factors, including the number of intermediate states and their spin-parity characteristics. We identify an optimal closure energy of approximately 3.7 MeV for the $0\nu\beta\beta$ decay of $^{136}$Xe that reproduces the nonclosure NME using the closure approach. The calculated total NME for the light neutrino-exchange $0\nu\beta\beta$ decay of $^{136}$Xe is 2.06 with the CD-Bonn short-range correlation (SRC). These results can be valuable for future experimental investigations into the $0\nu\beta\beta$ decay of $^{136}$Xe.

nucl-th↗

arXiv2024

Pulsed loading of a magneto-optical trap on atom chip for fast pressure recovery in ultrahigh vacuum environment

This study presents investigations on pulsed loading of a magneto-optical trap (MOT) on an atom chip in an UHV environment. Using three parallel resistively heated Rb-metal dispensers activated by pulsed current supply, approximately 3.0 $\times$ $10^{7}$ cold $^{87}Rb$ atoms were loaded into the MOT. A current pulse of $\sim$ 24 A with duration of $\sim$ 10 s raised the pressure in the chamber from 2.0 $\times$ $10^{-10}$ Torr to 3.3 $\times$ $10^{-10}$ Torr. Remarkably, the pressure recovery time after switching off the dispensers current was found to be $\sim$ 600 ms, making a significant advancement in achieving fast recovery of UHV environment surrounding the MOT region. This study is very useful for laser cooling and magnetic trapping / evaporative cooling of atoms on atom chip in the same UHV chamber.

physics.atom-ph↗

arXiv2024

MoEDAL search in the CMS beam pipe for magnetic monopoles produced via the Schwinger effect

We report on a search for magnetic monopoles (MMs) produced in ultraperipheral Pb--Pb collisions during Run-1 of the LHC. The beam pipe surrounding the interaction region of the CMS experiment was exposed to 184.07 \textmu b$^{-1}$ of Pb--Pb collisions at 2.76 TeV center-of-mass energy per collision in December 2011, before being removed in 2013. It was scanned by the MoEDAL experiment using a SQUID magnetometer to search for trapped MMs. No MM signal was observed. The two distinctive features of this search are the use of a trapping volume very close to the collision point and ultra-high magnetic fields generated during the heavy-ion run that could produce MMs via the Schwinger effect. These two advantages allowed setting the first reliable, world-leading mass limits on MMs with high magnetic charge. In particular, the established limits are the strongest available in the range between 2 and 45 Dirac units, excluding MMs with masses of up to 80 GeV at 95\% confidence level.

nucl-ex↗

arXiv2024

Spin-Mediated Direct Photon Scattering by Plasmons in BiTeI

We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmon in Raman scattering.

cond-mat.str-el↗

arXiv2023

Laboratory realization of relativistic pair-plasma beams

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black holes and neutron star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with such pair plasmas. Their behaviour is quite different from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components and their role in the dynamics of such compact objects is believed to be fundamental. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies which are rather limited. We present first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN's Super Proton Synchrotron (SPS) accelerator. The produced pair beams have a volume that fills multiple Debye spheres and are thus able to sustain collective plasma oscillations. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations.

physics.plasm-ph↗

arXiv2023

Allowed $β^-$ decay of bare atoms with A$\sim$ 60-80 in stellar environments

We have calculated $β^-$ decay rates to the continuum and bound states of some fully ionized atoms in the stellar s-process environment having free electron density and temperature in the range $n_e = 10^{26} $ cm$^{-3} - 10^{27} $ cm$^{-3}$ and $T = 10^8$ K - $5 \times 10^{8}$ K, respectively. The presence of bare atoms in these particular situations has been confirmed by solving Saha ionization equation taking into account the ionization potential depression (IPD). At these temperatures, low lying excited energy levels of parent nuclei may have thermal equilibrium population and those excited levels may also decay via $β^-$ emission. The Nuclear Matrix Element (NME) of all the transitions of the set of 15 nuclei is calculated using nuclear shell-model. These NME are then used to calculate the comparative half-life ($ft_{1/2}$) of the transitions. Calculated terrestrial half-lives of the $β^-$ decays are in good agreement with the experimental results in most of the cases. Decay to bound and continuum states of bare atoms from ground/isomeric levels and excited nuclear levels have been calculated separately. The ratio of bound state to continuum state decay rates as a function of IPD modified $Q$-value reveals that bound state $β^-$ decay rate may compete and even dominate for $Q$-value $<$ 100 keV. The importance of the bound state $β^-$ decay in stellar situations has been shown explicitly. We have calculated total $β^-$ decay rates (bound state plus continuum state) taking into account IPD corrected neutral atom $Q$-value as a function of density and temperature. We have also presented results for the stellar $β^-$ half-lives and compared the ratio of neutral atom to bare atom half-lives for different density and temperature combinations. These results may be useful for s-process nucleosynthesis calculations.

nucl-th↗

arXiv2023

Effect of Spin-Dependent Short-Range Correlations on Nuclear Matrix Elements for Neutrinoless Double Beta Decay of $^{48}$Ca

The neutrinoless double beta decay is a pivotal weak nuclear process that holds the potential to unveil the Majorana nature of neutrinos and predict their absolute masses. In this study, we delve into examining the impact of spin-dependent short-range correlations (SRC) on the nuclear matrix elements (NMEs) for the light neutrino-exchange mechanism in neutrinoless double beta ($0νββ$) decay of $^{48}$Ca, employing an extensive interacting nuclear shell model. All computations are performed employing the effective shell model Hamiltonian GXPF1A, encompassing the entire $fp$ model space through the closure approximation. Our investigation examines the NMEs' dependencies on factors such as the number of intermediate states, coupled spin-parity attributes of neutrons and protons, neutrino momentum, inter-nucleon separation, and closure energy. This scrutiny is performed with respect to both the conventional Jastrow-type approach of SRC, employing various parameterizations, and the spin-dependent SRC paradigm. Our findings illuminate a discernible distinction in NMEs induced by spin-dependent SRC, differing by approximately 10-20\% from those computed through the conventional Jastrow-type SRC, incorporating distinct parameterizations.

nucl-th↗

arXiv2023

A Nanocar and Rotor in One Molecule

Depending on its adsorption conformation on the Au(111) surface, a zwitterionic single-molecule machine works in two different ways under bias voltage pulses. It is a unidirectional rotor while anchored on the surface. It is a fast-drivable molecule-vehicle (nanocar) while physisorbed. By tuning the surface coverage, the conformation of the molecule can be selected to be either rotor or nanocar. The inelastic tunneling excitation producing the movement is investigated in the same experimental conditions for both the unidirectional rotation of the rotor and the directed movement of the nanocar.

physics.chem-ph↗

arXiv2023

The high-Reynolds-number stratified wake of a slender body and its comparison with a bluff-body wake

The high-Reynolds number stratified wake of a slender body is studied using a high-resolution hybrid simulation. The wake generator is a 6:1 prolate spheroid with a tripped boundary layer, the diameter-based body Reynolds number is $Re= U_\infty D/ν= 10^5$, and the body Froude numbers are $Fr=U_\infty/ND=\{2,10,\infty\}$. The wake defect velocity ($U_d$) decays following three stages with different wake decay rates \citep{Spedding1997} as for a bluff body. However, the transition points among stages do not follow the expected $Nt = Nx/U_\infty$ values. Comparison with the wake of a circular disk in similar conditions \citep{Chongsiripinyo2020} quantifies the influence of the wake generator - bluff versus slender - in stratified flow. The strongly stratified $Fr=2$ wake is in a resonant state. The steady lee waves strongly modulate the mean flow and, relative to the disk, the $Fr=2$ spheroid wake shows an earlier transition from the non-equilibrium (NEQ) stage to the quasi-two-dimensional (Q2D) stage. The NEQ-Q2D transition is followed by a sharp increase in the turbulent kinetic energy and horizontal wake meanders. At $Fr=10$, the start of the NEQ stage is delayed. Transfers between kinetic energy and potential energy reservoirs (both mean and turbulence) are analyzed and the flows are compared in phase space (local Froude and Reynolds number as coordinates). Overall, the results of this study point to the difficulty of finding a universal framework for stratified wake evolution, independent of the features of the body, and provide insights into how buoyancy effects depend on the wake generator.

physics.flu-dyn↗

arXiv2022

The high-Re far wake of a slender body and the effects of density stratification

The high-Reynolds-number wake of a slender body with a turbulent boundary layer is investigated using a hybrid simulation. The wake generator is a 6:1 prolate spheroid and the Reynolds number based on the diameter $D$ is $Re = 10^5$. The transition of the unstratified wake to a state of complete self-similarity is investigated by looking for the first time into the far field of a slender body wake. Unlike bluff-body wakes, here the flow is not dominated by vortex shedding in the near wake. Instead, the recirculation region is very small, the near wake is quasi-parallel and is characterized by the presence of broadband turbulence. The simulations show that these particularities alter the decay rate of the wake significantly and challenge the notion of universality -- the idea that far from the body all turbulent wakes decay at the same rate. The influence of stratification in the wake is also studied at Froude numbers $Fr=U_\infty/ND=2$ and $10$. The results imply that the onset of buoyancy effects depends on the wake generator and that the far wake retains information of the initial conditions.

physics.flu-dyn↗

arXiv2022

Disk wakes in nonlinear stratification

Nonlinearity of density stratification modulates buoyancy effects. We report results from a body-inclusive large eddy simulation (LES) of a wake in nonlinear stratification, specifically for a circular disk at diameter-based Reynolds number ($Re$) of $5000$. Five density profiles are considered; the benchmark has linear stratification and the other four have hyperbolic tangent profiles to model a pycnocline. The disk moves inside the central core of the pycnocline in two of those four cases and, in the other two cases with {a} shifted density profile, the disk moves partially/completely outside the pycnocline. The maximum buoyancy frequency ($N_{max}$) for all the profiles is the same. The first part of the study investigates the centered cases. Nonuniform stratification results in increasing wake turbulence relative to the benchmark owing to reduced suppression of turbulence production as well as the wave trapping in the pycnocline. Steady lee waves are also quantified to understand limitations of linear theory. The second part pays attention to the effect of a relative shift between the pycnocline and the disk. The wake defect velocity decays faster in the cases with a shift. The effect of disk location on the Kelvin wake waves (a family of steady waves within the pycnocline) and its modal form is obtained and explained by solving the Taylor-Goldstein equation. The family of unsteady internal gravity waves that are generated by the wake is also studied and the effect of disk shift is quantified.

physics.flu-dyn↗

arXiv2022

Flow past an inclined spheroid in homogeneous and stratified environments

Large-eddy simulations (LES) are performed to study the flow past a 6:1 prolate spheroid placed at an angle of incidence of $α= 10^{\circ}$. The diameter-based Reynolds number ($Re = U_{\infty}D/ν$) is set to a value of $5\times10^{3}$ and four values of diameter-based Froude numbers ($Fr = U_\infty/ND$) are analyzed: $Fr = \infty, 6, 1.9,$ and $1$. Visualizations of coefficient of pressure ($C_p$) and friction ($C_f$) contours reveal asymmetry in the $Fr = \infty$ and $6$ flows while, at $Fr = 1$ and $Fr = 1.9$, the flow over body does not have any visible asymmetry. This finding is further corroborated through the analysis of force coefficients on the body. The changes in the pressure coefficients ($C_p$), friction coefficients ($C_f$), and drag coefficients ($C_d$) with the Froude number are described in detail for $α= 10^{\circ}$. We also present the analyses of forces on the body at $α= 0^{\circ}$ angle of incidence for comparison with the $α= 10^{\circ}$ cases.

physics.flu-dyn↗

arXiv2022

Search for Highly-Ionizing Particles in pp Collisions at the LHC's Run-1 Using the Prototype MoEDAL Detector

A search for highly electrically charged objects (HECOs) and magnetic monopoles is presented using 2.2 fb-1 of p - p collision data taken at a centre of mass energy (ECM) of 8 TeV by the MoEDAL detector during LHC's Run-1. The data were collected using MoEDAL's prototype Nuclear Track Detector array and the Trapping Detector array. The results are interpreted in terms of Drell-Yan pair production of stable HECO and monopole pairs with three spin hypotheses (0, 1/2 and 1). The search provides constraints on the direct production of magnetic monopoles carrying one to four Dirac magnetic charges (4gD) and with mass limits ranging from 590 GeV/c^2 to 1 TeV/c^2. Additionally, mass limits are placed on HECOs with charge in the range 10e to 180e, where e is the charge of an electron, for masses between 30 GeV/c^2 and 1 TeV/c^2.

hep-ex↗