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Anikesh Pal

Publications and source records attributed to Anikesh Pal.

At least 19 recordsLinked to original sources
arXiv2026

Ventilated Cavitation around a sphere through surface air injection at Re = 10,000

We perform DNS of ventilated cavitation over a sphere at subcritical (Re=10,000). The flow is modeled as a homogeneous mixture of water and air, with the interface tracked using the volume-of-fluid method. Unlike previous numerical investigations, surface tension is explicitly included. Air is injected through a circular strip on the sphere at three locations selected based on the bulk flow dynamics: front (FR025, (18^\circ\leq\theta\leq63^\circ)), mid (MD025, MD050, (75^\circ\leq\theta\leq104^\circ)), and back (BK025, (120^\circ\leq\theta\leq180^\circ)), with (C_q=0.2) and (0.4). The injection location strongly governs cavity inception and stability and alters flow separation in the front- and mid-injection cases. Front injection produces an unsteady bubbly cavity due to vigorous puffing, which fragments the injected air and increases drag by (\sim56%) relative to the single-phase case. In contrast, mid- and back-injection produce stable cavities with distinct leading-edge dynamics. In the mid-injection cases, puffing dominates and, despite delayed flow separation, the cavity detaches from the leading edge of the injection patch. For back injection, the cavity detaches upstream of the injection patch owing to the adverse pressure gradient induced by cavity formation. Kelvin--Helmholtz instabilities and divot formation characterize the back-injection cavity. Stable cavities exhibit strong air entrainment and close through a re-entrant jet. The mid-injection cases achieve (35%) and (25%) drag reduction at (C_q=0.2) and (0.4), respectively, while back injection yields a maximum drag reduction of (46%) relative to the single-phase case.

physics.flu-dyn↗
arXiv2026

Ultimate regime in Rayleigh-Darcy Convection

DNS of Rayleigh-Darcy convection in a 3D porous domain is performed at Ra $\in [10^3, 10^6]$ to investigate heat-transfer scaling, thermal boundary-layer dynamics, and flow-structure evolution in the unexplored ultimate regime. The Nu exhibits an approximately linear dependence on Ra throughout the investigated range. However, a distinct change in slope is observed at $Ra \approx 4\times10^5$, indicating the onset of the ultimate regime. For $Ra \leq 2.5\times10^5$, our scaling is 6.25% lower than that reported by \cite{hewitt2014high}, while for $Ra \geq 4\times10^5$ our results are within 1.24% of the extrapolated ultimate-regime prediction of \cite{pirozzoli2021towards}. Analysis of thermal structure reveals formation of near-wall protoplumes that merge into large-scale columnar megaplumes. With increasing Ra, the size of the protoplumes decreases, whereas the numbers increase, thus enhancing boundary-layer convection and heat transport. The thermal boundary-layer thickness scales as ~ Ra^{-1} and ~ Nu^{-1}, corroborating the persistence of linear heat-transfer scaling in the ultimate regime. The thermal dissipation is found to be increasingly shifting from the boundary layer to the bulk with increasing $Ra$, further indicating that the finer protoplumes efficiently transport heat from walls to bulk. The flow structures are quantified using the dominant length scale using the mean wavenumber ($\overline{k}$). It exhibits linear variation with $Ra$ for near-wall structures, with a higher slope in the ultimate regime, signifying finer protoplumes. At the mid-plane, a weaker scaling suggests that the megaplumes also become finer with increasing $Ra$ in the ultimate regime, thus leading to efficient heat transport in the bulk.

physics.flu-dyn↗
arXiv2024

Inverse cascade in zonal flows

Zonal winds on Jovian planets play an important role in governing the cloud dynamics, transport of momentum, scalars, and weather patterns. Therefore, it is crucial to understand the evolution of the zonal flows and their sustainability. Based on studies in two-dimensional (2D) $\beta$ plane setups, zonal flow is believed to be forced at the intermediate scale via baroclinic instabilities, and the inverse cascade leads to the transfer of energy to large scales. However, whether such a process exists in three-dimensional (3D) deep convection systems remains an open and challenging question. To explore a possible answer, we perform Large Eddy Simulations at the geophysically interesting regime of $Ra=$$10^{12}$, $Ek=$$10^{-6}$,$10^{-7}$ and $10^{-8}$ in horizontally rotating Rayleigh-B\'enard convection setup and discover the existence of natural forcing through buoyancy and inverse cascade. The turbulent kinetic energy budget analysis and the spectral space assessment of the results corroborate the emanation of a strong mean flow from chaos.

physics.flu-dyn↗
arXiv2024

Evolution of the rotating Rayleigh-Taylor instability under the influence of magnetic fields

The combined effects of imposed vertical mean magnetic field (B0) and rotation on heat transfer phenomenon driven by the Rayleigh-Taylor instability are investigated using DNS. In the hydrodynamic (HD) case (B0 = 0), as the rotation rate f increases from 4 to 8, the Coriolis force suppresses the growth of mixing layer height (h) and u3', leading to a reduction in heat transport. The imposed B0 forms vertically elongated thermal plumes that exhibit larger u3' and efficiently transport heat between hot and cold fluid. Therefore, we observe an enhancement in heat transfer in the initial regime of unbroken elongated plumes in f=0 MHD cases compared to the corresponding HD case. In the mixing regime, the flow is collimated along the vertical magnetic field lines due to imposed B0, resulting in a decrease in u3' and an increase in growth of h compared to f=0 HD case. This increase in h enhances heat transfer in the mixing regime of f=0 MHD over the corresponding HD case. When rotation is added along with imposed B0, the growth and breakdown of vertically elongated plumes are inhibited by the Coriolis force, reducing h and u3'. Consequently, heat transfer is also reduced in rotating MHD cases compared to corresponding f=0 MHD cases. The heat transfer in rotating MHD cases remains higher than in corresponding rotating HD cases. This also suggests that B0 mitigates the instability-suppressing effect of the Coriolis force. The t.k.e. budget reveals the conversion of t.k.e., generated by the buoyancy flux, into t.m.e..

physics.flu-dyn↗
arXiv2023

Development of Indigenous Pulse-Shape Discrimination Algorithm for Organic Scintillation detectors

The use of programmable hardware devices is imperative for digital based pulse shape discrimination (PSD) to differentiate between various types of radiation. This work reports the development of a PSD algorithm based on tail area and total area, eliminating the need for programmable hardware. The pulses were collected using BC501 detector and Pu-Be source from a digitizer in the oscilloscope mode. The algorithm performs crucial functions such as pulse normalization, shaping, identification and removal of multiple peaks and threshold determination. The algorithm provides neutron and gamma-ray counts, scatter plot, and FoM. In order to test the efficacy of our proposed algorithm, pulses were collected from a different source-detector setup comprising BC501A detector and an Am-Be source from a digitizer in the oscilloscope mode and Charge Integration (CI) mode. The results obtained from our proposed algorithm and CI method clearly indicates a good agreement in terms of number of neutrons and gamma-rays and Figure-of-Merit (FoM), thus providing cost-effective alternative method for neutron and gamma-ray discrimination, offering flexibility and accuracy without specialized hardware.

physics.ins-det↗
arXiv2023

Numerical Investigation of Water Entry of Hydrophobic Spheres

We perform numerical simulations to study the dynamics of the entry of hydrophobic spheres in a pool of water using ANSYS. To track the air-water interface during the translation of the sphere in the pool of water, we use the volume of fluid (VOF) model. The continuum surface force (CSF) method computes the surface tension force. To simulate the hydrophobic surface properties, we also include wall adhesion. We perform simulations with different diameters and impact speeds of the sphere. Our simulations capture the formation of different types of air cavities, pinch-offs of these cavities, and other finer details similar to the experiments performed at the same parameters. Finally, we compare the coefficient of drag among the different hydrophobic cases. We further perform simulations of hydrophilic spheres impacting the pool of water and compare the drag coefficient with the analogous hydrophobic cases. We conclude that the spheres with hydrophobic surfaces have a lower drag coefficient than their hydrophilic counterparts. This lower drag of the hydrophobic spheres is attributed to the formation of the air cavity by the hydrophobic surfaces while translating through the pool of water, which reduces the area of the sphere in contact with water. In contrast, no such air cavity forms in the case of hydrophilic spheres.

physics.flu-dyn↗
arXiv2023

Thickness Dependent Sensitivity of GAGG:Ce Scintillation detectors for Thermal Neutrons: GEANT4 Simulations and Experimental Measurements

In the present work, we report extensive GEANT4 simulations in order to study the dependence of sensitivity of GAGG:Ce scintillation crystal based detector on thickness of the crystal. All the simulations are made considering a thermalised Am-Be neutron source. The simulations are validated, qualitatively and quantitatively, by comparing the simulated energy spectra and sensitivity values with those obtained from experimental measurements carried out using two different thicknesses of the crystal from our own experiment (0.5mm and 3mm) and validated with three other thicknesses (0.01mm, 0.1 mm and 1 mm) from literature. In this study, we define sensitivity of GAGG:Ce as the ratio of area under 77 keV sum peak to 45 keV peak. The present studies clearly confirm that, while it requires about 0.1 mm thickness for the GAGG:Ce crystal to fully absorb thermal neutrons, it requires about 3 mm to fully absorb the thermal neutron induced events. Further, we propose an equation, that can be used to estimate the thickness of the GAGG:Ce crystal directly from the observed sensitivity of the GAGG:Ce crystal. This equation could be very useful for the neutron imaging community for medical and space applications, as well as for manufactures of cameras meant for nuclear security purposes.

physics.ins-det↗
arXiv2023

Neutron-Gamma Pulse Shape Discrimination for Organic Scintillation Detector using 2D CNN based Image Classification

This study shows an implementation of neutron-gamma pulse shape discrimination (PSD) using a two-dimensional convolutional neural network. The inputs to the network are snapshots of the unprocessed, digitized signals from a BC501A detector. By exposing a BC501A detector to a Cf-252 source, neutron and gamma signals were collected to create a training dataset. The realistic datasets were created using a data-driven approach for labeling the digitized signals, having classified snapshots of neutron and gamma pulses. Our algorithm was able to successfully differentiate neutrons and gammas with similar accuracy as the CI approach. Additionally, the independent dataset accuracy for our suggested 2D CNN-based PSD approach is 99%. In contrast to the traditional charge integration method, our suggested algorithm with data augmentation, is capable of extracting features from snapshots of the raw data based on the signal structures, making it computationally more efficient and also appropriate for other types of neutron detectors.

physics.ins-det↗
arXiv2023

Numerical investigation of viscous fingering in a three-dimensional cubical domain

We perform three-dimensional numerical simulations to understand the role of viscous fingering in sweeping a high-viscous fluid (HVF). These fingers form due to the injection of a low-viscous fluid (LVF) into a porous media containing the high-viscous fluid. We find that the sweeping of HVF depends on different parameters such as the Reynolds number ($Re$) based on the inflow rate of the LVF, the P\'eclet number ($Pe$), and the logarithmic viscosity ratio of HVF and LVF, $\mathfrak{R}$. At high values of $Re$, $Pe$, and $\mathfrak{R}$, the fingers grow non-linearly, resulting in earlier tip splitting of the fingers and breakthrough, further leading to poor sweeping of the HVF. In contrast, the fingers evolve uniformly at low values of $Re$, $Pe$, and $\mathfrak{R}$, resulting in an efficient sweeping of the HVF. We also estimate the sweep efficiency and conclude that the parameters $Re$, $Pe$ and $\mathfrak{R}$ be chosen optimally to minimize the non-linear growth of the fingers to achieve an efficient sweeping of the HVF.

physics.flu-dyn↗
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↗
arXiv2023

Direct numerical simulations of turbulent mixing driven by the Faraday instability in rotating miscible fluids

The effect of the rotation on the turbulent mixing of two miscible fluids of small contrasting density, induced by Faraday instability, is investigated using direct numerical simulations (DNS). We quantify the irreversible mixing which depicts the conversion of the available potential energy (APE) to the background potential energy (BPE) through irreversible mixing rate $\mathcal{M}$. We demonstrate that at lower forcing amplitudes, the turbulent kinetic energy ($t.k.e.$) increases with an increase in the Coriolis frequency $f$ till $\left(f/\omega\right)^2<0.25$, where $\omega$ is the forcing frequency, during the sub-harmonic instability phase. This enhancement of $t.k.e.$ is attributed to the excitement of more unstable modes. The irreversible mixing sustains for an extended period with increasing $\left(f/\omega\right)^2$ till $0.25$ owing to the prolonged sub-harmonic instability phase and eventually ceases with instability saturation. When $\left(f/\omega\right)^2 > 0.25$, the Coriolis force significantly delays the onset of the sub-harmonic instabilities. The strong rotational effects result in lower turbulence because the bulk of the APE expends to BPE, decreasing APE that converts back to $t.k.e.$ reservoir for $\left(f/\omega\right)^2 > 0.25$. Since the instability never saturates for $\left(f/\omega\right)^2 > 0.25$, conversion of APE to BPE via $\mathcal{M}$ continues, and we find prolonged irreversible mixing. At higher forcing amplitudes, the instability delaying effect of rotation is negligible, and the turbulence is less intense and short-lived. Therefore, the irreversible mixing phenomenon also ends quickly for $\left(f/\omega\right)^2<0.25$. However, when $\left(f/\omega\right)^2>0.25$, a continuous irreversible mixing is observed.

physics.flu-dyn↗
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-Bénard 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

Effect of rotation on turbulent mixing driven by the Faraday instability

The effect of the rotation on the turbulent mixing of two miscible fluids of small contrasting density, produced by Faraday instability, is investigated using direct numerical simulations (DNS). We demonstrate that at lower forcing amplitudes, the t.k.e. increases with an increase in f till (f/ω\right)^2<0.25, where ωis the forcing frequency, during the sub-harmonic instability phase. The increase in t.k.e. increases B_V, which increases the total potential energy (TPE). A portion of TPE is the APE. Some parts of APE can convert to $t.k.e.$ via B_V, whereas the rest converts to internal energy, increasing BPE through ϕ_i. The remaining TPE also converts to BPE through the diapycnal flux ϕ_d resulting in irreversible mixing. With the saturation of the instability, irreversible mixing ceases. When (f/ω\right)^2 > 0.25, the Coriolis force significantly delays the onset of the sub-harmonic instabilities. During this period, the initial concentration profile diffuses to increase TPE, which eventually expends in BPE. The strong rotational effects suppress t.k.e.. Therefore, B_V and APE become small, and the bulk of the TPE expends to BPE. Since the instability never saturates for $\left(f/ω\right)^2 > 0.25$, the $B_V$ remains non-zero, resulting in a continuous increase in TPE. Conversion of TPE to BPE via $ϕ_d$ continues, and we find prolonged irreversible mixing. At higher forcing amplitudes, the stabilizing effect of rotation is negligible, and the turbulence is less intense and short-lived. Therefore, the irreversible mixing phenomenon also ends quickly for $\left(f/ω\right)^2<0.25$. However, when $\left(f/ω\right)^2>0.25$ a continuous mixing is observed. We find that the turbulent mixing is efficient at lower forcing amplitudes and rotation rates of (f/ω)^2 > 0.25.

physics.flu-dyn↗
arXiv2022

The onset and saturation of the Faraday instability in miscible fluids in a rotating environment

We investigate the influence of rotation on the onset and saturation of the Faraday instability in a vertically oscillating two-layer miscible fluid using a theoretical model and direct numerical simulations (DNS). Our analytical approach utilizes the Floquet analysis to solve a set of the Mathieu equations obtained from the linear stability analysis. The solution of the Mathieu equations comprises stable and harmonic, and sub-harmonic unstable regions in a three-dimensional stability diagram. We find that the Coriolis force stabilizes the flow and delays the onset of the sub-harmonic instability responsible for turbulent mixing at lower forcing amplitudes. However, at higher forcing amplitudes, the flow is energetic enough to mitigate the stabilizing effect of rotation, and the evolution of the turbulent mixing zone is similar in both rotating and non-rotating environments. These results are corroborated by DNS at different Coriolis frequencies and forcing amplitudes. We also observe that for $\left(f/ω\right)^2<0.25$, where $f$ is the Coriolis frequency, and $ω$ is the forcing frequency, the instability, and the turbulent mixing zone size-$L$ saturates. When $\left(f/ω\right)^2\geq0.25$, the turbulent mixing zone size-$L$ never saturates and continues to grow.

physics.flu-dyn↗
arXiv2022

Dynamics of Hanging Droplets from Liquid Interfaces

The impact of a heavier droplet into a deep pool of lighter liquid is investigated using three-dimensional numerical simulations. Unprecedented to any numerical simulations, we demonstrate that the heavier droplets can hang from the surface of a lighter liquid using surface tension. The impact phenomenon and the evolution of the heavier droplet as a function of its size and release height are explored. A theoretical model is also formulated to understand the role of different forms of energies associated with the hanging droplet. We further solve the force balance equations for the hanging droplets analytically and demonstrate that the results obtained from our simulations match very well with the analytical solution. This research offers opportunities in many areas, including drug and gene delivery, encapsulation of biomolecules, microfluidics, soft robots, and remediation of oil spills.

physics.flu-dyn↗
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

Buoyancy effects on film boiling heat transfer over a sphere at low velocities

A theoretical model is developed for the forced convection film boiling phenomenon over a heated sphere moving vertically downwards in the water. Unprecedented to the previous analytical studies, this model accounts for the buoyancy effects while solving the momentum, and energy equations in the vapor phase to obtain the velocity, and the temperature distribution in terms of the vapor boundary layer thickness. To calculate the vapor boundary layer thickness an energy balance is applied at the vapor-liquid interface. The flow of liquid around the sphere is considered to be governed by the potential theory, and the energy equation in liquid is then solved for the known velocity distribution. We find that the vapor boundary layer thickness increases with an increase in the sphere, and the bulk water temperature, and a decrease in the free stream velocity. This further results in a decrease in the film boiling heat transfer coefficient. The present study concludes that at low free stream velocities ($< 0.5$ m/s) buoyancy becomes significant in delaying the separation, and when the velocity is further reduced the separation angle approaches $180^{\circ}$.

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↗