arXiv subjects

Anubhab Roy

Publications and source records attributed to Anubhab Roy.

At least 19 recordsLinked to original sources

arXiv2026

Collision efficiency of rapidly settling particle pairs in a turbulent flow

We investigate the collision dynamics of hydrodynamically interacting inertialess spherical particle pairs sedimenting in a homogeneous isotropic turbulent flow. The analysis focuses on the rapid-settling limit, in which the particle settling time across a Kolmogorov eddy is much shorter than the Kolmogorov time scale. We also consider continuum breakdown during lubrication interactions, which is important when the separation the particles is comparable to the $O(100)$ nm mean-free path of a gaseous media. Owing to the sub-Kolmogorov particle sizes considered here, we approximate the local flow field in the vicinity of a particle pair as a stochastic linear flow induced by the background turbulence. In the rapid-settling regime, the cumulative effect of turbulent strain fluctuations is weak, and the relative particle motion may therefore be described as a diffusive process. In addition, hydrodynamic interactions generate a net relative drift between the particle pairs. We obtain the hydrodynamic diffusivity and relative drift velocity from the Lagrangian autocorrelation function of the fluid velocity gradient evaluated along the settling trajectory. The rapid-settling assumption further enables us to relate the autocorrelation function to the turbulence energy spectrum. Using these results, we solve the advection-diffusion equation for the pair probability density function to determine the collision rate. We show that the ideal collision rate increases monotonically with increasing relative strength of gravity to turbulence, whereas the collision efficiency decreases monotonically over the same range.

physics.flu-dyn↗

arXiv2026

An inertial slender-body theory

We present a fully inertial slender-body theory (SBT) that incorporates the effect of fluid inertia on the scale of the length (the "outer" region) as well as the characteristic diameter (the "inner" region) of a steadily translating slender particle. This is achieved by matching the solution of the quasi-two-dimensional full Navier-Stokes equations in the inner region to an outer solution that consists of a superposition of a solution of the linearized Navier-Stokes equations driven by a line of forces and a potential flow solution driven by a line distribution of sources and source dipoles. The drag and lift forces result from the distribution of Oseen force singularities. These Oseenlets also predominantly govern the torque at small Reynolds numbers and large aspect ratios. However, the potential flow singularities play a crucial role in yielding a torque that grows with increasing Reynolds number at large Reynolds numbers and finite aspect ratios. By comparing the forces and torque on the steadily translating particle with those obtained from a finite difference Navier-Stokes solution, we demonstrate the accuracy of the resulting inertial SBT for $\mathrm{Re}_D$ up to 10, where $\mathrm{Re}_D$ is the Reynolds number based on the smallest dimension, i.e., the characteristic cross-sectional diameter of the slender particle.

physics.flu-dyn↗

arXiv2026

Radiation Forces and Torques on Janus Cylinders

We investigate radiation-induced drag, lift, and torque on circular Janus cylinders under transverse-magnetic plane-wave illumination, considering metallo-dielectric and purely dielectric configurations. The lattice Boltzmann method (LBM) is employed with absorption neglected, isolating scattering as the sole momentum-transfer mechanism. For metallo-dielectric Janus cylinders, analytical expressions for radiation force and torque are derived and used to validate the LBM, showing excellent agreement across a wide range of dielectric constants and interface orientations. For dielectric Janus cylinders, material inhomogeneity induces asymmetric scattering giving rise to nonzero lift and torque under plane-wave illumination, with non-monotonic dependence on interface orientation and dielectric contrast. Two mechanisms govern the observed variations: resonance-driven energy amplification and scattered field redistribution. The computed force and torque maps serve as design diagrams for predicting the optomechanical response. Coupling these with viscous dynamics at low Reynolds number reveals diverse particle trajectories, including curved paths during reorientation and nearly straight motion once torque-free equilibria are reached. The system is externally actuated and results represent scattering-dominated dynamics under idealized conditions, providing physical insight into optomechanical responses of Janus particles with implications for trajectory shaping in optofluidic systems.

cond-mat.soft↗

arXiv2026

Orientation dynamics of a settling spheroid in simple shear flow: bifurcations and stochastic alignment

We investigate the orientation dynamics of a settling spheroid in simple shear flow, combining a deterministic dynamical-systems analysis with a stochastic Fokker-Planck treatment. The dynamics is governed by the competition between the Jeffery torque from the background shear and the inertial torque from settling. For configurations in which gravity lies in the shear plane, the azimuthal dynamics reduces to overdamped motion in a tilted periodic potential controlled by a single effective parameter $\mathcal{R}$ that combines the particle shape anisotropy and the settling strength. A saddle-node bifurcation on an invariant circle (SNIC) at $\mathcal{R}=1$ governs the transition from sustained rotational motion to steady equilibrium, with the rotation period diverging as $(1-\mathcal{R})^{-1/2}$. When gravity is parallel to the vorticity axis, the attractor is a periodic orbit for all settling strengths. The stochastic analysis reveals that noise plays a fundamentally different role depending on whether settling-induced potential barriers are present: in the classical Jeffery problem it diffuses over the orbit constant, whereas with settling it drives Kramers-type phase slips whose rate is exponentially sensitive to the Péclet number, defined as the ratio of diffusive to convective time scales. Langevin simulations confirm the predicted intermittent dynamics, with phase slips becoming progressively rarer as the barrier height or Péclet number increases. Asymptotic results in both the small- and large-$\mathrm{Pe}$ limits, together with numerical solutions of the Fokker-Planck equation at arbitrary $\mathrm{Pe}$, quantify the orientation moments across all regimes.

physics.flu-dyn↗

arXiv2026

Evidence of an inertialess Kapitza instability due to viscosity stratification

The classical Kapitza instability of a gravity-driven falling film requires finite inertia to operate. We show that a surface-mode instability can arise in the complete absence of inertia when the film possesses a continuous viscosity stratification, a feature relevant to particle-laden films with shear-induced migration, thermally stratified coatings, and concentration-graded flows. The viscosity field, prescribed as a linear profile across the film thickness, evolves through an advection-diffusion equation characterized by a P$é$clet number. Using long-wave asymptotics and Chebyshev spectral computations, we solve the coupled eigenvalue problem for the perturbation streamfunction and viscosity fields and demonstrate that viscosity stratification destabilizes the surface mode in the zero-inertia (Stokes) limit. The instability is confined to a finite window of P$é$clet numbers. Increasing the stratification parameter lowers the critical P$é$clet number, broadens the range of unstable wavenumbers, and increases the growth rate. The instability mechanism is traced to the phase relationship between perturbation vorticity and the interface displacement: viscosity stratification shifts the vorticity to a lagging configuration, which reinforces interface deformation, following the framework of Hinch (1984). The mechanism bears a structural resemblance to the surfactant-driven Marangoni instability in creeping two-layer flows, extending this class of scalar-mediated, inertialess instabilities to bulk viscosity stratification.

physics.flu-dyn↗

arXiv2026

Lattice Boltzmann Method for Electromagnetic Wave Scattering

In this work, the lattice Boltzmann method (LBM) is assessed as a time-domain numerical approach for electromagnetic wave scattering. Owing to its explicit formulation and suitability for parallel computation on structured grids, LBM provides an alternative framework for solving Maxwell's equations. The formulation is first validated using canonical benchmarks, including reflection and refraction at a planar dielectric interface and two-dimensional scattering from infinitely long circular cylinders, where the computed angular scattering intensities are compared with analytical Lorenz-Mie solutions. Additional comparisons are performed for circular cylinders with varying dielectric constants to examine performance across different material contrasts. The framework is then extended to three-dimensional scattering from dielectric spheres, representing the most computationally demanding case considered in this work, and the resulting angular scattering intensities are compared with exact Lorenz-Mie solutions. To further examine performance for non-circular geometries, scattering from an infinitely long hexagonal dielectric cylinder is investigated and benchmarked against results obtained using the Discretized-Mie Formalism. Across all cases, the LBM predictions show close agreement with analytical and semi-analytical reference solutions over a range of size-to-wavelength ratios.

physics.optics↗

arXiv2026

Shear-induced self-diffusivity in dilute suspensions with repulsive interactions

In a dilute non-Brownian suspension undergoing simple shear, pairwise hydrodynamic interactions are fore-aft symmetric at zero Reynolds number and produce no net cross-streamline displacement. A weak central repulsive force between particles breaks this symmetry, deflecting trajectories and generating irreversible transverse displacements that cumulatively yield a shear-induced self-diffusivity. We derive, via matched asymptotic expansions in the limit of weak repulsion, closed-form scaling laws for the gradient and vorticity components of this diffusivity. The gradient component exhibits a logarithmic enhancement relative to the vorticity component, a structural anisotropy that persists for all monotonically decaying repulsive potentials. The specific interaction enters only through integral functionals of the force profile weighted by hydrodynamic mobility functions, establishing that the scaling is universal across physically distinct mechanisms, such as electrical double-layer repulsion, steric interactions, or any other short-range central force. We validate the asymptotic predictions against full numerical trajectory integration for the representative case of electrostatic repulsion, modelled using the Gouy-Chapman description of the electrical double layer, and find excellent agreement in the expected regime.

cond-mat.soft↗

arXiv2025

Transport and orientation of anisotropic particles settling in surface gravity waves

We study the translation and orientation dynamics of an anisotropic particle settling in monochromatic linear surface gravity waves. Recent work has shown that a neutrally buoyant spheroid attains a preferred mean orientation in such wave fields, independent of its initial state and determined solely by its aspect ratio. Comparing the settling parameter $\mathrm{Sv}$, the ratio of settling speed to wave speed, with the asymptotically small wave steepness $ε$, we investigate the long time dynamics of a negatively buoyant particle. We examine the transition from aspect ratio-dependent equilibrium orientation in the weak settling regime ($\mathrm{Sv} \ll ε^2$) to initial-condition-dependent alignment in the strong settling limit ($\mathrm{Sv} \gg 1$). Since translation and orientation are coupled for anisotropic particles, we use orientation dynamics to predict net horizontal transport. Fluid inertia induces an inertial torque that breaks the Stokesian degeneracy and drives broadside alignment. We analyze the influence of this torque on drift and alignment rate as functions of settling and wave parameters. Finally, we evaluate finite-size effects through the parameter $σ$, showing that a neutrally buoyant finite-size spheroid exhibits $σ$-dependent drift, validating the finite-size approximation when the spheroid size approaches the wavelength.

physics.flu-dyn↗

arXiv2025

Electrostatic enhancement of particle collision rates in atmospheric flows

Collisional growth of tiny particles is a fundamental process governing the growth of cloud droplets and the aggregation of ash particles in volcanic plumes, with direct implications for precipitation formation, cloud lifetime, and ash plume dynamics. The particles in these scenarios often carry electric charges. In this study, we investigate the collision dynamics of a pair of like charged dielectric spheres subjected to a uniaxial compressional flow, an important linear flow that captures key features of atmospheric straining motions. Finite particle size leads to electrostatic interactions that deviate from the point charge approximation, resulting in far field repulsion and near-field attraction, which in turn generate nontrivial particle trajectories and critical collision thresholds. For certain combinations of charge and size, the interplay between hydrodynamic and electrostatic forces creates strong radially inward particle relative velocities that substantially alter particle pair dynamics and modify the conditions required for contact. For uncharged particles, collision efficiency increases monotonically with particle size ratio. However, in the presence of electrostatic forces with high charge ratio values, the collision efficiency exhibits a nonmonotonic dependence, attaining a maximum at small size ratios and decreasing as the ratio increases, with a crossover beyond which larger particles become less favorable for collision. These results demonstrate that the same polarity charges on finite sized atmospheric particles do not necessarily inhibit collisions. Instead, they can enhance collisional growth for specific charge and size ratio combinations, revealing counterintuitive pathways relevant to cloud microphysical processes and volcanic ash aggregation in electrified atmospheric environments.

physics.flu-dyn↗

arXiv2025

On the generation of free-surface waves by instabilities in quadratic shear flows

This paper investigates the generation of free-surface waves in a liquid layer driven by linear instabilities in Couette-Poiseuille (quadratic) shear flows. The base velocity profiles are characterized by a curvature parameter, and two-dimensional viscous and inviscid perturbations are analyzed across a wide parameter space of curvature, wavenumber, and Reynolds number, for fixed Froude and Bond numbers. In the inviscid limit, analytical solutions of the Rayleigh equation reveal that velocity profiles ranging from Nusselt to linear flows remain stable against the rippling instability, with long-wave growth occurring only under strong interfacial forcing, whereas weaker forcing produces well-defined stability boundaries. For the viscous problem, Orr-Sommerfeld computations and asymptotic analyses reveal that a slight convex curvature of the shear flow suppresses long-wave instabilities, while a slight concave curvature suppresses short-wave instabilities, so even small deviations from a linear profile produce qualitatively different behaviors. Furthermore, we observe that strongly forced long waves are more unstable at large $Re$ than the inviscid value they latch on to as $Re \to \infty$. Growth-rate maps highlight smooth transitions between long-wave and rippling modes and reveal an additional shear instability near the linear profile at high Reynolds numbers. Based on energy transfers and eigenfunction structures, five distinct instability types are identified: shear, rippling, long-wave interfacial, short-wave interfacial, and a composite mode that combines features of shear, rippling and long-wave interfacial instabilities at large Reynolds numbers.

physics.flu-dyn↗

arXiv2025

Electric field effects on the collision efficiency of uncharged water droplets in a linear flow

We study the dynamics of collisions between a pair of uncharged conducting droplets under the influence of a uniaxial compressional flow and an external electric field. The near-field asymptotic expression for the electric-field-induced attractive force demonstrate that surface-to-surface contact in finite time is facilitated by overcoming lubrication resistance. We demonstrate the significant role of the external electric field on the relative trajectories of two droplets in a compressional flow and provide estimates of the correlation between collision efficiency and the forces induced by the electric field. For droplet collisions in clouds, continuum lubrication approximations become inadequate to capture collision dynamics, and thus we incorporate non-continuum lubrication interactions into our analysis to address this complexity. Our findings reveal the dependence of collision efficiency on the strength of the electric field, geometry of the two interacting droplets, non-continuum effects, and van der Waals forces.

physics.flu-dyn↗

arXiv2025

Trapping and Transport of Inertial Particles in a Taylor-Green Vortex: Effects of Added Mass and History Force

We investigate the dynamics of small inertial particles in a two-dimensional, steady Taylor-Green vortex flow. A classic study by Taylor (2022) showed that heavy inertial point particles (having density parameter R = 1) are trapped by the flow separatrices when the particle Stokes number St, which measures the particle's inertia, is less than 1/4. Here, we consider finitely dense particles, incorporating the previously neglected effects of added mass and the Boussinesq-Basset history force. Using linear stability analysis near stagnation points, we determine the critical parametric conditions in the St-R plane that leads to particle trapping within vortex cells. We identify additional stagnation points perceived by inertial particles, beyond the traditional ones at vortex cell corners, when the added mass effect is included, and we analyze their stability. Numerical analysis of the full nonlinear system confirms the existence of distinct particle behaviours--trapped, diffusive, and ballistic--depending on initial conditions, consistent with Nath et al. (2024), with modifications due to added mass effect. We delineate the regions in the St-R plane where these behaviours dominate based on the prominent particle dynamics. However, when both the history force and added mass effect are included, all particles exhibit ballistic motion regardless of St and R.

physics.flu-dyn↗

arXiv2025

Gravity-induced collisions of uncharged cloud droplets in an electric field

We investigate the collisions of uncharged, conducting droplets settling under gravity in the presence of an external electric field. Previous studies have derived a near-field asymptotic expression for the electric-field-induced attraction, suggesting that this force can overcome lubrication resistance and drive surface-to-surface contact between two spherical conductors within a finite time. However, for droplets moving in air, traditional lubrication theory breaks down when the inter-droplet gap approaches the mean free path of air molecules. To account for this, we incorporate non-continuum hydrodynamic effects to estimate the gravity-driven collision efficiency under electric-field-induced forces. This study examines how an external electric field influences the trajectories of settling droplet pairs of unequal sizes. By analyzing their motion, we compute collision efficiencies and explore their dependence on droplet size ratio, electric field strength, the angle between the field and gravity, and key dimensionless parameters governing electric-field-induced and van der Waals forces. Our findings reveal that electric-field-induced forces significantly enhance collision efficiency, highlighting their critical role in droplet coalescence dynamics.

physics.flu-dyn↗

arXiv2025

Electrostatic interactions between anisotropic particles

We investigate the electrostatic interactions between two charged anisotropic conductors using a combination of asymptotic and numerical methods. For widely separated particles, we employ the method of reflections to analyze the interactions. Although the formulation applies to conductors of arbitrary shapes, it is specifically implemented for spheroid-sphere systems to capture anisotropy effects in a simple configuration. In near-contact cases with axisymmetric configurations, the lubrication approximation is used to extend the analysis. Additionally, we develop a Boundary Integral Method (BIM) to study particle interactions at arbitrary separations, validating the results with asymptotic solutions for both near and far fields. We derive analytical expressions for the electrostatic force and torque on a spheroid due to another spheroid in the far-field regime. When combined with hydrodynamic effects, the electrostatic torque competes with the hydrodynamically favourable alignments of a pair of settling spheroids in certain regions while reinforcing them in others. Consequently, the inclusion of electrostatic effects may influence the instability observed in dilute suspensions of spheroids.

cond-mat.soft↗

arXiv2024

Clustering and chaotic motion of heavy inertial particles in an isolated non-axisymmetric vortex

We investigate the dynamics of heavy inertial particles in a flow field due to an isolated, non-axisymmetric vortex. For our study, we consider a canonical elliptical vortex - the Kirchhoff vortex and its strained variant, the Kida vortex. Contrary to the anticipated centrifugal dispersion of inertial particles, which is typical in open vortical flows, we observe the clustering of particles around co-rotating attractors near the Kirchhoff vortex due to its non-axisymmetric nature. We analyze the inertia-modified stability characteristics of the fixed points, highlighting how some of the fixed points migrate in physical space, collide and then annihilate with increasing particle inertia. The introduction of external straining, the Kida vortex being an example, introduces chaotic tracer transport. Using a Melnikov analysis, we show that particle inertia and external straining can compete, where chaotic transport can be suppressed beyond a critical value of particle inertia.

physics.flu-dyn↗

arXiv2024

The flow field due to a sphere moving in a viscous, density stratified fluid

We study the flow field induced by a sphere translating in a viscous density-stratified ambient, specifically, in the limit of small Reynolds $(Re = ρU a/μ\ll 1)$, and viscous Richardson numbers $(Ri_v = γa^3 g/μU\ll 1)$, and large Peclet number $(Pe = Ua/D\gg 1)$. Here, $a$ is the sphere radius, $U$ its translational velocity, $ρ$ an appropriate reference density within the Boussinesq framework, $μ$ the ambient viscosity, $γ$ the absolute value of the background density gradient, and $D$ the diffusivity of the stratifying agent. For the scenario where buoyancy forces first become comparable to viscous forces at large distances, corresponding to the Stokes-stratification regime defined by $Re \ll Ri_v^{1/3} \ll 1$ for $Pe \gg 1$, important flow features such as a vertical reverse jet and a horizontal wake, on scales larger than the primary screening length of $\mathcal{O}(aRi_v^{-1/3})$, have been identified by Varanasi and Subramanian (2022). Here, we show that the reverse jet is only the central portion of a columnar structure with multiple annular cells. In the absence of diffusion this columnar structure extends to downstream infinity with the number of annular cells diverging in this limit. We provide expressions for the boundary of the structure, and the number of cells within, as a function of the downstream distance. For small but finite diffusion, two additional length scales emerge - a secondary screening length of $O(aRi_v^{-1/2}Pe^{1/2})$, where diffusion starts to smear out density variations across cells, leading to exponentially decaying flow field; and a tertiary screening length, of $O(aRi_v^{-1/2}Pe^{1/2}\ln(Ri_v^{-1}Pe^3))$, beyond which the columnar structure ceases to exist and the downstream disturbance field reverts from an exponential to eventual algebraic decay, analogous to that prevalent at large distances upstream.

physics.flu-dyn↗

arXiv2024

Instability of a dusty shear flow

We study the instability of a dusty simple shear flow where the dust particles are distributed non-uniformly. A simple shear flow is modally stable to infinitesimal perturbations. Also, a band of particles remains unaffected in the absence of any background flow. However, we demonstrate that the combined scenario -- comprising a simple shear flow with a localised band of particles -- can exhibit destabilisation due to their two-way interaction. The instability originates solely from the momentum feedback from the particle phase to the fluid phase. Eulerian-Lagrangian simulations are employed to illustrate the existence of this instability. Furthermore, the results are compared with a linear stability analysis of the system using an Eulerian-Eulerian model. Our findings indicate that the instability has an inviscid origin and is characterised by a critical wavelength below which it is not persistent. We have observed that increasing particle inertia dampens the unstable modes, whereas the strength of the instability increases with the strength of the coupling between the fluid and particle phases.

physics.flu-dyn↗

arXiv2024

Evidence of Chaotic Mixing in the Alveolar Region of the Lung

We report an experimental and numerical investigation to study the role of asymmetry in the expansion-contraction of the acinar wall on the particle transport in the acinus. We model the acinar flow feature using a T-section by appropriately matching the dimensionless numbers to that in the acinus of healthy human subjects. We show that asymmetry in the expansion-contraction process (quantified by $ϕ$) is required for chaotic advection. We show the stretch and fold process leading to chaos for a range of $ϕ$ and scaled oscillation frequency $Sr$. We show a regime map in this generalize $ϕ$ and $Sr$ space and show that most mammalian lungs fall at the boundary of chaotic regime.

physics.bio-ph↗