arXiv subjects

Pijush Patra

Publications and source records attributed to Pijush Patra.

6 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

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↗

arXiv2026

Stochastic coupling of climate variables and ice volume over the Late Pleistocene glacial cycles

Understanding the interactions between ice sheets and global climate forcings over geological timescales is essential for projecting their future. Previous studies have highlighted the role of ice dynamics and climate interactions in establishing the 100,000-year glacial cycles, particularly regarding the growth of the North American ice sheet. Researchers have reconstructed consistent time series for ice volume, temperature, and carbon dioxide by applying inverse forward modeling to benthic oxygen isotope records. Here we model the stochastic behavior of paleoclimate time series to evaluate the coupling between climate variables during the Pleistocene glacial cycles. We quantify the behavior of these time series using multifractal time-weighted detrended fluctuation analysis, which differentiates between near-red-noise and white-noise behavior below and above the 100,000-year glacial cycle, respectively, in all records. This study builds upon the work of Keyes et al. [Chaos vol. 33, 093132 (2023)] by incorporating ice volume into a five-variable model that includes carbon dioxide, methane, nitrous oxide, and temperature, along with intervariable coupling terms to capture potential relationships among these variables. Our analysis shows that ice volume, carbon dioxide, and temperature have a stabilizing effect upon each other. To test our model, we compute response functions for each pair of variables and compare these with empirical data, confirming our predictions regarding intervariable stability and coupling. This study provides a comprehensive overview of glacial-interglacial dynamics and highlights the role of cryosphere-climate feedbacks in shaping Earth's climate evolution.

physics.ao-ph↗

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

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

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↗