arXiv subjects

Saini Jatin Rao

Publications and source records attributed to Saini Jatin Rao.

9 recordsLinked to original sources

arXiv2026

Vortex rings as hydrodynamic dispersants for mitigating underwater oil spills

The usage of chemical dispersant to promote the breakup of oil droplets remains one of the crucial response strategy towards controlling the rapidly rising droplets during an underwater oil spillage disaster like the Deepwater horizon. However, the detrimental effects of dispersant on the marine life are now well recognized. Here, we demonstrate that vortex ring presents an environmental friendly alternative to the debatable usage of chemical dispersant that primarily aids droplet break-up. We propose two mitigating strategies through the application of vortex rings: (i) atomization of large oil droplets into smaller droplets, which enhances natural dispersion and biodegradation (ii) targeted relocation of spilled oil droplets from ecologically sensitive regions to safer remote locations during minor spill events. This proof-of-concept study demonstrates that vortex rings can efficiently atomize underwater oil droplets while simultaneously transporting them over long distances. The efficiency strongly depends on the droplet size distribution and the strength of the vortex ring. Using time scale analysis and force balances arguments, we propose droplet response and atomization criteria by a traversing vortex ring. Beyond atomization and droplet transport which are center to many engineering processes, the work finds high relevance in the area of bubble turbulence, agricultural industry, multiphase reactors, combustion process, atmospheric fluid dynamics and oil extraction from rocks, to name a few.

physics.flu-dyn↗

arXiv2026

Self-similar Features in Secondary Breakup of a Droplet and Ligament Mediated Fragmentation under Extreme Conditions

Droplet formation is relevant in many applications spanning natural and artificial settings. A physical understanding of aerobreakup or air-assisted secondary atomization and predicting size distributions in these applications is non-trivial. We show that extreme airflow speeds induce catastrophic breakup, which, although chaotic and seemingly obscure, is not hopelessly unstructured. In the present study, through shockwave-induced breakups, we investigate the associated intermediate processes at smaller spatiotemporal scales at very high Weber numbers ($We \sim 10^3-10^4$). We show that microscale protrusions decorate the disintegrating droplet interface and eventually fragment, resulting in the generation of daughter droplets. We discover these undulations to follow breakup patterns (sub-secondary breakup) that resemble a scaled-down version of secondary atomization. The consistent topology across a vast range of scales $(10^{-6}m-10^{-2}m)$ suggests a self-similar mechanism bridged by local Weber number. The normalized size distribution of the resultant droplets exhibits universality and $We$ invariance within the investigated range of extreme conditions, including transient statistics for subsequent time periods. This conforms to a modified gamma distribution characterized by ligament shape factors, which tend toward the limiting behavior associated with the maximum corrugations physically possible. Scaling laws based on the $We$ are derived for the averaged diameter as $\sim {We}^{-1/3}$, using a high-energy aerodynamic breakup mechanism and subsequently used to derive a time-integrated distribution. These observations reinforce the idea of a self-similar mechanism for the catastrophic droplet breakups, encompassing multiscale deformation cascades, self-similar sub-secondary breakups, maximally corrugated ligaments, and universal droplet size distributions.

physics.flu-dyn↗

arXiv2026

Vortex Dynamics During Pinch-off of Micro-Droplets

Micro-droplets are extensively used in chemical, biological, and medical research, primarily for conducting various tests on samples, including living organisms, using a microfluidic framework. Recent studies have shown that the physiology of bacteria can be significantly altered when subjected to shear and/or extensional stresses. With this motivation, we perform experiments to understand the vortex dynamics involved during the pinch-off process in a cross flow droplet generator, using particle image velocimetry (PIV) to visualize the vortical structures and to quantitatively measure the associated stresses developed inside droplets. The process of pinching off inherently leads to bi-directional acceleration of fluid in the rapidly thinning capillary bridge, resulting in a vortex in the separated droplet as well as in the retracting ligament. We propose scaling laws for the vortical flow inside the droplet post pinch-off and predict the maximum circulation production inside droplet. Further, we discuss the vortex dynamics inside the droplet, the retracting ligament and the advancing ligament and examine the stress fields associated with this transient phenomenon.

physics.flu-dyn↗

arXiv2025

Blast wave induced unsteady flow at the shock tube opening

Shock tubes have been a crucial device, facilitating studies across a wide range of practical applications. An open-ended shock tube employing the wire-explosion technique with a rectangular cross section is used in the present study to generate blast waves over a Mach number range of 1.2-1.8, enabling detailed investigation of unsteady compressible flow at the tube opening. The blast wave produces a complex flow field comprising a compressible vortex ring with a trailing jet, and several transient structures, including embedded shocks, inward-moving shock or reverse shocks, shear layers, and Prandtl-Meyer expansion fans. An approximate model based on a power-law density profile describes the blast evolution inside and outside the tube, with the equivalent source deduced from measured shock trajectories. The blast wave-tube exit interaction is analyzed using the method of characteristics with alternate exit boundary conditions. A steady-pressure outlet best reproduces experimental observations, predicting supersonic efflux, embedded shocks, expansion waves, and circulation production. Several previously unreported unsteady features, including reverse shock or "reshock" formation and embedded shock shedding, are documented. The findings highlight the intricate dynamics of various features associated with such highly transient, blast-driven flows emanating from an open-ended shock tube.

physics.flu-dyn↗

arXiv2024

Interaction of vortex ring with perforated plate at different included angles

Experiments are performed to investigate the interaction of a vortex ring (Reynolds number based on circulation (Re_Gamma = 10500) with perforated surface (open area ratio, phi_1 = 0.24 and phi_2 = 0.44) with different included angles (theta = 60deg-180deg). The phenomenon is characterized using techniques like planer laser-induced fluorescence (PLIF) imaging and particle image velocimetry (PIV). Lagrangian analysis using finite-time Lyapunov exponents (FTLE) and Gamma_2 vortex identification methods are utilized to understand flow physics. We observe the development of mushroom-like structures at the holes, driven by the induced flow of the vortex ring. These formations, together with Kelvin-Helmholtz instability, introduce initial instability to the emerging jets. We discern a sequential emergence of the vortex ring in the form of jets at lower theta value that diminishes at higher values. Notably, a single vortex ring is split into two distinct vortex rings for theta less than equal to 120deg. On either side of the perforated plate, the sense of circulation after interaction does not show bias towards the sense of flow on the upstream region for some values of theta. We further show the evolution of circulation by jets in the downstream region aligns with the proposed cumulative slug flow model using the centre line peak velocity of individual jets.

physics.flu-dyn↗

arXiv2024

Insights into Spatio-temporal dynamics during shock -- droplet flame interaction

The study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow that is imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed unique miniature shock generation apparatus that generates blast waves using the wire-explosion technique which facilitates a wide range of shock Mach number (1.03 < Ms < 1.8). The experiments are performed in two configurations: Open field blast wave and focused blast wave. The charging voltage and the configuration determine the shock Mach number (Ms) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by re-ignition and full extinction. Simultaneously, the droplet also interacts with the flow imposed by the blast wave exhibiting different modes of response ranging from pure deformation, Rayleigh-Taylor piercing bag breakup, and shear-induced stripping. The KH instability is exhibited along the windward side interface of the droplet during the interaction with the blast wave decay profile which gets aggravated when the induced flow interaction ensues. Increasing the Mach number (Ms > 1.1) makes the droplet flame more vulnerable to extinction. However, the flame exhibits stretching and shedding, followed by re-ignition at lower Mach numbers (Ms < 1.06). In all cases, the flame base lifts off in response to the imposed flow, and the advection of the flame base interacting with the flame tip results in flame extinction. The entire interaction occurs in two stages: 1) interaction with the blast wave and the decaying velocity profile associated with it, and 2) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). The criteria for partial and complete extinction of flame have been postulated which is in good agreement with the experiments.

physics.flu-dyn↗

arXiv2024

Dynamics of Soap Bubble Inflation

Bubbles have always captivated our curiosity with their aesthetics and complexities alike. While the act of blowing bubbles is familiar to everyone, the underlying physics of these fleeting spheres often eludes reasoning. In this letter, we discuss the dynamics of inflating a soap bubble using controlled airflow through a film-coated nozzle. We assess and predict the rate of inflation by varying the source pressure. Visualising the previously unexplored internal flow reveals that air enters the bubble as a round jet, emerging from the nozzle opening and impinges on the expanding concave bubble interface to form a toroidal vortex. Several scaling laws of the associated vortical flow spanning the entire bubble and the vortex core are reported. The observed dynamics of this bubble-confined vortex ring formation indicate universality in certain aspects when compared to the free laminar vortex rings.

physics.flu-dyn↗

arXiv2023

Depth from Defocus Technique: A Simple Calibration-Free Approach for Dispersion Size Measurement

Particle size measurement is crucial in various applications, be it sizing droplets in inkjet printing or respiratory events, tracking particulate ejection in hypersonic impacts, or detecting floating target markers in free surface flows. Such systems are characterised by extracting quantitative information like size, position, velocity and number density of the dispersed particles, which is typically non-trivial. The existing methods like phase Doppler or digital holography offer precise estimates at the expense of complicated systems, demanding significant expertise. We present a novel volumetric measurement approach for estimating the size and position of dispersed spherical particles that utilises a unique 'Depth from Defocus' (DFD) technique with a single camera. The calibration free sizing enables in-situ examination of hard to measure systems, including naturally occurring phenomena like pathogenic aerosols, pollen dispersion or raindrops. The efficacy of the technique is demonstrated for diverse sparse dispersions, including dots, glass beads, spray droplets, and pollen grains. The simple optical configuration and semi-autonomous calibration procedure make the method readily deployable and accessible, with a scope of applicability across vast research horizons.

physics.flu-dyn↗

arXiv2022

Depth from Defocus Technique Applied to Unsteady Shock-Drop Secondary atomization

The two-sensor depth from defocus (DFD) technique for the measurement of drop sizes in a spray is further developed to achieve higher spatial and temporal resolution, to improve estimates of size and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. The technique and these improvements are demonstrated using the case of secondary atomization when a shock wave interacts with a single drop. This is an application in which both high spatially and temporally resolved number density and size distributions of secondary droplets generated in the wake of the original drop are necessary.

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