arXiv subjects

Kirti Chandra Sahu

Publications and source records attributed to Kirti Chandra Sahu.

At least 37 records · Page 2Linked to original sources

arXiv2024

Cracking of submerged beds

We investigate the phenomena of crater formation and gas release caused by projectile impact on underwater beds, which occurs in many natural, geophysical, and industrial applications. The bed in our experiment is constructed of hydrophobic particles, which trap a substantial amount of air in its pores. In contrast to dry beds, the air-water interface in a submerged bed generates a granular skin that provides rigidity to the medium by producing skin over the bulk. The projectile's energy is used to reorganise the grains, which causes the skin to crack, allowing the trapped air to escape. The morphology of the craters as a function of impact energy in submerged beds exhibits different scaling laws than what is known for dry beds. This phenomenon is attributed to the contact line motion on the hydrophobic fractal-like surface of submerged grains. The volume of the gas released is a function of multiple factors, chiefly the velocity of the projectile, depth of the bed and depth of the water column.

cond-mat.soft↗

arXiv2024

Application of deep learning and inline holography to estimate the droplet size distribution

We examine five machine learning-based architectures to estimate the droplet size distributions obtained using digital inline holography. The architectures, namely, U-Net, R2 U-Net, Attention U-Net, V-Net, and Residual U-Net are trained using synthetic holographic images. Our assessment focuses on evaluating the training, validation, and prediction performance of these architectures. We found that U-Net and R2 U-Net to be the most proficient, displaying consistent performance trends and achieving the highest Intersection Over Union (IOU) scores compared to the other three architectures. We employ additional training using experimental holographic images for the two top-performing architectures to validate their efficacy further. Subsequently, they are employed to segment an experimental dataset illustrating the bag breakup phenomenon, facilitating the extraction of size distribution. The extracted size distribution from U-Net and R2 U-Net segmentation is then compared with the analytical model proposed by \cite{jackiw2022prediction} by employing the gamma and log-normal distributions. Our findings indicate that the gamma distribution provides a more accurate prediction of the multi-modal size distribution than the log-normal distribution owing to its long exponential tail. The present study offers valuable insights into the effectiveness of machine learning architectures in estimating particle/droplet sizes, highlighting their practical application in real-world experimental scenarios.

physics.flu-dyn↗

arXiv2024

Coalescence of non-spherical drops with a liquid surface

We employ three-dimensional numerical simulations to explore the impact dynamics of non-spherical drops in a deep liquid pool by varying the aspect ratios $(A_r)$ and Weber numbers $(\We)$. We observe that when a non-spherical drop is gently placed on a liquid pool, it exhibits a partial coalescence phenomenon and the emergence of a daughter droplet for $A_r>0.67$. In contrast to the prolate $(A_r<1)$ and spherical drops $(A_r=1)$, an oblate $(A_r>1)$ drop with a high aspect ratio encapsulates air in a ring-like bubble within the pool and emerges a liquid column that undergoes Rayleigh-Plateau capillary instability, leading to the formation of two daughter droplets with complex shapes. When the parent drop is impacted with finite velocity, our observations indicate that increasing the Weber number leads to elevated crater heights on the free surface for all aspect ratios. A prolate drop produces a less pronounced wave swell and exhibits a prolonged impact duration owing to its negligible impact area. Conversely, an oblate drop generates a much wider wave swell than spherical and prolate drops. We analyze the relationship between rim formation dynamics and the kinetic and surface energies of the system. Finally, we establish an analogy by comparing the dynamics of a freely falling non-spherical drop, undergoing topological oscillations during its descent from a height, with the impact dynamics of parent drops of various shapes striking the liquid surface with an equivalent velocity. Our investigation involving non-spherical drops contrasts the extensive studies conducted by various researchers on the impact of a parent spherical drop just above the free surface of a liquid pool.

physics.flu-dyn↗

arXiv2023

Collision of two drops moving in the same direction

The collision dynamics of two drops of the same liquid moving in the same direction has been studied numerically. A wide range of radius ratios of trailing drop and leading drop ($R_r$) and the velocity ratios ($U_r$) have been deployed to understand the collision outcomes. A volume of fluid (VOF) based open-source fluid flow solver, Basilisk, has been used with its adaptive mesh refinement feature to capture the nuances of the interface morphology. The simulations are analyzed for the evolving time instances. Different collision outcomes, such as coalescence and reflexive separation with and without the formation of satellite drops, have been observed for various combinations of $U_r$ and $R_r$. The study analyzes the evolution of kinetic energy and surface energy before and after the collision for plausible outcomes. The collision outcomes are depicted on a regime map with $U_r-R_r$ space, highlighting distinct regimes formed due to variations in relevant governing parameters.

physics.flu-dyn↗

arXiv2023

Modeling binary collision of evaporating drops

We investigate the interactions between two drops in a heated environment and analyze the effect of evaporation on bouncing, coalescence and reflexive separation phenomena. A reliable mass transfer model is incorporated in a coupled level-set and volume-of-fluid framework to accurately model the evaporation process and the evolution of drop interfaces during the interactions. The numerical technique is extensively validated against the benchmark problems involving the evaporation of a single drop. We analyze the contour plots of temperature and vapor mass fraction fields for each collision outcome. Our numerical simulations reveal that vapor entrapment during the separation process, with high-velocity vapor manages to escape. Increasing evaporation rates result in slower post-collision drop separation. Furthermore, the differences in kinetic energy and surface energy are analyzed for different Stefan numbers. The coalescence of drops exhibits energy oscillations until dissipation, while the bouncing and reflexive separations lack such oscillations. In the reflexive separation regime, the kinetic energy of the drops becomes zero after detachment.

physics.flu-dyn↗

arXiv2023

Freezing of sessile droplet and frost halo formation

The freezing of a sessile droplet unveils fascinating physics, characterised by the emergence of a frost halo on the underlying substrate, the progression of the liquid-ice interface, and the formation of a cusp-like morphology at the tip of the droplet. We investigate the freezing of a volatile sessile droplet, focusing on the frost halo formation, which has not been theoretically explored. The formation of the frost halo is associated with the inherent evaporation process in the early freezing stages. We observe a negative evaporation flux enveloping the droplet in the initial stages, which indicates that vapour produced during freezing condenses on the substrate close to the contact line, forming a frost halo. The condensate accumulation triggers re-evaporation, resulting in a temporal shift of the frost halo region away from the contact line. Eventually, it disappears due to the diffusive nature of the water vapour far away from the droplet. We found that increasing the relative humidity increases the lifetime of the frost halo due to a substantial reduction in evaporation that prolonged the presence of net condensate on the substrate. Increasing liquid volatility increases the evaporation flux, and condensation occurs closer to the droplet, as a higher amount of vapour is in the periphery of the droplet. We also found that decreasing the thermal conductivity of the substrate increases the total freezing time. The slower freezing process is accompanied by increased vaporized liquid, resulting in condensation with its concentration reaching supersaturation.

physics.flu-dyn↗

arXiv2023

Universal scaling law for electrified sessile droplets on a lyophilic surface

Electrified sessile droplets on solid surfaces are ubiquitous in nature as well as several practical applications. Although the influence of electric field on pinned sessile droplets and soap bubbles have been investigated experimentally, the theoretical understanding of the stability limit of generic droplets remains largely elusive. By conducting a theoretical analysis in the framework of lubrication approximation, we show that the stability limit of a sessile droplet on a lyophilic substrate in the presence of an electric field exhibits a universal power-law scaling behaviour. The power-law exponent between the critical electric field and the droplet volume is found to be -1. The existence of this scaling law is further explained by virtue of minimization of the total free energy of the electrified droplet.

physics.flu-dyn↗

arXiv2023

An investigation on the impact of two vertically aligned drops on a liquid surface

The dynamics of two vertically coalescing drops and a pool of the same liquid have been investigated using a Coupled Level Set and Volume of Fluid (CLSVOF) method. Such a configuration enables us to study the dynamic interaction of an arbitrary-shaped liquid conglomerate, formed owing to drop-drop coalescence, with a pool. Similar to drop-pool and drop-drop interactions, partial coalescence is observed when a conglomerate interacts with a pool. The presence of the pool below the father drop is found to influence the coalescence characteristic of the two drops. At the same time, the movement of the capillary waves resulting from the interaction of two drops governs the coalescence dynamics of the conglomerate with the pool. As liquid interfaces interact and generate capillary waves at multiple locations, complex trajectories of capillary waves are observed, which play a crucial role in determining the pinch-off characteristics of the satellite during conglomerate-pool interaction. We examine the effect of the ratio of the diameters of the lower/father drop to the upper/mother drop (D_r) on the coalescence dynamics while maintaining the size of the mother drop constant. The variation in the coalescence dynamics due to change in $D_r$ is quantified in terms of the residence time (tau_r), pinch-off time (tau_p) and the satellite diameter to conglomerate diameter ratio (Ds/Dc). The coalescence dynamics of the conglomerate is then compared with that of an equivalent spherical drop of the same volume and also with that of a drop initialized with the same shape as that of the conglomerate. Finally, the regions of complete and partial coalescence for the conglomerate-pool interactions are demarcated on the Weber number - diameter ratio (We-Dr) space.

physics.flu-dyn↗

arXiv2023

Dynamics of an evaporating drop migrating in a Poiseuille flow

The evaporation of a liquid drop of initial diameter (Ddrop) migrating in a tube of diameter (D0) is investigated using the coupled level set and volume of fluid (CLSVOF) method focusing on determining the heat and mass transfer coefficients for a deforming drop. A robust phase change model is developed using an embedded boundary method under a finite difference framework to handle vaporizing flows. The model is extensively validated through simulations of benchmark problems such as arbitrary evaporation of a static drop and reproduction of psychrometric data. The results show that the Sherwood number (Sh) and the Nusselt number (Nu) reach a steady value after an initial transient period for the drop subjected to Hagen-Poiseuille flow. A parametric study is conducted to investigate the effect of drop deformation on the rate of evaporation. It is observed that Stefan flow due to evaporation has a negligible impact on the drop deformation dynamics. We also observed that, for different values of Ddrop/D0, the Sh follows a linear correlation with Re^{1/2}Sc^{1/3}.

physics.flu-dyn↗

arXiv2023

Effect of viscosity on the dynamics of a non-equilibrium bubble in free-field and near a free-surface

The effect of viscosity on the behaviour of a non-equilibrium bubble is investigated experimentally, in two scenarios; firstly, when the bubble is generated in the bulk of the fluid (termed as ``free-field'' bubble) and secondly when the bubble is generated near a free-surface (termed as ``free-surface'' bubble). The bubble is created using a low-voltage spark circuit and its dynamics is captured using a high-speed camera with back-lit illumination. The viscosity of the surrounding fluid is varied by using different grades of silicone oil. For a ``free-field'' bubble, the bubble oscillates radially and as the viscosity of the liquid increases, the number of oscillations, as well as the time-period of each oscillation, are increased. At high viscosities, the bubble also becomes stable and does not disintegrate into smaller bubbles. For ``free-surface'' bubbles, two parameters, namely, the initial distance of the bubble from the free-surface and the viscosity of the surrounding fluid are varied. It is observed that beyond a certain initial distance of the bubble from the free-surface, the bubble behaves as a ``free-field'' bubble with negligible influence of the free-surface on its dynamics. This limiting initial distance decreases as the liquid viscosity is increased and is not dependent on the bubble radius. For these bubbles, different behaviours of the free-surface in each liquid are also presented as a function of the two parameters.

physics.flu-dyn↗

arXiv2023

An experimental investigation of droplet morphology in swirl flow

The interaction of a droplet with a swirling airstream is investigated experimentally by shadowgraphy and particle image velocimetry techniques. In swirl flow, the droplet experiences oppose-flow, cross-flow, and co-flow conditions depending on its ejection location, the velocity of the airstream, and swirl strength, which results in distinct droplet morphologies as compared to the straight airflow situation. We observe a new breakup phenomenon, termed as `retracting bag breakup', as the droplet encounters a differential flow field created by the wake of the swirler's vanes and the central recirculation zone in swirl airflow. A regime map demarcating the various modes, such as no breakup, vibrational breakup, retracting bag breakup, and bag breakup modes, is presented for different sets of dimensionless parameters influencing the droplet morphology and its trajectory. In contrast to the straight flow, the swirl flow promotes the development of the Rayleigh-Taylor instability, enhancing the stretching factor in the droplet deformation process, resulting in a larger number of fingers on the droplet's surface. In order to gain physical insight, a modified theoretical analysis based on the Rayleigh-Taylor instability is proposed for the swirl flow. The experimental behaviour of droplet deformation phenomena in swirl flow conditions can be determined by modifying the stretching factor in the theoretical model.

physics.flu-dyn↗

arXiv2023

Physics of drying complex fluid drop: flow field, pattern formation, and desiccation cracks

Drying complex fluids is a common phenomenon where a liquid phase transforms into a dense or porous solid. This transformation involves several physical processes, such as the diffusion of liquid molecules into the surrounding atmosphere and the movement of dispersed phases through evaporation-driven flow. As a result, the solute forming a dried deposit exhibits unique patterns and often displays structural defects like desiccation cracks, buckling, or wrinkling. Various drying configurations have been utilized to study the drying of colloids, the process of their consolidation, and fluid-flow dynamics. This review focuses on the drying of colloids and the related phenomena, specifically the drying-induced effects observed during sessile drop drying. We first present a theoretical overview of the physics of drying pure and binary liquid droplets, followed by drying colloidal droplets. Then, we explain the phenomena of pattern formation and desiccation cracks. Additionally, the article briefly describes the impact of evaporation-driven flows on the accumulation of particles and various physical parameters that influence deposit patterns and cracks.

physics.flu-dyn↗

arXiv2023

Size distribution of a drop undergoing breakup at moderate Weber numbers

The size distribution of child droplets resulting from a dual-bag fragmentation of a water drop is investigated using shadowgraphy and digital in-line holography techniques. It is observed that parent drop fragmentation contributes to the atomization of tiny child droplets, while core drop disintegration predominantly results in larger fragments. Despite the complexity associated with dual-bag fragmentation, we demonstrate that it exhibits a bi-modal size distribution. In contrast, the single-bag breakup undergoes a tri-modal size distribution. We employ the analytical model developed by Jackiw and Ashgriz (2022) for dual-bag fragmentation that convincingly predicts the experimentally observed droplet volume probability density. We also estimate the temporal evolution of child droplet production in order to quantitatively illustrate the decomposition into initial and core breakups. Furthermore, we confirm that the analytical model adequately predicts the droplet size distribution for a range of Weber numbers.

physics.flu-dyn↗

arXiv2023

Stability and retention force factor for binary-nanofluid sessile droplets on a inclined substrate

We investigate the retention force factor of sessile droplets of pure (ethanol) and binary (water-ethanol) fluids laden with alumina nanoparticles placed on a critically inclined substrate. It is observed that while the critical angle of an ethanol droplet increases with an increase in nanoparticles concentration, for water-ethanol binary droplets, it reaches to plateau and decreases slightly after 0.6 wt.\% nanoparticle loading. The effect of composition and concentration of nanoparticles on the retention force factor is studied, and correlations are proposed for the retention force factor and critical angle for pure and binary droplets. Infrared images of evaporating droplets of pure and binary fluids reveal richer hydrothermal waves in droplets with nanoparticles loading than in droplets without loading, and these waves are more intense in pure ethanol droplets. On an inclined substrate, the body force caused the droplets to elongate more toward the receding side, which led to an earlier breakup of the droplet at the receding side. To the best of our knowledge, our study is a first attempt to investigate the retention force factor for the droplets loaded with nanoparticles on an inclined substrate.

physics.flu-dyn↗

arXiv2022

Droplet size distribution in a swirl airstream using in-line holography technique

We investigate the morphology and size distribution of satellite droplets resulting from the interaction of a freely falling water droplet with a swirling airstream of different strengths by employing shadowgraphy and deep-learning-based digital in-line holography techniques. We found that the droplet exhibits vibrational, retracting bag and normal breakup phenomena for the no swirl, low and high swirl strengths for the same aerodynamic field. In the high swirl scenario, the disintegrations of the nodes, rim, and bag-film contribute to the number mean diameter, resulting in smaller satellite droplets. In contrast, in the low swirl case, the breakup of the rim and nodes only contributes to the size distribution, resulting in larger droplets. The temporal variation of the Sauter mean diameter reveals that for a given aerodynamic force, a high swirl strength produces more surface area and surface energy than a low swirl strength. The theoretical prediction of the number-mean probability density of tiny satellite droplets under swirl conditions agrees with experimental data. However, for the low swirl, the predictions differ from the experimental results, particularly due to the presence of large satellite droplets. Our results reveal that the volume-weighted droplet size distribution exhibits two (bi-modal) and three (multi-model) peaks for low and high swirl strengths, respectively. The analytical model that takes into account various mechanisms, such as the nodes, rim, and bag breakups, accurately predicts the shape and characteristic sizes of each mode for the case of high swirl strength.

physics.flu-dyn↗

arXiv2022

Counter-intuitive evaporation in nanofluids droplets due to stick-slip nature

We experimentally investigate the evaporation characteristics of a sessile ethanol droplet containing Al$_2$O$_3$ and Cu nanoparticles of sizes 25 nm and 75 nm on a heated substrate using shadowgraphy and infrared imaging techniques. Our results demonstrate that the droplet contact line dynamics resulting from the presence of various nanoparticles plays a dominant role in the evaporation process. This is in contrast to the widely-held assumption that the enhanced evaporation rate observed in sessile nanofluid droplets is due to the higher thermal conductivity of the added nanoparticles. We observe that even though the thermal conductivity of Al$_2$O$_3$ is an order of magnitude lower than that of Cu, droplets containing 25 nm-sized Al$_2$O$_3$ exhibit pinned contact line dynamics and evaporate much more rapidly than droplets containing Cu nanoparticles of both sizes and 75 nm Al$_2$O$_3$ nanoparticles that exhibit stick-slip behaviour. We also found that the droplets with different nanoparticles display distinct thermal patterns due to the difference in contact line behaviour, which alters the heat transfer inside the droplets. We establish this counter-intuitive observation by analysing the temporal variations of the perimeter, free surface area, and deposition patterns on the substrate.

physics.flu-dyn↗

arXiv2021

Two-layer channel flow involving a fluid with time-dependent viscosity

A pressure-driven two-layer channel flow of a Newtonian fluid with constant viscosity (top layer) and a fluid with a time-dependent viscosity (bottom layer) is numerically investigated. The bottom layer goes through an aging process in which its viscosity increases due to the formation of internal structure, which is represented by a Coussot-type relationship. The resultant flow dynamics is the consequence of the competition between structuration and destructuration, as characterized by the dimensionless timescale for structuration (tau) and the dimensionless material property (beta) of the bottom fluid. The development of Kelvin-Helmholtz type instabilities (roll-up structures) observed in the Newtonian constant viscosity case was found to be suppressed as the viscosity of the bottom layer increased over time. It is found that, for the set of parameters considered in the present study, the bottom layer almost behaves like a Newtonian fluid with constant viscosity for tau greater than 10 and beta greater than 1. It is also shown that decreasing the value of the Froude number stabilizes the interfacial instabilities. The wavelength of the interfacial wave increases as the capillary number increases.

physics.flu-dyn↗

arXiv2020

Lifetime of sessile saliva droplets in the context of SARS-CoV-2

Spreading of respiratory diseases, such as COVID-19, from contaminated surfaces is dependent on the drying time of the deposited droplets containing the virus. The evaporation rate depends on environmental conditions, such as ambient temperature and relative humidity and physical properties (e.g., droplet volume, contact angle and composition). The respiratory droplets contain salt (NaCl), protein (mucin), and surfactant (dipalmitoylphosphatidylcholine) in addition to water, which are expected to influence the evaporation in a big way. A diffusion-based theoretical model for estimating the drying time is developed which takes into account the dynamic contact angle of saliva droplets laden with salt and insoluble surfactants. The effect of the initial volume, contact angle, salinity, surfactant concentration, ambient temperature and relative humidity on the drying time of droplets is investigated.

cond-mat.soft↗