arXiv subjects

Swetaprovo Chaudhuri

Publications and source records attributed to Swetaprovo Chaudhuri.

14 recordsLinked to original sources

arXiv2026

Effects of fuel and soot concentrations on the inception and development of contrails

Fundamental questions related to the roles of fuel type, combustion parameters, and turbulence transport interactions in the inception and growth of contrails have remained intractable in remote sensing and in-flight measurements. Consequently, we developed a novel laboratory-scale facility for studying the inception, growth and persistence of contrails for aircraft-relevant conditions. The set of exhaust conditions, generated using an inverted co-flow soot generator at a set of global equivalence ratio for two fuels - ethylene and propane, is supplied to the contrail tunnel which then mixes with an ambient flow emulating long-haul aircraft cruise conditions (\SI{20.8}{kPa} and \SI{190}{K}). Detailed soot characterization using a scanning mobility particle sizer and transmission electron microscopy is coupled with measurements of instantaneous and averaged scattering intensities from the generated contrails. The experimental results are complemented by numerical simulations of the contrail tunnel using solutions of the Favre-averaged Navier-Stokes (FANS) equation and a two-equation model for handling particulate matter, including soot and ice. Results show the first experimental snapshots of a contrail cross section, highlighting the interaction of turbulent mixing and microphysical growth scales involved in ice nucleation across the shear layers. As expected, the average scattering intensities of contrails increase with soot number concentrations and water vapor content. Comparisons between ethylene and propane exhausts indicate that the scattering propensity of contrails is more sensitive to exhaust water vapor content than to soot concentrations. Finally, depolarization measurements are used to show asphericity in ice crystal habits. Thus, our study present a unique window into contrail formation, theoretical modeling and simulation.

physics.flu-dyn↗

arXiv2024

How "mixing" affects propagation and structure of intensely turbulent, lean, hydrogen-air premixed flames

Understanding how intrinsically fast hydrogen-air premixed flames can be rendered much faster in turbulence is crucial for systematically developing hydrogen-based gas turbines and spark ignition engines. Here, we present fundamental insights into the variation of flame displacement speeds by investigating how the disrupted flame structure affects speed and vice-versa. Three DNS cases of lean hydrogen-air mixtures with $Le$ from 0.5 to 1 and $Ka$ from 100 to 1000 are analyzed. Suitable comparisons are made with the closest canonical laminar flame configurations at same mixture conditions and their suitability and limitations in expounding turbulent flame properties are elucidated. Since near zero-curvature surface locations are most probable and representative of the average flame geometry in such large $Ka$ flames, this study focuses on the statistical variation of flame displacement speed and the concomitant change in flame structure at those locations. Relevant flame properties are averaged normal to the zero-curvature isotherm regions to obtain the conditional mean flame structures. In the smallest $Le$ case, downstream of the most probable zero-curvature regions, the temperature exceeds that of the standard laminar flame, leading to enhanced local thermal gradient and flame speed. This is due to increased heat-release rate contribution by differential diffusion in positive curvatures downstream of the zero-curvature locations. Furthermore, locally, the flame structure is broadened for all cases due to a reversal in the direction of the flame speed gradient. This reversal is caused by cylindrical flame-flame interactions upstream of the zero-curvature regions, resulting in localized scalar mixing within the flame structure. These non-local effects, in combination, define the mean flame structure and the associated variation in local flame speed in turbulent premixed flames.

physics.flu-dyn↗

arXiv2023

On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction

Given the need to develop zero-carbon combustors for power and aircraft engine applications, $S_d$ of a turbulent premixed flame, especially for H$_2$-air, is of immediate interest. The present study investigates 3D DNS cases of premixed H$_2$-air turbulent flames at varied pressures for different $Re_t$ and $Ka$ with detailed chemistry to theoretically model $S_d$ at negative curvatures. Prior studies at atmospheric pressure showed $\widetilde{S_d}$ to be enhanced significantly over $S_L$ at large negative $κ$ due to flame-flame interactions. 1D simulations of an imploding cylindrical H$_2$-air laminar premixed flame used to represent the local flame surfaces undergoing flame-flame interaction in a turbulent flame at the corresponding pressure conditions are performed to understand the interaction dynamics. These simulations emphasized the transient nature of the flame structure during flame-flame interactions and enabled analytical modeling of $\widetilde{S_d}$ at these regions of extreme negative $κ$ of the 3D DNS. The JPDF of $\widetilde{S_d}$ and $κ$ and the corresponding conditional averages from 3D DNS showed a negative correlation between $\widetilde{S_d}$ and $κ$. The model successfully predicts the variation of $\langle\widetilde{S_d}|_κ\rangle$ with $κ$ for the regions on the flame surface with $κδ_L \! \ll \! -1$ at all pressures, with good accuracy. This shows the aforementioned configuration to be fruitful in representing local flame-flame interaction in 3D turbulent flames. Moreover, at $κ=0$, on average $\widetilde{S_d}$ can deviate from $S_L$, manifested by the internal flame structure, controlled by turbulence transport in the large $Ka$ regime. Thus, the correlation of $\langle\widetilde{S_d}\rangle/S_L$ with $\langle|\widehat{\nabla c}|_{c_0}\rangle$ at $κ=0$ is explored.

physics.flu-dyn↗

arXiv2022

Mean-field synchronization model for open-loop, swirl controlled thermoacoustic system

Open-loop control is known to be an effective strategy for controlling self-excited thermoacoustic oscillations in turbulent combustors. In this study, we investigate the suppression of thermoacoustic instability in a lean premixed, laboratory-scale combustor using experiments and analysis. Starting with a self-excited thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate transitions the system from thermoacoustic instability to the suppressed state through a state of intermittency. To model such transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of mean-field phase oscillators and the basis expansion of the acoustic pressure governing equation. The assumption that coupling strength among the oscillators is a linear combination of acoustic and swirler rotation frequency is justified \textit{a posteriori}. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model replicates the bifurcation characteristics, time series, probability density function (PDF), and power spectral density (PSD) of the various dynamical states observed during the transition to the suppressed state, to excellent accuracy. Specifically, the model captures the change in the PDF of pressure and heat release rate fluctuations from a bimodal distribution during thermoacoustic instability to a unimodal distribution during suppression. Finally, we discuss the global and local flame dynamics and show that the model qualitatively captures various aspects of spatio-temporal synchronization that underlies the transition.

physics.flu-dyn↗

arXiv2022

Mean-field synchronization model of turbulent thermoacoustic transitions

Thermoacoustic instabilities observed in turbulent combustion systems have disastrous consequences and are notoriously challenging to model, predict and control. Here, we introduce a mean-field model of thermoacoustic transitions, where the nonlinear flame response is modeled as the amplitude weighted response of an ensemble of phase oscillators constrained to collectively evolve at the rhythm of acoustic fluctuations. Starting from the acoustic wave equation coupled with the phase oscillators, we derive the evolution equations for the amplitude and phase and obtain the limit cycle solution. We show that the model captures abrupt and continuous transition to thermoacoustic instability observed in disparate combustors. We obtain quantitative insights into the model by estimating the model parameters from the experimental data using parameter optimisation. Importantly, our approach provides an explanation of spatiotemporal synchronization and pattern-formation underlying the transition to thermoacoustic instability while encapsulating the statistical properties of desynchronization, chimeras, and global phase synchronization. We further show using the model that continuous and abrupt transitions to limit cycle oscillations in turbulent combustors corresponds to synchronization transitions of \textit{second-order} and \textit{first-order}, respectively. The present formulation provides a highly interpretable model of thermoacoustic transitions: changes in empirical bifurcation parameters which lead to limit cycle oscillations amounts to an increase in the coupling strength of the phase oscillators, promoting global phase synchronization. The generality of the model in capturing different types of transitions and states of pattern-formation highlights the possibility of extending the present model to a broad range of fluid-dynamical phenomena beyond thermoacoustics.

physics.flu-dyn↗

arXiv2022

Turbulent flame speed based on the mass flow rate: theory and DNS

Starting with an integral formulation of mass flow rate through an ensemble of isotherms constituting a statistically planar, turbulent premixed flame, a scaling for the corresponding turbulent flame speed is derived without invoking Damk{ö}hler's hypotheses. Major approximations and interim results are validated using a large Karlovitz number, unity Lewis number, Direct Numerical Simulation (DNS) dataset of n-heptane/air mixture, computed with reduced chemistry. A new length scale quantifying the fluctuation distance of the isotherms within the premixed flame structure is introduced.

physics.flu-dyn↗

arXiv2021

Penetration and Secondary Atomization of Droplets Impacted on Wet Facemasks

Face covering, commonly known as facemask, is considered to be one of the most effective Personal Protective Equipments (PPEs) to reduce transmissions of pathogens through respiratory droplets - both large drops and liquid aerosol particles. Face masks, not only inhibit the expulsion of such respiratory droplets from the user, but also protects the user from inhaling pathogen-laden potentially harmful droplets or their dried nuclei. While the efficacies of various dry face masks have been explored in the recent past, a comprehensive investigation of a wet mask is lacking. Yet, users wear masks for a long period of time and during this period, owing to respiratory droplets released through multiple respiratory events, the mask matrix becomes wet. We, herein, present an experimental study on the dynamics of sequential impacts of droplets on masks to understand how wetness affects possible penetration and secondary atomization of the impacted droplet. Two different types of masks, hydrophobic and hydrophilic, were used in this study to evaluate the underlying physical mechanism that controls the penetration in each of them.

physics.flu-dyn↗

arXiv2021

Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence

Comprehensive knowledge of local flame displacement speed, $S_d$, in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, $S_d$ is enhanced much beyond its corresponding unstretched, planar laminar value $S_L$. In this study, the effect of high Karlovitz number ($Ka$) turbulence on density-weighted flame displacement speed, $\widetilde{S_d}$, in a H$_2$-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of $\widetilde{S_d}$ from $S_L$, for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model's applicability to intensely turbulent flames characterized by large $Ka$. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to $\widetilde{S_d}$ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced $\widetilde{S_d}$ is driven by interactions among complete flame structures, $\widetilde{S_d}$ enhancement in high $Re_t$ and high $Ka$ flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in $\widetilde{S_d}$ in regions of large negative curvature occurs as a result of these interactions, evincing that the interaction model is useful for high $Ka$ turbulent premixed flames as well.

physics.flu-dyn↗

arXiv2020

On Secondary Atomization and blockage of surrogate cough droplets in single and multi-layer face masks

By now it is well-understood that the usage of facemasks provides protection from transmission of viral loads through exhalation and inhalation of respiratory droplets. Therefore, during the current Covid-19 pandemic the usage of face masks is strongly recommended by health officials. Although three-layer masks are generally advised for usage, many commonly available or homemade masks contain only single and double layers. In this study, we show through detailed physics based analyses and high speed imaging that high momentum cough droplets on impingement on single- and double-layer masks can lead to significant partial penetration and more importantly atomization into numerous much smaller daughter droplets, thereby increasing the total population of the aerosol, which can remain suspended for a longer time. The possibility of secondary atomization of high momentum cough droplets due to impingement, hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the mask has not been explored before. However, this unique mode of aerosol generation poses a finite risk of infection as shown in this work. We also demonstrate that in single layer masks close to 70 % of a given droplet volume is atomized and only 30 % is trapped within the fibers. The entrapped volume is close to 90 % for double layer masks which still allows some atomization into smaller droplets. We however found that a triple-layer surgical mask permits negligible penetration and hence should be effective in preventing disease transmission.

physics.app-ph↗

arXiv2020

Analyzing the dominant SARS-CoV-2 transmission routes towards an ab initio SEIR model

Identifying the relative importance of the different transmission routes of the SARS-CoV-2 virus is an urgent research priority. To that end, the different transmission routes, and their role in determining the evolution of the Covid-19 pandemic are analyzed in this work. Probability of infection caused by inhaling virus-laden droplets (initial, ejection diameters between $0.5-750μm$) and the corresponding desiccated nuclei that mostly encapsulate the virions post droplet evaporation, are individually calculated. At typical, air-conditioned yet quiescent indoor space, for average viral loading, cough droplets of initial diameter between $10-50 μm$ have the highest infection probability. However, by the time they are inhaled, the diameters reduce to about $1/6^{th}$ of their initial diameters. While the initially near unity infection probability due to droplets rapidly decays within the first $25s$, the small yet persistent infection probability of desiccated nuclei decays appreciably only by $\mathcal{O} (1000s)$, assuming the virus sustains equally well within the dried droplet nuclei as in the droplets. Combined with molecular collision theory adapted to calculate frequency of contact between the susceptible population and the droplet/nuclei cloud, infection rate constants are derived ab-initio, leading to a SEIR model applicable for any respiratory event - vector combination. Viral load, minimum infectious dose, sensitivity of the virus half-life to the phase of its vector and dilution of the respiratory jet/puff by the entraining air are shown to mechanistically determine specific physical modes of transmission and variation in the basic reproduction number $\mathcal{R}_0$, from first principle calculations.

q-bio.PE↗

arXiv2020

Insights on drying and precipitation dynamics of respiratory droplets in the perspective of Covid-19

We isolate a nano-colloidal droplet of surrogate mucosalivary fluid to gain fundamental insights into the infectivity of air borne nuclei during the Covid-19 pandemic. Evaporation experiments are performed with salt-water solutions seeded with a viral load of inactive nanoparticles in an acoustic levitator. We seek to emulate the drying, flow and precipitation dynamics of such air borne mucosalivary droplets. Observations with the surrogate fluid are validated by similar experiments with actual samples from a healthy subject. A unique feature emerges with regards to the final crystallite dimension; it is always 20-30% of the initial droplet diameter for different sizes and ambient conditions. The preserved precipitates from levitated droplets show that 15% of the total virion population remain dispersed on the outer surface of air-desiccated air borne nuclei. This fraction increases to ~90% if the respiratory droplets (of larger initial size) settle on a surface and then evaporate in the sessile mode.

cond-mat.soft↗

arXiv2020

Flame dynamics during intermittency and secondary bifurcation to longitudinal thermoacoustic instability in a swirl-stabilized annular combustor

In this experimental study on a laboratory-scale turbulent annular combustor with sixteen swirl-stabilized burners, we study the flame-flame and flame-acoustic interactions during different dynamical states associated with the longitudinal mode of the combustor. We simultaneously measure the acoustic pressure and CH* chemiluminescence emission of the flame using a high-speed camera. Upon increasing the equivalence ratio, the combustor undergoes the following sequence of transition: combustion noise (CN) to low amplitude longitudinal thermoacoustic instability (TAI) through the state of intermittency (INT), and from low amplitude to high amplitude longitudinal TAI through a secondary bifurcation. We report the first evidence of secondary bifurcation from low amplitude TAI to high amplitude TAI for a turbulent thermoacoustic system which allows us to test the flame response at two different amplitude of perturbation in a natural setting. We find a significant difference in the dynamics of the flame interactions during the periodic part of intermittency and low and high amplitude TAI. Specifically, during the periodic part of intermittency, the phase difference between the local heat release rate (HRR) measured from various burners show significant phase slips in time. During low amplitude TAI, there are fewer phase slips among the HRR response of the burners, which result in a state of weak synchronization among the flames. During high amplitude TAI, we find that the flames are in perfect synchrony amongst themselves and with the pressure fluctuations. We then quantify the degree of temporal and spatial synchronization between different flames, and flames and pressure fluctuations using the Kuramoto order parameter and the phase-locking value. We show that synchronization theory can be conveniently used to characterize and quantify flame-acoustic interactions in an annular combustor.

physics.flu-dyn↗

arXiv2020

Modeling ambient temperature and relative humidity sensitivity of respiratory droplets and their role in Covid-19 outbreaks

One of the many unresolved questions that revolves around the Covid-19 pandemic is whether local outbreaks can depend on ambient conditions like temperature and relative humidity. In this paper, we develop a model that tries to explain and describe the temperature and relative humidity sensitivity of respiratory droplets and their possible connection in determining viral outbreaks. The model has two parts. First, we model the growth rate of the infected population based on a reaction mechanism - the final equations of which are similar to the well-known SIR model. The advantage of modeling the pandemic using the reaction mechanism is that the rate constants have sound physical interpretation. The infection rate constant is derived using collision rate theory and shown to be a function of the respiratory droplet lifetime. In the second part, we have emulated the respiratory droplets responsible for disease transmission as salt solution droplets and computed their evaporation time accounting for droplet cooling, heat and mass transfer and finally crystallization of the salt. The model output favourably compares with the experimentally obtained evaporation characteristics of levitated droplets of pure water and salt solution, respectively, ensuring fidelity of the model. Droplet evaporation/desiccation time is indeed dependent on ambient temperature and relative humidity, considered at both outdoor and indoor conditions. Since the droplet evaporation time determines the infection rate constant, ambient temperature and relative humidity are shown to impact the outbreak growth rates.

physics.flu-dyn↗

arXiv2013

Turbulent Flame Speed Scaling for Expanding Flames with Markstein Diffusion Considerations

In this work we clarify the role of Markstein diffusivity on turbulent flame speed and it's scaling, from analysis and experimental measurements on constant-pressure expanding flames propagating in near isotropic turbulence. For all C0-C4 hydrocarbon-air mixtures presented in this work and recently published C8 data from Leeds, the normalized turbulent flame speed data of individual mixtures approximately follows the recent theoretical and experimental $Re_{T,f}^{0.5} $ scaling, where the average radius is the length scale and thermal diffusivity is the transport property. We observe that for a constant $Re_{T,f} $, the normalized turbulent flame speed decreases with increasing Markstein Number. This could be explained by considering Markstein diffusivity as the large wavenumber, flame surface fluctuation dissipation mechanism. As originally suggested by the theory, replacing thermal diffusivity with Markstein diffusivity in the turbulence Reynolds number definition above, the present and Leeds dataset could be scaled by the new $Re_{T,M}^{0.5} $irrespective of the fuel considered, equivalence ratio, pressure and turbulence intensity for positive Mk flames over a large range of Damköhler numbers.

physics.flu-dyn↗