Abhishek Saha

Droplet impact on mildly heated liquid films is important in applications such as cooling and printing. However, most previous studies on heated substrates have focused on superheated conditions, in which the substrate temperature exceeds the liquid saturation temperature, and evaporation strongly alters the impact dynamics. In this study, we experimentally investigate droplet impact on heated subcooled films, where the film temperature remains below saturation, with particular emphasis on how heating modifies the critical conditions for the transition from noncoalescence to coalescence outcomes. Regime maps obtained at different temperatures are used to characterize these transitions. We further derive a scaling relation for interfacial gas layer drainage that incorporates phase-change effects. The combined scaling analysis and experiments with different liquids reveal the key physical mechanisms governing the critical impact speed required for achieving complete coalescence on mildly heated films.

The influence of flow non-uniformity and unsteadiness on premixed flames is of considerable interest due to its relevance to practical combustion systems. The steady counterflow flame has long served as a canonical configuration for investigating flame dynamics under controlled, spatially non-uniform conditions. A commonly studied variation, referred to as the unsteady counterflow, introduces a temporal perturbation to the otherwise steady flow from the nozzles, thereby enabling the systematic examination of the coupled effects of unsteadiness and non-uniformity. Prior investigations have focused on flame dynamics along the line of symmetry, where the reduced dimensionality of the problem facilitates analysis. In the present study, we extend this perspective by experimentally examining flame behavior at off-center locations, where multi-dimensional effects of non-uniformity and unsteadiness are more pronounced. Results reveal markedly different dynamics away from the centerline, characterized by a dominant contribution from higher harmonic responses. Further analysis of the associated vortex dynamics in the shear layer demonstrates that the radial variations in the intensity of these vortical structures directly govern the variations in the strength of the observed higher harmonics, and thereby the altered flame behavior. Novelty and significance statement While the counterflow configuration is a widely used canonical model for studying flames subjected to unsteady strain rates, prior investigations have primarily focused on centerline or symmetry-plane behavior. This study expands that framework by systematically examining both centerline and off-center flame dynamics, revealing pronounced spatial variations in the spectral response. In particular, the results uncover distinct spectral signatures associated with the coupling between imposed unsteadiness and vortex shedding in off-center regions, which are not observable from centerline analyses alone. These off-center perspectives extend the relevance of counterflow studies to other canonical flame configurations, such as bluff-body-stabilized and jet flames, where flame-vortex interactions play a central role in stabilization. The explored unsteady dynamics of off-center locations are also relevant for practical combustors, where flames are often asymmetric and highly unsteady.

Hydrogen-ammonia blends are increasingly considered as promising carbon-free fuels for future combustion systems. In this study, we investigate the dynamics of cellular instabilities in hydrogen-ammonia Bunsen flames. While previous studies have examined such instability phenomena in hydrocarbon and pure hydrogen flames, the influence of ammonia addition, particularly under flow-driven configuration, remains less explored. Using chemiluminescence and Mie-scattering imaging, we characterize the effect of ammonia concentration on the transition between stable and unstable flame regimes and on the morphology of the resulting polyhedral structures. Our results show that increasing ammonia content narrows the range of unstable operating conditions due to its lower chemical reactivity, whereas higher hydrogen fractions promote flashback, thereby limiting stable flame operation. The observed transitions and modifications in flame morphology are further interpreted using scaling analyses derived from the corresponding dispersion relations. Novelty and significance statement: While hydrogen-ammonia blends are widely regarded as promising carbon-free fuels, important questions remain regarding their suitability for practical engine operation. Pure hydrogen flames, especially under fuel-lean conditions, are susceptible to cellular instabilities that induce intrinsic self-acceleration and thereby, elevated propagation speeds. This study demonstrates how the addition of ammonia to hydrogen modulates the transition between stable and unstable regimes. While cellular instabilities are most often studied in freely expanding flames, we explore them in a burner-stabilized configuration. To the authors’ best knowledge, this is the first study to systematically investigate how varying the degree of ammonia blending in hydrogen affects the morphology, stability, and transition dynamics of polyhedral flames. In addition, the study also confirms that intense local stretch near the cusps of polyhedral ridges can trigger localized extinction. Beyond its fundamental significance, the experimental observations and accompanying scaling analysis offer a practical framework for identifying operating conditions that suppress such instabilities.

In fluid flow imaging, intensity gradients are a good measure of spatial variations in scalar properties, which play an important role in controlling transport processes. However, current flow imaging techniques exhibit system-limited spatial resolutions, thus inhibiting the ability to accurately detect intensity gradients. To address this challenge, we present a method and system, inspired by Structured Illumination Microscopy (SIM), which can be implemented in dynamic flow imaging to enhance pixel resolution and, thereby, the estimation of scalar gradients. We utilize sub-pixel-scale patterned light matching the system pixel scale and multi-frame imaging that creates quasi-static images over four frames, with scalability for high-speed imaging. These multi-frame images are then processed using a bespoke recombination algorithm that produces a new image with twice the pixel resolution compared to the original images. The sub-pixel spatial-resolution enhancement capabilities are shown with static images and dynamic fluid flow, for which enhancement in the flow gradient is demonstrated.

The Rijke tube is widely used in the literature to study combustion dynamics, offering a simple, self-excited setup for laboratory-scale experiments. While many studies utilize laminar multi-flame burners, most analyses focus on changes in pressure and global heat release rate, often overlooking interactions between individual flames. This study experimentally investigates the role of individual flame dynamics and their influence on neighboring flames in shaping overall pressure fluctuations. By varying the hydrogen percentage in premixed hydrogen/propane/air flames, we demonstrate how the system transitions from periodic oscillations to quasi-periodic oscillations and, ultimately, to half-integer subharmonic oscillations. Through high-speed imaging of individual flames in a seven-flame burner, we further reveal the emergence of an alternating oscillation pattern from interactions between flames and vortex shedding. This effect intensifies with increasing hydrogen content. Additionally, we compare the fundamental modes of the air column in the Rijke tube and the harmonics of flame oscillations to illustrate how energy within the pressure dynamics is redistributed among different frequencies. Novelty and Significance Statement Previous studies with Rijke tube often used these multi-flame configurations to create complex combustion dynamics, yet the genesis of this complexity remained unexplored. This research explores such flames from different perspectives. The novelty in our approach is that we visualized and analyzed the dynamics of individual flames, while previous studies focused only on collective dynamics. This paved the way for investigating local interactions, which unveiled that the interactions between vortices shed by neighboring flames lead to subharmonic bifurcations, a critical transition process for combustion dynamics. • YW designed the study parameters, led the experiments, data collection, and analysis, and wrote the manuscript. • YZ supported the experiments, data collection, analyses, and manuscript writing. • AS conceptualized the study, performed limited experiments, supervised the project, secured funding, and reviewed and revised the manuscript.

A gas bubble in an unbounded liquid medium undergoing heat and mass transfer deforms as the bubble surface expands or contracts. Thermocapillary instabilities on the surface of bubbles can lead to convective flow in the near-field of the bubble. If the evaporation-induced thermocapillary instability is large enough, the interfacial tension gradients can sustain the instability and lead to the emergence of localized zones of cooler temperatures, herein referred to as thermal islands. This work uses detailed finite element numerical simulations to accurately resolve the transport and fluid dynamics around the gas–liquid interface. The interaction between shape oscillations, heat and mass transfer, and the fluid flow around the bubble is analyzed. We establish a correlation between the tangential velocities and the temperature gradient along the interface, thus emphasizing the role of the thermocapillary effect. We show how heat transfer in one direction may lead to stability with suppression of the thermal islands, yet the system becomes unstable when the direction is reversed. A discussion on the effect of appropriate dimensionless parameters on the stability of the perturbations is also presented. This investigation provides new insight into the dynamic behavior of nonequilibrium bubbles undergoing heat and mass transfer. It has practical implications for precisely controlling the transfer at gas–liquid interfaces.

Abstract Mycobacterium tuberculosis (Mtb) employs multiple virulence factors, including cell wall-associated proteins, to evade host immune responses. PE11, a cell wall-localized esterase, contributes to Mtb persistence by facilitating cell wall remodelling and resistance to acidic and antibiotic stress. Herein we describe a novel role of PE11 in subverting host autophagy through disruption of TFEB-mediated lysosomal function. PE11 promotes FLCN-dependent depletion of intracellular lactate to destabilize TFEB and thereby downregulate genes essential for autophagic flux and lysosomal acidification. Using a PE11-deficient Mtb strain, we demonstrate that PE11 targets the FLCN–lactate axis to regulate TFEB stability. Exogenous lactate supplementation restored TFEB stability, enhanced lysosomal acidification, and significantly reduced intracellular bacterial burden. Lactate also synergized with frontline anti-tubercular drugs to improve Mtb clearance. These findings establish PE11 as a key immune evasion factor and highlight lactate as a promising host-directed therapeutic to enhance bacterial killing and reduce antibiotic-associated toxicity.

Abstract Acmella paniculata (AP), an ethnomedicinal plant, is valued for the management of pain, inflammation, arthritis, infection, diabetes and as tribal cuisine The study focused on validating AP against inflammatory pain, rheumatoid arthritis (RA), and hyperglycemia. Hydro‐ethanolic extract of AP flower (HEFeAP) was prepared via maceration, followed by ultrasonication. Physicochemical characterization was done using Thin Layer Chromatography (TLC), Liquid Chromatography‐Mass Spectrometry (LC‐MS), Ultra‐violet Spectroscopy (UV), and Fourier Transform Infrared Spectroscopy (FTIR). Major phytochemicals were assessed against inflammatory mediators using molecular docking and Molecular Dynamics (MD) simulations. The antioxidant potential of HEFeAP was assessed using the 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) assay (31.22 ± 0.53 µg mL −1 ) and hydrogen peroxide (H₂O₂) scavenging assay (40.36 ± 0.47 µg mL −1 ). The anti‐inflammatory activity, protective role against RA, and anti‐hyperglycaemic activities were evaluated via egg‐albumin denaturation, MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide) assay against human‐fibroblast cells and α‐amylase inhibition, showing IC 50 of 27.61, 65.75, and 85.43 µg mL −1 , respectively. Tumor Necrosis Factor‐alpha (TNF‐α), and nitric‐oxide (NO) levels were significantly downregulated in LPS‐stimulated PBMC‐derived macrophages (IC 50 = 152 and 164.56 µg mL −1 ). Phytochemical analysis revealed coumarins, flavonoids, polyphenols, steroids, and local‐anaesthetic alkylamides. Active phytomolecules in HEFeAP exhibited strong and stable binding with inflammatory biomarkers. These findings suggest HEFeAP can be used as a potential candidate for inflammatory pain, RA, and diabetes.

Further enhancements in aircraft engine development will rely in significant part on facile control of fuel spray characteristics like droplet size, size distribution, and intact jet-to-droplet length. Aircraft engines suffer from extreme trade-offs when exceptional performance is needed. For example, high fuel consumption, poor propulsive efficiency, and a significant increase in both noise and a radar-visible exhaust gas plume often accompany high-thrust engine states. We present a novel nozzle pintile design that utilizes a single-crystal piezoelectric ultrasonic device to induce more rapid and complete fuel atomization. Preliminary results of experiments where the liquid sheets are injected into quiescent air show that the device increases the efficiency of fuel burning even in harsh environments resembling that of a combustion engine. Specifically, we show that at Weber numbers applicable to fuel flow rates, the device increases the amount of smaller droplets and shrinks the distribution of droplet sizes. Furthermore, these results appear as the liquid's surface tension decreases. With these promising results, we plan to test design variations.

Abstract Droplet impact on substrates is the cornerstone of several processes relevant to many industrial applications. Imposing substrate oscillation modifies the impact dynamics and can, therefore, be used to control the ensuing heat, mass, and energy transfer between the substrate and the impacting droplet. Previous research has shown that substrate oscillation strongly influences the spreading behavior of the droplet. In this study, we extend this understanding to examine how substrate oscillations can further modulate the retraction dynamics of the droplet, consequently affecting its long-term behavior, with a particular focus on induced jetting and subsequent breakup. We systematically examine the breakup of jets formed by the recoiling droplet through experimental investigations across a range of oscillation frequencies and amplitudes. Our findings reveal two distinct jet breakup modes: early and late, each governed by different time scales. Subsequently, we present a mechanistic description of the jetting process. Furthermore, we derive a simple scaling analysis based on energy balance to identify the critical condition required for jet breakup. Finally, we compare the experimental data with the scaling analyses to show its efficacy.

The lipolytic enzymes of Mycobacterium tuberculosis play a critical role in immunomodulation and virulence. Among these proteins, PE11 which also belongs to the PE/PPE family, is the smallest (∼10.8 kDa) and play a significant role in cell wall remodelling and virulence. PE11 is established to be an esterase , but its enzymatic and structural properties are not yet characterized. In this study, using homology modelling we deduced the putative structure which shows the presence of both α-helix and β-sheet structures which is in close agreement with that observed by CD spectra of the purified protein. PE11 was found to contain a Gx 3 Sx 4 G motif homologous to canonical ‘GxSxG’ motif present in many serin hydrolases . The catalytic triad appears to be located within this motif as substitution of Serine 26 and Glycine 31 residues abrogated its enzymatic activity . Gel-filtration chromatography data indicate that PE11 possibly exists as dimer and tetramer showing positive cooperativity for binding its substrates . In addition, PE11 esterase activity was found to be critical for cell wall remodelling, antibiotic resistance and conferring survival advantages to M. tuberculosis . Our data suggest that PE11 can be targeted for designing potential therapeutic strategies.

Abstract Rheumatoid arthritis (RA) and associated inflammatory complications are the most prevalent illnesses and can turn into fatal conditions if left untreated. Allopathic medicine is not satisfactory for curing RA. Scientific literature reports reveal that several phyto‐compounds viz . flavonoids, saponins, and terpenoids, can heal joints and organs from auto‐inflammatory rheumatoid arthritis and pain. Gene ontology, gene network analysis, molecular clustering, and literature review were used to optimise RA‐specific highly expressed genes. In‐silico molecular docking was performed to short‐out potential phytomolecules (Neohesperidin dihydrochalcone (NHDC)) from 1000 datasets‐library against RA and validate using MD simulation running at 100 ns. In‐vitro anti‐inflammatory assays of NHDC inhibited egg‐albumin denaturation, IC 50 of 47.739±0.51 μg/ml. The ex‐vivo MTT assay with NHDC rendered 67.209 % inhibition at 100 μM against fd‐FLS‐cells. NHDC downregulated pro‐inflammatory cytokine IL‐17 A production by 61.11 % and 50 % at 300 and 200 μM, respectively. Thus, this Studies recommend that NHDC may be highlighted as a novel multi‐target PADI4 and JAK3 inhibitor with better efficacy and minimal toxicity in RA warranted to In‐Vivo and clinical investigation. The current findings have uncovered remarkable genes and signalling pathways linked to RA, which could enhance our existing comprehension of the molecular mechanisms that drive its development and progression.

Abstract The lipolytic enzymes of Mycobacterium tuberculosis play a critical role in immunomodulation and virulence. Among these proteins, PE11 which also belongs to the PE/PPE family, is the smallest (∼10.8 kDa) and play a significant role in cell wall remodelling and virulence. PE11 is established to be an esterase, but its enzymatic and structural properties are not yet characterized. In this study, using homology modelling we deduced the putative structure which shows the presence of both α-helix and β-sheet structures which is in close agreement with that observed by CD spectra of the purified protein. PE11 was found to contain a GX3SX4G motif homologous to canonical ‘GxSxG’ motif present in many serin hydrolases. The catalytic triad appears to be located within this motif as substitution of Serine 26 and Glycine 31 residues abrogated its enzymatic activity. Gel-filtration chromatography data indicate that PE11 possibly exists as dimer and tetramer showing positive cooperativity for binding its substrates. In addition, PE11 esterase activity was found to be critical for cell wall remodelling, antibiotic resistance and conferring survival advantages to M. tuberculosis . Our data suggest that PE11 can be targeted for designing potential therapeutic strategies.

Droplet impact on oscillating substrates is important for both natural and industrial processes. Recognizing the importance of the dynamics that arises from the interplay between droplet transport and substrate motion, in this work, we present an experimental investigation of the spreading of a droplet impacting a sinusoidally oscillating hydrophobic substrate. We focus particularly on the maximum spread of droplets as a function of various parameters of substrate oscillation. We first quantify the maximum spreading diameter attained by the droplets as a function of frequency, amplitude of vibration, and phase at the impact for various impact velocities. We highlight that there can be two stages of spreading. Stage I, which is observed at all impact conditions, is controlled by the droplet inertia and affected by the substrate oscillation. For certain conditions, a Stage II spreading is also observed, which occurs during the retraction process of Stage I due to additional energies imparted by the substrate oscillation. Subsequently, we derive scaling analyses to predict the maximum spreading diameters and the time for this maximum spread for both Stage I and Stage II. Furthermore, we identify the necessary condition for Stage II spreading to be greater than Stage I spreading. The results will enable optimization of the parameters in applications where substrate oscillation is used to control the droplet spread, and thus heat and mass transfer between the droplet and the substrate.

The local interactions between the flame-front and turbulence control the dynamics, morphology, and propagation of a premixed turbulent flame. To investigate such complex dynamics of a flame–turbulence interaction, we present an experimental exposition of a premixed turbulent Bunsen flame. Several quantities have been evaluated to assess the flame–turbulence interaction. We first measured the statistics of the flowfield adjacent to the flame and compared it with the cold flow. This allowed us to evaluate the effect of the flame on the upstream turbulence. Subsequently, we performed statistical analyses of the local values of various stretch rates and quantified how their distribution changes with turbulence intensity and flame temperature. We also evaluated the pairwise relation among various stretch rates to assess their dependence on each other. Finally, we used flame particles to evaluate the Lagrangian evolution of stretch rates conditioned on flame-fronts. All the analyses presented in this work point out Karlovitz number as a key factor in determining the flame–turbulence interaction. Specifically, we observe a stronger influence of turbulent eddies on flames with increasing Karlovitz number, as evidenced by the reduced effect of flame on upstream flow, wider probability distribution functions of stretch rates, and increased persistence timescales for stretches.

Background and Aim: As per World Health Organization (WHO), ~80% of COVID-19 infections are mild-to-moderate or asymptomatic; 15% develop severe disease and 5% have a critical disease with complications. In the initial part of the pandemic, there was no antiviral specific to COVID-19 available. Multiple different therapeutic options like antimalarial, HIV medications, antivirals, antihelminthics, and steroids have been repurposed for the management of COVID-19 in various phases of the pandemic and studies have been undertaken to estimate their efficacy. Similarly, various homeopathic medicines were also suggested for prophylaxis and treatment of COVID-19, and research studies were undertaken. This study evaluates the clinical efficacy of adjunct individualized homeopathic care along with the standard of care in hospitalized COVID-19-positive patients.

Droplet impact on oscillating substrates is important for both natural and industrial processes. Recognizing the importance of the dynamics that arise from the interplay between droplet transport and substrate motion, in this work, we present an experimental investigation of the spreading of a droplet impacting a sinusoidally oscillating hydrophobic substrate. We particularly focus on the maximum spread of droplets as a function of various parameters of substrate oscillation. We first quantify the maximum spreading diameter attained by the droplets as a function of frequency, amplitude of vibration, and phase at the impact for various impact velocities. We highlight that there can be two stages of spreading. Stage-I, which is observed at all impact conditions, is controlled by the droplet inertia and affected by the substrate oscillation. For certain conditions, a Stage-II spreading is also observed, which occurs during the retraction process of Stage-I due to additional energies imparted by the substrate oscillation. Subsequently, we derive scaling analyses to predict the maximum spreading diameters and the time for this maximum spread for both Stage-I and Stage-II. Furthermore, we identify the necessary condition for Stage-II spreading to be greater than Stage-I. The results will enable optimization of the parameters in applications where substrate oscillation is used to control the droplet spread and, thus, heat and mass transfer between the droplet and the substrate.

View Video Presentation: https://doi.org/10.2514/6.2023-3463.vid Turbulent combustion is critical in aviation industry due to its omnipresence in aircraft and rocket engines. Overall dynamics of turbulent flame is controlled by the local interactions between the flamefront and turbulence, and their influence on each other. To investigate such complex dynamics, we present an experimental investigation of a premixed Bunsen burner. First, the burner was characterized by evaluating the flow-field and turbulence properties using high-speed particle image velocimetry (HS-PIV). Next, we measured the flow field adjacent to the flame and compared it with the cold flow. We find that, there is reduction in turbulence strength when the flame is present, and the degree of reduction was found to be dependent on the Karlovitz number (defined as the ratio of flame time scale and turbulence time scale). Subsequently, we performed statistical analyses of local values of various stretch rates. We found that, with increasing Karlovitz number, the variance of stretch rates increase. Furthermore, our analysis showed that the three components of flame stretch rate are pairwise linked, which dictates the possible boundaries in joint probability distribution functions. Next, to understand the local evolution of the flamefront and its properties, we adopted a Lagrangian approach of studying ''flame particles''. By analyzing the instantaneous location of these flame particles, and examining the temporal evolution of local stretch rates, we have shown some key behavior of premixed turbulent flames.

Abstract We present a phenomenological reduced-order model to capture the transition to thermoacoustic instability in turbulent combustors. Based on the synchronization framework, the model considers the acoustic field and the unsteady heat release rate from turbulent reactive flow as two nonlinearly coupled sub-systems. To model combustion noise, we use a pair of nonlinearly coupled second-order ODEs to represent the unsteady heat release rate. This simple configuration, while nonlinearly coupled to another oscillator that represents the independent sub-system of acoustics (pressure oscillations) in the combustor, is able to produce chaos. Previous experimental studies have reported a route from low amplitude chaotic oscillation (i.e., combustion noise) to periodic oscillation through intermittency in turbulent combustors. By varying the coupling strength, the model can replicate the route of transition observed and reflect the coupled dynamics arising from the interplay of unsteady heat release rate and pressure oscillations.

In this Letter, the merging vs bouncing response of obliquely oriented colliding jets under elevated and reduced gaseous environment pressures was experimentally examined. Experiments with water and n-tetradecane confirmed that the collision outcome transitions from merging to bouncing and then to merging again, when the impact velocity was increased. This behavior which was previously reported for atmospheric pressure has now also been observed at elevated and reduced pressures. New results also show that there exists a critical pressure (0.9 bar for n-tetradecane and 5 bar for water) below which increasing pressure promotes bouncing (expands the bouncing regime), while beyond this, merging is promoted (reduces the bouncing regime) instead. This leads to a non-monotonic influence of pressure on the non-coalescence outcomes of collisional jets, which was not previously reported. The study provides evidence of new behaviors in colliding jets at reduced and elevated pressures, which differs from well-studied droplet–droplet collisions.

The dynamics of an evaporating droplet in an unsteady flow is of practical interest in many industrial applications and natural processes. To investigate the transport and evaporation dynamics of such droplets, we present a numerical study of an isolated droplet in an oscillating gas-phase flow. The study uses a one-way coupled two-phase flow model to assess the effect of the amplitude and the frequency of a sinusoidal external flow field on the lifetime of a multicomponent droplet containing a non-volatile solute dissolved in a volatile solvent. The results show that the evaporation process becomes faster with an increase in the amplitude or the frequency of the gas-phase oscillation. The liquid-phase transport inside the droplet also is influenced by the unsteadiness of the external gas-phase flow. A scaling analysis based on the response of the droplet under the oscillating drag force is subsequently carried out to unify the observed evaporation dynamics in the simulations under various conditions. The analysis quantifies the enhancement in the droplet velocity and Reynolds number as a function of the gas-phase oscillation parameters and predicts the effects on the evaporation rate.