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IIT Hyderabad

Department of Mechanical and Aerospace Engineering

Many complex systems undergo critical transitions. A thermoacoustic system is one such system which exhibits a catastrophic transition to a state of oscillatory instability known as thermoacoustic instability. So far, several early warning signals have been devised to detect the transition from stable operation to thermoacoustic instability in both laminar and turbulent systems. We focus on these early warning signals, and the challenges faced while detecting oscillatory instabilities in practical systems. In contrast to the transition from a fixed point to limit cycle oscillations in laminar systems, turbulent systems exhibit a transition from a chaotic state to limit cycle via a state of intermittency. In this review, we highlight the importance of the inherent fluctuations in the system in providing early warning signals for critical transitions. © 2021, The Author(s), under exclusive licence to EDP Sciences, Springer-Verlag GmbH Germany, part of Springer Nature.

World University root node

European Physical Journal: Special Topics

Critical transitions and their early warning signals in thermoacoustic systems

Fulltext

Scientific Reports

Author Correction: Data suggest COVID-19 affected numbers greatly exceeded detected numbers, in four European countries, as per a delayed SEIQR model (Scientific Reports, (2021), 11, 1, (8106), 10.1038/s41598-021-87630-z)

The present paper discusses impedance and mobility approach integrated with finite element method (FEM) for free vibration analysis of structural-acoustic coupled systems for irregular geometry. In this method, coupled natural frequencies of any irregular geometry with flexible surfaces with different boundary conditions can be estimated. The proposed hybrid method helps to understand the underlying physics of structural-acoustic coupling in complex geometries. The formulation being in impedance and mobility form, gives more insight in understanding coupled modes of the complex geometries. It is for three dimensional irregular geometries, so it can be used in most of the practical applications where coupled natural frequencies play a vital role. The proposed method is demonstrated for regular and irregular geometry with one wall flexible configuration. The results are corroborated with existing literature data for regular geometry and numerical models for irregular geometry. © 2020 Amogh Nalawade, et al.

Journal of Vibroengineering

Free vibration analysis of structural-acoustic coupled system using hybrid impedance and mobility approach

In this study, a Green's function-based semi-analytical method is presented to predict the transmission loss (TL) of a circular chamber having concentric perforated screens. Initially, the Green's function is developed for a single-screen configuration as the summation of eigenfunctions of the inner pipe in the absence of the mean flow. The inlet and the outlet ports are modeled as oscillating piston sources. A transfer matrix is formulated from the velocity potential generated by the piston sources. The results obtained from the proposed method are validated with the numerical and analytical models and with the experimental results available in the literature. Later, the method has been extended to the double-screen configuration. The effect of the additional perforated screen on the TL is studied in terms of the surface impedance of the chamber. Along with grazing flow considerations, guidelines are provided to incorporate more concentric perforated screens into the formulation. © 2020 by ASME.

Journal of Vibration and Acoustics, Transactions of the ASME

A Green's Function Solution for Acoustic Attenuation by a Cylindrical Chamber with Concentric Perforated Liners

We investigate the mechanics of the detachment of an inclined micro-pillar adhered to a dissimilar substrate when subjected to a combination of an axial load and end moment. When the micro-pillar has adhered to the substrate, singular stress fields exist at the bi-material corners. The order of singularity is estimated using asymptotic analysis. The first two terms in the asymptotic expansion lead to singular stress fields. The magnitude of the singularity is evaluated in terms of the elastic mismatch between the pillar and substrate and the micro-pillar inclination. The asymptotic stress due to the moment loading is more sensitive to the micro-pillar inclination when compared to that due to the axial loading. They are insensitive to the micro-pillar inclination when the micro-pillar is rigid when compared to the substrate. A short interfacial crack is further assumed to exist at the bi-material corner. This crack is embedded in the corner singularity region and is loaded by the singular fields due to axial and bending loads. A boundary layer analysis is performed on the singular zone to estimate the stress intensity factor when a short crack embedded in it is subjected to the singular fields. The stress intensity factors are also calculated for a long interfacial crack at the bi-material corner, which extends beyond the singular zone. By using the aforementioned results, we investigate the detachment of the inclined micro-pillar under the combination of an axial load and end moment. © 2021 by ASME

Journal of Applied Mechanics, Transactions ASME

The detachment of an inclined micro-pillar adhered to a dissimilar substrate

Wet anisotropic etching is extensively employed in silicon bulk micromachining to fabricate microstructures for various applications in the field of microelectromechanical systems (MEMS). In addition, it is most widely used for surface texturing to minimize the reflectance of light to improve the efficiency of crystalline silicon solar cells. In wet bulk micromachining, the etch rate is a major factor that affects the throughput. Slower etch rate increases the fabrication time and therefore is of great concern in MEMS industry where wet anisotropic etching is employed to perform the silicon bulk micromachining, especially to fabricate deep cavities and freestanding microstructures by removal of underneath material through undercutting process. Several methods have been proposed to increase the etch rate of silicon in wet anisotropic etchants either by physical means (e.g. agitation, microwave irradiation) or chemically by incorporation of additives. The ultrasonic agitation during etching and microwave irradiation on the etchants increase the etch rate. However, ultrasonic method may rupture the fragile structures and microwave irradiation causes irradiation damage to the structures. Another method is to increase the etching temperature towards the boiling point of the etchant. The etching characteristics of pure potassium hydroxide solution (KOH) is studied near the boiling point of KOH, while surfactant added tetramethylammonium hydroxide (TMAH) is investigated at higher temperature to increase the etch rate. Both these studies have shown a potential way of increasing the etch rate by elevating the temperature of the etchants to its boiling point, which is a function of concentration of etch solution. The effect of various kinds of additives on the etch rate of silicon is investigated in TMAH and KOH. In this paper, the additives which improve the etch rate have been discussed. Recently the effect of hydroxylamine (NH2OH) on the etching characteristics of TMAH and KOH is investigated in detail. The concentration of NH2OH in TMAH/KOH is varied to optimize the etchant composition to obtain improved etching characteristics especially the etch rate and undercutting which are important parameters for increasing throughput. In this article, different methods explored to improve the etch rate of silicon have been discussed so that the researchers/scientists/engineers can get the details of these methods in a single reference. © 2021, The Author(s).

Department of Physics

Micro and Nano Systems Letters

High speed silicon wet anisotropic etching for applications in bulk micromachining: a review

The study is aimed to analyse the comparative behaviour of the high-temperature abrasive wear of H13 steel surfaces modified by laser melting and cladding with Stellite 6 and Stellite 6 + 30 wt% WC. 3-body abrasive tests were conducted at room temperature, 450 °C, 550 °C, and 650 °C. The microstructural evolution, microhardness, wear surface morphology and mechanisms, and various phases formed during laser surface modifications were also studied. The laser remelting of H13 steel surface increased its room temperature microhardness to 750 ± 35 HV0.01, whereas laser cladding of Stellite 6 powder yielded hardness of around 600 ± 20 HV0.01 in the clad layer; and Stellite 6/WC composite clad layer had marginally higher hardness than the Stellite 6 clad layer in the matrix and much higher hardness of ~3000 HV0.01 at the sporadically distributed WC particle sites. Though the room temperature microhardness of laser remelted H13 surface is the highest, the volumetric wear loss in it was comparable to that of the Stellite 6 cladding. However, Stellite 6/WC composite layer recorded a relatively less volumetric loss as WC particles resisted the abrasive wear. With increasing temperature, the wear loss in laser remelt surfaces increased at a fast rate, while that in Stellite 6 and composite clad layers varied marginally with no definite trend. Overall, Stellite 6/WC composite cladding performed better than others in the current temperature range. © 2021 Elsevier B.V.

Surface and Coatings Technology

High-temperature abrasive wear characteristics of H13 steel modified by laser remelting and cladded with Stellite 6 and Stellite 6/30% WC

This work presents the development and demonstration of a novel underwater wet laser welding technique, where unlike the conventional underwater wet laser welding, a water-jet parallel to the weld pool surface was introduced inside the water environment. The water flow was found to enhance the removal of water vapour formed at the laser-water-workpiece interface without creating hindrance to the molten weld bead, resulting in lower energy loss due to absorption and scattering, thus enhancing the laser energy coupling efficiency to the material. The process was studied for bead-on-plate welding on 4 mm stainless steel (SS) sheets using a 2 kW Yb-Fiber laser (wavelength 1.07 µm), with water column height of 5–15 mm, and a parallel water jet of speed 2–3 m/s, under continuous (CW) and pulsed mode (modulated power) laser operation. The major objective of the work was to study the effect of water flow and mode of laser operation on the formation and growth of vapour layer at the processing zone and investigate its effect on laser energy coupling efficiency. Further, their effect on weld bead geometry and microstructure was investigated. Welding operation with modulated laser power was found to produce higher weld depth compared to CW mode with same average laser power due to lower scattering loss and better energy coupling under pulsed mode operation. The micro-structural investigation has showed the formation of finer grains and oxide or carbide precipitation in case of underwater welding, indicating the enhanced cooling phenomena in this technique. © 2021 Elsevier GmbH

Optik

An investigation on physical phenomena of water-jet assisted underwater wet laser welding technique under continuous and pulsed mode operation

Performance of a gasoline direct injection engine significantly depends on the fuel injection phenomenon. It has been shown that variation in the fuel thermo-physical properties and in-cylinder thermodynamic conditions can adversely affect engine performance. Spray collapse due to flash boiling is the consequence of such varying in-cylinder thermodynamics. It has also been observed that gasoline direct injection engines have higher particulate matter emissions compared to port fuel injection engines. One of the possible reasons for this observation may be fuel impingement on the piston head and spray collapse at high fuel injector temperature. In the present work, experiments have been performed to understand spray characteristics under flash boiling conditions and ultimately its effect on in-cylinder combustion quality for the three different fuels: n-butanol, iso-butanol and iso-octane. To mimic in-cylinder conditions, hot fuel was introduced inside an optically accessible engine. It was observed that fuel temperature and their thermo-physical properties have a significant effect on piston head wetting and pool fire on the piston top. Butanol isomers showed significant reduction in sooty combustion with increase in fuel temperature. However, iso-octane showed higher wall wetting at elevated fuel temperature due to spray collapse. © IMechE 2020.

International Journal of Engine Research

Optical investigation of flash boiling and its effect on in-cylinder combustion for butanol isomers and iso-octane

We report a familial cluster of 24 individuals infected with SARS-CoV-2. The index case had a travel history &spent twenty four days in the house before being tested and was asymptomatic. Physical overcrowding in the house provided a favourable environment for intra-cluster infection transmission. Restriction of movement of family members due to countrywide lockdown limited the spread in community. Among the infected, only 4 individuals developed symptoms. The complete genome sequences of SARS-CoV-2 was retrieved using next-generation sequencing from eight clinical samples which demonstrated a 99.99% similarity with reference to Wuhan strain and the phylogenetic analysis demonstrated a distinct cluster, lying in the B.6.6 pangolin lineage. © 2021 Cambridge University Press. All rights reserved.