Department of Applied Mechanics

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

The collisions in a dilute polydisperse suspension of sub-Kolmogorov spheres with negligible inertia settling in a turbulent flow and interacting through hydrodynamics including continuum breakdown on close approach are studied. A statistically significant decrease in ideal collision rate without gravity is resolved via a Lagrangian stochastic velocity-gradient model at Taylor microscale Reynolds number larger than those accessible by current direct numerical simulation capabilities. This arises from the difference between the mean inward velocity and the root-mean-square particle relative velocity. Differential sedimentation, comparable to the turbulent shear relative velocity, but minimally influencing the sampling of the velocity gradient, diminishes the Reynolds number dependence and enhances the ideal collision rate i.e. the rate without interactions. The collision rate is retarded by hydrodynamic interactions between sphere pairs and is governed by non-continuum lubrication as well as full continuum hydrodynamic interactions at larger separations. The collision efficiency (ratio of actual to ideal collision rate) depends on the relative strength of differential sedimentation and turbulent shear, the size ratio of the interacting spheres and the Knudsen number (defined as the ratio of the mean-free path of the gas to the mean radius of the interacting spheres). We develop an analytical approximation to concisely report computed results across the parameter space. This accurate closed form expression could be a critical component in computing the evolution of the size distribution in applications such as water droplets in clouds or commercially valuable products in industrial aggregators. © The Author(s), 2021. Published by Cambridge University Press.

World University root node

Journal of Fluid Mechanics

Collision rate of bidisperse, hydrodynamically interacting spheres settling in a turbulent flow

The Kapitza resistance (Rk) at the water-carbon nanotube (CNT) interface, with water on the inside of the nanotube, was investigated using molecular dynamics simulations. We propose a new equilibrium molecular dynamics (EMD) method, also valid in the weak flow regime, to determine the Kapitza resistance in a cylindrical nanoconfinement system where nonequilibrium molecular dynamics (NEMD) methods are not suitable. The proposed method is independent of the correlation time compared to Green-Kubo-based methods, which only work in short correlation time intervals. Rk between the CNT and the confined water strongly depends on the diameter of the nanotube and is found to decrease with an increase in the CNT diameter, the opposite to what is reported in the literature when water is on the outside of the nanotube. Rk is furthermore found to converge to the planar graphene surface value as the number of water molecules per unit surface area approaches the value in the graphene surface and a higher overlap of the vibrational spectrum. A slight increase in Rk with the addition of the number of CNT walls was observed, whereas the chirality and flow do not have any impact. ©

Department Of Civil Engineering

Langmuir

Nanoconfinement Effects on the Kapitza Resistance at Water-CNT Interfaces

Molecular Dynamic simulations and experiments have been performed to investigate the evaporative behavior of a liquid droplet in the presence of nanoparticles on a smooth, heated flat plate. Nanofluids of different volume fractions with CuO and Al2O3 nanoparticles were prepared to obtain liquid droplets. Experiments were conducted for droplets of both nanofluids and base fluid (DI water). The results showed that in the pinned region of evaporation, droplets of nanofluids with higher volume fractions of dense nanoparticles evaporated by keeping their base pinned at the triple point contact line. It is also observed that the evaporation rate of nanofluid droplets with nanoparticles having higher density is slower and promoted a typical pattern formation. Molecular Dynamic simulations were further performed to analyse the influence of various nanoparticle-liquid interaction strengths (εnp−l) on evaporative behavior. The simulation results suggest that in the pinned evaporation stage, for higher values of εnp−l, the droplet undergoes a delay on evaporation while subjected to higher heat flux on the substrate. © 2021 Elsevier B.V.

Department of Metallurgical and Materials Engineering

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Influence of Nanoparticles on the Evaporation of a Nanodroplet from Solid Substrates: An Experimental and Molecular Dynamics Investigation

We study the generation of vorticity at a water–air interface due to the spreading of ethanol–water droplets for ethanol concentrations in the drop ranging from 20% ≤ Ce≤ 100% on the surface of a 50 mm deep water layer. On deposition onto the water layer, the lighter, miscible ethanol droplet spreads as a film on the water surface due to the ethanol–water surface tension difference. Simultaneous to the film spreading, an expanding vortex ring is found below the tip of the spreading film front. The phenomenon in the water layer is visualized using particles, and 2D PIV is used to obtain the velocity field. Vortex regions are identified using the λ2 method for various time instants. The average vorticity generation in between the time instants is calculated. A scaling law is proposed for the dimensionless vorticity at a given instant based on the experimental observations. © 2021, Springer Nature Singapore Pte Ltd.

Lecture Notes in Mechanical Engineering

Vorticity Generation Due to Surface Tension-Driven Spreading

Nowadays, adequate and accurate representation of the microvascular flow resistance constitutes one of the major challenges in computational haemodynamic studies. In this work, a theoretical, porous media framework, ultimately designed for representing downstream resistance, is presented and compared against an in vitro experimental results. The resistor consists of a poro-elastic tube, with either a constant or variable porosity profile in space. The underlying physics, characterizing the fluid flow through the porous media, is analysed by considering flow variables at different network locations. Backward reflections, originated in the reservoir of the in vitro model, are accounted for through a reflection coefficient imposed as an outflow network condition. The simulation results are in good agreement with the measurements for both the homogenous and heterogeneous porosity conditions. In addition, the comparison allows identification of the range of values representing experimental reservoir reflection coefficients. The pressure drops across the heterogeneous porous media increases with respect to the simpler configuration, whilst flow remains almost unchanged. The effect of some fluid network features, such as tube Young's modulus and fluid viscosity, on the theoretical results is also elucidated, providing a reference for the invitro and insilico simulation of different microvascular conditions. © 2021 Elsevier Ltd

Journal of Biomechanics

On the poro-elastic models for microvascular blood flow resistance: An in vitro validation

A non-planar or a bilateral mixing-layer is studied by means of a series of direct numerical simulations (DNSs). This mixing-layer forms at the interface of two co-current plane Couette flows of different Reynolds numbers. The current DNS study determined the conditions for the onset of shear-layer instability at the interface. The influence of different Reynolds number (of the co-current plane Couette flows) and their Reynolds number ratio on the mixing-layer is studied. A critical Reynolds number of about 500 (or more particularly one of the co-current plane Couette flows must be turbulent) and a Reynolds number ratio greater than 2 is required for the genesis of this bilateral shear-layer instability. Independent of the Reynolds number and the Reynolds number ratio, the temporal evolution of the shear-layer instability followed the same pattern. In addition, the oscillation frequency of the instability was found to increase with increasing Reynolds number and increasing Reynolds number ratio. Further, influence of instability on the local skin friction and the two-point correlation is elaborated on. © 2021 Elsevier Inc.

International Journal of Heat and Fluid Flow

Onset of shear-layer instability at the interface of parallel Couette flows

Quenching of limit-cycle oscillations (LCOs), either through mutual coupling or external forcing, has attracted wide attention in several fields of science and engineering. However, the simultaneous utilization of these coupling schemes in quenching of LCOs has rarely been studied despite its practical applicability. We study the dynamics of two thermoacoustic oscillators simultaneously subjected to mutual coupling and asymmetric external forcing through experiments and theoretical modeling. We investigate the forced response of both identical and nonidentical thermoacoustic oscillators for two different amplitudes of LCOs. Under mutual coupling alone, identical thermoacoustic oscillators display the occurrence of partial amplitude death and amplitude death, whereas under forcing alone, asynchronous quenching of LCOs is observed at nonresonant conditions. When the oscillators are simultaneously subjected to mutual coupling and asymmetric forcing, we observe a larger parametric region of oscillation quenching than when the two mechanisms are utilized individually. This enhancement in the region of oscillation quenching is due to the complementary effect of amplitude death and asynchronous quenching. However, the forced response of coupled nonidentical oscillators shows that the effect of forcing is insignificant on attaining synchronization and quenching of oscillations in the oscillator that is not directly forced. Finally, we qualitatively capture the experimental results using a reduced-order theoretical model of two Rijke tubes that are coupled through dissipative and time-delay coupling and asymmetrically forced. We believe that these findings offer fresh insights into the combined effects of mutual and forced synchronization in a system of coupled nonlinear oscillators. © 2021 American Physical Society.

Department of Aerospace Engineering

Fulltext

Physical Review Applied

Dynamics of Coupled Thermoacoustic Oscillators under Asymmetric Forcing

Compressed sensing (CS) has been adapted to synthetic aperture (SA) ultrasound imaging to improve the frame-rate of the system. Recently, we proposed a novel CS framework using Gaussian under-sampling to reduce the number of receive elements in multi-element synthetic transmit aperture (MSTA) imaging. However, that framework requires different receive elements to be chosen randomly for each transmission, which may add to practical implementation challenges. Modifying the scheme to employ the same set of receive elements for all transmissions of MSTA leads to degradation of the recovered image quality. Therefore, this work proposes a novel sampling scheme based on a genetic algorithm (GA), which optimally chooses the receive element positions once and uses it for all the transmission of MSTA. The CS performance using GA sampling schemes is evaluated against the previously proposed CS framework on in-vitro and in-vivo datasets. The obtained results suggest that not only does the GA-based approach allows the use of the same set of sparse receive elements for each transmit, but also leads to the lowest CS recovery error (NRMSE) and 14% overall improvement in image contrast, in comparison to the previously-proposed Gaussian sampling scheme. Thus, using the CS framework along with GA, can potentially reduce the complexity in implementation of CS-framework to MSTA based systems. © 2021 Elsevier B.V.

Department of Mechanical Engineering

Ultrasonics

Towards practical implementation of the compressed sensing framework for multi-element synthetic transmit aperture imaging

Carbon mineral trapping is commonly believed to be insignificant relative to other trapping mechanisms, yet, the role of cm-scale intraformational baffles is rarely considered in reservoir scale assessments. The present study aims to reveal conditions governing carbon mineral trapping and its quantification for a range of rock compositions in intraformational baffles. A series of 2-D multiphase reactive transport simulations were set-up to model the geochemical processes occurring at the interface of a baffle and a reservoir rock for a period of 1000 years. The study shows that the presence of baffles with certain rock properties within the reservoir rock can enhance or reduce carbon mineral trapping capacity relative to homogeneous reservoir rocks depending on clay and carbonate mineral content in baffles. The analysis shows that the precipitation of secondary carbonates occurs at the interface of the two rock types. Furthermore, it was found that higher clay and lower carbonate content of the baffle contribute towards enhancing mineral trapping while the mineralogy of reservoir rock does not have an important influence. The results are further utilized to derive averaged mineral composition for the integration of fine scale processes in coarser scale simulations. © 2021 Elsevier Ltd

Department of Mathematics

International Journal of Greenhouse Gas Control

Conditions and processes controlling carbon mineral trapping in intraformational baffles

Electrotactile displays provide tactile feedback using electrical currents. However, prolonged usage of them causes skin irritation and burns. This study presents a novel method to use the electrotactile display at a lower threshold by applying two subthreshold electrotactile stimuli placed nearby. This method is tested on fourteen subjects over a frequency range from 20 to 1280 Hz in the tuning curve. Two Psychophysical experiments have been conducted to measure their thresholds for individual stimuli and presented together. In the first experiment, the frequencies of both the stimuli were the same, and it showed 36% to 18% reduction in EPT of the second stimulus when the first stimulus was at 10% to 50% of its threshold. In the second experiment, Meissner and Pacinian were selectively stimulated by applying stimuli at their resonant frequencies (30 and 250 Hz, respectively), results showed slightly more reduction in the case of 250 Hz compared to 30 Hz. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.