Department of Mechanical Engineering

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

Focusing of ultrasound waves is critical to a number of clinical and industrial applications including biomedical and underwater imaging, nondestructive evaluation and material processing. This paper discusses the use of a novel 'add-on' gradient refractive index (GRIN) metamaterial structure made of concentric shells, to focus ultrasonic waves generated by conventional transducers. Analysis based on the Huygen's principle and numerical simulations is used to design the geometric and material properties of the proposed structure, whose working is demonstrated through experiments. Varying the shell material or thickness is shown to offer an elegant and straightforward way to tailor the focal spot inside the target material. The concentric-shell GRIN lens proposed here has a simple design, and has a potential to be used in dynamic focusing without advanced lenses or electronic steering. © 2021 Elsevier B.V.

World University root node

Ultrasonics

Concentric shell gradient index metamaterials for focusing ultrasound in bulk media

In the present work, three-dimensional numerical investigations are carried out to study the influence of top wall confinement on unsteady flow and heat transfer characteristics of sinusoidal wavy channels. Different channel confinements considered in the present study are channels having sinusoidal bottom wall and plane, in-phase, and out-of-phase configurations of the top wall. Computations are carried out by using open-source computational fluid dynamics package OpenFOAM. Numerical results are validated against the experimental results reported in the literature. The onset of unsteady flow and three-dimensionality in the channel has been identified by monitoring velocity at a point in the domain. Steady and unsteady flow characteristics have been presented with the help of temporal variation of velocity signals, phase space trajectories, power spectral density, instantaneous streamlines, and iso Q-surfaces. Heat transfer characteristics have been presented with the help of instantaneous Nusselt number contours and time-averaged values of Nusselt number. © 2021 Taylor & Francis Group, LLC.

Heat Transfer Engineering

Effect of Top Wall Configuration on Unsteady Flow and Heat Transfer Characteristics of Sinusoidal Wavy Channels

Three dimensional computations have been carried out using Lattice Boltzmann Method (LBM) and Finite Volume Method (FVM) for the case of flow past a stationary sphere. The Reynolds number of the mean flow have been varied from 50 to 300 during which flow transitions from steady state to unsteady state. The LBM simulations were carried out using the open source solver Palabos developed in the University of Geneva while the FVM computations were performed using the commercial solver ANSYS Fluent 17.2. Qualitative results like streamlines and three dimensional vortex structures in the wake of the sphere, obtained from the two different techniques have been compared alongside results reported in literature. Quantitative results such as, variation of velocity in the sphere wake and length of wake bubble have also been compared. The present study also tabulates the computational time required to complete the flow simulation for the both the solvers while utilizing the same computational resource. The results from the present study highlights the advantages of LBM solver for simulation of unsteady three dimensional flow but also indicate that FVM technique has definitive benefits when it comes to steady flows. © 2021 IEEE.

Proceedings of 2nd IEEE International Conference on Computational Intelligence and Knowledge Economy, ICCIKE 2021

Comparison of Lattice Boltzmann and Finite Volume method for flow past sphere

Present work describes a numerical investigation concerning the response of oscillatory thermo-capillary convection to free surface heat loss in a cylindrical half floating zone of high Prandtl number liquid (Pr=67) under microgravity conditions. Unsteady and three-dimensional computations have been carried out in the liquid domain using the finite volume method (FVM) to reveal the oscillatory thermo-fluid dynamic regimes at the onset of instability. The influence of different free surface heat loss conditions on the bifurcation from the basic steady axisymmetric state has been discussed in terms of critical azimuthal wavenumber, m. The dynamic evolution of the temperature disturbances at supercritical conditions are characterized by ‘pulsating’ and ‘rotating’ modes of spatio-temporal convection. The development of counter-propagating hydrothermal waves in azimuthal direction is found to be followed by an even mode of convection characterised by m = 2. The liquid bridge system exhibiting the well-known transition from periodic (m=2) to quasi-periodic behaviour (m=1) with an increase in Biot number, Bi has been analysed from both spatial and temporal points of view. The existence of both symmetric and asymmetric modes of the supercritical state for different Bi are presented. The dominant fluid dynamic information of 3D oscillatory state are extracted from a specific set of disturbance temperature data by employing a data-driven computational technique ‘dynamic mode decomposition’ (DMD). Various neutral, stable and unstable dynamic modes responsible for bifurcation to the 3D supercritical state are depicted with the help of DMD spectrum and the frequency spectrum. © 2021 COSPAR

Advances in Space Research

On three-dimensional flow structures and oscillatory instability due to free surface heat loss in half-floating zones under microgravity

The potential of higher frequency ultrasonic guided wave mode cluster (HOMC) waves to be used for remote inspection of notch defects in plate-like structures is investigated, at room and elevated temperatures. Quantitative studies of HOMC interaction with notch defects ranging from 5% to 50% of plate thickness are performed using 2D finite element simulations, and are validated by controlled experiments performed, firstly at room temperature. Analysis using reciprocity-based relations is used to uncover for the first time, how the constituent modes of HOMC play a vital role in their scattering processes. Further experiments are used to show that the results are stable up to 300 ̊C, thereby demonstrating the feasibility of short range higher-resolution remote inspection of notch defects using non-dispersive higher frequency mode clusters in industrial conditions. © 2021

Department of Electrical Engineering

NDT and E International

Scattering of Higher Order Mode Clusters (HOMC) from surface breaking notches in plates with application to higher temperature gradients

In this work, grain refinement and deformation mechanisms of ultrafine-grained AA 6061 alloy have been investigated using uniaxial tensile tests with stress relaxation. Initially, solutionized AA 6061 alloy samples with 3 mm thickness are subjected to constrained grove pressing (CGP) with an effective plastic strain of 1.16. The CGP samples are eventually rolled at room temperature (cold rolled) to produce ultrafine-grained thin sheets of 1 mm thickness. Due to the intense plastic strain applied during CGP and cold rolling (CR), it is practically challenging to quantify the grain size refinement by optical methods. Therefore, stress relaxation, which is an alternate transient mechanical test, is used to estimate the average degree of grain refinement for a larger length scale. In this method, the transient data from uniaxial tensile tests with stress relaxation is used to calculate the activation volume, which helps to understand the deformation mechanisms of UFG structured materials. In the present work, uniaxial tensile tests with controlled single and repeated stress relaxation are performed to determine apparent and actual activation volume in different materials conditions (solutionized, CGPed, and CGP + CR). Stress–strain curves obtained from the stress relaxation tests are compared with the monotonic tensile stress–strain curves. The results showed that the activation volume determined from single and repeated relaxation tests substantially decreased after the CGP and CGP + CR processes. The results also indicated that the grain boundary sliding is the possible deformation mechanism in CGP + CR samples and dislocation-dislocation interactions in CGP and SL samples. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.

Lecture Notes in Mechanical Engineering

Stress Relaxation Study of Ultrafine-Grained AA 6061 Alloy Processed Through Combined Constrained Groove Pressing and Cold Rolling

The 6xxx series aluminium alloys are extensively used in the structural applications of automobile and aerospace. The strength of these alloys can be further improved by severe plastic deformation. The severely deformed aluminium alloys may experience severe thermal and mechanical process during development of structural components. Therefore, it is important to develop processing map for such severely deformed 6xxx alloys to understand its workability under various temperature and strain rate. In the current work, hot deformation of severely deformed (cryorolled-CR) AA6063 alloy was studied via hot tensile and hot compressive testing in temperature range of 300°C – 450°C and at strain rate of 0.001 s−1 – 10 s−1. The safe processing regions were established via processing map with the help of dynamic materials model (DMM). Also, the constitutive model was established and the activation energies were known to understand the deformation mechanism of CR AA6063 alloy in both tensile and compressive mode. Stable and unstable zones were identified from processing maps in both tensile and compressive mode of deformation. In tensile mode dynamic recovery and in compressive mode simultaneous action of dynamic recovery and dynamic recrystallization were primary mechanisms of deformation in stable region. However, instability region was identified by stress concentration cracking in both tension and compression modes of deformation. The average activation energy in compressive mode (196.57 kJ/mol) of deformation was found to be higher compared to tensile (151.55 kJ/mol) defamation. Microstructural evolution was used to establish a correlation between constitutive model and processing maps. © 2020

International Journal of Mechanical Sciences

Hot deformation behavior and processing map development of cryorolled AA6063 alloy under compression and tension

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications. © 2021 American Chemical Society.

Langmuir

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves

Single-cell analysis has emerged as a powerful method for genomics, transcriptomics, proteomics, and metabolomics characterisation at the individual cell level. Here, we demonstrate a technique for the detection and selective isolation of target cells encapsulated in microdroplets in single-cell format. A sample containing a mixed population of cells with fluorescently labelled target cells can be focused using a sheath fluid to direct cells in single file toward a droplet junction, wherein the cells are encapsulated inside droplets. The droplets containing the cells migrate toward the centre of the channel owing to non-inertial lift force. The cells present in the droplets are studied and characterised based on forward scatter (FSC), side scatter (SSC), and fluorescence (FL) signals. The FL signals from the target cells can be used to activate a selective isolation module based on electro-coalescence, using suitable electronics and a program to sort droplets containing the target cells in single-cell format from droplets containing background cells. We demonstrated the detection and isolation of target cells (cancer cells: HeLa and DU145) from mixed populations of cells, peripheral blood mononuclear cells (PBMC) + cervical cancer cells (HeLa) and PBMC + human prostate cancer cells (DU145), at a concentration range of 104-106 ml-1 at 300 cells per s. The performance of the device is characterised in terms of sorting efficiency (>97%), enrichment (>1800×), purity (>98%), and recovery (>95%). The sorted target cells were found to be viable (>95% viability) and showed good proliferation when cultured, showing the potential of the proposed sorting technique for downstream analysis. © 2021 The Royal Society of Chemistry.

Analyst

An optomicrofluidic device for the detection and isolation of drop-encapsulated target cells in single-cell format

Option discovery and skill acquisition frameworks are integral to the functioning of a hierarchically organized Reinforcement learning agent. However, such techniques often yield a large number of options or skills, which can be represented succinctly by filtering out any redundant information. Such a reduction can decrease the required computation while also improving the performance on a target task. To compress an array of option policies, we attempt to find a policy basis that accurately captures the set of all options. In this work, we propose Option Encoder, an auto-encoder based framework with intelligently constrained weights, that helps discover a collection of basis policies. The policy basis can be used as a proxy for the original set of skills in a suitable hierarchically organized framework. We demonstrate the efficacy of our method on a collection of grid-worlds evaluating the obtained policy basis on downstream tasks and demonstrate qualitative results on the Deepmind-lab task. © 2021, Springer Nature Switzerland AG.