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

Viscosity variation of solvent in local regions near a solid surface, be it a biological surface of a protein or an engineered surface of a nanoconfinement, is a direct consequence of intermolecular interactions between the solid body and the solvent. The current coarse-grained molecular dynamics study takes advantage of this phenomenon to investigate the anomaly in a solvated protein's rotational dynamics confined using a representative solid matrix. The concept of persistence time, the characteristic time of structural reordering in liquids, is used to compute the solvent's local viscosity. With an increase in the degree of confinement, the confining matrix significantly influences the solvent molecule's local viscosity present in the protein hydration layer through intermolecular interactions. This effect contributes to the enhanced drag force on protein motion, causing a reduction in the rotational diffusion coefficient. Simulation results suggest that the direct matrix-protein non-bonded interaction is responsible for the occasional jump and discontinuity in orientational motion when the protein is in very tight confinement. © 2021 IOP Publishing Ltd.

World University root node

Nanotechnology

Rotational dynamics of proteins in nanochannels: Role of solvent's local viscosity

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs. © 2020 Acta Materialia Inc.

Department of Metallurgical and Materials Engineering

Scripta Materialia

Composite of medium entropy alloys synthesized using spark plasma sintering

The compression after high-velocity impact (CAI) behavior of pseudo-elastic shape memory alloy (PE-SMA) embedded glass/epoxy composite laminates manufactured via vacuum-assisted resin infusion (VARI) technique was investigated experimentally. High-velocity impact tests were performed on the glass/epoxy composites using an in-house build gas-gun set-up at four different velocities (70, 85, 95, and 105 m/s). To elucidate, the series of damage events undergone on the composites under the high-velocity impact, a high-speed camera was used. Quasi-static compression-after-impact (CAI) tests were performed to evaluate the damage tolerance of different composites subjected to high-velocity impact. Real-time damage progression during the CAI as well as the extent of damage was characterized by using a 2D Digital Image Correlation (DIC) technique. Three different types of embedment configurations of PE-SMA wire, namely, with length variation (35, 70, and 150 mm) and also with and without anchors, were used. The impact test results show that the ballistic limit of anchored PE-SMA composites with an embedment length of 35 mm is 72.72% higher than that of the homogeneous glass/epoxy composites. CAI results indicate that the PE-SMA based composites have better damage tolerance with respect to the homogeneous ones at 70 m/s. In contrast, at higher velocities, the delamination area decreases in the homogenous glass/epoxy composites due to more localized damage and present a higher compressive strength. © 2020 Elsevier Ltd

Department of Engineering Design

Composite Structures

Compression after high-velocity impact behavior of pseudo-elastic shape memory alloy embedded glass/epoxy composite laminates

The numerical investigation of 2D insect wing kinematics in an inclined stroke plane is carried out using an immersed boundary solver. The effect of vortex shedding and dipole jet on the vertical force generation by the flapping wing due to change in the stroke plane angle is investigated in the vicinity of the ground. The results of instantaneous force and vorticity contours reveal the underlying lift enhancement mechanisms for the inclined stroke plane flapping wing. Moreover, they aid in the understanding of the wake-ground interaction and the associated shear layers. The calculated average vertical force delineates different force trends for the inclined stroke plane flapping near the ground. Furthermore, the dipole jet patterns are analyzed for different heights. These patterns are found to be a better tool to assess the kinematics for the vertical force enhancement and reduction, especially at intermediate heights. Vertical force enhancement is the critical parameter in the design of the micro aerial vehicle (MAV). Through this study, it is certain that the dipole jet has the potential to be used as a lift modification mechanism in MAVs. In summary, the study gives a holistic view of the physics of the inclined plane kinematics near the ground and serves as the basis for the design of MAVs. © 2021 IOP Publishing Ltd.

Bioinspiration & biomimetics

Can the ground enhance vertical force for inclined stroke plane flapping wing?

Surface coatings have proved to be one of the most effective methods to protect the concrete structures in aggressive environments. However, coating the structures with polymer after 28 days of curing/hydration to get the best interfacial performance has little scientific backing. The measured surface free energy of the hydrating cement paste showed that the surface energy is high for a 2-day hydrated paste compared to cement paste cured for 7, 28 days. This opens up the possibility for exploration into coating an acrylic based polymer on a concrete surface before it reaches 28 days of hydration, which has been investigated in this study. The practical work of adhesion between the hydrating cement paste and polymer as measured from macro mechanical tests showed to have a dependency on the surface evolution of the cement paste. However, it was also observed that it depends equally on the polymer functional groups. Poly (methyl methacrylate) (PMMA), when coated on a 2-day hydrated cement paste, showed higher interfacial strength than when coated on later days (7, 14, 28 days). In the case of epoxy, irrespective of when the polymer was coated, it had the same interfacial strength. The influence of the evolving cement paste and the polymer functional group (PMMA in particular) on the interfacial strength at the micro- and molecular-length scales are discussed. It is found that the free/adsorbed calcium form a metal-complex with the ester functional group of the polymer in addition to the physical interactions, resulting in higher interfacial strength. The observations shed light on the importance of interfacial science and act as a guideline for the design of effective polymer-cement coatings. © 2020

Cement and Concrete Composites

Interfacial adhesion mechanism between organic polymer coating and hydrating cement paste

The current study provides a discussion on two aspects of size-effects which is known to be a characteristic behavior of micro-scaled structures. First it provides the experimentalists with a novel micro-bend technique wherein the micro-cantilever is loaded at different points along its length. This eases the experimental process by reducing the efforts of the experimentalists in sample preparation (as the same sample can be used for obtaining results corresponding to different L/H ratio). Also, precision cutting and mounting of the samples to obtain the micro-cantilever of desirous overhang length is not required. Further, the above described technique is used to test nickle micro-cantilevers of three thicknesses (15, 25, 50 μm) and different L/H (=200, 300, 400) ratios. They are elastically bent to undergo small deflections. Apart from reporting the obvious size-effects for smaller thicknesses and fitting the experiments with a model for determining the length scale parameter, size-effects for beams with different thicknesses and varying L/H ratios are presented. Secondly, a discussion on the widely fluctuating nature of numerical values that the length scale parameter assumes is presented. And it is postulated that the numerical value of length scale parameter is a function of material, model and geometry of beam cross-section. © 2021 Taylor & Francis Group, LLC.

Mechanics of Advanced Materials and Structures

Novel nickle foil micro-bend tests and the need for a relook at length scale parameter’s numerical value

In turbulent premixed flames, the fractal dimension of flame iso-surface is argued to be D=7/3 for Damköhler's large-scale limit (Da&gt;&gt;1) and D=8/3 for Damköhler's small-scale limit (Da∼O(1)) based on heuristic scaling arguments. However, such scaling arguments do not consider the effect of the intermittent nature of turbulent kinetic energy dissipation on the flame surface. In this paper, we account for the effects of intermittent dissipation on the fractal dimension of low Da turbulent premixed flames. Intermittent dissipation leads to variability in the inner cut-off, which then affects the scalar flux and total interface area. We account for these variabilities through two approaches: coarse-grained approach based on the moments of the dissipation and fine-scale analysis by adopting the multifractal formalism. We derive two corrections to the upper-limit of fractal dimension – D=8/3+3/4(1−D1/4) and D=8/3+2/3(3−D1/3). We further show that the second correction leads to an explicit dependence of the fractal dimension (D) on the scaling exponent (ξ) of the velocity structure function through the relation: D=7/3+ξ. Thus, we explicitly quantify the effect of the intermittent nature of turbulence upon low Da premixed combustion. © 2020 The Combustion Institute

Department of Aerospace Engineering

Combustion and Flame

Fractal dimension of premixed flames in intermittent turbulence

Collisions in a dilute polydisperse suspension of spheres of negligible inertia interacting through non-continuum hydrodynamics and settling in a slow uniaxial compressional flow are studied. The ideal collision rate is evaluated as a function of the relative strength of gravity and uniaxial compressional flow and it deviates significantly from a linear superposition of these driving terms. This non-trivial behaviour is exacerbated by interparticle interactions based on uniformly valid non-continuum hydrodynamics, that capture non-continuum lubrication at small separations and full continuum hydrodynamic interactions at larger separations, retarding collisions driven purely by sedimentation significantly more than those driven purely by the linear flow. While the ideal collision rate is weakly dependent on the orientation of gravity with the axis of compression, the rate including hydrodynamic interactions varies by more than with orientation. This dramatic shift can be attributed to complex trajectories driven by interparticle interactions that prevent particle pairs from colliding or enable a circuitous path to collision. These and other important features of the collision process are studied in detail using trajectory analysis at near unity and significantly smaller than unity size ratios of the interacting spheres. For each case analysis is carried for a large range of relative strengths and orientations of gravity to the uniaxial compressional flow, and Knudsen numbers (ratio of mean free path of the media to mean radius). © 2021 The Author(s),. Published by Cambridge University Press.

Journal of Fluid Mechanics

Collision rate of bidisperse spheres settling in a compressional non-continuum gas flow

In this paper, the development of modified beamforming methods, Filtered Delay Weight Multiply and Sum (F-DwMAS) and Filtered Delay Euclidian-Weighted Multiply and Sum (F-DewMAS), are reported. These methods were investigated on a minimum-redundancy synthetic aperture technique, called as 2 element Receive Synthetic Aperture Focusing Technique (2R-SAFT), which uses one element on transmit and two consecutive elements on receive, for reducing hardware complexity without compromising much on the image quality. The performance of the developed F-DwMAS and F-DewMAS methods were compared with Delay and Sum (DAS) and recently introduced F-DMAS beamforming methods. Notably, in the proposed methods, an additional aperture window function is designed and incorporated to the F-DMAS method. The different methods of F-DwMAS, F-DewMAS, F-DMAS and DAS were compared in terms of the resulting image quality metrics, Lateral Resolution (LR), Axial Resolution (AR), Contrast Ratio (CR) and contrast-to-noise ratio (CNR), in simulation and experiments on tissue-mimicking phantoms. Experimental results show that (F-DwMAS) and F-DewMAS resulted in improvements of AR by (46.32% and 23.51%), 43.56% and 17.78%, LR by (47.81% and 30.27%), 44.26% and 26.14% and CR by (45.68% and 17.15%), 42.16% and 9.87% compared to those obtained using DAS and F-DMAS, respectively. However, CNR of F-DwMAS and F-DewMAS was found to be 31.19% and 21.16% less compared to DAS, but 4.89% and 18.64% more than F-DMAS, respectively. Hence, it can be concluded that the image quality improved by both F-DwMAS and F-DewMAS compared to DAS and F-DMAS. Also, between F-DwMAS and F-DewMAS, the later has the advantage of ready applicability to different acquisition schemes and settings compared to the former also having an additional advantage of better CNR compared to both F-DMAS and F-DewMAS. © 2020 Elsevier B.V.

Ultrasonics

Weighted non-linear beamformers for low cost 2-element receive ultrasound imaging system

Ground resonance is an aero-mechanical instability in helicopters that use soft in-plane rotors. Traditionally, ground resonance is mitigated by using passive lead–lag dampers that provide sufficient in-plane damping. However, these dampers because of their passive nature cannot adapt to all operating conditions. In this work, a magnetorheological fluid–based semi-active lead–lag damper is proposed to offer controllable damping. Two nonlinear control strategies are reported to operate the voltage to be supplied to the magnetorheological damper. The first strategy is a model-based control using dynamic inversion. The second is a fuzzy logic control integrated with a particle swarm optimization algorithm to optimize the control parameters. Both control strategies are shown to be effective in eliminating ground resonance. Unlike bang–bang control, the prescribed control algorithms can make use of complete voltage level available in the magnetorheological damper with smooth voltage updates. A comparative study of the controller performances is made through appropriate performance indices and system responses. Finally, the most optimum control strategy to mitigate ground resonance is inferred. © The Author(s) 2021.