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IIT Hyderabad

Department of Mechanical and Aerospace Engineering

Silicon-based microelectromechanical system (MEMS) and nanoelectromechanical systems (NEMS) have been used to design and fabricate sensitive sensors and actuators. Recent research trends show that graphene and carbon nanotubes (CNTs) have been used to change the surface properties of silicon-based MEMS and NEMS to improve different mechanical, optical and electrical properties of silicon-based composites. In this paper, we focus on analyzing the vibrational characteristics of silicon-based devices when the surface of silicon is coated with single-layer graphene and horizontally aligned carbon nanotubes (HACNTs). To perform the analysis, we use multi-scale finite element approach for developing graphene–silicon nanocomposites (GSNCs) and carbon nanotube-silicon nanocomposites (CSNC) composites in which interface layer of silicon with graphene or CNT is modeled using bonded contact element. Subsequently, we performed modal analysis to find the first transverse mode frequency of GSNC and CSNC composites for beam with smaller as well as longer lengths. The numerical model is compared with classical beam theory with and without surface effect. For GSNCs composites, we take a fixed-free case with lengths in the range of (20 Å–120 Å) and (400 Å–2000 Å), respectively. For CSNC composites, CNT diameter is varied from (5 Å–30 Å) for single walled nanotube. Subsequently, we analyze the influence of HACNTs-on-silicon on its vibrational characteristics. The analysis presented in the paper demonstrate that GSNCs offer a higher bending stiffness compared to single layer graphene (SLGs) and isolated silicon nanosheet which lead to higher natural frequency. A similar trend is found in the case of HACNTs on silicon NS when the number of tubes increases. © 2019 Elsevier Inc.

World University root node

Applied Mathematical Modelling

Frequency analysis of carbon and silicon nanosheet with surface effects

Finite element method is extensively used for the analysis of impact response in complex materials. The prediction from finite element model may exhibit significant difference from that of experiments due to uncertainties in model, experimental measurements, and parameters that are derived based on experiments for model development. The quantification of parametric uncertainties, such as parameters in constitutive relation, associated with the numerical model is an important aspect that needs to be investigated for a credible computational prediction. This work considers uncertainty quantification in finite element modeling of nanoscale dynamic impact problems. A viscoplastic power law constitutive model is obtained from nanoscale impact experiments on Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) samples. The constitutive model is used in a finite element model to simulate impact experiments. The measured response from impact experiment and FEM simulation is used to quantify the parametric uncertainties in the constitutive model for the analyzed HTPB-AP sample. © The Society for Experimental Mechanics, Inc. 2019.

Conference Proceedings of the Society for Experimental Mechanics Series

Uncertainty quantification in nanoscale impact experiment in energetic materials

Interfaces in the materials are known entities since last century described as early as in the interfacial excess energy formulations by Gibbs (Bobelmann et al. 2007). The interface effect (or surface effect) is also widely referred to as the interface stress (or surface stress) that consists of two parts, both arise from the distorted atomic structure near the interface (or surface): the first part is the interface (or surface) residual stress which is independent of the deformation of solids, and the second part is the interface (or surface) elasticity which contributes to the stress field related to the deformation. Plastic deformation, in particular, the initial yielding point (i.e., the yield surface), is sensitive to the local stress (or local strain) of a heterogeneous material, which includes both the local (surface/interface) residual stress and local stress-strain relationship. The plastic deformation at the interfaces also considers the tension and compression along the interface and stress mismatch because of the material property differences. In the nanomaterials, the surface and interface stresses become even more important owing to the nanoscale size of the particles and interface areas. © Springer Nature Switzerland AG 2019. All rights reserved.

Handbook of Nonlocal Continuum Mechanics for Materials and Structures

Properties of material interfaces: Dynamic local versus nonlocal

We investigate the route to self-excited thermoacoustic instability in a laminar flow multiple flame matrix burner. With an increase in the equivalence ratio, the thermoacoustic system that is initially quiet (stable operation) transitions to limit cycle oscillations through two distinct dynamical states, namely, bursting oscillations and mixed mode oscillations. The acoustic pressure oscillations transition from quiescence to large amplitudes during bursting oscillations. Such high amplitude bursting oscillations that occur well ahead of the onset of limit cycle oscillations can potentially cause structural damage. The thermoacoustic system exhibits hysteresis. The transition to limit cycle oscillations is replicated in a phenomenological model containing slow-fast time scales. © 2019 Author(s).

Fulltext

Chaos

Bursting and mixed mode oscillations during the transition to limit cycle oscillations in a matrix burner

An approach to account for combined convection during droplet evaporation and study of pure component droplets and mono-dispersed sprays is presented. The classical gas-phase and infinite conductivity liquid-phase model are extended, with the use of an effective Reynolds number, to conflate the combined effects of forced and natural convection. The current model, after validation with experimental and numerical data using an independent code, is incorporated into a commercial CFD software, ANSYS Fluent, via user-defined functions with the Eulerian–Lagrangian numerical scheme. A validation study is carried out by comparing with available experimental, numerical, and analytical data on pure component droplets and a mono-dispersed spray, respectively, for without and with droplet dynamics. The results are shown in terms of mass fraction, droplet velocity, droplet diameter square, and droplet temperature. The validation shows reasonably good match between the present numerical data and experimental and analytical data, respectively, for initial Red/Grd 0.09, 2.12, and 60 evaporating droplets/sprays. It is concluded that the biofuels, for example, ethanol with a lower latent heat of vaporization, burn much like mono-component droplets and the blowing effect can be important in their modeling in the spray combustion. © Springer Nature Singapore Pte Ltd. 2019.

Lecture Notes in Mechanical Engineering

Numerical modeling and study of vaporization of single droplet and mono-dispersed spray under mixed convection conditions

Flexibility of graphene (Gr) is partially credited to its two-dimensional (2D) molecular structure which resembles like interconnected unit of hexagonal carbon atoms ring. Therefore, to increase the flexibility of micromechanical beams, we analyze the influence of hexagonal shape of microcantilever beam without and with hexagonal hole of varying side length. To carry out the analysis, we first fabricate the hexagonal structure and perform measurement using laser vibrometer. Based on the measurement, we found that the presence of hexagonal holes make the beam flexible by two order of magnitude more than that of a conventional beam without holes. However, we found that the frequency of microcantilever beam with hexagonal shape decreases with increase in hole size. To undo the effect of hexagonal holes on microbeam frequency, we cover the microbeam with an electrospun cellulose acetate nanofiber mat (CAF). Based on the measurements, we found that the frequency can be increased more than that of a solid rectangular microbeam. Finally, we propose a finite element based methodology to model microbeam and nanofiber mat to compute their frequencies. Thus, based on the understanding developed in this work, this hierarchical structure of microcantilever beam with electrosun nanofibers can be used as a best candidate for utilizing it for the development of flexible sensors based on MEMS devices. © 2019 IOP Publishing Ltd.

Department of Chemical Engineering

Department of Physics

Materials Research Express

Frequency analysis of hexagonal microbeam with 2D nanofiber mat

Ditch crossing is one of the essential capabilities required for a biped robot in disaster management and search and rescue operations. Present work focuses on crossing a wide ditch with landing uncertainties by an under-actuated planar biped robot with five degrees of freedom. We consider a ditch as wide for a robot when the ankle to ankle stretch required to cross it is at least equal to the leg length of the robot. Since locomotion in uncertain environments requires real-time planning, in this paper, we present a new approach for generating real-time joint trajectories using control constraints not explicitly dependent on time, considering impact, dynamic balance, and friction. As part of the approach, we introduce a novel concept called the point of feasibility for bringing the biped robot to complete rest at the end of ditch crossing. We present a study on the influence of initial posture on landing impact and net energy consumption. Through simulations, we found the best initial postures to efficiently cross a wide ditch of width 1.05 m, with less impact and without singularities. Finally, we demonstrate the advantage of the proposed approach to cross a wide ditch when the surface friction is not same on both sides of the ditch. © Cambridge University Press 2018.

Robotica

Generating real-time trajectories for a planar biped robot crossing a wide ditch with landing uncertainties

Two-fluid flow blurring atomization is characterized by the backflow recirculation of the air phase in the liquid pipe by bifurcation of the liquid and airflow. Most of the primary spray process is completed in the injector due to the penetration of air into the liquid tube. Thus, the majority of the liquid ligaments are converted into a fine spray at the outlet of the nozzle. Experiments were performed with two different air to liquid ratios (0.6 and 1) by mass, where water is considered as the liquid and airflow was kept constant (0.2 g/s). To change the ALR, the liquid flow rate was changed. Particle image velocimetry (PIV) diagnostic technique provides the full-field velocity of the spray droplets (discrete phase). It may be noted that sprays are self-seeded and PIV measurements reflect the droplet velocities instead of air velocity. To understand the effect of the spatial resolution of PIV on spray droplet velocity; experiments were conducted at three different spatial resolutions (11.8, 16.4 and 23.22 µm/pixel) for each ALR. As the ALR is increased, the mass of the liquid in the spray decreases, resulting in finer atomization and velocity of the spray droplets. This means that finer droplets are generated for the same mass of air at a lower liquid flow rate as compared to higher liquid flow rate. Note that Reynolds stresses provide an indication of the turbulent breakup of the droplet and larger magnitudes observed for higher ALR indicate finer atomization. Copyright © 2019 ASME

ASME 2019 Gas Turbine India Conference, GTINDIA 2019

Experimental investigation of flow blurring atomizer at near field using particle image velocimetry

The effect of tempering temperature on surface hardened AISI H13 tool steel by laser remelting process using an Yb-fiber laser has been investigated. Single remelting tracks were produced in argon environment at different laser power and scan speed in the range of 400–600 W and 200–1600 mm/min respectively, maintaining the laser spot diameter fixed at 3 mm. Their effects on geometrical aspects, microstructure and microhardness were analyzed considering the thermal history of molten pool recorded using an IR pyrometer. Microstructure was found to be associated with cooling rate and melt pool lifetime, estimated from the temperature signal. Thereafter, remelted surface with 30% overlapping tracks was generated and subjected to 1 h tempering cycle at different temperatures in 500–900 °C range followed by hardness and wear tests. Hardness was retained fully up to 500 °C and significant softening occurred at 600 °C and 700 °C. At 800 °C and higher temperatures, effects of laser remelting were impaired completely, but substrate got hardened through martensite formation in a conventional manner. Wear resistance followed the trend of hardness. Changes in microstructure and formation of various phases were found to be the reasons behind the modifications in hardness. © 2019 Elsevier B.V.

Surface and Coatings Technology

Effect of tempering on laser remelted AISI H13 tool steel

The depletion of natural resources coupled with the alarming increase of pollution level is the driving force to use diesel in an environmentally benevolent manner. Research done till now is mainly focused upon the utilization of lower alcohols and information on higher alcohol blend is limited. The present study aims to investigate the effect of using a ternary blend of diesel, biodiesel, and decanol on a diesel engine. Tests were performed using decanol blended with biodiesel and diesel. The concentration of ternary blends were 10%, 15%, 20%, 30%, and 40% of decanol by volume while the diesel concentration was maintained to be 50% throughout. The study revealed that among the ternary blend, brake thermal efficiency increases with increase in the concentration of decanol. The brake specific fuel, and energy consumption decrease with an increase in alcohol content. Thus 40% decanol blend shows the least brake specific fuel consumption. NOx emission increased with the increase of alcohol concentration, whereas the emission levels of CO, HC, and smoke opacity decreased. Peak pressure is also found to be highest for 40% decanol blend and least for 10% decanol blend. With the addition of decanol content in the ternary blend, heat release rate is found to be increased whereas the cumulative heat release rate is observed to be reduced during the end phase of combustion. The study concluded that 40% blend of decanol, biodiesel and diesel can optimize engine performance and emission without performing any modifications in the CI engine. © 2018 Elsevier Ltd