Indian Institute of Science Bangalore
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Machine learning and density functional theory assisted insights into the mechanical and oxidation properties of nickel-based superalloys
No full textDue to global warming and increasing fuel costs, there is a constant thrust toward increasing fuel efficiency and reducing the emissions of gas-turbine engines, which are made out of superalloys. New superalloy materials, which possess higher temperature capabilities and better oxidation resistance, are desperately needed to address these issues. Using machine learning (ML) and density functional theory (DFT), this work attempts to provide an alternate route for gaining insights that could help develop new superalloys. We develop machine learning-based approaches to predict the mechanical properties of nickel-based superalloys, including yield strength, ultimate tensile strength, creep rupture life, and fatigue life. The ML models developed are highly accurate, and the analysis of the ML models reveals significant trends to optimize the mechanical properties. The ML models also successfully predict physical phenomena, such as the yield point anomaly, in excellent agreement with the physical models. In addition, ML modeling is performed to study the oxidation of nickel-based superalloys. Mass gain due to oxidation and parabolic oxidation rates are predicted using a combined supervised and unsupervised learning approach. The oxidation rates are optimized (minimized) as a function of composition and operational parameters via the genetic algorithm. The approach yielded a set of compositions with improved oxidation resistance while maintaining excellent mechanical properties. Furthermore, we establish structure-property linkages by introducing a new methodology to estimate the Vickers hardness by employing compositions and microstructures. Featurization through image processing and statistics lead to the development of highly accurate ML models for Vickers hardness. We identify essential elements and microstructural features through feature engineering to enhance the hardness. The creep and fatigue properties depend heavily on the interfacial characteristics of the superalloys. Further, we study the interface between nickel-based superalloys' γ and γ' phases. The effects of vacancy formation and doping with different elements are studied at the interface. The solid solution strengtheners are stable at both sides of the interface and increase strength, while corrosion-resisting elements are unstable at the interface and drastically decrease the interface strength. We also study the diffusion across the interface and find that rhenium has the highest barrier for diffusion and inhibits the diffusion of surrounding atoms. Finally, using DFT, we also design high entropy alloys having properties similar to nickel-based superalloys. The problem of the site occupancy of iron in the γ' phase is settled conclusively. The planar fault energies of pristine and Fe-doped γ′ phases are calculated using DFT, and the results are compared to the SEM micrographs to determine the dominant deformation mechanism at room temperature and high temperature. The results presented in this work improve the understanding of the underlying mechanisms of strengthening and could accelerate the development of novel superalloys with enhanced mechanical and corrosion properties
Precambrian ocean, life, and syn to post-collisional records in the Himalayas (Kumaun)
No full textThe Ph.D. thesis focuses on the central sector of the Himalayas, i.e., Kumaun. Here, an attempt has been made to understand the poorly understood genesis of magnesium carbonates (sparry magnesite) of the Precambrian (Neoproterozoic) Deoban Formation in the Lesser Himalayas, and poorly constrained post-collisional metamorphism and anatexis in Kumaun and its impact on the Lesser Himalayas carbonates. Some of the key findings during the doctoral work are 1) First time identification of Neoproterozoic oceanwater droplets from the sparry magnesite crystals of Kumaun, 2) Identifying missing links between Snowball Earth glaciation, magnesite formation and oxygenation of Earth’s oceans and atmosphere, 3) Identification of structural and temporal relations between metamorphisms, anatexis, and tectonics, that shaped the overall architecture of the orogenic belt.
The sparry magnesite-bearing horizons of the Deoban Formation preserve evidence of rampant growth of the photosynthetic stromatolites during the upward transition of dolomite to magnesite horizons (sparry magnesite), which is identified as an ecological response of the microbial mats (oligotrophs) towards oligotrophic conditions (nutrition deficient, Ca poor) during Snowball Earth glaciations potentially arrived due to low riverine input of dissolved product of weathering (freezing of rivers). The trapped fluid inclusions in the magnesite represent the Neoproterozoic seawater, glacial meltwater, and their interaction. Hence, long-lived global glacial events favour magnesite precipitation, population expansion of the cyanobacteria, and, through positive feedback, contribute to large oxygen production. The Deoban basin potentially contributed to increased oxygen levels in the Neoproterozoic—the Neoproterozoic Oxygenation Event and the sparry magnesites are potential time capsules of the paleo-oceans. The central sector also preserves an immediate (~ 49 Ma monazite) and peak (~ 21 Ma) metamorphic response towards the India-Asia collision (52- 50 Ma). The Greater Himalayan Sequence (GHS) of Kumaun show an inverted metamorphism, i.e., middle to upper (~ 610 to 680 oC at ~ 4.5 to 7 kbar) at the top and lower to middle amphibolite facies metamorphism (~ 550 to 590 oC at ~ 4.5 to 7.5) at the base. The klippes in the Lesser Himalayan Zone (LHZ) are texturally and metamorphically (~ 360 to 580 oC at ~ 1 to 6.5 kbar) related to the base of the Main Crystalline Zone (MCZ). The ~ 21 Ma peak metamorphism is potentially related to the activation (23-17 Ma) of the Main Central Thrust (MCT), and most probably, the klippes reached LHZ through southward thrusting along the Main Central Thrust. The MCT activation and peak metamorphism are synchronous to the post-collisional anatexis in the Kumaun. The field relations, feldspar exsolution, and mineral chemistry suggest that the Higher Himalayan Leucogranites (HHL) crystallized under hypersolvus conditions. The maximum solidus temperature reached ~ 650 oC (two mica granite) to ~ 750 oC (tourmaline granite). The MgO and TiO2 in mica suggest a different melting source, i.e., the tourmaline granite is possibly formed due to the melting of muscovite-rich and the two-mica granite from biotite-rich metapelites. The ~ 22 Ma HHL emplacement is synchronous to peak metamorphism and the Main Central Thrust activation. Suggesting the activation of MCT played an important role in the metamorphism, HHL generation and overall tectonic evolution in the Himalayas.
Furthermore, the metamorphism and anatexis mostly impacted the Greater Himalayas. In contrast, the carbonates in the Lesser Himalayas have undergone very little or no change. As a result, the primary information of these carbonates remains intact, providing valuable insights into the sedimentation, life, and ecology of the Precambrian period
Investigating Instability Mitigation through Flame Oscillator Synchronization
No full textStringent emission regulations for reducing NOx and soot emissions are driving gas turbine
combustors manufacturers to adopt lean premixed combustion technology. However, the
rich dynamics of turbulent, lean premixed combustion inside the gas turbine combustor
could pose challenges like thermoacoustic instabilities, blow-off, and flashback. As such
the unsteady heat release rate can couple in-phase with the combustor duct acoustics to
generate large amplitude oscillations known as thermoacoustic instabilities. During such
instability, the combustor structure undergoes multiple cycles of thermal and mechanical
loading which could increase the chances of a premature fatigue failure. Furthermore, when
the fuel flow rate drops or the air flow rate suddenly increases due to upstream dynamics,
the premixed flame could experience a blow-off event where a part or in the worst case the
entire flame extinguishes. Such events drastically affect the performance of the gas turbine
combustors, leading to severe safety risks for aviation gas turbine engines. It remains essential
to understand the sources and factors causing thermoacoustic instabilities and blow-off to
design improved next-generation engines where such problems are mitigated.
To that end in this thesis, we seek to deepen our understanding on the mitigation of the self excited thermoacoustic instabilities with our in-house designed and developed rotating swirler
burner experimental setup. This combustor is novel in its implementation of rotating the
otherwise static swirler meant to stabilize the lean premixed flame to mitigate the instability.
In such a combustor with inherent, high-amplitude thermoacoustic instability is realized
while the swirler is static. However, we show that increasing the swirler rotation upto certain
x speeds can progressively reduce the instability amplitude. We use simultaneous high speed
stereo Particle Image Velocimetry (sPIV), high speed chemiluminescence measurement
in the vertical (r −y) plane, alongside time resolved pressure measurement to investigate
the flow-flame dynamics as the combustion transitions from a thermoacoustically unstable
condition to a stable state. During such transition, we observe the pressure amplitude
does not decrease uniformly but instead bursts of large-amplitude oscillations appear more
sparsely in the low-amplitude noisy data as the stable state is approached. This is known as
intermittency, which is mostly observed while transitioning from stable to unstable state by
varying flow Reynolds number. To model such intermittent dynamics phenomenologically,
we discretize the swirling turbulent flame into an ensemble of flame oscillators arranged
circumferentially around the center-body of the swirler, oscillating in the r −y plane. To
simulate the synchronization between these oscillators we modify the Kuramoto model to
emulate the flame oscillator dynamics. The proposed model can qualitatively reproduce the
time-averaged and intermittent dynamics while transitioning from unstable to stable states.
Next, we move on to high speed chemiluminescence imaging of the horizontal (r −θ)
plane with simultaneous pressure measurement to identify the synchronization dynamics
of flame oscillators in the combustor. Furthermore, we vary the tube lengths to excite the
flame at different acoustic frequencies to test the proposed model at different conditions. As
we change swirler rotational rate, the system goes through intermittency while transitioning
from unstable to stable state. We observe the system reaches to the state of combustion noise
at lower swirler rotation rate, as the tube length increases. At higher Reynolds number, the
swirler rotation rate required for instability mitigation increases. From the Rayleigh index
map we find the source of flame-acoustic coupling to be distributed in the r−θ plane. As the
flame images are transformed into the r −θ co-ordinate, we observe the flame at different
azimuthal location oscillate together during combustion instability. During combustion noise
these oscillations becomes asynchronous, which proves the location of the oscillators in
the azimuthal plane. With this evidence, we proceed to modify the synchronization model
by incorporating the feedback mechanism between the duct acoustics and heat release rate
oscillations. The heat release rate oscillations are modelled with Kuramoto model with
flamelet oscillators. We used a linearized Helmholtz equation with heat source to model the
duct acoustics of the setup. The feedback mechanism synchronizes the flamelet oscillators
with the pressure oscillations in the Kuramoto model whereas the heat source term in the
Helmholtz equation is generated from the ensemble of the flame oscillators. This model is
used to predict the heat release rate oscillations for the different experimental conditions. It
showed qualitative match with the experimental data, and reproduced the different states of
the thermoacoustic systems with good accuracy.
In the last part of the thesis, as an appendix, the preliminary investigations on blow-off
precursors of a model gas turbine combustor with three interacting swirling premixed flame,
is included. We focus on the origin of the flame extinction that leads to a complete flame
blow-off. We use high speed OH* chemiluminescence to locate the origin of the flame
extinction. Using low-speed simultaneous stereo Particle Image Velocimetry and Planar
Laser Induced Fluorescence (sPIV-PLIF) measurements, we identify the reasons behind the
flame extinction through strain rate measurement
Synthesis and X-ray Crystallographic Studies of Salts, Co-crystals, Solvates of some Amine containing Active Pharmaceutical Ingredients and In-situ Cryo Crystallographic Studies on some Organic Liquids
No full textChapter I describes the general introductory note to describe the common terms like crystalline solids, properties, and their uses pharmaceutical industry. The definition of common terms like Salts, Co-crystal, solvates were introduced along with factor affecting crystallization specially on pharmaceutical Co-crystals. The general discussion on hydrogen bonds, halogen bonds and chalcogen bonds and study of weak interaction through in-situ Cryo-crystallization were introduced.
Chapter II emphasizes the importance of intermolecular interactions in pharmaceutical co-crystal and in formation of unusual base pairs are investigated. Efforts have been made to obtain details on the preferences of formation of salts and co-crystal based on the pKa value, the salt and Co-crystal of trimethoprim and sulfamethazine along some heterocyclic coformers were synthesized, thorough analysis of inter and intra molecular hydrogen bonding features with molecular geometry have been studied.
Chapter III, we have synthesized three salt of cytosine molecule with coformers of gallic acid, salicylic acid via Aspirin and thiazole carboxylic acid viz., CYT.GAL, CYT.SAL, CYT.TCA. the strong electrostatic force of attraction between the ionic species by shifting the proton from coformers to cytosine has been characterised by the intermolecular interactions and hydrogen bonding pattern.
Chapter IV emphasis on the solvated polymorphs quinidine, in which the solvents participated in the structural part of the unit cell that have specific crystalline arrays, also designated as pseudo polymorphs of quinidine. Three solvates were synthesised and involvement of their interaction with quinidine through hydrogen bonds were studied.
Chapter V, Section A describes the in situ Cryo-crystallization studies of low melting organic liquids of three isomeric compounds, namely ortho, meta, and para-trifluoromethyl anilines. Their crystal structures have been analysed for various intermolecular interactions, particularly weak hydrogen bonding interactions. In Section B dimerized 4-amino thiophenol and selenophenol crystal structures have been analyzed for various chalcogen, hydrogen bonds along with other weak interaction.
Chapter VI, Section A provides the importance in capturing of CO2 from atmospheric flue gas under the ambient condition by using Primary amines. Three aromatic amine carbamates viz., benzylamine, phenylethylamine, and 3-phenyl propyl iso-structurally and packing pattern were discussed through hydrogen bonding interaction. In Section B, the structure of ethylene diamine carried out by in-situ cryo crystallization, and adsorption of CO2 under the ambient condition as ethylene diamine carbamate have been analysed for its structural features
Computational analysis of polymer nanocomposites across length scales
No full textPolymer nanocomposites (PNC) exhibit excellent mechanical properties making them a promising material for several engineering applications. The enhancement in properties is primarily attributed to the toughening and stiffening mechanisms provided by nanofillers. Computational techniques have proven to be a powerful tool for studying the properties and behavior of materials. Since the dimensions of the components of PNC ranges from the nano to macroscale, a computational study on PNCs should account for mechanics across multiple length scales. The primary objective of this thesis is to investigate the fundamental mechanisms that govern the mechanical properties of PNCs. A better understanding of these mechanisms can facilitate the efficient design of PNCs for desired applications. To this end, this thesis is structured into two parts. First, we employ molecular dynamics (MD) simulations to study the effect of functionalization and agglomeration on the mechanical properties of PNCs. Second, we investigate the relevance of multiscale simulations and higher-order continuum theories as alternatives to computationally expensive MD simulations.
Functionalization and agglomeration are two key factors which influence the mechanical properties — especially fracture properties — of PNCs. Through a suite of MD simulations, we studied the influence of varying degrees of functionalization on the elastic and fracture properties. Interestingly, it was found that there exists an optimal degree of functionalization corresponding to maximum enhancement in elastic property, tensile strength, ductility, and fracture toughness. The underlying mechanics behind this optimality is identified through careful studies on crack propagation mechanisms, including crack arresting and the formation of new crack surfaces. Moreover, the improvement in interfacial interaction induced by functionalization was explained from an atomistic perspective. A sequence of similar studies on ag- glomerated PNCs revealed that even functionalization does not improve fracture toughness. The agglomeration results in crack to completely propagate through the polymer matrix. Thus the CNTs do not participate in resisting the crack. Furthermore, the shear strain distribution along the CNT surface revealed that the CNTs and the polymer surrounding them act as a monolithic unit, resulting in a reduction in fracture toughness and ductility. The agglomerated CNTs are also found to contribute towards strain accumulation and failure of PNC at a lower global strain level. These atomistic studies provide valuable insights into the overall mechanical behavior of PNCs. However, the computational cost of MD simulations hinders its application to large systems.
In the second part of the thesis, we study possible alternatives to expensive MD simulations. An MD-informed hierarchical multiscale method was adopted to estimate the effective elastic properties. Through this method, the elastic properties of PNC and hierarchical PNC were determined at a low volume percentage of CNT reinforcement. The results were in good agreement with the published experimental values. Attaining such a low volume fraction using MD simulations is impossible due to the computational cost. Following the idea of reducing the computational cost, the relevance of higher-order continuum theories in predicting the mechanical response of nanotubes was investigated. However, these theories have unknown length scale parameters which need to be computed. For this purpose, we used MD and ML in tandem. Initially, MD simulations were used to generate a dataset consisting of different variables and the corresponding length scale parameter values. Then ML-based techniques were used to make further predictions. The method proved to be effective in eliminating further MD simulations within the limit of the dataset generated. However, the alternatives to MD simulation presented in this thesis are confined to estimating the elastic properties.
In general, this thesis provides a comprehensive computational study at different length scales of a PNC. Several atomic-level mechanisms instigated due to functionalization and agglomeration are presented. A few alternatives to elude the computational cost of MD simulations are also discussed
Studying the role of small GTPase Rab14 in the regulation of endosomal function
No full textThe endomembrane system and their associated membrane trafficking are fundamental
features of eukaryotic cell biology. These processes are regulated by small GTP-binding
proteins that acts as molecular switches to control signalling based on the type of bound
nucleotide. Small GTPases function to exert spatial and temporal regulation on intracellular
trafficking. Amongst the Ras superfamily of small GTPases, the Rab subfamily consists of
around 70 members. Rabs serve as markers for membrane/organelle identity and function as
regulatory factors in vesicle transport. In this study, we attempted to characterize the
endosomal function of the small GTPase Rab14.
Rab14 was identified as a low molecular weight GTP-binding protein (Elferink et al., 1992).
This protein localizes to trans-Golgi, early and recycling endosomes and has been shown to
control the transport from the Golgi and endosomes. Rab14 has been implicated in multiple
cellular processes including development (Ueno et al., 2011) and immune response. Multiple
studies have described the mechanism of Rab14 function in endosomal recycling. Here, we
attempted to investigate its function in the biogenesis of tubular recycling endosomes (TREs).
Objective : Studying the regulation of recycling endosomes by Rab14
Rab14 has been shown to localize to endosomal compartments in addition to trans-Golgi
(Junutula et al., 2004). Although it is involved in endocytic recycling, its precise role in
regulating the recycling of cargo such as transferrin has not been completely understood.
Expression of GFP-Rab14 in HeLa cells showed its localization to early endosomes (EEA1)
and recycling endosomes (TfR). Further, constitutive active mutant of GFP-Rab14 also
showed its localization to these compartments. However, the dominant negative mutant of
GFP-Rab14 did not show its effect as expected but localized to trans-Golgi as shown by
other groups. Multiple Rabs are known to localize to endosomal compartments. However,
whether and how Rab14 regulates other endosomal structures labelled by Rabs have not been
well studied. To study the effect of Rab14 on endo-lysosomal pathway, we co-expressed
GFP-Rab14WT and its mutants with mCherry-tagged Rab11A or Rab22A to represent REs
and Rab7A to mark late endosomes/lysosomes. Our studies show that Rab14 co-localizes
highly with RE localized Rabs Rab11A and Rab22A, and to a lesser extent with Rab7A.
Consistently, overexpression of constitutive active mutant of Rab14 causes increased co localization with co-expressed Rabs. To study the possible regulation of Rab14 on recycling
endosomes, we employed a siRNA-mediated knockdown approach. Depletion of Rab14
levels did not significantly affect the length or the number of tubules labelled by the kinesin-3
motor KIF13A or the small GTPase Rab22A. We also checked the status of the CIE cargo,
CD147, which traffics through tubular recycling endosomes. However, we noticed that
Rab14 knockdown significantly decreased the percentage of cells showing endogenous
Rab11A tubules. Interestingly, we also observed that Rab11A staining of Rab14-depleted
cells under methanol fixation conditions displayed enlarged Rab11A vesicular structures,
which was not observed in control cells.
Rab11A tubules can carry both CIE as well as CME cargo. To confirm if Rab11A-labelled
tubules were broadly affected, we also assessed the status of CIE marker CD55 as well as
CD147 in Rab14-depleted conditions. To study if Rab11A knockdown can affect Rab14
localization or Rab14 compartments, we knocked down Rab11A and expressed GFP-tagged
Rab14. However, we did not observe any significant effect of Rab11A knockdown on Rab14.
These observations suggest that Rab14 possibly may work upstream and/or regulate a subset
of Rab11A-positive tubules. For recycling of cargo to the plasma membrane, endosomal
tubules are either generated from early endosomes or from the endocytic recycling
compartment (ERC). To investigate which of these two spatially distributed REs are
regulated by Rab14, we stained Rab14 knockdown cells with a combination of cargo (marked
by TfR)/organelle markers to study if Rab14 regulates Rab11A-dependent cargo localization
and transport from early endosomes to recycling endosomes or recycling towards the plasma
membrane. Our results show that Rab14 does not significantly affect cargo localization and
transport to and from Rab11A endosomes, indicating the possibility of alternate redundant or
parallel pathways to ameliorate block in the Rab14-dependent recycling pathway.
We also tested if Rab14 levels may affect late endosomal-lysosomal compartments using
markers specific for late endosomal and lysosomal markers. Our results do not show
significant changes in endo-lysosomes suggesting that Rab14 is primarily associated with
early and recycling endosomes and may not directly be involved in regulating late
endosomal-lysosomal compartments.
A number of molecules have been shown in literature to regulate Rab11A tubules. Based on
literature and our observations, we hypothesized that Rab14 may recruit sorting nexins,
probably SNX4 or SNX1 (which are known membrane sculpting and remodelling
molecules), to modulate Rab11A endosome tubules downstream. We overexpressed SNX4
and SNX1 in Rab14 depleted cells and made observations on its cellular distribution and
localization. Our results suggest that Rab14 may work through different mechanisms or
effectors to modulate Rab11A tubules. Overall, our results suggest that Rab14 acts upstream
of Rab11A and regulates Rab11A endosomal compartments or a subset of Rab11A
structures
A Unified Modeling Approach for Design and Performance Improvement of Triple Active Bridge Converter
No full textTriple Active Bridge (TAB) converter is a multi-port DC-DC converter. This converter is an extension of the popular Dual Active Bridge converter. It features desirable traits of the DAB converter, such as high power density, galvanic isolation, and bi-directional power flow between any of the ports. As in other multi-port converters, redundant power conversion is minimized through component sharing among the ports in a TAB converter. All the switches in a TAB converter can undergo soft-switching over a wide range of operating points, reducing switching losses and the size of auxiliary components. The multiple degrees of freedom in modulating a TAB converter offer several design and operational flexibilities.
However, this converter has yet to come into the limelight despite these advantages. One of the reasons is the lack of a unified analytical framework for the design and operation of this converter. The existing models for the TAB converter are limited in scope and cannot be easily used for the design and operational optimization of the converter. This work focuses on developing simple, unified models for analyzing the TAB converter.
Firstly, the popular Fundamental Harmonic Approximated (FHA) large-signal and small- signal models are evaluated to understand their limitations. Improved large-signal and small-signal Generalised Harmonic Models (GHM) are developed by incorporating the impact of higher-order harmonics. While the GHM is shown to be superior for small-signal analysis of the converter and the design of a closed-loop control system, it is not suitable to analyze the soft-switching bounds of the TAB converter. To overcome the limitations of GHM, a Unified Model that incorporates the impact of the magnetising inductance of the three-winding transformer is proposed. The Unified Model can accurately predict the AC port currents at the switching instants and is used to study the soft-switching bounds of the TAB converter. The GHM and Unified Model are validated through extensive switching circuit simulations and experimental results from a 1 kW hardware prototype developed in the laboratory.
Further, a new design algorithm for the TAB converter is proposed. The proposed algorithm leverages the FHA model’s simplicity and the Unified Model’s accuracy. Finally, a new modulation scheme based on Penta Phase Shift with five degrees of freedom is proposed to achieve soft-switching across the operational range of the TAB converter
Shear Behaviour of GCL-Sand Interrfaces under Static and Dynamic Conditions
No full textGeosynthetic clay liners (GCL) are unique geocomposites that combine the beneficial
properties of bentonite clay and geosynthetics in providing effective hydraulic barriers in
landfill systems. In landfills, GCLs form interfaces with soils and geosynthetics, resulting in
inhomogeneity of the system. Inadequate shear strength mobilization at the interfaces results
in translational failures in Geosynthetic Clay Liner (GCL). Interface shear strength of GCLs
with the sand particles is predominantly influenced by the surface characteristics of the GCL,
size and shape of the sand particles and their interaction mechanisms. These mechanisms
change drastically with the hydration of GCLs and under repeated and dynamic loading
conditions. This thesis examines the GCL-sand interactions and quantifies the interface shear
strength under static and dynamic conditions with dry and hydrated conditions in sand.
Illegal sandmining has resulted in the depletion of natural river sand and its scarcity for
various constructional activities. To combat this issue, this study proposes the use of
Manufactured sand (Msand) as a suitable subgrade or cover soil in landfills and evaluates its
performance as an interfacing material with GCLs and compares it with the performance of
river sand. Since the particle shapes of natural river sand and Msand are significantly different,
a part of this thesis is focused on quantifying the shape parameters of the sands and
investigating the effects of particle shape on the interaction mechanisms and shear strength of
different GCL-sand interfaces.
This thesis presents three different types of interface shear tests – modified direct shear
tests, inclined plane tests and shaking table tests on GCL-sand interfaces with a natural sand
and a manufactured sand under dry and hydrated conditions. Gradation of natural sand and
Msand was kept identical to eliminate the particle size effects. The static shear strength of
GCL-sand interfaces was evaluated through modified direct shear tests at higher normal
stresses and inclined plane tests at lower normal stresses. The dynamic frictional properties
were estimated using shaking table tests conducted at different g-levels, normal stresses, and
excitation frequencies under dry and saturated conditions. Results from the interface shear tests
were analysed in the light of shape analyses of sand particle and digital image analysis of
sheared GCL surfaces. Particle shape parameters were obtained using computational
algorithms applied to digital images of particles in MATLAB. To investigate the performance
of GCLs under repeated shear conditions, modified direct shear tests were carried out for eight
cycles of shearing in dry and hydrated conditions. The GCL used in the current study has a
nonwoven geotextile as the carrier layer and a woven geotextile as the cover layer. Interface
shear studies were carried out on both woven and nonwoven geotextiles interfacing with natural
and manufactured sands under dry and hydrated conditions.
Manufactured sand particles are less spherical and less rounded compared to river sand
particles and their roughness is about twice to that of the roughness of the river sand particles.
Results from the experimental and image studies showed that manufactured sand provides
better particle-fibre interlocking compared to river sand under all test conditions, due to the
favourable shape of its grains. Hence the natural sand interfacing with GCL in liners and
capping components of landfills can be replaced with manufactured sand, with added benefits.
Digital image analysis of GCL specimens exhumed after the shear tests provided important
clues to the microscopic interactions that govern the overall shear strength of the interfaces.
Surface changes to woven and nonwoven geotextiles due to shearing against natural and
manufactured sand are compared in terms of percentage area of sand particle entrapment,
extent of bentonite extrusion at different normal stresses under hydrated conditions and damage
to the geosynthetic fibres during repeated shear. Shaking table studies showed that the dynamic
friction angle of GCL-sand interfaces is only one third of the static friction angle, indicating
the need for choosing materials that provide higher interface resistance for the construction of
landfills in locations prone to earthquakes
Vision-driven Tele-Operation for Robot Manipulation
No full textIt’s worth the time to acknowledge just how amazingly well we humans can perform tasks
with our hands. Starting from picking up a coin to buttoning up our shirts. All these tasks for
robots are still at the very forefront of robotics research & require significant interactions between
vision, perception, planning & control. Becoming an expert in all of them is quite a challenge.
Tele-operation augments the robot’s capability for performing complex tasks in unstructured en-
vironments and unfamiliar objects with human support. It offers the robots reasoning skills,
intuition, and creativity for performing these tasks in unstructured environments and unfamiliar
objects.
However, most Tele-operation techniques either use some sort of sensor/gloves or expensive
cameras to capture the gestures of the human, making the operation bulky as well as expensive.
We present a vision-based Tele-operation of the KUKA IIWA industrial robot arm that imitates
in real-time the natural motion of the human operator seen from a depth camera.
First, we will discuss about Wahba’s algorithm, which was used to estimate the 6-d hand pose
of the operator’s hand. Wahba’s algorithm uses the predicted 3d location of the 21 hand landmarks
from google’s mediapipe to estimate this 6-DoF hand pose. The hand orientation estimated above
is used to tele-operate the 7-DoF KUKA IIWA manipulator in master-slave as well as in semi-
autonomous mode. Then we will talk about how an object’s orientation is estimated and used in
the semi- autonomous mode of operation. The object of interest for manipulation is picked by
the operator’s pointing to the object in a video stream. The focused object is then detected and
segmented, and the object’s pose is estimated based on its geometry of surface normals.
Finally, the object’s 6-DoF pose is estimated using hand-eye calibration and robot motion
is planned with a B-spline trajectory. After combining all these techniques, two modes of tele-
operations for KUKA IIWA are proposed. These methods give efficient operation of robot imitating
human motion as well as gesture based operation for the semi-autonomous mode of operation
Development of Spatio-Temporal Multi-Task Assignment Approaches for Perimeter Defense Problem
No full textRapidly evolving technologies in the autonomous operation of Uninhabited Aerial Vehicles (UAVs) and associated developments in low-cost sensors have created significant interest among researchers in using them for various civil and military applications. With the autonomy and presence of various sensing equipment, onboard UAVs lead to problems in the privacy, safety, and security of many safety-critical infrastructures. A critical infrastructure that needs to be protected is approximated by the convex region and called territory. A team of UAVs that protects the territory is called the defenders and UAVs which try to enter the territory are called the intruders. A team of defenders operates inside and, on the perimeter, and protects the territory from intruders by capturing intruders on the perimeter is referred as the Perimeter Defense Problem (PDP). The velocity of intruders is used to predict the arrival location on the perimeter and arrival time. In this way, each intruder generates a spatio-temporal task for the defenders to reach tha= t specific location at a specific time to neutralize that intruder. So, PDP is formulated as the spatio-temporal multi-task assignment (STMTA) problem. In the STMTA problem, some minimum number of defenders (robots) are required to execute the given spatio-temporal tasks; without this minimum number of defenders, STMTA problem is ill-posed. The proposed Dynamic REsource Allocation with Multi-task assignment (DREAM) algorithm solves the bottleneck issue of iterative computation for the required number of robots and provides the two-step solution to compute the required minimum number of robots and their optimal assignments to execute given spatio-temporal tasks. Next, the trajectory generation algorithm has been developed to compute the trajectory of each defender. Furthermore, it is proved that all the computed trajectories of homogeneous agents, operating in the convex region, are collision-free.
For highly maneuvering intruders, the errors in the prediction of tasks deteriorate the performance of DREAM. In the P-DREAM approach, a dedicated defender is assigned to each prioritized intruder by enforcing the prioritized intruder as a first task. A prioritized intruder must be delegated to the reserve defender before it becomes infeasible for the reserve defender. The static design for PDP computes the minimum number of reserve stations, their optimal location, priority region, monitoring region, and the minimum number of defenders required for monitoring. The quantification of priority and monitoring region will be helpful in practical implementations. For protecting a large territory, more defenders are required, also each defender has a limited sensing range to detect and track intruders. To address these issues of partial observability and scalability the decentralized spatio-temporal multi-task assignment approach is proposed. A modified consensus-based bundle algorithm has been proposed to solve the STMTA problem. Finally, the thesis demonstrates the working of the DREAM approach for heterogeneous pick-up and just-in-time delivery (PJITD) problems. Just-in-time tasks have been used to improve operational efficacy for static (priorly known) tasks. The non-iterative solution of modified DREAM overcomes the bottleneck problem of the iterative (and hence offline) solution and provides a real-time implementable solution. The cost function is modified to include the traveling time, operating time, and heterogeneous skills required to execute the tasks. The working of modified DREAM is illustrated using high-fidelity ROS2-GAZEBO simulations and lab-scale hardware experiments