Indian Institute of Science Bangalore
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Novel L12 precipitate hardened Co-base alloys
No full textConventional cobalt base superalloys relied on solid-solution and carbide precipitate based strengthening. They lacked high temperature (more than 800 ˚C) creep strength as compared to Ni-base superalloys which are strengthened by γ′ precipitates having L12 crystal structure coherent with the γ matrix. The blades of land-based gas turbines for electricity generation, need to possess hot corrosion resistance from Sulphur in the gasified coal used as fuel. This motivates the present work, which is to investigate Co–Ti–V alloys to develop Co-base superalloys possessing (γ + γ′) two-phase microstructure similar to Ni-base superalloys.
Co–Ti system has thermodynamically stable γ′ phase but the lattice misfit of γ–γ′ phases is unacceptably high. Vanadium being smaller in atomic size as compared to titanium can be added to minimize the misfit. Therefore, the objective of this thesis was to investigate the effect of vanadium addition to cobalt-titanium system on the physical and mechanical properties.
The Co–Ti–V alloys were vacuum arc melted and cast into rod form, followed by heat treatments and microstructural characterization. Mechanical testing was carried out to evaluate the strength from room temperature to high temperature. First principles density functional theory calculations, finite element modelling, and discrete dislocation dynamics were performed to analyze the effects of various parameters on physical and mechanical properties.
The addition of V to Co–Ti system decreases the γ′ solvus but increases the solidus and liquidus temperatures. Thus, it improves the homogenisation temperature window. The γ′ precipitate morphology changes from cuboidal to cuboidal with round corners with an increase in V concentration. The composition of γ′ phase suggests Ti as better γ′ phase former than V. The extent of discontinuous precipitation at the grain boundaries in Co–Ti system decreases with V addition. The constrained lattice parameter misfit of γ–γ′ phases decreases with V addition. The γ′ phase is off-stoichiometric with Co antisite defects in 20% of Ti sublattice in Co3(Ti, V).
Co–Ti–V ternary alloys possess improved strength over Co–Ti binary alloy. The strength at room temperature initially decreases with V addition followed by an increase at higher concentration of V. The alloys show yield stress anomaly; increase of strength with increase in temperature with the maximum strength observed at 750 ˚C. The peak high temperature strength at 750 ˚C decreases with V addition to Co–Ti system. Dislocations are predicted to shear the γ′ precipitates due to very narrow γ matrix channels. The γ′ precipitates are predicted to be semi-coherent in the Co–Ti alloy due to high lattice misfit. The anti-phase boundary (APB) energy and complex stacking fault (CSF) energy on {111} plane are predicted to increase with V addition to Co3Ti. Whereas the stacking fault energy in both phases is predicted to decrease with V addition. The yield stress anomaly is possibly due to anisotropy in anti-phase boundary (APB) energy on {111} and {100} planes as well as elastic constants anisotropy.
The creep strength of Co–Ti–V alloys at 700 ˚C is found to be better than γʹ phase strengthened Co–Ti alloy and conventional solid-solution strengthened Co-base superalloys. Grain boundary cracking is observed as the creep damage mechanism. The creep strength decreases for high V addition probably due to deviation of γ′ precipitate morphology from perfect cuboidal and decrease of extent of grain boundary precipitates.
Theory and Algorithms for sequential non-Gaussian Bayesian filtering and estimation
No full textSeamless integration of dynamical system models with sparse measurements, called as Data Assimilation, is important in many applications like weather forecasting, socio-economics, navigation, and beyond. In order to produce accurate and efficient state forecasts using data assimilation, one needs to account for non-linearities in the dynamics of state space, and the interaction between probabilistic information of both sensor measurements and state space. From a computational viewpoint, it is desirable to have schemes that require low computing cost and are easy to implement on a code. In this thesis, we develop an efficient non-intrusive sequential Data assimilation scheme that utilizes Stochastic collocation-based Polynomial Chaos expansion (PCE) to propagate the uncertainty in a non-linear dynamic system and Gaussian Mixture Model (GMM) priors to represent the statistics of PC expansion forecasts. First, we represent the uncertainty in a dynamical system using PCE and propagate it using the stochastic collocation method until an assimilation time. Then, we convert the polynomial basis prior to its equivalent Karhunen-Loeve (KL) form, fit a GMM in the subspace and perform a Bayesian filtering step. Thereafter, the posterior polynomial basis is recovered from the posterior GMM in the KL form, and uncertainty propagation is continued using the stochastic collocation method. The derivation and new equations required for the above conversions are presented. We apply the new scheme to an illustrative population growth dynamics application and a complex fluid flow problem for demonstrating its capabilities. In both cases, our filter accurately captures the non-Gaussian statistics compared to the Polynomial Chaos - Ensemble Kalman Filter and the Polynomial Chaos - Error Subspace Statistical Estimation.MHR
Nature-inspired Algorithms for Automated Deployment of Outdoor IoT Networks
No full textA vast majority of the Internet of Things (IoT) devices will be connected in a topology where the edge-devices push data to a local gateway, which forwards the data to a cloud for further processing. In sizeable outdoor deployment regions, the edge-devices may experience poor connectivity due to their distant locations and limited transmission power. Repeaters or relays must be placed at a few locations to ensure reliable connectivity to either a gateway or another node in the network. A big challenge in achieving reliable connectivity and coverage is the outdoor propagation environment being heterogeneous. Engineers often deploy networks based on resource-intensive field visits, detailed surveys, measurements, initial test deployments, followed by fine-tuning. For scalability to large scale IoT deployments, automated network planning tools are essential. Such tools should predict connectivity based on the edge-device locations using available Geographical Information System (GIS) data, identify the need for relays/repeaters, and, if needed, suggest the number of relays needed with their locations. Furthermore, such tools should also be extended to suggest the minimum number and locations of base stations to maximise coverage.
In this thesis, we discuss coverage estimation procedure for the deployment of out- door Internet of Things (IoT). In the first part of the thesis, a data-driven coverage estimation technique is proposed. The estimation technique combines multiple machine- learning-based regression ideas. The proposed technique achieves two purposes. The first purpose is to reduce the bias in the estimated received signal strength arising from estimations performed only on the successfully received packets. The second purpose is to exploit commonality of physical parameters, e.g. antenna-gain, in measurements that are made across multiple propagation environments. It also provides the correct link function for performing a nonlinear regression in our communication systems context. Next, a method to use readily available geographic information system (GIS) data (for classifying geographic areas into various propagation environments) followed by an algorithm for estimating received signal strength (which is motivated by the initial section of the thesis) is proposed. Together they enable quick and automated estimation of coverage in outdoor environments.
In the second part of the thesis, we propose an automated network deployment frame- work. Our proposed methodology uses either Ant Colony Optimisation (ACO) or Differential Evolution (DE) to identify the number and locations of relays for meeting specified quality of service constraints using a black box, received signal strength estimation oracle, that provides signal strength estimates between candidate pairs of transceiver locations in a heterogeneous deployment region. We discuss adaptations of our techniques to handle scenarios with multiple gateways. Further, we show the effectiveness of these algorithms to find suitable candidate base station locations to provide coverage in a heterogeneous propagation environment that meets the specified quality of service constraints. We then demonstrate the effectiveness of our algorithms in two deployment regions. It is anticipated that these will lead to faster and more efficient deployment of outdoor Internet of Things
Non-contact Breathing and Heartbeat signals monitoring using FMCW radar
No full textNon-contact breathing and heartbeat signals monitoring are the tasks of extracting them
without contact sensors. It became even more critical in COVID 19, and hence it is
crucial to estimate them correctly. FMCW (Frequency Modulated Continuous Wave)
radar is employed to estimate these two signals without contact. Radar captures chest
displacement and body movement. Because of this, breathing and heartbeat signals are
distorted. The reduction of false peaks and peak estimation is crucial for breathing rate
calculation. So in this thesis, firstly, we propose a novel way for tracing body movement
and eliminating the traced segment for breathing and heart rate calculation. In the second
part, we efficiently reduced false peaks using maximal overlap discrete wavelet transform
(MODWT) to decompose and reconstruct the filtered breathing signal for estimating
breathing rate. The heartbeat signal is estimated using bandpass filtering of unwrapped
phase. We also compared our algorithm with the task force monitoring (TFM) device as
a reference and discussed its performance. Also, our proposed method for breathing rate
estimation has an accuracy of 92.43% and heartrate estimation it is 85.16%
Nonlinear Optical Enhancement Studies in Silicon-based Resonant Metasurfaces
No full textMetasurfaces are two dimensional arrangements of building blocks called meta-atoms which have been found to be useful to manipulate amplitude, phase, polarization of light at the nanoscale. Using these properties, functional nanoscale devices for light manipulation have been built with wide-ranging applications in the various fields of sensing, metrology, optical communication, quantum computation etc. The subwavelength thick devices along with the possibility of manipulating properties of a light beam at will has made metasurfaces a promising alternative to conventional bulky optical components in various fields. Recently metasurfaces made of dielectric materials have been finding increasing research interest with the excitation of various resonance effects including Mie resonance, guided mode resonance, electromagnetically induced transparency, bound state in the continuum etc.
Nonlinear optics is the field of study of light matter interaction where the material response to an incident light beam depends on higher powers of the light field amplitude. The applications of this field of study virtually spans all the optical domains, especially the fields of harmonic generation, wave mixing processes, sensing, switching, quantum optics etc. The enhancement of electric field near the resonances along with relaxed phase matching requirements makes metasurfaces a promising platform to realize these devices. In particular dielectric metasurfaces have been shown to be attractive in realizing these devices due to their field confinement inside the dielectric, high damage threshold, excitation of magnetic resonances etc. Silicon is the preferred dielectric because of its CMOS compatibility along with high third order nonlinear optical susceptibility. However, second order nonlinear optical effect is vanishing in silicon due to centro-symmetry. Due to this research interest has also been directed towards other materials with high second order susceptibilities particularly III-IV materials, two dimensional materials including transition metal dichalcogenides, gallium selenide, hexagonal boron nitride etc.
In the first part, resonant third harmonic generation (THG) is demonstrated from amorphous silicon (a-Si) nanodisk arrays arranged in hexagonal lattice. The resonances occurring in such structures can be explained based on isolated Mie resonances and collective guided mode resonances in the array. Resonant THG enhancement ≈ 500 times corresponding to the fundamental resonance wavelength of 1510 nm is obtained compared to un-patterned a-Si film. Along with this we study spatial, spectral and intensity dependence of the THG process in a nonlinear microscopy set up. Intensity dependent reversal of THG image contrast is also demonstrated.
Next, large area four wave mixing (FWM) microscopy on a silicon-on-insulator (SOI) based partially etched zero-contrast gratings (ZCG) metasurface is performed. The etch depth is a useful parameter which can be used to tune spectral positions along with Q factor of the resonances. Signal resonance is designed to occur at 1580 nm in sub-wavelength zeroth order diffraction region while the pump wavelength is fixed at 1040 nm higher order diffraction region resulting in the FWM wavelength of 775 nm. Maximum FWM enhancement ≈ 450 times is obtained as the signal beam is scanned through the fundamental resonance wavelength. Such structures can have potential applications in wavelength conversions across widely separated wavelength bands using wave mixing processes.
In the next part, polarization-independent resonant enhancement of second harmonic generation (SHG) from multilayer gallium selenide (GaSe) on silicon-based resonant metasurface is performed in the presence of fundamental field depolarization and higher order diffraction effects. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement of ~22 times on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component.
In the final part, electromagnetic design and analysis of hybrid metasurfaces composed of multi-layer GaSe coupled to silicon holey disk arrays is presented for achieving high spectral contrast chiral SHG. The silicon holey-disk structures are designed to support EIT-like optical resonances in the 1.5-1.8 µm wavelength range in the vicinity of electric and toroidal dipole scattering modes. The fundamental electric field enhancement above the silicon structures shows RCP and LCP-like characteristics at distinct wavelengths resulting in two prominent peaks for the LCP and RCP resolved SHG from the GaSe layer above the holey-disks. This results in high contrast SHG degree-of-circular-polarization spanning -1 to +1 over the fundamental excitation spectral range considered. The chiral SHG response is also found to be strongly influenced by the unit-cell lattice arrangement with square or hexagonal lattice exhibiting very different chiral SHG response. Finally, circular polarization resolved SHG and THG microscopic studies are also performed on a resonant metasurface of partially etched a-Si nanodisk arrays integrated with multilayer GaSe
C-H Bond Functionalization and Tandem Cyclization with Alkynoates and Maleimides Using Directing Group Strategy
No full textThe thesis presents manganese‒ and rhodium‒catalyzed C‒H bond activation and tandem cyclization reactions. These reactions lead to the construction of carbon‒carbon bonds using directing group strategy. The thesis is divided into two sections. Section‒A is presented in three chapters, which describes the C‒H bond functionalization of N‒pyrimidylindoles, imines, and benzoic/acrylic acids with 4‒hydroxy‒2‒alkynoate as a coupling partner. This alkynoate is a highly functionalized unsymmetrical internal alkyne that can undergo regioselective alkenylation/annulation and lactonization after C‒H bond activation. The developed reactions are highly efficient, step economical, and require a single set of reaction conditions. Section‒B is presented in two chapters employing maleimides as a coupling partner with 2‒alkenylphenols and sulfoximines for [5+1] and diastereoselective [4+1] spirocyclization reactions, respectively, using Rh(III)‒catalytic system
Mesonic screening mass at zero and finite chemical potential
No full textThe finite temperature analysis of QCD is challenging since perturbation theory is not applicable in that regime. A non-perturbative method like lattice QCD is thus required to obtain an estimate of the necessary observables. In this Ph.D. thesis, we study the symmetries of the chiral phase transition and its transition temperature. This is undertaken by measuring the screening masses of those mesons that rotate amongst themselves under the symmetries.
Firstly, we discuss the regime of zero chemical potential where it is easy to obtain the correlators using which we calculate the meson screening masses. We discuss the different computational techniques used for calculating the screening masses on lattice following which we do their continuum extrapolate, to obtain their physical values. Using them, we notice the temperatures at which the symmetries get restored.
Finally, we expand our analysis to finite chemical potential. Including the non-zero value of chemical potential to the action makes it imaginary resulting in what is called the sign problem. This imaginary action cannot be used to simulate the system on a lattice. We avoid this problem by expanding the screening correlators in a Taylor series expansion in our analysis. Thus, this gives us an estimate of the screening mass at a small finite chemical potential
Opportunistic Beamforming and Asymptotic Throughput Analysis of Hybrid Analog-Digital mmWave Multi-User MIMO Systems
No full textIn this thesis, we address the problem of large training/feedback overhead and the requirement
of computationally intensive algorithms to determine the phase angles of the analog
precoder for downlink data transmission in hybrid analog-digital (A-D) multi-user (MU)
millimeter wave (mmWave) systems. We investigate the use of opportunistic beamforming
(OBF) using a dumb analog precoder as a solution to these issues. The OBF based
schemes work as follows. The BS transmits a known pilot symbol over a random analog
precoding vector. Using this, all the users in the system estimate their effective channels,
and the best user efficiently feeds back its SNR to the BS, e.g., using a timer-based scheme.
Then, the BS schedules the best user using the chosen analog precoding vector. Since the
randomly chosen precoding vector is likely to be optimal to a subset of the users, and since
the best user is selected in each time slot, deep fades at any given user are avoided, and
the overall system throughput improves. Note, also, that this scheme requires very little
feedback and no optimization of the analog precoding vector is necessary.
This thesis has two parts. In the first part, we consider a single radio frequency (RF)
chain at the base station (BS). We analyze two schemes of OBF, called fully random
precoder (full RP) and channel structure-aware random precoder (CSA-RP). In the full
RP scheme, we use random phase angles across the antenna array. In the CSA-RP scheme,
we consider the use of structured random beams, where the phase angles of the analog precoder are chosen so as to have the same structure as the array response of the geometric
channel model of mmWave systems. For the CSA-RP scheme, we derive the asymptotic
scaling laws of the average throughput as a function of the number of users, number of
antennas, and the SNR, using extreme value theory, as the number of users gets large.
Our simulation results show that the second scheme achieves near-optimal throughput,
i.e., close to that achieved using coherent beamforming with best user selection (called the
maximum rate baseline (max-rate BL) scheme, since the user obtaining the maximum rate
is scheduled to be served), via opportunistic selection among fewer users in the system
compared to the first scheme. Next, we use M pilot symbols with M different random
orthonormal analog precoding vectors instead of a single pilot symbol, to further reduce
the number of users required to obtain near-optimal throughput. We derive the scaling
law of the average throughput for OBF using M random orthonormal precoding vectors
also.
In the second part, we consider the case where the BS is equipped with multiple RF
chains. We propose two schemes (called greedy high and greedy low) with OBF to simultaneously
serve multiple users, with very low feedback overhead. The BS randomly
generates an analog precoding matrix whose columns form a set of random orthonormal
precoding vectors, and transmits pilot symbols using the precoding matrix. Each user measures
the SINR for each of the random precoding vectors. The two schemes we consider
differ in terms of how many beams are assigned to the users in each round of feedback: the
greedy high scheme entails a larger number of rounds of feedback compared to the greedy
low scheme, but is purportedly better in its performance. Through simulations, we find
that the two schemes offer nearly identical throughputs, and thus conclude that the greedy low scheme with lower feedback overhead is more attractive for practical implementation.
Furthermore, we derive the average throughput scaling laws using extreme value theory
when many users exist in the system for both schemes.
The results in this thesis show that as long as there are a reasonable number of users in
the system, OBF is an attractive, low-complexity, and low-feedback approach for practical
implementation of MU mmWave MIMO communication
Epigenetic modulation of Foam cell generation during Mycobacterium tuberculosis/ Cryptococcus neoformans infection
No full textFoamy macrophages or Foam cells are a critical cellular component of the granuloma formed during pulmonary infection. These lipid rich cells generally contain neutral lipids, Cholesteryl Esters (CE) and/or Triglycerides (TAGs) which not only serve as the carbon source for the pathogen to thrive in the hostile environment of host, but also act as a substrate for various immunomodulatory enzymes such as COX-2 etc. Lipid accretion is a result of various cellular processes involved in lipid uptake and synthesis that are fine-tuned by various transcription factors and chromatin modifiers. The thesis submitted is focused around the epigenetics of foam cell formation during pulmonary infection stemming from Mycobacterium tuberculosis and Cryptococcus neoformans. Mycobacterium tuberculosis, which is the causative agent of tuberculosis induces the formation of foamy macrophages in the host to permit its own growth. In the present context, we have highlighted the role of WNT-responsive epigenetic modifiers, G9a (H3K9 methyltransferase) and SIRT6 (H3K9 deacetylase) in fine-tuning the expression level of genes involved in cholesterol biosynthesis and efflux. Moreover, augmented levels of cholesterol was observed to fuel anti-oxidative response as depletion of cholesterol or G9a/SIRT6 elevated the oxidative response and eventually reduced bacterial survival. Similarly, LncRNAs have recently been showed to play a cardinal role in regulating the gene expression via modulating the activity of various transcription factors and chromatin remodellers. In the present study, we have attempted to underpin the role of Malat1 in governing Mtb pathogenesis. High throughput analysis including RNA sequencing and ATAC sequencing revealed Malat1-dependent global change in transcriptome and chromatin accessibility, respectively. Furthermore, In vivo study utilising both WT and Malat1 KO mice study ascertained the pivotal role of Malat1 in regulating Mtb burden as Malat1 KO mice showed reduced bacillary burden and improved lung pathology. Moreover, macrophages devoid of Malat1 showed reduced lipid content and enhanced necroptosis and elevated extracellular burden. In addition to above, thesis submitted also elucidates the molecular players involved in perturbing lipid content during C. neoformans infection. Enhanced lipid content has been demonstrated to support intracellular growth of C. neoformans. In this respect, Pyk2- cRaf axis-driven WNT signalling was found to be critical in regulating the levels of Lysine Specific Demethylase-1 (LSD1). LSD1 owing to its demethylase activity removes H3K9me2 repressive marks over the promoters of the genes involved in lipid homeostasis, leading to their enhanced transcription. Moreover, LSD1 sustains elevated lipid levels by inhibiting host lipophagy and hence preventing lipid turnover
Vaned Diffuser Effect on Centrifugal Compressor Performance and Stall
No full textA centrifugal compressor is a mechanical device that helps to achieve higher pressure ratios at lower mass flow rates. It is used in many applications such as turbochargers, cruise missiles, and turbojet engines where compact high pressure ratio compressors are required. The performance of a centrifugal compressor is greatly influenced by instabilities like stall and surge, which decrease the operating range and the efficiency of a compressor. These instabilities are effected by the geometry of the vaned diffuser used and hence, the present study aims to see both computationally and experimentally the effect of vaned diffuser parameters like the number of vanes (solidity) and the vane setting angle (α) on the performance and stall of a centrifugal compressor. Computational steady-state study is performed for different vaned diffuser geometries with changes in solidity and vane setting angle to understand their effects on performance. These studies show large variations in the performance curves with changes in surge points, choking mass flow rates and efficiencies. A new rotating centrifugal compressor facility is used for the experimental study. This facility uses a connected turbine driven by compressed air to achieve the required rotation rates, with the compressor RPM being controlled by valves in the turbine inlet and the load on the compressor being controlled by valves at the compressor outlet. This facility enables studies of both performance and stall of centrifugal machines as a function of the vaned diffuser geometry, and preliminary experiments have been done to establish the facility