PhD Theses (Mechanical Engineering)

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Numerical and Experimental Investigations of Nonlinear Dynamics and Heat Transfer Deterioration in Supercritical Natural Circulation Loop
(2024) Srivastava, Tanuj
The natural circulation loop (NCL) efficiently transfers energy from a high-temperature source to a low-temperature sink without direct contact. The key driving force in natural circulation systems is buoyancy, caused by density differences. Single-phase NCLs face limitations from saturation temperature and low flow rates, while two-phase loops risk dry-out and complex flow regimes. Supercritical fluids offer an alternative, combining the benefits of both single- and two-phase systems. The concept of a supercritical natural circulation loop (sNCL) is important for Generation-IV nuclear reactors. This thesis evaluates sNCLs using CO2 through both numerical and experimental methods. Numerical studies analyze the steady-state and transient behavior of sNCL, using 3D simulations for steady-state and 2D/1D models for transient cases. Buoyancy and friction forces determine system behavior. As heating power increases, buoyancy dominates, leading to a rise in flow rate and heat transfer. However, friction eventually takes over, reducing flow rate and leading to flow-induced heat transfer deterioration (FiHTD). This phenomenon, which can be delayed but not avoided, is key to ensuring safe operation. Based on fluid temperature, four heat transfer zones were identified: low power, enhanced heat transfer, transition, and deteriorated heat transfer. 3D simulations using ANSYS Fluent confirmed the boundary of safe operation, with data fitting a power-law curve. Changes in friction factor along the heater section also signal the onset of FiHTD. The dissertation further explores static and dynamic instability in sNCL. Steady-state circulation shows a sharp decline, consistent with previous analyses, and exhibits Ledinegg instability at intermediate power levels. Both static and dynamic instabilities were identified, with results aligning across simulations. Under varying heat input, sinusoidal heating caused chaotic oscillations, while ramp heating remained stable due to gradual buoyancy generation. A 2D model explored startup transients, revealing the complex behavior of sNCL near the pseudocritical point. The system's bulk motion is influenced by phenomena like the piston effect, Rayleigh-Taylor instability, and adiabatic heating, which create hot fluid packets that drive system dynamics. Flow reversals and chaotic behavior result from intermittent fluid packet generation and disappearance. Experiments using CO2 examined the effects of sink temperature, pressure, tilt angle, and heating power for FiHTD. Flow rate peaked before declining, with the highest rates in vertical loops. No instability was observed under the test conditions, but the mass flow trends closely matched the simulations.
Experimental and Computational Analysis of Interface Fracture using Extrinsic and Intrinsic Cohesive Zone Modelling
(2024) Saikia, Pran Jyoti
In recent years, material interfaces have become part of numerous engineering and structural applications. Interface failure comprising both cohesive and adhesive failure is one of the shortcomings of bonded structures during service loading conditions. Therefore, predicting interface failures is essential for ensuring the reliability, safety, and cost-effectiveness of systems and processes across various industries. The cohesive zone model (CZM) is a widely used computational technique for analyzing the interface fracture phenomenon within computational fracture mechanics studies. The main objective of the present thesis is to expand the applicability of the CZM for a wide range of material interfaces, ranging from adhesively bonded joints to laminated composites. Additionally, the experimental crack growth studies of isotropic and orthotropic material interfaces augment the proposed numerical methodology within the finite element framework.
Non-conventional plasma machining and nano-polishing of optics
(2023) Krishna, Enni
In this study, a medium-pressure plasma flow system was designed and developed for plasma machining of fused silica optics. The results showed that the plasma flow system can be used for long machining times (more than 60 minutes) with a material removal rate that is 300% higher than the earlier studied confined system. The study also found that there is an improvement in surface integrity, as identified by Raman microscopy, and surface topography, as determined by the 3D profiler. In this study, a non-invasive method was developed using an optical emission spectrometer to predict MRR during polishing. The Comsol® simulation was utilized tooptimize the plasma chamber configuration, resulting in a custom-made chamber with a Vshaped groove that showed uniform reactive radical distribution for polishing free-form optics. Confocal Raman microscopy was used to quantify the depth of damage on ground fused silica. This methodology was adopted to optimize the rotary ultrasonic machining parameters for shaping fused silica hemispherical resonator shells (HRG). To investigate the cause of Ring Laser Gyroscope device failure, a photoluminescence spectrometer was utilized for analysis and verification. Furthermore, the SSD depth at the nanometer scale was quantified for the ultrasmooth prism substrate using Secondary Ion Mass Spectrometer (SIMS). A process flow was established for removing 350 nm depth of material without affecting the surface finish of the ultra-fine fused silica substrate, using plasma processing followed by chemical leaching to enhance the surface integrity of the prism substrate. In this study, a non-invasive method was developed using an optical emission spectrometer to predict MRR during polishing. The Comsol® simulation was utilized to optimize the plasma chamber configuration, resulting in a custom-made chamber with a V-shaped groove that showed uniform reactive radical distribution for polishing free-form optics. Confocal Raman microscopy was used to quantify the depth of damage on ground fused silica. This methodology was adopted to optimize the rotary ultrasonic machining parameters for shaping fused silica hemispherical resonator shells (HRG). To analyze and verify the plausible mechanism of Ring Laser Gyroscope device failure, a photoluminescence spectrometer was used. Furthermore, the SSD depth at the nanometer scale was quantified for the ultra-smooth prism substrate using Secondary Ion Mass Spectrometer (SIMS). A process flow was established for removing 350 nm depth of material without affecting the surface finish of the ultra-fine fused silica substrate, using plasma processing followed by chemical leaching to enhance the surface integrity of the prism substrate. This study compares plasma etching versus wet chemical etching and suggests plasma machining as a safer alternative.
Fabrication and Post Processing of Additively Manufactured Biomedical Implants through Hybrid Electrochemical Magnetorheological Finishing
(2023) Rajput, Atul Singh
Additive Manufacturing (AM) or 3D printing provides the benefits of individualizing the implant per patient requirements. However, the poor surface quality of additively manufactured components is a major limitation as it increases its wear rate on their tribological interaction. Hybrid Electrochemical Assisted Magnetorheological (H-ECMR) utilizes the synergic action of mechanical abrasion and electrochemical reaction to enhance the surface quality of the parts without affecting their surface topography. The electrochemical reaction forms a uniform and thick oxide layer on the Ti-6Al-4V surface as layer thickness increases to 78 nm from its initial value of 8 nm, further improving its corrosion resistance. This work details the working principle of the H-ECMR finishing process with an analysis of the impact of process parameters on the reduction in surface roughness. The H-ECMR finishing process effectively applies to parts with initial surface roughness (Ra) in the sub-micron range. Hence, chemical etching or milling operation is used as an intermediated process after fabricating the Ti-6Al-4V biomedical implants by Laser Powder Bed Fusion (LPBF) or Selective Laser Melting (SLM) to reduce the surface roughness in the sub-micron range. The surface finishing operation is performed on the Ti-6Al-4V femoral head and bone plate to improve its surface quality and biocompatibility. Moreover, Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), and optical profilometer are used to examine the change in the surface quality before and after post-processing of the LPBF fabricated femoral head. The laser scanning study confirms that the femoral head's dimensional accuracy remains intact during the H-ECMR finishing process. The average surface roughness (Ra) value is reduced to 33.14 nm from its initial surface roughness value of 14.67 μm after the H-ECMR finishing to produce a mirror-like polished femoral head surface. The wear, corrosion, and wettability tests signify that the biocompatibility of the fabricated parts is enhanced after post-processing. The corrosion rate for the LPBF manufactured Ti-6Al-4V is 0.081 mm/year, further reduced to 0.0103 mm/year after the chemical etching as surface irregularities of LPBF fabricated surface are very high, creating the grooves for confined corrosion products. The wear rate value on the final polished surface is further reduced to 0.0046 mm/year as an electrochemical reaction during the H-ECMR finishing process, providing a uniform and thick passive oxide layer on the surface of Ti-6Al-4V. The wear rate corresponding to the LPBF fabricated, chemically etched, and polished surfaces are 18.86×10-5, 6.36×10-5, and 0.96×10-5 mm3/min, respectively.
Design and Development of Electrospun Piezo-electric Nanocomposite for Sensing Applications
(2023) Kumar, Mukesh
Piezoelectric material based sensors, actuator are widely used in many engineering fields such as aerospace, medical, marine, consumer sports, etc. Many research groups have been extensively studying piezoelectric based nanofiber mats over the years due to their higher sensing capability. Flexible piezoelectronics are the key components in various micro, nano-scale energy harvesting devices, structural health monitoring, and medical devices. A wide range of devices have been developed specifically for energy harvesting applications. Moreover, these devices can be used to power small electronic devices such as sensors, capacitors, light-emitting diodes, watches, etc. Piezoelectric materials such as lead zirconate titanate PZT-5A and PZT-5H are highly brittle due to which complex shapes, robust stringent loading, and boundary conditions limits their application for developing nano or microdevices for sensing or actuation purpose. Hence, piezoelectric polymer-ceramic based nanofibers are better option for the sensing and energy harvesting applications. The sensitivity of nanofiber composite mats depends upon the manufacturing/fabrication method. Fiber mats can be manufactured using solution casting, thermal evaporation, spin coating, hydrothermal, and electrospinning, etc. Electrospinning is the most suitable fabrication method due to its ability to fabricate nanostructures with novel properties such as small diameter, long length, diversified composition, high surface area to volume ratio, inter/intra fibrous porosity, flexibility in surface functionalities, and self poling. Electrospun nanofibers have been used in various areas such as tissue engineering, wound dressing, filtration, drug delivery systems, desalination, protective clothing fabrication, optical electronics, personal care, sound absorption, and biosensors. The sensing and actuating capacity of these fibers significantly depends on various parameters, which affect the morphology of fabricated nanofibers. The aim of this research is to develop P(VDFTrFE) based flexible piezoelectric mats for energy harvesting and sensing applications. Since pure P(VDF-TrFE) based mats have low power output. Some piezoceramic nanoparticles such as ZnO, BaTiO3, and TiO2 are added to enhance the power output of the electrospun mats. P(VDFTrFE)/ZnO nanofiber membranes are synthesized by optimizing the electrospinning parameter. Electrospun PVDF/BaTiO3 functionally graded webs are fabricated for the energy harvesting application. Further, hybrid nanocomposite mats are synthesized and incorporated as wearable devices. P(VDF-TrFE)/TiO2 based hybrid nanogenerators comprised of piezoelectric and triboelectric nanogenerators are also designed for energy harvesting and impact sensor application. The optimization of process parameters (applied voltage, ow rate, spinning distance, shape of spinneret), solution parameters (molecular weight, polymer concentration, viscosity, conductivity), and ambient parameters (humidity, temperature, and type of atmosphere), which affect the nanofiber power output are also studied. Electrospun nanofiber composites are then investigated for their energy capturing and sensing capability by varying the matrix and reinforcing fillers concentration. Subsequently, the surface morphology, mechanical behavior, crystallinity, fraction of beta phase, thermal stability, rheological properties, storage modulus, loss modulus, damping factor, and piezoelectric performance of various fiber composite have been analyzed. Piezoelectric nanogenerator (PENG) devices are designed using nanofiber mats as an active layer placed between the top and bottom electrodes for energy harvesting and sensing applications. These devices are subjected to pressing, bending, tapping, and impact load, and output is recorded using a digital storage oscilloscope (DSO). These devices can be used as biomechanical sensors, impact sensors, and energy harvesting applications. The present research output can be a basis for the futuristic development of wearable medical and energy harvesting devices.
Development of a Partially Saturated Cells based Lattice Boltzmann Solver for Thermo-Fluidic Applications
(2023) Majumder, Sambit
This thesis is devoted to the development of a robust and accurate partially saturated cells (PSC) based lattice- Boltzmann (LB) solver for incompressible flows and its application to conjugate heat transfer process. The lattice- Boltzmann (LB) method has emerged as a promising alternative to the conventional Navier-Stokes solvers in the last two decades. Despite certain promising advantages, the conventional treatment of boundary conditions on the curved surfaces using LB approaches suffer staircase representation of the surface. This disparity gets exacerbated in moving boundary problems with continuous interchange of fluid and solid lattice nodes necessitating refilling algorithms at each time, and leads to an increased computational expense. Hence, the need for a fast and robust computational framework for moving body problems has led to the emergence of non-body conformal methods, like the coupled immersed boundary-lattice Boltzmann (IB-LB) method, over the last two decades. Recently, one such variant of the IB-LB method, namely the partially saturated computational cells (PSC) method, has evolved as a promising numerical framework for several moving boundary applications. The key advantage of the technique is that, it employs a unified evolution equation for all media (solid and fluid) present in the computational domain comprising a weighting function based on solid volume fraction and an additional solid collision operator. The latter two elements of the PSC technique are responsible for affecting the boundary conditions to be imposed on the solid body, which may be stationary or moving.
Design, Processing and Characterization of High Entropy Alloys for High Temperature Applications
(2023) Das, Sujit
High entropy alloys (HEAs) are a novel class of alloys that contain multiple principal elements. HEAs possess high mixing entropy that inhibits the formation of multiple phases in them by reducing their Gibbs free energy, especially at high temperature. Refractory high entropy alloys (RHEAs) generally have BCC structure and possess very high mechanical strength at elevated temperatures. Transition metal HEAs contain transition elements. In this research work, two RHEAs such as WMoVCrTa and W23Mo23V17Cr8Ta7Fe22, and one Al and Cu contained transition metal HEA such as (Al)10(FeCoNiCu)90 were designed based on alloy design criteria of Guo et al. and Takeuchi et al., and fabricated subsequently. To minimize the difficulties in processing the alloys, mechanical alloying by ball milling was done prior to the consolidation. Milling characteristics of the alloys were established. The milled WMoVCrTa powder revealed two BCC phases. The milled W23Mo23V17Cr8Ta7Fe22 powder revealed a single BCC phase. The milled (Al)10(FeCoNiCu)90 powder revealed two FCC phases such as FCC1 and FCC2. The as-milled WMoVCrTa and W23Mo23V17Cr8Ta7Fe22 powders were cold compacted followed by vacuum arc melted and heat treated, whereas the milled (Al)10(FeCoNiCu)90 powder was cold compacted and sintered to fabricate the alloy ingots. The structure, mechanical and tribological properties of the alloys were determined. The two RHEAs show very high mechanical strength at room and elevated temperatures. WMoVCrTa and W23Mo23V17Cr8Ta7Fe22 respectively exhibited yield strength retention of 79% and 90% of the room temperature value, at 1000 °C. They also have reasonable ductility both at room and high temperature, and very high hardness. The Al10(FeCoNiCu)90 exhibits high mechanical strength and ductility. The two RHEAs have higher wear resistance than that of high-speed steel (HSS). The good mechanical and tribological properties possessed by these alloys indicate that they are viable materials for high temperature applications.
Dynamic Deformation Behaviour of a Dual-Phase High Entropy Alloy
(2023) Tamuly, Samrat
A novel dual-phase high entropy alloy comprising of FCC and BCC phases is processed at a large scale under industrial environment to understand the castability and scalability of high entropy alloys with a special attention towards strength-ductility balance and its potential use in high strain rate applications. Subsequently, appropriate experiments using split Hopkinson pressure bar were conducted and deformation behaviour from microstructural aspects were analysed. To understand the effect of cooling rate of solidification on the mechanical properties, the alloy was remelted and rapidly solidified at small-scale suction-cast laboratory environment. Furthermore, friction stir processing was also employed as a viable and effective rapid mass-production based thermo-mechanical process and the processed samples were compressed under a range of strain rates. For comparative analysis of the largescale as-cast alloy, the suction-cast alloy and the friction stir processed alloy, the hardness, compressive strength, strain rate sensitivity, strain hardening behaviour and deformation mechanisms of all the alloy samples under both quasi-static and dynamic loading were examined. The friction stir processed samples exhibited the highest strength and the lowest strain rate sensitivity among all the conditions of the alloy. The as-cast alloy samples exhibited the highest strain rate sensitivity among all the alloy samples and excellent strain hardening at high strain rate of loading. The rapidly cooled suction-cast alloy displayed higher strength than the as-cast condition but lower strength than the friction stir processed samples. Similarly, the strain rate sensitivity of the suction-cast alloy was higher than the friction stir processed alloy and lower than the as-cast alloy. All the alloy samples were characterised by distinct deformation mechanisms under different strain rate regimes. The as-cast alloy resulted in wavy deformation bands under quasi-static loading conditions and deformation twins near interphase boundary under dynamic loading conditions. The suction-cast alloy under quasi-static compression displayed clear strain partitioning between FCC and BCC phases at moderate strains and the deformation became homogeneous without any strain partitioning at higher strains. The dynamic deformation in the suction-cast alloy resulted in the formation of deformation twins and bands. The friction stir processing of the as-cast alloy led to very high FCC to BCC phase transformation and grain refinement, and the deformation of the processed alloy further exhibited moderate stress induced phase transformation effect under both quasistatic and dynamic conditions. However, distinct twins were formed in friction stir processed alloy under dynamic loading which were almost absent under quasi-static compression.
Impression Creep and Fatigue Crack Growth behaviour of Aluminium alloy Friction Stir Weldments
(2023) Das, Jayashree
Joining Aluminium alloys by Friction stir welding (FSW) results in different weld zones viz, nugget zone (NZ), thermos-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base metal (BM). Each zone exhibits different microstructures and mechanical properties across the weld leading to poor mechanical attributes for the weldment. An insight in to the creep deformation and fatigue crack growth through these weld zones will enlighten the research community with the usage of these materials for elevated temperature applications as well as for conditions of dynamic loading. The present work was therefore taken up with the objective of joining defect free joints of Aluminium alloys by FSW and investigating the (i) creep behaviour of different weld zones at elevated temperatures by impression creep (IC) tests and (ii) fatigue crack growth (FCG) behaviour through different weld zones of the weldments. The materials chosen for the study are commercial Aluminium alloys viz, AA2014 and AA6061-T6. For obtaining the best quality joints of the two Aluminium alloys based on the mechanical properties the process parameters selected were low tool rotational speed, low welding speed, low plunge depth and moderate shoulder diameter of square pin (SQ) tool profile. The effect of process parameters on the mechanical properties viz, ultimate tensile strength (UTS), yield strength (YS), % elongation, Flexural strength (FS) and bend angles of the weldments were studied. The best AA6061-T6 weldments exhibited 97%, 98% and 98% of the UTS, YS and FS of the base metal respectively. In the case of AA2014 alloy the corresponding properties were 86%, 80% and 90% of the base metal. The results of the microstructure and fractography under field emission scanning electron microscope (FESEM) are studied and discussed.
Event-Triggered Adaptive Position-Force Control of Robotic Manipulators in Medical and Cooperative Industrial Applications
(2023) Abbas, Mohamed
Motivated by human dexterity and coordination, robotic manipulators have drawn the attention of researchers in recent years. These manipulators can mimic the human manipulation property and provide assistance at improved accuracies far exceeding those of human operators. Therefore, the adoption of robotic manipulators is demanded in diverse medical and industrial scenarios to fulfill several tasks such as ultrasound scans, rehabilitation exercises, and cooperative manipulation. On the other hand, networked control systems (NCSs) have become very popular in the last few years, introducing several benefits such as flexibility, reliability, and ease of maintenance for practical robotic applications. However, the implementation of such communication poses different constraints, including limited bandwidth channels. Moreover, parametric uncertainties and interaction forces are crucial to consider in the human-robot and robot-environment interaction tasks. Therefore, designing an appropriate controller to achieve the desired position and force tracking for robotic manipulators under the network-induced limited bandwidth, parametric uncertainties, and interaction forces is challenging and open to research. This thesis proposes a few simultaneous position-force control schemes to overcome the aforementioned challenges and maintain the performance and stability of the robotic manipulators in different medical and cooperative industrial applications.
Numerical and Experimental Investigation of Minichannel Heat Sinks with Supercritical Working Fluids
(2023) Kumar, Nitesh
The rapid advancement of high performance tiny electronic devices has been sparked by the growing dependence of modern human life on digitization and artificial intelligence. To ensure dependable performance during the full designated operating regime as well as a satisfactory life term, designers now face the additional problem of thermal management due to the sharp increase in the power density requirements for such equipment. Heat generation is an irreversible process, and it must be removed for components to function continuously. Since the temperature rise in the circuits is the primary cause of component failures, the thermal energy generated during operation needs to be effectively reduced for the components to operate continuously. As a result, a significant amount of study has been focused on the evaluation of alternative working fluids as well as the creation and augmentation of effective cooling strategies over the past ten years. The high area-to-volume ratio of a miniaturised or mini-channel heat sink (MCHS) and the favourable thermophysical properties of the medium have been identified as two factors that make this option particularly enticing.
Nonlinear dynamics and active/passive control of parametrically excited slender structures
(2022) Reddy, Rajidi Shashidhar
This dissertation deals with the nonlinear dynamics and active/passive control of parametrically excited beam-like slender structures using piezoelectric actuators, viscoelastic damping materials and functionally graded materials (FGMs). First, the dynamics of smart slender beams is studied where the main focus is to investigate the usefulness of shear mode and extensional mode piezoelectric actuators in active control of complex nonlinear dynamics of the smart beams in pre- and post-buckled states. The subsequent study is carried out to investigate the usefulness of viscoelastic damping materials in passive control of complex dynamics of parametrically excited beams. However, for the corresponding dynamic analysis, a formulation of the HBM-based full-order FE model (FOM) is derived by introducing three new strategies, namely (a) a special factorization of the nonlinear strain-displacement matrix, (b) exploitation of orthogonality of Fourier basis functions and (c) reduction of various viscoelastic constitutive relations into a generalized mathematical form for the time-periodic stress/strain. Further, to achieve reduced computational time in evaluating nonlinear transient/frequency responses, the nonlinear FOMs are subsequently reduced to reduced-order FE models (ROMs). The formulation of nonlinear ROMs is carried out at the elemental level without involving the full-order system matrices/vectors, where the primary contribution lies in the formulation of the nonlinear memory-load vector. Besides the reduced computational time, the accuracy of the nonlinear ROMs is achieved by proposing a new methodology for computation of appropriate reduced basis vectors (RBVs) using normal vibration modes (NMs), static modal derivatives (MDs) and proper orthogonal decomposition (POD) method. Finally, an FGM is taken as the material of a beam-like slender structure, where the main focus is to investigate the influence of graded material properties of the slender FGM structure on its nonlinear dynamic characteristics under the parametric excitation. This study is performed considering a pinned-pinned vertical/inclined FG pipe conveying hot fluid with the steady/pulsatile flow velocity.
Design and Development of a Sterilization Box and its Variants with Different Functionalities
(2024) Mahanta, Nilkamal
The purpose of design and development of a new product is to transform ideas into tangible products. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prompted the researchers to design and develop a sterilization box for the prevention of coronavirus disease 2019 (COVID-19). In this work, first a sterilization box was designed with combined Ultraviolet-C (UV-C) radiation and incandescent bulb heating facilities. For performance assessment, initially unfolding of Immunoglobulin G (IgG) and antibacterial efficacy for the clinically relevant E. coli bacteria as well as for bacteria collected from daily use items were assessed. UV-C exposure 70 °C for 15 min unfolded the protein and killed all the bacteria. As a product diversification strategy, another sterilization box with Infrared (IR) and UV-C facilities was designed; its disinfection ability was tested against S. aureus and S. typhi bacteria and SARS-CoV-2 spike and RNase A proteins. For the broad-spectrum antibacterial activity, the optimum condition was UV-C exposure at 65.61 °C for 13.54 min. For understanding the effect on N95 masks, five disinfection methods, viz., incandescent bulb heating, UV-C irradiation, air drying after alcohol immersion, exposing to steam, and incandescent bulb heating combined with UV-C irradiation were studied. Alcohol treated and UV-C treated fibers became wider and rougher than the untreated fibers. Incandescent bulb heating combined with UV-C irradiation caused the least reduction in ultimate tensile strength (UTS) and scratch resistance. Further, sterilization box was transformed into a pedagogical gadget for product diversification. Five pedagogical heat transfer experiments with the help of developed gadget were described.
Design and Development Of Bambusa tulda reinforced Bio-composites for Structural Applications
(2024) Saha, Abir
Traditional materials like metals, ceramics, and polymers cannot provide the unique combination of qualities needed to match advancements in modern technologies, leading to a focus on composites. Due to environmental concerns and the disadvantages of synthetic fibers, research is increasingly focused on natural fibers to develop sustainable, biodegradable composites that are suitable for automotive, aerospace, and construction because of their excellent strength-to-weight ratio. Using bamboo-based biocomposites as an example of eco-friendly and sustainable material development, the present study focused on designing and developing eco-friendly and sustainable biomaterials. This study investigates the physical, mechanical, structural, and thermal properties of Bambusa tulda fiber and its reinforced green composites. Initial investigations were conducted on fibers extracted from the inner, middle, and outer parts of bamboo culms, evaluating their physical, chemical, mechanical, and thermal properties. Physical and tensile properties of the fibers were analyzed using Weibull's statistical approach. The investigation revealed that technical fibers extracted from the outer part (external technical fibers) of the bamboo culm had higher cellulose content (58.13 ± 3.51%), higher crystallinity index (60.142%), greater tensile strength (365.014 ± 50.441 MPa), modulus (14.098 ± 1.763 GPa), lower moisture absorption capacity, and higher thermal stability than fibers from the middle and inner parts of the bamboo. The extracted external technical fibers were then chemically treated with different concentrations of sodium hydroxide (NaOH). Various characterization processes were used to examine the effects of chemical treatment. Single-fiber tensile testing, fiber pull-out testing, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and atomic force microscopy were performed to examine the impact of these treatments. The investigation found that fibers treated with 6% NaOH exhibited a tensile strength of 526.452 ± 17.509 MPa and a tensile modulus of 24.055 GPa, both higher than those of untreated fibers. These treated fibers also had higher cellulose content and greater surface roughness, which improved interfacial interaction with the polymer matrix. Furthermore, green composite samples were fabricated with various fiber weight fractions (10%, 20%, 30%, and 40%).
Rayleigh-Taylor instability in viscosity stratified fluid medium
(2023) Jaiswal, Shatrughan Prasad
In the present work, study of Rayleigh-Taylor instability in viscosity stratified fluid medium is carried out. The instability is studied for two dimensional rectangular as well as axisymmetric coordinate configurations using numerical simulations. In both the configurations, two viscous fluid layers, a heavier fluid layer is superposed over a lighter fluid layer, are sandwiched between two parallel isothermal horizontal solid walls. Out of the two walls, one wall is isothermally heated, while the other wall is isothermally cooled. In particular, the effect of stratification due to viscosity varying exponentially with temperature on the instability is studied. Marangoni effect on the instability is studied under the influence of viscous stratification for both the configurations. The studies carried out in the present work consist six parts.
Exploring Self-Sensing in Shape Memory Alloy Wire Actuators under Practical Loading Conditions
(2023) Mohan, Sagar
Shape Memory Alloy (SMA) wire actuators offer large force and displacement capabilities following a non-linear, hysteretic behaviour. This necessitates sophisticated feedback controllers, making the system bulkier. It can be avoided by utilizing the self-sensing capability of SMA, wherein the change in the electrical resistance of the SMA wire can be used to estimate the system output. In literature, various empirical relations are derived based on the experimental results or system model, and are used for the same. These approaches are system specific and needs to be repeated for the change in system parameters or loading conditions. To obviate this, few state estimation-based models have been developed to predict the system output from the change in electrical property of the SMA actuator. To develop the system model, the SMA constitutive model of Boyd and Lagoudas (1996) is modified, to simulate the minor loop response of SMA. Based on the modified SMA constitutive model, an Extended Kalman Filter and a Particle Filter model are developed for the SMA wire actuated linear and nonlinear systems. The linear system comprises of a linear spring biased SMA wire actuator and the nonlinear one consists of SMA wire actuated single degree of freedom manipulator. Correspondingly two experimental setups are fabricated to obtain electrical resistance variation in the SMA wire actuators of these systems for a set of given time varying voltage signals. The voltage signals are taken as inputs and electrical resistance data are taken as measured data and the corresponding response of these systems are estimated using the developed EKF and PF models and the same are compared with the experimental outcome. Under natural convection, the estimation accuracy is found to be reasonably good. Next, forced cooling of varying durations and magnitude is introduced into the system to mimic practical scenario, for which the estimation accuracy is found to deteriorate significantly. The filters are then modified to estimate the convective heat transfer coefficient as well in addition to the systems’ response. In this approach, estimation accuracy improved significantly particularly with the PF model. Similar performance is observed in case of SMA actuated single DOF manipulator, in case the stress and temperature of the SMA wire varies non-monotonically. Finally, an LSTM-based Deep Neural Network models are also developed for these systems and are found to be capable of yielding appreciable accuracy while harnessing self-sensing feature of the SMA wire actuators.
Prediction of Textured Journal Bearing Performance Characteristics implementing Mass Conserving Boundary Conditions using Progressive Mesh Densification Method
(2023) Rasool, Syed Nayab
Surface texturing is proven to be a feasible technique for enhancement of the performance characteristics of the hydrodynamic bearings. A considerable amount of numerical and experimental study is carried out on textured hydrodynamic bearings. The surface texturing onto the bearing surface can be done using different micro-fabrication techniques such as laser surface texturing (LST), chemical etching techniques, novel dressing techniques, Additive manufacturing, abrasive jet machining, photolithography, focused ion beam, micro-electric discharge machining, electrochemical texturing, ultrasonic machining, thermal implantation etc. The surface texture of regular geometries, either dimpled or protruded shapes, can be produced on the bearing surface with the above-mentioned manufacturing techniques
Analysis and Identification of Dynamic Transmission Error Parameters in a Geared Rotor Using Full Spectrum
(2023) Rao, Bhyri Rajeswara
Geared rotor bearing systems are widely used torque carrying components in most of the mechanical and electrical equipment in the automotive, aerospace and ship building industries. A dominant source of vibration in geared rotor systems are the gear mesh dynamic transmission error (TE) parameters. These include the static TE, the mesh stiffness and damping, and the gear runout. One needs to know additionally these dynamic parameters along with the gear macro-geometry (for examples, the number of teeth, pitch circle diameter, pressure angle, and tooth geometries), load and rotating speed, for the vibration-based diagnosis of geared rotor power transmission units.
Numerical and Experimental Thermo-Mechanical analysis of Sheet Forming by Laser Line Heating
(2024) Das, Biplab
In this study, a detailed strategy for sheet forming by laser line heating has been presented. The work presented in this thesis is concerned with the evaluation of the effect of process parameters on thermal history, residual deformation, residual stresses, and strains associated with the process of laser line heating. Residual deformation is induced on the metallic sheet due to the heating effect of a laser beam when irradiated over a suitable heating path. The deformation generally takes place due to the combined effect of yielding and temperature distribution across the thickness of the metallic sheet. Both numerical and experimental analyses on laser line heating were carried out to investigate the thermal history and residual deformation.
(A) numerical investigation on augmented heat flux in non-standard variants of turbulent Rayleigh-Bénard convection
(2023) Chand, Krishan
The present work investigates non-standard variants (roughness-aided and tilted convection) of Rayleigh-Bénard convection (RBC) to augment heat flux for a fixed working fluid (Prandtl number = 0.7) over a wide Rayleigh number range (106 ≤ Ra ≤ 1010). For both 2D and 3D, the study focuses on the coherent structures and heat transfer mechanisms in different configurations of Rayleigh-Bénard convection (RBC), considering thermal plumes, boundary layers, and large-scale rolls (LSR). In the smooth case, the absence of lateral direction results in the entrapped thermal plumes, which are subsequently emitted as thermal jets into the bulk. The Nusselt number (Nu) quantifies the heat carried by thermal plumes across the isothermal walls. A positive correlation between vertical velocity and temperature fluctuations is used to quantify thermal plumes. The impact of surface roughness on heat flux is investigated, highlighting the influence of irregular roughness geometries. The study identifies an onset of enhanced heat flux regime and explores the role of bulk-plume interaction and fluid mixing. With increasing Rayleigh number, transformation from a double-roll state to multiple-roll state is associated with the onset of enhanced heat flux regime for the taller configuration. On the other hand, presence of huge number of roughness elements is responsible for enhanced heat flux in the smaller configuration. Near-wall dynamics and the penetration of peaks into the thermal boundary layer are studied, revealing the significance of secondary vortices and the tendency of plume emission. The investigation extends to three-dimensional RBC with conical roughness configurations, emphasizing the role of coherent structures and intense thermal plumes in enhancing heat flux. The study provides insights into the influence of roughness on flow strength and the orientation of large-scale rolls. The effect of inclination angles in tilted RBC is examined, indicating shifts in heat transport effectiveness and early onset of turbulence with increased roughness height. In the smooth case, inclined convection (IC) enhances heat flux below Ra = 108, while above this value, normal RBC yields the highest heat flux. However, for rough surfaces, the effectiveness of IC to transport heat shifts to lower Ra as the roughness height increases, leading to an early onset of turbulence. The maximum heat flux in the smooth case is achieved at a tilt of 75° for Ra ≤ 108, while in roughness cases, it depends on both Ra and the roughness configurations. The study reports a maximum increase of 25% in Nusselt number for roughness-aided tilted convection. Additionally, as Ra increases, the onset of thermal stratification is delayed in the smooth case, while an increase in roughness height results in a similar delay in rough configurations, indicating an early onset of turbulence even at larger inclination angles.

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