Structure-tribological property relations have been studied for five high entropy alloys (HEAs). Microhardness, room and elevated (100°C and 300°C) temperature sliding friction coefficients and wear rates were determined for five HEAs: Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4; Co Cr Fe Ni Al0.25 Ti0.75; Ti V Nb Cr Al; Al0.3CoCrFeNi; and Al0.3CuCrFeNi2. Wear surfaces were characterized with scanning electron microscopy and micro-Raman spectroscopy to determine the wear mechanisms and tribochemical phases, respectively. It was determined that the two HEAs Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4 and Ti V Nb Cr Al exhibit an excellent balance of high hardness, low friction coefficients and wear rates compared to 440C stainless steel, a currently used bearing steel. This was attributed to their more ductile body centered cubic (BCC) solid solution phase along with the formation of tribochemical Cr oxide and Nb oxide phases, respectively, in the wear surfaces. This study provides guidelines for fabricating novel, low-friction, and wear-resistant HEAs for potential use at room and elevated temperatures, which will help reduce energy and material losses in friction and wear applications.

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This thesis presents a torque control mechanism for switched reluctance machine (SRM) drives. The proposed mechanism is capable of maintaining ripple free torque control while minimizing the copper loss or mode-0 radial force or both at a fixed switching frequency. In the proposed approach, the torque control problem is addressed by splitting it into two parts. The first part consists of identification of optimum phase current references while the second part incorporates the design of an efficient current controller. For the identification of optimum phase current references, three algorithms are presented in the form of a developmental process. The nature of the online optimization problem is demonstrated using a simple 2-dimensional gradient descent method. Subsequently, a parametric form gradient descent algorithm is presented which transforms the original optimization problem into two 1-dimensional problems, viz. torque error minimization and identification of optimum search direction. This method yields improved computational efficiency and accuracy. The third algorithm incorporates projection using equality constraint on the phase torque contributions to achieve a 1-dimensional solution process. Although this algorithm takes more iteration as compared to the parametric form gradient descent algorithm, it demonstrates greater accuracy and computational efficiency. A comparative analysis of these algorithms is performed in at different operating conditions in terms of the torque ripple magnitude and computational effort. The thesis also presents a comprehensive analysis of well known control techniques for application in SRM current control in discrete-time domain. This analysis also presents a comparative evaluation of these control techniques under different operating conditions. On account of this analysis, several recommendations pertaining to the performance improvement are presented. Finally, a digital sliding-mode based model-free current controller suitable for fixed switching frequency operation is presented. The proposed controller is capable of providing a consistent dynamic response over wide operating range without utilizing any model information. The reference current tracking performance of this controller is verified through simulation studies in MATLAB/Simulink® environment and over a 1.2kW, 100V, 2500RPM, 12/8 experimental SRM drive. / Thesis / Doctor of Philosophy (PhD)

This thesis examines the time resolved heat transfer to drops rolling or sliding along inclined, subcritical heated non-wetting surfaces. Results were experimentally obtained using IR imaging for a smooth hydrophobic surface and post as well as rib structured superhydrophobic surfaces of varying solid fraction (f_s = 0.06 - 0.5). Tests were performed at varying inclination angle (α = 10, 15, 20, and 25°), drop volume (12, 20, 30, and 40 μL), and surface temperature (T_w = 50, 65, and 80 °C). Rib structured superhydrophobic surfaces were explored for drops moving parallel and perpendicular to the rib structures. The findings indicate that transient heat transfer is predominantly influenced by the surface’s solid fraction and the velocity of the drops, with a secondary dependence on drop volume. Surfaces with low solid fraction show a significant reduction in initial heating rate (up to 80% reduction) to the drop, when compared with that of the smooth surface. The drop velocity depends on surface solid fraction and inclination angle, with drop volume exerting smaller influence. Rib structured surfaces impact heat transfer by enhancing heat transfer rate for drops that move along the rib direction compared with drops that move perpendicular to the ribs. The difference is likely due to increased drop velocity that exists for the parallel rib orientation.

Although instabilities and large oscillations are traditionally considered as conditions to be avoided in structures, a new design philosophy based on their exploitation towards the achievement of innovative mechanical features has been initiated in the last decade. In this spirit, instabilities are exploited towards the development of systems that can yield designed responses in the post-critical state. Further, the presence of oscillating constraints may allow for a stabilization of the dynamic response. These subjects entail a rich number of phenomena due to the non-linearity, so that the study of such mechanical systems becomes particularly complex, from both points of view of the mechanical modeling and of the computational tools. Two elastic structures are studied. The first consists of a flexible and extensible rod that is clamped at one end and constrained to slide along a given profile at the other. This feature allows one to study the effect of the axial stiffness of the rod on the tensile buckling of the system and on the compressive restabilization. A very interesting effect is that in a region of parameters double restabilization is found to occur, involving four critical compressive loads. Also, the mechanical system is shown to work as a novel force limiter that does not depend on sacrificial mechanical elements. Further, it is shown that the system can be designed to be multi-stable and suitable for integration in metamaterials. The second analyzed structure is a flexible but inextensible rod that is partially inserted into a movable rigid sliding sleeve which is kept vertical in a gravitational field. The system is analytically solved and numerically and experimentally investigated, when a horizontal sinusoidal input is prescribed at the sliding sleeve. In order to model the system, novel computational tools are developed, implementing the fully nonlinear inextensibility and kinematic constraints. It is shown that the mathematical model of the system agrees with the experimental data. Further, a study of the inclusion of dissipative terms is developed, to show that a steady motion of the rod can be accomplished by tuning the amplitude or the frequency of the sliding sleeve motion, in contrast with the situation in which a complete injection of the rod inside the sleeve occurs. A special discovery is that by slowly decreasing the frequency of the sleeve motion, the length of the rod outside the sleeve can be increased significantly, paving the way to control the rod’s end trajectory through frequency modulation.

The primary objective of this study is to develop a sliding interface method for simulations involving relative rotational grid motion suitable for unstructured grid topologies. The present method alleviates computationally expensive grid deformation, remeshing, and hole cutting procedures. Rotational motion is accomplished by rigidly rotating a subdomain representing the moving component. At the subdomain interface boundary, the faces along the interfaces are extruded into the adjacent subdomain to create new volume elements and provide a one-cell overlap. These new volume elements close the control volumes for the nodes on the interface surface and allow a flux to be computed across the subdomain interface. An interface flux is computed independently for each subdomain. The values of the solution variables and other quantities for the nodes created by the extrusion process are found by interpolation. The extrusion is done so that the interpolation will maintain information as localized as possible. A parallel implementation of the neighbor search is used to find the extruded points in the adjacent subdomain. The method has been implemented in a parallel, node-centered finite volume, high-resolution viscous flow solver. The method does not impose any restrictions on the subdomain interface aside from the axisymmetric limitation required for rotational motion. In addition, the grid on the subdomain interface is arbitrary. The boundary surfaces between the two subdomains can have independent grids from one another. They do not have to connect in a one-to-one manner and there are no symmetry or pattern restrictions placed on the surface grid. A variety of numerical simulations were performed on several small-scale model problems to examine conservation of the interface flux. Overall flux conservation errors were found to be comparable to that for fully connected and fully conservative simulations. In addition, excellent agreement was obtained with both theoretical and experimental results. Three large-scale applications were also used to validate the method and highlight some of the advantages of the sliding interface method compared to the current state-of- the-art for unstructured grid applications. This sliding interface method requires no geometric modifications and has significantly shorter run times Furthermore, there were no apparent adverse effects on the numerical solutions by not strictly enforcing flux conservation at the subdomain boundary.

Yes / Heat-treatment is a frequently used technique for modifying the physical and chemical properties of materials. In this study, the effect of heat-treatment on the mechanical properties, thermal stability and surface morphology of two types of electrodeposited coatings (pure-Ni and Ni/Al2O3) were investigated. The XRD analyses showed that the crystal structure of the as-deposited coating changes from slightly amorphous to crystalline as the heat-treatment temperature increases. The heat-treatment of both the pure-Ni and the Ni/Al2O3 coating caused an increase of the grain size within the coatings. However, the unreinforced Ni coating experienced a faster growth rate than the Ni/Al2O3 coating, which resulted in a larger average grain size. The temperature-driven changes to the microstructure of the coatings caused a reduction in the hardness and wear resistance of the coatings. The presence of nanoparticles within the Ni/Al2O3 coating can successfully extend the operational temperature range of the coating to 473 K by pinning grain boundaries.

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