<p dir="ltr">Recent regulatory changes have been proposed to phase down the use of hydrofluorocarbon</p><p dir="ltr">(HFC)-based refrigerants in air conditioning and refrigeration systems. The proposed</p><p dir="ltr">low global warming potential alternatives (low-GWP) are characterized by lower volumetric</p><p dir="ltr">capacities, which require either higher displacements or higher speeds to meet compressor</p><p dir="ltr">loads. In order to address this, the coupled optimization of a compressor system and its electric</p><p dir="ltr">drive has been proposed. The primary goal of this thesis is to establish tools that can be</p><p dir="ltr">used to assess the impact that alternative low-GWP refrigerants have on the sizing and performance</p><p dir="ltr">of electrically driven compressors. Toward this goal, a method-of-moments-based</p><p dir="ltr">model has been established and structured to enable rapid evaluation of the electromagnetic</p><p dir="ltr">performance of V-shaped interior magnet machines. Contributions to the model formulation</p><p dir="ltr">include the use of a judicious combination of point and pulse basis functions to evaluate</p><p dir="ltr">machine behavior under saturation of stator and rotor steels. Also included is a straightforward</p><p dir="ltr">means to include multiple operating points with minimal additional computational</p><p dir="ltr">expense. Coupled to the electromagnetic model is a thermal equivalent circuit model that</p><p dir="ltr">includes conductive heat transfer between slot winding bundles and stator steel. It also includes</p><p dir="ltr">convective heat transfer from the stator to the rotor through the airgap. The proposed</p><p dir="ltr">models have been validated using commercial finite-element based software. Subsequently,</p><p dir="ltr">they have been applied in design optimization studies used to compare the efficiency and size</p><p dir="ltr">(mass) between machines designed for a common HFC refrigerant (R410A) and a proposed</p><p dir="ltr">alternative (R454B).</p>

This thesis developed a linear shaped charge (LSC) separation mechanism capable of severing the interstage skin for first stage separation of the Ares I launch vehicle. The derived LSC design solution was found using available data on Explosive Technology’s Jetcord LSC and from National Aeronautics and Space Administration (NASA) Marshall Space Flight Center’s (MSFC) desired characteristics. Mechanism components are designed after Minuteman III’s separation mechanism for first stage separation and NASA MSFC’s desired characteristics. Mechanism severance is verified through the use of the numerical method capability smoothed particle hydrodynamics that the hydrocode Autodyn offers. Three simulations are conducted to determine feasibility: the first of only the LSC exploding, to numerically validate the explosion process; the second of the LSC penetrating the target, to numerically validate the penetration process and failure mechanisms; and the last of the entire mechanism, to obtain information about the explosion, penetration, failure, and debris generated.

No description available.

This paper is an attempt to study the behavior of axially loaded concrete filled steel tubular (CFST) stub columns with special-shaped cross-sections, i.e. triangular, fan-shaped, D-shaped, 1/4 circular and semi-circular. A total of forty-four specimens including CFST stub columns and reference hollow steel tubular stub columns were tested. The effects of the changing steel tube wall thickness and the infill of concrete on the behavior of the composite columns were investigated. The results showed that the tested special-shaped CFST stub columns behaved in a ductile manner, and the composite columns showed an outward local buckling model near the middle section. Generally, the failure modes of these five kinds of special-shaped specimens were similar to those of the square CFST stub columns. Finally, simplified model for predicting the cross-sectional strength of the special-shaped CFST sections was discussed and proposed.

Active structural acoustic control has previously been used to reduce low-frequency sound radiation from relatively simple laboratory structures. However, significant implementation issues have to be addressed before active control can be used on large, complex structures such as an aircraft fuselage. The purpose of this project is to extend decentralized structural control systems from individual bays to more realistic airframe structures. In addition, to make this investigation more applicable to industry, potential control strategies are evaluated using a realistic aft-cabin disturbance identified from flight test data. This work focuses on decentralized control, which implies that each control unit is designed and implemented independently. While decentralized control systems are relatively scalable, performance can be limited due to the destabilizing interaction between neighboring controllers. An in-depth study of this problem demonstrates that the modeling error introduced by neighboring controllers can be expressed as the product of the complementary sensitivity function of the neighboring control unit multiplied by a term that quantifies the diagonal dominance of the plant. This understanding can be used to improve existing control strategies. For instance, decentralized performance can often be improved by penalizing control effort at the zeros of the local control model. This stabilizes each control unit and reduces the modeling error induced on neighboring controllers. Additional analyses show that the performance of decentralized model-based control systems can be improved by augmenting the structural damping using robust, low-authority control strategies such as direct velocity feedback and positive position feedback. Increasing the structural damping can supplement the performance of the model-based control strategy and reduce the destabilizing interaction between neighboring control units. Instead of using low-authority controllers to stabilize the decentralized control system, another option is to modify the model-based design. Specifically, an iterative approach is developed and validated using real-time control experiments performed on a structural-acoustic system with poles close to the stability boundary, non-minimum phase zeros, and unmodeled dynamics. Experiments demonstrate that the iterative control strategy, which combines frequency-shaped linear quadratic Gaussian (LQG) control with loop transfer recovery (LTR), is capable of achieving 12dB peak reductions and a 3.6dB integrated reduction in radiated sound power from a rib-stiffened aluminum panel. / Ph. D.

Fouling properties of materials such as polyethylene glycol (PEG) have been extensively studied over the past decades. Traditionally, the factors believed to result in protein adsorption resistance have included i) steric exclusion arising from the compression of longer chains and ii) grafting density contribution which may provide shielding from the underlying material. Recent studies have suggested that PEG interaction with water may also play a role in its ability to resist protein adsorption suggesting that steric exclusion may not be the only mechanism occurring during PEG/protein interactions. Star-shaped PEG polymers have been utilized in protein adsorption studies due to their high PEG segment concentration, which allows to increase the PEG chain grafting density compared to that achieved with linear PEG chains. Most studies that have investigated the interactions of tethered linear and star-shaped PEG layers with proteins have considered linear PEG molecules with molecular weights several orders of magnitude smaller than those considered for star-shaped PEG molecules (i.e. 10 000 g/mol vs. 200 000 g/mol, respectively). Additionally, the star-shaped PEG molecules which have been considered in the literature had up to ~70 arms and were therefore modeled by hard-sphere like structures and low chain densities near the surface due to steric hindrance. This resulted in some difficulties to achieve grafted PEG chain overlap for star molecules. Here, triethoxysilane end-functionalized linear PEG molecules have been synthesized and utilized to form star-shaped PEG derivatives based on ethoxy hydrolysis and condensation reactions. This resulted in PEG stars with up to ~4 arms, which were found to result in grafted star-shaped PEG chains with significant chain overlap. Linear PEG derivatives were synthesized so that their molecular weight would match the overall molecular weight of the star-shaped PEG molecules. These 2 PEG molecular architectures were covalently self-assembled to hydroxylated silicon wafers and the thickness, grafting density, and conformation of these films were studied. The adsorption of human albumin (serum protein) on linear and star-shaped PEG films was compared to that obtained on control samples, i.e. uncoated silicon wafers. Both film architectures were found to significantly lower albumin adsorption. / Ph. D.

This work is motivated by the pressing need for operational control in the fabrication of Microelectromechanical systems or MEMS. MEMS are miniature three-dimensional integrated electromechanical systems with the ability to absorb information from the environment, process this information and suitably react to it. These devices offer tremendous advantages owing to their small size, low power consumption, low mass and high functionality, which makes them very attractive in applications with stringent demands on weight, functionality and cost. While the system''s "brain" (device electronics) is fabricated using traditional IC technology, the micromechanical components necessitate very intricate and sophisticated processing of silicon or other suitable substrates. A dearth of fabrication facilities with micromachining capabilities and a lengthy gestation period from design to mass fabrication and commercial acceptance of the product in the market are factors most often implicated in hampering the growth of MEMS. These devices are highly application specific with low production volumes and the few fabs that do possess micromachining capabilities are unable to offer a complete array of fabrication processes in order to be able to cater to the needs of the MEMS R&D community. A distributed fabrication network has, therefore, emerged to serve the evolving needs of this high investment, low volume MEMS industry. Under this environment, a central facility coordinates between a network of fabrication centers (Network of MEMS job shops -- NMJS) containing micromachining capabilities. These fabrication centers include commercial, academic and government fabs, which make their services available to the ordinary customer. Wafers are shipped from one facility to another until all processing requirements are met. The lengthy and intricate process sequences that need to be performed over a network of capital intensive facilities are complicated by dynamic job arrivals, stochastic processing times, sequence-dependent set ups and travel between fabs. Unless the production of these novel devices is carefully optimized, the benefits of distributed fabrication could be completely overshadowed by lengthy lead times, chaotic routings and costly processing. Our goal, therefore, is to develop and validate an approach for optimal routing (assignment) and sequencing of MEMS devices in a network of stochastic job shops with the objective of minimizing the sum of completion times and the cost incurred, given a set of fabs, machines and an expected product mix. In view of our goal, we begin by modeling the stochastic NMJS problem as a two-stage stochastic program with recourse where the first-stage variables are binary and the second-stage variables are continuous. The key decision variables are binary and pertain to the assignment of jobs to machines and their sequencing for processing on the machines. The assignment variables essentially fix the route of a job as it travels through the network because these variables specify the machine on which each job-operation must be performed out of several candidate machines. Once the assignment is decided upon, sequencing of job-operations on each machine follows. The assignment and sequencing must be such that they offer the best solution (in terms of the objective) possible in light of all the processing time scenarios that can be realized. We present two approaches for solving the stochastic NMJS problem. The first approach is based on the L-shaped method (credited to van Slyke and Wets, 1969). Since the NMJS problem lacks relatively complete recourse, the first-stage solution can be infeasible to the second-stage problem in that the first stage solution may either violate the reentrant flow conditions or it may create a deadlock. In order to alleviate these infeasibilities, we develop feasibility cuts which when appended to the master problem eliminate the infeasible solution. Alternatively, we also develop constraints to explicitly address these infeasibilities directly within the master problem. We show how a deadlock involving 2 or 3 machines arises if and only if a certain relationship between operations and a certain sequence amongst them exists. We generalize this argument to the case of m machines, which forms the basis for our deadlock prevention constraints. Computational results at the end of Chapter 3 compare the relative merits of a model which relies solely on feasibility cuts with models that incorporate reentrant flow and deadlock prevention constraints within the master problem. Experimental evidence reveals that the latter offers appreciable time savings over the former. Moreover, in a majority of instances we see that models that carry deadlock prevention constraints in addition to the reentrant flow constraints provide at par or better performance than those that solely carry reentrant flow constraints. We, next, develop an optimality cut which when appended to the master problem helps in eliminating the suboptimal master solution. We also present alternative optimality and feasibility cuts obtained by modifying the disjunctive constraints in the subproblem so as to eliminate the big H terms in it. Although any large positive number can be used as the value of H, a conservative estimate may improve computational performance. In light of this, we develop a conservative upper bound for operation completion times and use it as the value of H. Test instances have been generated using a problem generator written in JAVA. We present computational results to evaluate the impact of a conservative estimate for big H on run time, analyze the effect of the different optimality cuts and demonstrate the performance of the multicut method (Wets, 1981) which differs from the L-shaped method in that the number of optimality cuts it appends is equal to the number of scenarios in each iteration. Experimentation indicates that Model 2, which uses the standard optimality cut in conjunction with the conservative estimate for big H, almost always outperforms Model 1, which also uses the standard optimality cut but uses a fixed value of 1000 for big H. Model 3, which employs the alternative optimality cut with the conservative estimate for big H, requires the fewest number of iterations to converge to the optimum but it also incurs the maximum premium in terms of computational time. This is because the alternative optimality cut adds to the complexity of the problem in that it appends additional variables and constraints to the master as well as the subproblems. In the case of Model 4 (multicut method), the segregated optimality cuts accurately reflect the shape of the recourse function resulting in fewer overall iterations but the large number of these cuts accumulate over the iterations making the master problem sluggish and so this model exhibits a variable performance for the various datasets. These experiments reveal that a compact master problem and a conservative estimate for big H positively impact the run time performance of a model. Finally, we develop a framework for a branch-and-bound scheme within which the L-shaped method, as applied to the NMJS problem, can be incorporated so as to further enhance its performance. Our second approach for solving the stochastic NMJS problem relies on the tight LP relaxation observed for the deterministic equivalent of the model. We, first, solve the LP relaxation of the deterministic equivalent problem, and then, fix certain binary assignment variables that take on a value of either a 0 or a 1 in the relaxation. Based on this fixing of certain assignment variables, additional logical constraints have been developed that lead to the fixing of some of the sequencing variables too. Experimental results, comparing the performance of the above LP heuristic procedure with CPLEX over the generated test instances, illustrate the effectiveness of the heuristic procedure. For the largest problems (5 jobs, 10 operations/job, 12 machines, 7 workcenters, 7 scenarios) solved in this experiment, an average savings of as much as 4154 seconds and 1188 seconds was recorded in a comparison with Models 1 and 2, respectively. Both of these models solve the deterministic equivalent of the stochastic NMJS problem but differ in that Model 1 uses a big H value of 1000 whereas Model 2 uses the conservative upper bound for big H developed in this work. The maximum optimality gap observed for the LP heuristic over all the data instances solved was 1.35%. The LP heuristic, therefore, offers a powerful alternative to solving these problems to near-optimality with a very low computational burden. We also present results pertaining to the value of the stochastic solution for various data instances. The observed savings of up to 8.8% over the mean value approach underscores the importance of using a solution that is robust over all scenarios versus a solution that approximates the randomness through expected values. We, next, present a dynamic stochastic scheduling approach (DSSP) for the NMJS problem. The premise behind this undertaking is that in a real-life implementation that is faithful to the two-stage procedure, assignment (routing) and sequencing decisions will be made for all the operations of all the jobs at the outset and these will be followed through regardless of the actual processing times realized for individual operations. However, it may be possible to refine this procedure if information on actual processing time realizations for completed operations could be utilized so that assignment and sequencing decisions for impending operations are adjusted based on the evolving scenario (which may be very different from the scenarios modeled) while still hedging against future uncertainty. In the DSSP approach, the stochastic programming model for the NMJS problem is solved at each decision point using the LP heuristic in a rolling horizon fashion while incorporating constraints that model existing conditions in the shop floor and the actual processing times realized for the operations that have been completed. The implementation of the DSSP algorithm is illustrated through an example problem. The results of the DSSP approach as applied to two large problem instances are presented. The performance of the DSSP approach is evaluated on three fronts; first, by using the LP heuristic at each decision point, second, by using an optimal algorithm at each decision point, and third, against the two-stage stochastic programming approach. Results from the experimentation indicate that the DSSP approach using the LP heuristic at each decision point generates superior assignment and sequencing decisions than the two-stage stochastic programming approach and provides solutions that are near-optimal with a very low computational burden. For the first instance involving 40 operations, 12 machines and 3 processing time scenarios, the DSSP approach using the LP heuristic yields the same solution as the optimal algorithm with a total time savings of 71.4% and also improves upon the two-stage stochastic programming solution by 1.7%. In the second instance, the DSSP approach using the LP heuristic yields a solution with an optimality gap of 1.77% and a total time savings of 98% over the optimal algorithm. In this case, the DSSP approach with the LP heuristic improves upon the two-stage stochastic programming solution by 6.38%. We conclude by presenting a framework for the DSSP approach that extends the basic DSSP algorithm to accommodate jobs whose arrival times may not be known in advance. / Ph. D.

An experimental study was performed to measure surface Nusselt number and film cooling effectiveness on a film cooled first stage nozzle guide vane using a transient thin film gauge (TFG) technique. The information presented attempts to further characterize the performance of shaped hole film cooling by taking measurements on a row of shaped holes downstream of leading edge showerhead injection on both the pressure and suction surfaces (hereafter PS and SS) of a 1st stage NGV. Tests were performed at engine representative Mach and Reynolds numbers and high inlet turbulence intensity and large length scale at the Virginia Tech Transonic Cascade facility. Three exit Mach/Reynolds number conditions were tested: 1.0/1,400,000; 0.85/1,150,000; and 0.60/850,000 where Reynolds number is based on exit conditions and vane chord. At Mach/Reynolds numbers of 1.0/1,450,000 and 0.85/1,150,000 three blowing ratio conditions were tested: BR = 1.0, 1.5, and 2.0. At a Mach/Reynolds number of 0.60/850,000, two blowing ratio conditions were tested: BR = 1.5 and 2.0. All tests were performed at inlet turbulence intensity of 12% and length scale normalized by leading edge diameter of 0.28. Film cooling effectiveness and heat transfer results compared well with previously published data, showing a marked effectiveness improvement (up to 2.5x) over the showerhead only NGV and agreement with published showerhead-shaped hole data. NHFR was shown to increase substantially (average 2.6x increase) with the addition of shaped holes, with only a small increase (average 1.6x increase) in required coolant mass flow. Heat transfer and effectiveness augmentation with increasing blowing ratio was shown on the pressure side, however the suction side was shown to be less sensitive to changing blowing ratio. Boundary layer transition location was shown to be within a consistent region on the suction side regardless of blowing ratio and exit Mach number. / Master of Science

The exploration of Microelectromechanical systems (MEMS) represents a crucial aspect in the advancement of modern science and technology. They offer low-cost solutions to miniaturize numerous devices. The increasing use of MEMS applications in biological research has created a pressing need for reliable micromanipulation tools. In this context, microgrippers have emerged as promising tools for the precise handling and characterization of biological samples. This thesis presents a novel biocompatible microgripper that utilizes electrothermal actuation integrated with a rotary capacitive position sensor. To overcome the limited displacement possibilities associated with electrothermal actuators, this microgripper incorporates conjugate surface flexure hinges (CSFH). These hinges enhance the desired tweezers output displacement. The designed microgripper can in principle manipulate biological samples ranging in size from 15 to 120 μm. Based on the sensitivity calculation of the rotary capacitive position sensors, the sensitivity of the displacement measurement is 102 fF/μm. By employing a kinematics modeling approach based on the pseudo-rigid-body method (PRBM), an equation for the displacement amplification factor is developed, and this equation is subsequently verified through FEM-based simulations. By comparing the amplification ratio value obtained from the analytical modeling and simulations, there is an excellent match, with a relative difference of only ~1%, thus demonstrating the effectiveness of the PRBM approach in modeling the kinematics of the structure under investigation. In addition to this, by using analytical modeling based on finite elements method (FEM), the design of the electrothermal actuator and the heat dissipation mechanism is optimized. FEM-based simulations are used to validate the theoretical modeling, demonstrating good agreement between the displacements derived from analytical modeling and simulations. The temperature difference (∆T) across a range from room temperature to 278°C exhibits a relative difference of ~2.8%. Moreover, underpass technology is implemented to ensure that electrical signals or disturbances from other parts of the device, such as the electrothermal actuation system, do not interfere with the operation and integrity of the gripping mechanism. Ultimately, the microgripper is fabricated using conventional MEMS technology from a silicon-on-insulator (SOI) wafer through the deep reactive ion etching (DRIE) technique. The integration of theoretical modeling, simulations, and practical fabrication highlights a compelling approach that has the potential for transformative applications in the field of micromanipulation and biological sample handling. Furthermore, we propose a C-shaped structure with a curved beam mechanism to improve the movement provided by the thermal actuators. The design of experiment (DOE) method is used to optimize the geometrical parameters of our proposed device. Analytical modeling based on Castigliano's second theorem and finite element method (FEM) simulations are used to predict the behavior of the symmetrical C-shaped structure; the results are in good agreement. The MEMS-based rotational structures are fabricated on silicon-on-insulator (SOI) wafers using bulk micromachining and deep reactive ion etching (DRIE). The fabricated devices are tested; our findings reveal that our proposed MEMS rotational structure outperforms the symmetrical lancet structure by 28% in terms of delivered displacement. Furthermore, the experimental results agree well with those obtained through numerical analysis.

Advances in structural systems that resist extreme loading such as earthquake forces are important in their ability to reduce damages, improve performance, increase resilience, and improve the reliability of structures. Buckling resistant shear panels can be used to form new structural systems, which have been shown in preliminary analysis to have improved hysteretic behavior including increased stiffness and energy dissipating ability. Both of these characteristics lead to reduced drifts during earthquakes, which in turn leads to a reduction of drift related structural and nonstructural damage. Shear links are being used for seismic energy dissipation in some structures. A promising type of fuse implemented in structures for seismic energy dissipation, and seismic load resistance consists of a steel plate with cutouts leaving various shaped shear links. During a severe earthquake, inelastic deformation and damage would be concentrated in the shear links that are part of replaceable structural fuses, while the other elements of the building remain in the elastic state. In this study, by identifying the issues associated with general fuses previously used in structures, the behavior of the links is investigated and procedures to improve the behavior of the links are explained. In this study, a promising type of hysteretic damper used for seismic energy dissipation of a steel plate with cutouts leaving butterfly-shaped links subjected to shear deformations. These links have been proposed more recently to better align bending capacity with the shape of the moment diagram by using a linearly varying width between larger ends and a smaller middle section. Butterfly-shaped links have been shown in previous tests to be capable of substantial ductility and energy dissipation, but can also be prone to lateral torsional buckling. The mathematical investigations are conducted to predict, explain and analyze the butterfly-shaped shear links behavior for use in seismic structural fuses. The ductile and brittle limit states identified based on the previous studies, are mathematically explained and prediction equations are proposed accordingly. Design methodologies are subsequently conceptualized for structural shear links to address shear yielding, flexural yielding and buckling limit states for a typical link subjected to shear loading to promote ductile deformation modes. The buckling resistant design of the links is described with the aid of differential equations governing the links' buckling behavior. The differential equations solution procedures are developed for a useful range of link geometries and the statistical analysis is conducted to propose an equation for critical buckling moment. Computational studies on the fuses are conducted with finite element analysis software. The computational modeling methodology is initially verified with laboratory tests. Two parametric computational studies were completed on butterfly-shaped links to study the effect of varying geometries on the shear yielding and flexural yielding limit states as well as the buckling behavior of the different butterfly-shaped link geometries. It is shown that the proposed critical moment for brittle limit state has 98% accuracy, while the prediction equations for ductile limit states have more than 97% accuracy as well. Strategies for controlling lateral torsional buckling in butterfly links are recommended and are validated through comparison with finite element models. The backbone behavior of the seismic butterfly-shaped link is formulized and compared with computational models. In the second parametric study, the geometrical properties effects on a set of output parameters are investigated for a 112 computational models considering initial imperfection, and it is indicated that the narrower mid-width would reach to their limit states in lower displacement as compared to wider mid-width ones. The work culminates in a system-level validation of the proposed structural fuses with the design and analysis of shear link structural fuses for application in three buildings with different seismic force resisting systems. Six options for shear link geometry are designed for each building application using the design methodologies and predictive equations developed in this work and as guided by the results of the parametric studies. Subsequently, the results obtained for each group is compared to the conventional systems. The effect of implementation of the seismic links in multi-story structures is investigated by analyzing two prototype structures, with butterfly-shaped links and simple conventional beam. The results of the nonlinear response history analysis are summarized for 44 ground motions under Maximum Considered Event (MCE) and Design Basic Earthquake (DBE) ground motion hazard levels. It is shown that implementation of the butterfly-shaped links will lead to higher dissipated energy compared to conventional Eccentrically Braced Frame (EBF) systems. It is concluded that implementation of the seismic shear links significantly improves the energy dissipation capability of the systems compared to conventional systems, while the stiffness and strength are close in these two systems. / Ph. D. / Structural fuses are replaceable elements of a structure that are designed to yield and protect the surrounding members from damages, and then be accessible and replaceable after a major event. Several studies have indicated that steel plates with cutouts would have advantages for use in structural fuses. Having cutouts in a steel plate would make different shapes inside of the plate, which are called structural links. To have the same yielding condition all over the links, it is tried to better align the capacity of the links with the shape of the demand diagram caused by loading, which would be leading to the efficient implementation of the steel. In general, links are implemented to substantially increase the energy dissipation capacity of a structure and significantly reduce the energy dissipation demand on the framing members of a structure. For these purposes, various shapes have been proposed in this research study. The main feature of a replaceable link system is that the inelasticity is concentrated at the steel link while the beams and columns remain almost elastic. This study investigated the general behavior of the fuses, the applicability of them for space-constrained applications, the flexure, shear and buckling limit states affecting the behavior of the links. The computational analysis methodologies to model the links are explained and confirmed with the behavior of the different experiment tests as well as the proposed brittle limit state prediction equations. Subsequently, the two parametric studies are done to investigate the effect of geometrical properties on the links output results and establish prediction equations. The results from the analytical and computational studies for the seismic links are incorporated for seismic investigation of multi-story buildings. The results of seismic analysis of the two buildings are summarized for 44 ground motions.