A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Experimental Evaluation of Circulation Control Aerodynamics on a Cylindrical Body(1987) Ngo, Hieu Thien; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)In this study, an experimental investigation is conducted on a two-dimensional circulation control cylinder with blowing taking place from a single spanwise slot to determine its aerodynamic characteristics. The results include detailed pressure distributions (both chordwise and spanwise) for a range of momentum coefficients and slot locations. The measured results showed that the lift coefficients up to 4.8 were produced at momentum coefficients of 0.14 in a turbulent flow condition. The experimental results of lift coeffficients Were correlated satisfactorily with analytical results. The surface flow patterns were observed using the oil and smoke techniques. Also flow field surveys of the model Were obtained using total pressure, yaw and pitch probes. A color video display technique was used to present the results of the flow field surveys. Based on this evidence, a flow field model of the circulation control cylinder is presented.Item Interface Broadening and Radiation Enhanced Diffusion During Sputter Depth Profiling(1988) Chambers, George Paul; Rousch, Marvin; Chemical and Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The process of ion bombardment of solids has been investigated using Monte Carlo Computer Code simulation in conjunction with ultra-high vacuum experimental techniques. The computer code EVOLVE has been used to study the shape of the resultant collision cascade as well as the origins of sputtered particles while experimental studies of interface regions have been performed to elucidate the physical processes occurring during sputtering. The EVOLVE code models the target as an amorphous multicomponent semi-infinite solid. The target composition during ion bombardment is simulated. The study concludes that recoil activity grows in size and tends to move away from the target surface with increasing time. It is further concluded that the majority of sputtered atoms originate from early generations and are produced from sites near the entry point of the bombarding ion. Low energy noble gas ion bombardment of thin-film Cr/Ni multilayered structures has been performed in conjunction with Auger electron spectroscopy under UHV conditions. An accurate, reliable, and systematic parameterization of the interface region between metallic layers is presented. It is concluded from this study that the extent of the distortion of the interface region due to ion induced broadening is dependent not only on the material system used but on the experimental conditions employed as well. Lastly, radiation enhanced diffusion (RED) has been studied using Ag/Ni thin-film multilayered structures. A physical mathematical model of the radiation broadened Ag layer, capable of successfully deconvoluting the contributions to interface broadening due to RED from those due to cascade mixing and microstructure development, is presented and shown to be an accurate characterization of the interface region. It is concluded from the application of this model that RED can contribute substantially to interface broadening in multicomponent systems with low activation energies of diffusion. It is further concluded from this study that elevated temperatures, sustained during the depth profiling process, can cause the effects of RED to subside dramatically. This phenomenon is most probably due to the dispersion of complex defects responsible for the RED process.Item An Expert System for Helicopter Conceptual Design(1987) Babuska, Vit; Fabunmi, James A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The objective of this thesis is to demonstrate the applicability of expert systems in helicopter conceptual design by developing an expert assistant which aids the engineer in defining a feasible design configuration. The expert assistant combines some experiential knowledge of the design engineer with a typical conceptual design algorithm to guide the engineer to a reasonable baseline design. The expert assistant was developed on a personal computer using the expert system shell INSIGHT2+®. The design algorithm employed is SSPl, a helicopter weight and sizing program developed at the US Army Applied Technologies Laboratory. A set of heuristic rules was developed which attempts to simulate the thinking of an expert design engineer using SSP1 for helicopter conceptual design. The result, a Prototype expert assistant which aids an engineer in the conceptual design phase, demonstrates the feasibility of expert systems in helicopter design.Item The Stress Field Surrounding the Tip of a Crack Propagating in a Finite Body(1987) Chona, Ravinder; Irwin, George R.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The goal of this dissertation was to establish the relationship between a parameter descriptive of the trajectory of a smoothly curving crack, such as the curvature of the crack path, and the local stress state in the close vicinity of the crack tip. The behavior of fast -running cracks propagating along straight and smoothly curving paths in fracture specimens of various geometries was examined using dynamic photoelasticity and representations of the running crack stress field we redeveloped in terms of the coefficients of a set of infinite series, for both opening and shear mode loading conditions. Analysis of the isochromatic patterns, using local collocation methods based on this stress field representation, allowed the stress state in the neighborhood of the propagating crack-tip to be modelled with a high degree of accuracy and results were obtained for the variations with crack tip position of both the singular and leading non- singular stress field coefficients of interest. The results obtained for quasi-static and rapid crack propagation under opening mode conditions in a ring segment revealed the importance of retaining terms of order (at a minimum) r^1/2 even when only the singular term was to be determined accurately. Furthermore, it was found that the non-singular stress field coefficients varied similarly in both static and dynamic situations, with some variations in magnitude that could be attributed to crack speed. The results from the curved crack experiments also showed systematic variation of the non-singular terms, but more importantly, it was found that the instantaneous curvature of the crack path was related to the magnitude of the lowest order non-singular stress component (the coefficient of the r^1/2 term) associated with the local shear mode of deformation in the vicinity of the tip of the running crack. Furthermore, the results established that the only singularity associated with a crack propagating along a smoothly curving path in a brittle, isotropic material was that associated with the opening mode stress intensity factor, K1, and that the shear mode singularity, KII, was identically equal to zero.Item Expert System for Process Selection and Operation Optimization(1986) Kar, Amit; Pandelidis, Ioannis; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The working of an Expert System, PRODUCER, conceived as part of a CIM system for a plant manufacturing discreteparts, is presented. PRODUCER starts out by determining if the desired part can be produced. Based on part attributes it then decides on a specific manufacturing process, e.g., Welding, Casting, Forging or Machining. Having selected the process, PRODUCER establishes the particular operation. With the operation decided, PRODUCER proceeds to find all the feasible combinations, of equipment and tools, that could produce the desired component. The turning operation has been chosen to demonstrate PRODUCER's capabilities. PRODUCER then sets about the task of identifying the most optimal pair of machine-tool and cutting-tool, which will provide the highest Metal Removal Rate, MRR. This is accomplished at two levels. At the higher level, PRODUCER generates constraints, representing physical limitations of the cutting process, for each machine and tool combination. These constraints are then passed on to an Optimization program. This is a Fortran program, which operates at a lower level, and returns the optimum values of the process control variables, for each machine-tool and cutting-tool combination. PRODUCER finally yields the highest maximization of the MRR. In doing so it also identifies the particular machine-tool and cutting-tool associated with this global optimum. PRODUCER, essentially a knowledge-based production system, implemented in the First Order Predicate Logic language of Prolog, also enables intelligent adaptive control.Item Measurements of the Size Distribution and Aerodynamic Properties of Soot(1989) Cleary, Thomas George; Gentry, James W.; Chemical and Biomolecular Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The particle size distribution and aerodynamic properties of soot agglomerates formed by the combustion of acetylene is studied. A positive pressure soot generator was constructed which allowed for the characterization of the soot aerosol. Nearly monodisperse particles (10 to 20 nm in diameter) have been observed at low acetylene flow rates. The mean size and width of the distribution are confirmed with a diffusion battery and a differential mobility analyzer. Size distribution measurements of soot agglomerates have been obtained from optical and electron microscopy for a range of acetylene flow rates. The electron microscopy results are compared to model predictions of the cluster size distribution. Friction coefficient measurements from electrical mobility classified agglomerates have been made and are compared to fractal models for clusters. Aerodynamic diameters of impacted agglomerates are related to the geometric mean size form optical microscopy. These results suggest that the geometric mean size can replace the spherical diameter if a particle density of 0.1 g/cc is assumed.Item Dynamics of a Helicopter-Slung Load System(1980) Sampath, Prasad; Barlow, Jewel B.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Stability of a tandem rotor helicopter (347/HLH) carrying a slung cargo container has been investigated. Lagrange equations were used to write the equations of motion. The cables of the sling were modeled as massless rigid extensible rods, which collapse under compressive loads. Extensibility was provided by considering the rods as linear spring with viscous damping. Aerodynamics of the cable were neglected. Tabulated static aerodynamic data were considered for the helicopter as well as the load. The equations were divided into two sets, one representing the towing vehicle (referred to as Subsystem 1) and the other representing the slung load (referred to as Subsystem 2). Subsystem 2 corresponds to a wind tunnel model of a slung load.Item Design of Rigid Overlays for Airfield Pavements(1987) Rollings, Raymond Sydney; Witczak, Matthew W.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Existing rigid overlay pavement design methods are empirical and use a specified level of cracking as the defined failure condition. The existing empirical designs are based on tests run thirty years ago, and current analytical models provide greatly improved abilities to examine the overlay pavement structure. Emphasis by many agencies on life cycle cost analysis and more sophisticated maintenance and rehabilitation strategies require methods of predicting pavement performance rather than simply developing safe designs. A layered elastic analytical model was selected to evaluate stresses from applied loads in the pavement structure. Pavement performance was measured in terms of a Structural Condition Index which related the type, degree, and severity of pavement cracking and spalling on a scale of 0 to 100. Models were developed to represent the effect of cracking in base slabs under the overlay, to account for fatigue damage of previous traffic on the base pavement, and to account for the effects of substandard load transfer at slab joints. The predicted performance of overlays and pavements using this analysis was checked against the results of full-scale accelerated traffic tests conducted by the Corps of Engineers and against current overlay design methods and was found to provide reasonable agreement. This methodology using the layered elastic analytical model and analysis of fatigue and cracking in the base slab provides a method of predicting pavement and overlay deterioration in terms of a Structural Condition Index.