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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.


Mahmood, Osamah Ibrahim 01 January 2013 (has links)
When designing a bridge, serviceability usually controls and is a more important factor than the ultimate capacity of the bridge or the allowable stresses. Therefore, the behavior of the bridge girder deflection and camber must be predicted as accurately as possible. Therefore, excessive camber has become one of the most common problems when constructing concrete bridges. Different methods have been developed to overcome this problem. The most common and widely used is using haunch with adjustable pedestals to overcome the excessive camber. However, this method has limitations that must be considered. Therefore, this study is evaluating the effectiveness of using post tensioning jacking strands at the top flange of simply supported bridge girders to reduce the excessive camber and make it equal to the design camber.

Using Maturity to Predict Girder Camber

Bert, Stephen M. 17 June 2005 (has links)
The objective of this research was to determine if differential camber of prestressed concrete girders could be reduced by accurate prediction of initial camber at release of prestress. Maturity at prestress transfer was used to calculate modulus of elasticity for predicting camber at release. The research consists of a literature review of maturity methods, testing of a standard concrete mix to determine strength and modulus functions and measurement of girder camber and maturity. Both the Nurse-Saul and the Arrhenius maturity models were evaluated. Maturity relationships were developed for concrete mixes containing Type II and Type III cements. A relationship of modulus as a function of maturity was developed. Seven girders were tested. Camber predictions within 0 to ¼ in. of actual camber were obtained using modulus of elasticity calculated from a maturity based function. Comparison was made between maturity based modulus and standard strength based modulus models. Camber predictions based on modulus calculated based on field cured cylinder strengths were within 0 to ½ in. / Master of Science

Thin/cambered/reflexed airfoil development for micro-air vehicles at Reynolds numbers of 60,000 to 150,000 /

Reid, Michael R. January 2006 (has links)
Thesis (M.S.)--Rochester Institute of Technology, 2006. / Typescript. Includes bibliographical references (leaves 134-135).

Die Kammermusikwerke Fl. L. Gassmanns

Leuchter, Erwin, January 1926 (has links)
Thesis (doctoral)--Universität Wien.. / Thematic catalog: leaves 11-110. Includes bibliographical references (leaves 1-3).

Aerodynamic and Electromechanical Design, Modeling and Implementation Of Piezocomposite Airfoils

Bilgen, Onur 02 September 2010 (has links)
Piezoelectrics offer high actuation authority and sensing over a wide range of frequencies. A Macro-Fiber Composite is a type of piezoelectric device that offers structural flexibility and high actuation authority. A challenge with piezoelectric actuators is that they require high voltage input; however the low power consumption allows for relatively lightweight electronic components. Another challenge, for piezoelectric actuated aerodynamic surfaces, is found in operating a relatively compliant, thin structure (desirable for piezoceramic actuators) in situations where there are relatively high external (aerodynamic) forces. Establishing an aeroelastic configuration that is stiff enough to prevent flutter and divergence, but compliant enough to allow the range of available motion is the central challenge in developing a piezocomposite airfoil. The research proposed here is to analyze and implement novel electronic circuits and structural concepts that address these two challenges. <p> Here, a detailed theoretical and experimental analysis of the aerodynamic and electromechanical systems that are necessary for a practical implementation of a piezocomposite airfoil is presented. First, the electromechanical response of Macro-Fiber Composite based unimorph and bimorph structures is analyzed. A distributed parameter electromechanical model is presented for interdigitated piezocomposite unimorph actuators. Necessary structural features that result in large electrically induced deformations are identified theoretically and verified experimentally. A novel, lightweight electrical circuitry is proposed and implemented to enable the peak-to-peak actuation of Macro-Fiber Composite bimorph devices with asymmetric voltage range. <p> Next, two novel concepts of supporting the piezoelectric material are proposed to form two types of variable-camber aerodynamic surfaces. The first concept, a simply-supported thin bimorph airfoil, can take advantage of aerodynamic loads to reduce control input moments and increase control effectiveness. The structural boundary conditions of the design are optimized by solving a coupled fluid-structure interaction problem by using a structural finite element method and a panel method based on the potential flow theory for fluids. The second concept is a variable-camber thick airfoil with two cascading bimorphs and a compliant box mechanism. Using the structural and aerodynamic theoretical analysis, both variable-camber airfoil concepts are fabricated and successfully implemented on an experimental ducted-fan vehicle. A custom, fully automated low-speed wind tunnel and a load balance is designed and fabricated for experimental validation. The airfoils are evaluated in the wind tunnel for their two-dimensional lift and drag coefficients at low Reynolds number flow. The effects of piezoelectric hysteresis are identified. <p> In addition to the shape control application, low Reynolds number flow control is examined using the cascading bimorph variable-camber airfoil. Unimorph type actuators are proposed for flow control in two unique concepts. Several electromechanical excitation modes are identified that result in the delay of laminar separation bubble and improvement of lift. Periodic excitation to the flow near the leading edge of the airfoil is used as the flow control method. The effects of amplitude, frequency and spanwise distribution of excitation are determined experimentally using the wind tunnel setup.<p> Finally, the effects of piezoelectric hysteresis nonlinearity are identified for Macro-Fiber Composite bimorphs. The hysteresis is modeled for open-loop response using a phenomenological classical Preisach model. The classical Preisach model is capable of predicting the hysteresis observed in 1) two cantilevered bimorph beams, 2) the simply-supported thin airfoil, and 3) the cascading bimorph thick airfoil. / Ph. D.

Investigation of Time-Dependent Deflection in Long Span, High Strength, Prestressed Concrete Bridge Beams

Hinkle, Stephen Dock 14 September 2006 (has links)
Accurate camber prediction in prestressed concrete bridge beams is important to all parties involved in bridge design and construction. Many current prestress loss prediction methods, necessary for proper camber calculation, were developed many years ago and are predicated on assumptions that may no longer be valid as higher strength concrete, wider beam spacing, and longer span lengths become more commonplace. This throws into question which models are appropriate for use in camber calculation by the bridge engineers and contractors of today. Twenty-seven high-strength concrete modified 79 in. Bulb Tee beams with a design compressive strength of 9,000 psi were periodically measured to determine camber growth. Most available models for concrete creep and shrinkage were used to calculate creep and shrinkage strain. The modulus of elasticity equation of each model was used to predict modulus of elasticity of the studied mix. The Shams and Kahn compressive strength and modulus of elasticity equations were modified in order to approximate measured modulus of elasticity. The creep, shrinkage, and modulus of elasticity equations were used as inputs to an incremental time step method. The time-dependent change in beam curvature calculated by the time step method was used to calculate theoretical camber using the Moment-Area method. Predicted camber, using inputs from each considered model, was then compared with measured camber to determine the most accurate camber prediction models. Season of casting was also examined to determine what, if any, affect ambient temperature has on camber growth. For the studied beams, the Shams and Kahn Model for creep, shrinkage, and modulus of elasticity, used as inputs for an incremental time step analysis, were found to most accurately predict camber values. Lower concrete compressive strength was observed for test cylinders from beams cast in summer versus beams cast in winter. Differences in beam deflection based on season of casting showed mixed results. / Master of Science

The Short and Long-Term Effects of Temperature and Strain on a Concrete Bulb-Tree Girder Bridge

Pickett, Ethan 01 May 2017 (has links)
The Utah Transportation Center (UTC) as well as the Mountain Plains Consortium, sponsored a study to investigate the long-term performance of a deck bulb tee girder bridge. The bridge in question is located in Nibley, Utah and was erected in early 2016. Temperature and prestress losses were analyzed from embedded instrumentation placed within two of the bridge girders before casting. These two girders contained a total of 50 thermocouples and 16 vibrating wire strain gauges. These instruments were placed at the mid-span and end of an exterior girder and the mid-span, quarter-span, and end of a center girder in order to effectively monitor the bridge response in one quarter of the bridge superstructure. The monitoring performed with the thermocouples included the temperature of the girders during curing, weekly maximum and minimum temperatures compared to methods for predicting the average bridge temperature, maximum and minimum thermal gradients at each of the five selected cross sections compared to Code thermal gradients, and thermal camber by measured temperature compared to models to predict thermal gradients. The 16 strain gauges measured prestress losses at four girder cross-sections, which were compared to two predictive methods provided by AASHTO as well as a method by PCI. An additional comparison of the equations provided by AASHTO and a newly available equation used for determining the modulus of elasticity of concretes with a compressive strength of 6,000 – 12,000 psi was performed. Additional exterior instrumentation were provided by Bridge Diagnostics Inc. (BDI) in order to monitor short-term changes within the bridge. A total of 8 strain gauges were attached to the exterior of the girders with 6 attached at the bottom face of 6 girders and 2 attached at the centroid of 2 girders. These sensors as well as the software and wireless data acquisition provided a method to measure the magnitude and frequency of the ranges of strain experienced by the Nibley Bridge.

Structural Design And Evaluation Of An Adaptive Camber Wing

Sakarya, Evren 01 February 2010 (has links) (PDF)
This study presents a camber morphing concept as an alternative to existing plain flap or aileron type hinged control surfaces used in wings. Structural aspects of the concept are investigated with static nonlinear finite element analyses by using MSC Nastran. In order to assess the aerodynamic characteristics / CFD based 2D solutions are obtained using ANSYS Fluent. The camber morphing concept is applied to the full scale hingeless control surface and implemented in the adaptive camber wing. Hingeless control surfaces and adaptive camber wing are manufactured and changes made in manufacture stages are incorporated into finite element models. Finite element analyses of the wing are conducted with static and dynamic loading and comparison with experimental dynamic analyses are performed.

Multi-objective design optimization for high-lift aircraft configurations supported by surrogate modeling

Li, Daxin 12 1900 (has links)
Nowadays, the competition among airlines seriously depend upon the saving operating costs, with the premise that not to degrade its services quality. Especially in the face of increasingly scarce oil resources, reducing fleets operational fuel consumption, is an important means to improve profits. Aircraft fuel economy is determined by operational management strategies and application technologies. The application of technologies mainly refers to airplane’s engine performance, Weight efficiency and aerodynamic characteristics. A market competitive aircraft should thoroughly consider to all of these aspects. Transport aircraft aerodynamic performance mainly is determined by wing’s properties. Wings that are optimized for efficient flight in cruise conditions need to be fitted with powerful high-lift devices to meet lift requirements for safe takeoff and landing. These high-lift devices have a significant impact on the total airplane performance. The aerodynamic characteristics of the wing airfoil will have a direct impact on the aerodynamic characteristics of the wing, and the wing’s effective cruise hand high-lift configuration design has a significant impact on the performance of transport aircraft. Therefore, optimizing the design is a necessary airfoil design process. Nowadays engineering analysis relies heavily on computer-based solution algorithms to investigate the performance of an engineering system. Computational fluid dynamics (CFD) is one of the computer-based solution methods which are more widely employed in aerospace engineering. The computational power and time required to carry out the analysis increases as the fidelity of the analysis increases. Aerodynamic shape optimization has become a vital part of aircraft design in the recent years. Since the aerodynamic shape optimization (ASO) process with CFD solution algorithms requires a huge amount of computational power, there is always some reluctance among the aircraft researchers in employing the ASO approach at the initial stages of the aircraft design. In order to alleviate this problem, statistical approximation models are constructed for actual CFD algorithms. The fidelity of these approximation models are merely based on the fidelity of data used to construct these models. Hence it becomes indispensable to spend more computational power in order to convene more data which are further used for constructing the approximation models. The goal of this thesis is to present a design approach for assumed wing airfoils; it includes the design process, multi-objective design optimization based on surrogate modelling. The optimization design stared from a transonic single-element single-objective optimization design, and then high-lift configurations were two low-speed conditions of multi-objective optimization design, on this basis, further completed a variable camber airfoil at low speed to high-lift configuration to improve aerodynamic performance. Through this study, prove a surrogate based model could be used in the wing airfoil optimization design.

Analysis of High Angle of Attack Maneuvers to Enhance Understanding of the Aerodynamics of Perching

Lego, Zachary Michael January 2012 (has links)
No description available.

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