Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
Browse
13 results
Search Results
Item Experimental Investigation of the Mechanical Properties and Auxetic Behavior of Iron-Gallium Alloys(2009) Schurter, Holly Marie; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Iron-gallium alloys (known as Galfenol) are a unique material that have shown great potential for numerous applications. They exhibit a strong magneto-mechanical coupling, otherwise known as magnetostriction, which lends itself very well to transducer applications, from the nano-scale to macro scale. In addition, Galfenol is one of only a few metal alloys known to exhibit large auxetic or negative Poisson's ratio behavior. In order to develop any Galfenol-based applications, it will be necessary to understand its mechanical behavior. The goal of the research presented in this thesis therefore is to measure the elastic properties of Galfenol for a range of compositions in order to create a database, as well as present trends in the elastic properties. This is achieved through tensile testing of single-crystal Galfenol dogbone-shaped specimens and through resonant ultrasound spectroscopy (RUS) of small parallelepiped samples.Item QUASI-STATIC CHARACTERIZATION AND MODELING OF THE BENDING BEHAVIOR OF SINGLE CRYSTAL GALFENOL FOR MAGNETOSTRICTIVE SENSORS AND ACTUATORS(2009) DATTA, SUPRATIK; FLATAU, ALISON B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Iron-gallium alloys (Galfenol) are structural magnetostrictive materials that exhibit high free-strain at low magnetic fields, high stress-sensitivity and useful thermo-mechanical properties. Galfenol, like smart materials in general, is attractive for use as a dynamic actuator and/or sensor material and can hence find use in active shape and vibration control, real-time structural health monitoring and energy harvesting applications. Galfenol possesses significantly higher yield strength and greater ductility than most smart materials, which are generally limited to use under compressive loads. The unique structural attributes of Galfenol introduce opportunities for use of a smart material in applications that involve tension, bending, shear or torsion. A principal motivation for the research presented in this dissertation is that bending and shear loads lead to development of non-uniform stress and magnetic fields in Galfenol which introduce significantly more complexity to the considerations to be modeled, compared to modeling of purely axial loads. This dissertation investigates the magnetostrictive response of Galfenol under different stress and magnetic field conditions which is essential for understanding and modeling Galfenol's behavior under bending, shear or torsion. Experimental data are used to calculate actuator and sensor figures of merit which can aid in design of adaptive structures. The research focuses on the bending behavior of Galfenol alloys as well as of laminated composites having Galfenol attached to other structural materials. A four-point bending test under magnetic field is designed, built and conducted on a Galfenol beam to understand its performance as a bending sensor. An extensive experimental study is conducted on Galfenol-Aluminum laminated composites to evaluate the effect of magnetic field, bending moment and Galfenol-Aluminum thickness ratio on actuation and sensing performance. A generalized recursive algorithm is presented for non-linear modeling of smart structures. This approach is used to develop a magnetomechanical plate model (MMPM) for laminated magnetostrictive composites. Both the actuation and sensing behavior of laminated magnetostrictive composites as predicted by the MMPM are compared with results from existing models and also with experimental data obtained from this research. It is shown that the MMPM predictions are able to capture the non-linear magnetomechanical behavior as well as the structural couplings in the composites. Model simulations are used to predict optimal actuator and sensor design criteria. A parameter is introduced to demarcate deformation regimes dominated by extension and bending. The MMPM results offer significant improvement over existing model predictions by better capturing the physics of the magnetomechanical coupled behavior.Item Dynamic Modeling and Position Control of a Piezoelectric Flextensional Actuator(2008) Nickless, Benjamin John; Hubbard, James E; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many smart material actuators suffer in either insufficient force generation or displacement range, two important performance metrics in actuator design. Piezoelectric flextensional actuators were conceived to bridge the gap between displacement and force, offering acceptable performance in both categories. Their displacement range and load carrying capability make them suitable for many applications requiring micrometer-scale displacements. Typical applications require closed-loop control algorithms to achieve good resolution at these displacement levels. In this thesis, an open-loop model of a commercially available, piezoelectric flextensional actuator and drive system was designed. This model was used to design a feedback control system for reference tracking applications. The control system was built and verified with the physical actuator. Its performance was shown to agree with the model simulations. Both the model and the physical system had negligible overshoot, settling times of less than 30 milliseconds, and zero steady-state error in response to step inputs.Item The Role of Density Gradient in Liquid Rocket Engine Combustion Instability(2008-12-01) Ghosh, Amardip; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experimental and analytical studies were conducted to investigate key physical mechanisms responsible for flame-acoustic coupling during the onset of acoustically driven combustion instabilities in liquid rocket engines (LREs). Controlled experiments were conducted in which a turbulent hydrogen-oxygen (GH2-GO2) diffusion flame, established downstream of a two-dimensional model shear coaxial injector was acoustically forced by a compression driver unit mounted in a transverse direction and excited through a broad range of frequencies (200Hz-2000Hz) and amplitudes. Characteristic interactions between flame and acoustics visualized through OH* and CH* chemiluminescence imaging and dynamic pressure measurements obtained using high frequency dynamic pressure transducers indicated that small acoustic disturbances could be amplified by flame-acoustic coupling under certain conditions leading to substantial modulation in spatial heat release fluctuations. Density gradient between fuel and oxidizer was found to significantly affect the way acoustic waves interacted with density stratified flame fronts. The particular case of an asymmetric flame front oscillation under transverse acoustic forcing indicated that baroclinic vorticity, generated by the interactions between misaligned pressure gradient (across the acoustic wave) and density gradient (across the fuel oxidizer interface) could further amplify flame front distortions. Asymmetric interaction between flame and acoustics is shown to occur preferentially on flame fronts where controlled waves from the compression driver travel from lighter fluid to denser fluid and the amount of interaction between flame and acoustics is shown to depend strongly on the density ratio between the fluids on either sides of the flame front. This observation is in agreement with the baroclinic vorticity mechanism and a variant of the classical Rayleigh-Taylor instability mechanism. The results provide the first known experimental evidence that baroclinic vorticity could play a role in triggering flame-acoustic interactions associated with LRE shear coaxial injectors. Parametric studies investigating the sensitivity of flame-acoustic interaction on key physical parameters that govern shear coaxial injector operations (including density ratio, velocity ratio, momentum ratio and chemical composition of the fuel) were conducted by varying the parameter of interest independently while holding the other parameters relatively constant. Density ratios ranging from 1 to 16, velocity ratios ranging from 3.02 to 5.27, momentum ratios ranging from 0.67 to 2.12 and fuel mixtures ranging from pure hydrogen to 10%-90% GH2-GCH4 combination were tested. It is shown that in the ranges considered, flame-acoustic interaction is most sensitively affected by density ratio changes. Spectral measurements of flame front oscillations using local chemiluminescence measurements further revealed the non-linear nature of the interaction process : a flame system forced at 1150 Hz gave rise not only to 1150 Hz oscillations but also triggered flame oscillations occurring at substantially lower frequencies. Analytical models were developed to interpret and predict acoustic modes of a combustion chamber containing a density stratified flowfield subjected to transverse acoustic disturbances. Incorporating both the known phenomenon of jet mixing length and the new experimental result of preferential excitation, the models allow different resonant behaviors to occur for separate regions of the combustor bounded by sudden changes in density. For isothermal experiments where the flow temperature was known, calculated Eigen frequencies were in good agreement with measured frequencies. Overall, the identification of fuel-oxidizer density ratio as a critical parameter and the identification of baroclinic vorticity as a potential mechanism in flame acoustic coupling are significant because a reduction in the density gradient between fuel and oxidizer can be used as a control mechanism to improve flame stability in liquid rocket engines.Item Semi-Active Magnetorheological Seat Suspensions for Enhanced Crashworthiness and Vibration Isolation of Rotorcraft Seats(2007-09-27) Hiemenz, Gregory J; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research focuses on the use of magnetorheological (MR) dampers for enhanced occupant protection during harsh vertical landings as well as isolation of the occupant from cockpit vibrations. The capabilities of the current state-of-the-art in helicopter crew seat energy absorption systems are highly limited because they cannot be optimally adapted to each individual crash scenario (i.e. variations in both occupant weight and crash load level). They also present an unnecessarily high risk of injury by not minimizing the load transmitted to the occupant during a crash. Additionally, current rotorcraft seats provide no means of isolating the occupant from harmful cockpit vibrations. The objective of this research was to investigate and demonstrate the feasibility and benefits of an MR-based suspension for rotorcraft seats. As such, this research began with an in-depth investigation into design feasibility. Three MR seat suspension design cases are investigated: 1) for only vibration isolation, 2) for adaptive occupant protection, and 3) for combined adaptive occupant protection and vibration isolation. It is shown that MR-based suspensions are feasible for each of these cases and the performance benefits and tradeoffs are discussed for each case. Next, to further illustrate the occupant protection benefits gained with an MR-based suspension, three control strategies were developed and performance metrics were compared. It was shown that MR dampers can be controlled such that they will automatically adapt to the crash load level as well as occupant weight. By using feedback of sensor signals, MR dampers were adjusted to utilize the full stroke capability of the seat suspension regardless crash level and occupant weight. The peak load transmitted to the occupant and the risk of spinal injury, therefore, was always minimized. Because this control significantly reduced or eliminated injury risk during less severe landings, it is a significant advance over the current state-of-the-art rotorcraft seat suspensions which can provide no better than 20% risk of occupant injury. Finally, an MR-based seat suspension designed solely for the purposes of vibration isolation was designed, analyzed, and experimentally demonstrated. MR dampers were integrated into the current crashworthy SH-60 crew seat with minimal weight impact such that the original crashworthy capabilities were maintained. Then, utilizing semi-active control, experimental vibration testing demonstrated that the system reduced vertical cockpit vibrations transmitted to the occupant by 76%. This is a significant advance over current state-of-the-art rotorcraft seats which provide no attenuation of cockpit vibrations.Item Modeling, Simulating, and Controlling the Fluid Dynamics of Electro-Wetting On Dielectric(2007-08-06) Walker, Shawn; Shapiro, Benjamin; Nochetto, Ricardo H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work describes the modeling and simulation of a parallel-plate Electrowetting On Dielectric (EWOD) device that moves fluid droplets through surface tension effects. The fluid dynamics are modeled by Hele-Shaw type equations with a focus on including the relevant boundary phenomena. Specifically, we include contact angle saturation, hysteresis, and contact line pinning into our model. We show that these extra boundary effects are needed to make reasonable predictions of the correct shape and time scale of droplet motion. We compare our simulation to experimental data for five different cases of droplet motion that include splitting and joining of droplets. Without these boundary effects, the simulation predicts droplet motion that is much faster than in experiment (up to 10-20 times faster). We present two different numerical implementations of our model. The first uses a level set method, and the second uses a variational method. The level set method provides a straightforward way of simulating droplet motion with topological changes. However, the variational method was pursued for its robust handling of curvature and mass conservation, in addition to being able to easily include a phenomenological model of contact line pinning using a variational inequality. We are also able to show that the variational form of the time-discrete model satisfies a well-posedness result. Our numerical implementations are fast and are being used to design algorithms for the precise control of micro-droplet motion, mixing, and splitting. We demonstrate micro-fluidic control by developing an algorithm to steer individual particles inside the EWOD system by control of actuators already present in the system. Particles are steered by creating time-varying flow fields that carry the particles along their desired trajectories. Results are demonstrated using the model given above. We show that the current EWOD system at the University of California in Los Angeles (UCLA) contains enough control authority to steer a single particle along arbitrary trajectories and to steer two particles, at once, along simple paths. We also show that particle steering is limited by contact angle saturation and by the small number of actuators available in the EWOD system.Item Performance Measurement and Simulation of a Small Internal Combustion Engine(2007-04-30) Moulton, Nathan Lee; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes performance testing of 3W Modellmotoren's 100i-B2 which is a two-stroke gasoline engine presently being used to power a commercially produced Unmanned Air Vehicle (NAVMAR's Mako). Since the engine was originally manufactured for use in radio controlled model aircraft, the only performance information provided by the manufacturer is its rated power output of 9.3 Hp at 8500 RPM. However, much more detailed information is required for the UAV application in order to select propellers and engine operating points that maximize the range, endurance, and load-carrying capacity. This thesis reports the first detailed characterization of this engine's performance in the open literature that includes measurements of power output, specific fuel consumption, exhaust and cylinder head temperatures, and exhaust gas composition as a function of engine speed. The measurements show that the peak power output is 9.32 Hp at 8500 RPM with a brake specific fuel consumption of 0.797 lb/Hp-hr. The maximum BSFC of 0.668 lb/Hp-hr is achieved during ¼ throttle operation at 6500 RPM with a power output level of 5.08 Hp. Exhaust gas composition measurements indicate that the carburetor controls mixture ratio effectively across the entire operating range of the engine unlike smaller model engines. A preliminary attempt was also made to simulate the engine numerically in order to identify areas where the engine design could be improved. The simulation suggests that while the engine's performance is near optimal, it might be possible to gain additional power by decreasing the exhaust port duration.Item Mechanical Design of a Robotic Arm Exoskeleton for Shoulder Rehabilitation(2006-12-11) Liszka, Michael; Akin, David; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traditional shoulder therapy techniques involve the physical therapist controlling and measuring forces on the patient's arm to work particular muscles. The imprecise nature of this leads to inconsistent exercises and inaccurate measurements of patient progress. Some research has shown that robotic devices can be valuable in a physical therapy setting, but most of these mechanisms do not have enough degrees of freedom in the shoulder joint to be useful in shoulder therapy, nor are they able to apply forces along the arm limbs. Based upon the shortcomings of traditional physical therapy robots and low force exoskeletons designed for virtual reality applications, requirements were generated for a robotic arm exoskeleton designed specifically for rehabilitation. Various kinematic designs were explored and compared until a final design emerged. Options for actuation were discussed, and the selection process for actuator components was detailed. Sensors were addressed in their role in the control and safety architecture. A mechanical analysis was performed on the final design to determine various properties, such as torque output, range of motion, and frequency response. Finally, a list of future work was compiled based on the final design's deficiencies.Item DEVELOPMENT OF AN AERODYNAMIC SYNTHETIC JET ACTUATOR BASED ON A PIEZOCERAMIC BUCKLED BEAM(2006-12-08) Clingman, Dan John; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis documents the development of a synthetic jet actuator powered by an Enhanced Displacement (ED) motor based on piezoceramic bimorph beam operating in a post-buckled state. The motor is modeled using Lagrange's equations to capture the post-buckling beam dynamics and the dynamics of supporting elements of the motor. The fluid section of the actuator, including the air pressure and velocity within chamber, is modeled by a three-state adiabatic flow model. The motor and fluid models are coupled together to form a complete synthetic jet actuator model. An ED motor was fabricated and tested and shows that it produces eight times the energy compared to the same bimorph operated without a buckling load. Motor and model data agree well for both static and dynamic operation. The ED motor was installed in a synthetic jet actuator and demonstrated the ability to produce flows in excess of 15 m/sec with duty cycles varying between 2 Hz and 30 Hz. For these tests the drive signal used was a square wave and jet velocity was only mildly dependent of actuation frequency.Item Development of a Time-Accurate Viscous Lagrangian Vortex Wake Model for Wind Turbine Applications(2006-07-14) Gupta, Sandeep; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A second-order accurate model has been developed and validated for modeling the unsteady aerodynamics of a wind turbine. The free-vortex wake method consists of the Lagrangian description of the rotor flow field and viscous effects were incorporated using a viscous splitting approach. The wake geometry solution was then integrated with the rotor aerodynamics model in a consistent manner. The analysis was then used to predict the performance and airloads on a wind turbine in the upwind configuration under unyawed and yawed flow conditions. The present work has demonstrated the versatility and robustness of the free-vortex wake method for wind turbine applications. The understanding of the accuracy and the stability of the numerical method is very important in developing robust wake methodology. The accuracy of the straight-line segmentation method has been examined for a vortex ring and helical vortex, and it has been shown to be second-order accurate. However, a minimum discretization of ten degrees is shown to be required to obtain second-order accuracy and also keep the maximum error in the induced velocity field less than 10%. Linear and nonlinear numerical stability of various time-marching schemes were also examined, and a two-step backward differencing scheme was chosen. The overall numerical solution was demonstrated to converge with a second-order accuracy. The nonlinear unsteady aerodynamics of the blade section was modeled using the Leishman--Beddoes dynamic stall model modified for wind turbine applications. The numerical simulations captured the dynamics of the unsteady flow over the airfoil surface for both attached and stalled flow conditions. Validation of the numerical predictions of the aerodynamic force coefficients against measurements obtained for the S809 airfoil showed overall good agreement. It has been shown that with a proper representation of the static stall characteristics, this model can be used to predict dynamic stall for airfoil sections typical of those used for wind turbine applications. The unsteady airfoil model coupled with the blade model also adequately represented the three-dimensionality of the unsteady flow field for a parked blade, under both steady and unsteady flow conditions. The wake geometry solution integrated with the blade model was then used to predict the performance and airloads for a wind turbine tested under controlled conditions. It has been shown that it is important to accurately predict the transient wake aerodynamics to obtain accurate estimates of the unsteady airloads and power output. The skewed wake geometry behind an upwind wind turbine was successfully predicted in yawed flow conditions over a range of yaw angles and tip speed ratios. Measurements from the Phase {VI} of the NREL/NASA Ames wind tunnel test were used for validating the predictions of performance and airloads. The variation of the turbine thrust and the aerodynamic power output with wind speed was adequately predicted. Spanwise distributions of the aerodynamic coefficients were represented well, and encouraging agreement was obtained against the measured coefficients. The azimuthal variation of loads showed that the unsteady aerodynamic behavior of the the wind turbine was adequately represented, with some exceptions.