Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
Browse
4 results
Search Results
Item CHARACTERIZATION AND ANALYSIS OF FLUIDIC ARTIFICIAL MUSCLES(2021) Chambers, Jonathan Michael; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluidic artificial muscles (FAMs) are a form of soft actuator that have been applied to an expanding number of applications, due to their unique characteristics such as low weight, simple construction, inherent compliance, and high specific force and specific work capabilities. With energy sourced from a pressurized fluid, contractile FAMs provide a uniaxial contractile force, while their morphing geometry allows them to contract in length. In a design environment where actuators have tight spatial requirements and must provide precise force and position control, it is becoming more important than ever to have accurate mathematical representations of FAM actuation behavior and geometric characteristics to ensure their successful implementation. However, geometric models and force analyses for FAMs are still relatively crude. Geometric models of FAMs assume a cylindrical geometry, the accuracy of which is suspect because there are no documented methods for effectively measuring FAM shape. Actuation force analyses are also relatively inaccurate unless they are adjusted to fit to experimental response data. Research has continually pursued methods of improving the predictive performance of these analyses by investigating the complex working mechanisms of FAMs. This research improves these analyses by first, making improvements to the experimental characterization of a FAM's actuation response, and then using the more comprehensive data results to test long-held modeling assumptions. A quantitative method of measuring FAM geometry is developed that provides 0.004 in/pixel resolution measurements throughout a characterization test. These measurements are then used to test common assumptions that serve as sources of uncertainty: the cylindrical approximation of FAM geometry, and assumption that the FAM's braid is inelastic. Once these sources of modeling error are removed, the model's performance is then tested for potential improvements. Results from this research showed that the cylindrical approximation of the FAM's geometry resulted in overestimations of the FAM's average diameter by 4.7%, and underestimations of the FAM's force by as much as 37%. The inelastic braid assumption resulted in a maximum 4% underestimation of average diameter and a subsequent 5% overestimation in force, while the use of softer braid materials was found to have the potential for much larger effects (30% underestimation in diameter, 70% overestimation in force). With subsequent adjustments made to the force model, the model was able to achieve a fit with a mean error of only 2.8 lbf (0.3% of maximum force). This research demonstrates improvements to the characterization of a FAM's actuation response, and the use of this new data to improve the fidelity of existing FAM models. The demonstrated characterization methods can be used to clearly define a FAM's geometry to aid in the effective design and implementation of a FAM-actuated mechanism, or to serve as a foundation for further investigation into the working mechanisms and development of FAMs.Item Combustion Instability and Active Control: Alternative Fuels, Augmentors, and Modeling Heat Release(2016) Park, Sammy; 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 explore thermo-acoustic coupling during the onset of combustion instability in various air-breathing combustor configurations. These include a laboratory-scale 200-kW dump combustor and a 100-kW augmentor featuring a v-gutter flame holder. They were used to simulate main combustion chambers and afterburners in aero engines, respectively. The three primary themes of this work includes: 1) modeling heat release fluctuations for stability analysis, 2) conducting active combustion control with alternative fuels, and 3) demonstrating practical active control for augmentor instability suppression. The phenomenon of combustion instabilities remains an unsolved problem in propulsion engines, mainly because of the difficulty in predicting the fluctuating component of heat release without extensive testing. A hybrid model was developed to describe both the temporal and spatial variations in dynamic heat release, using a separation of variables approach that requires only a limited amount of experimental data. The use of sinusoidal basis functions further reduced the amount of data required. When the mean heat release behavior is known, the only experimental data needed for detailed stability analysis is one instantaneous picture of heat release at the peak pressure phase. This model was successfully tested in the dump combustor experiments, reproducing the correct sign of the overall Rayleigh index as well as the remarkably accurate spatial distribution pattern of fluctuating heat release. Active combustion control was explored for fuel-flexible combustor operation using twelve different jet fuels including bio-synthetic and Fischer-Tropsch types. Analysis done using an actuated spray combustion model revealed that the combustion response times of these fuels were similar. Combined with experimental spray characterizations, this suggested that controller performance should remain effective with various alternative fuels. Active control experiments validated this analysis while demonstrating 50-70\% reduction in the peak spectral amplitude. A new model augmentor was built and tested for combustion dynamics using schlieren and chemiluminescence techniques. Novel active control techniques including pulsed air injection were implemented and the results were compared with the pulsed fuel injection approach. The pulsed injection of secondary air worked just as effectively for suppressing the augmentor instability, setting up the possibility of more efficient actuation strategy.Item Modeling of Gas Turbine - Solid Oxide Fuel Cell Systems for Combined Propulsion and Power on Aircraft(2015) Waters, Daniel Francis; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the use of gas turbine (GT) engine integrated solid oxide fuel cells (SOFCs) to reduce fuel burn in aircraft with large electrical loads like sensor-laden unmanned air vehicles (UAVs). The concept offers a number of advantages: the GT absorbs many SOFC balance of plant functions (supplying fuel, air, and heat to the fuel cell) thereby reducing the number of components in the system; the GT supplies fuel and pressurized air that significantly increases SOFC performance; heat and unreacted fuel from the SOFC are recaptured by the GT cycle offsetting system-level losses; good transient response of the GT cycle compensates for poor transient response of the SOFC. The net result is a system that can supply more electrical power more efficiently than comparable engine-generator systems with only modest (<10%) decrease in power density. Thermodynamic models of SOFCs, catalytic partial oxidation (CPOx) reactors, and three GT engine types (turbojet, combined exhaust turbofan, separate exhaust turbofan) are developed that account for equilibrium gas phase and electrochemical reaction, pressure losses, and heat losses in ways that capture `down-the-channel' effects (a level of fidelity necessary for making meaningful performance, mass, and volume estimates). Models are created in a NASA-developed environment called Numerical Propulsion System Simulation (NPSS). A sensitivity analysis identifies important design parameters and translates uncertainties in model parameters into uncertainties in overall performance. GT-SOFC integrations reduce fuel burn 3-4% in 50 kW systems on 35 kN rated engines (all types) with overall uncertainty <1%. Reductions of 15-20% are possible at the 200 kW power level. GT-SOFCs are also able to provide more electric power (factors >3 in some cases) than generator-based systems before encountering turbine inlet temperature limits. Aerodynamic drag effects of engine-airframe integration are by far the most important limiter of the combined propulsion/electrical generation concept. However, up to 100-200 kW can be produced in a bypass ratio = 8, overall pressure ratio = 40 turbofan with little or no drag penalty. This study shows that it is possible to create cooperatively integrated GT-SOFC systems for combined propulsion and power with better overall performance than stand-alone components.Item One-Dimensional Analytical Model Development of a Plasma-Based Actuator(2014) Popkin, Sarah Haack; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation provides a method for modeling the complex, multi-physics, multi-dimensional processes associated with a plasma-based flow control actuator, also known as the SparkJet, by using a one-dimensional analytical model derived from the Euler and thermodynamic equations, under varying assumptions. This model is compared to CFD simulations and experimental data to verify and/or modify the model where simplifying assumptions poorly represent the real actuator. The model was exercised to explore high-frequency actuation and methods of improving actuator performance. Using peak jet momentum as a performance metric, the model shows that a typical SparkJet design (1 mm orifice diameter, 84.8 mm3 cavity volume, and 0.5 J energy input) operated over a range of frequencies from 1 Hz to 10 kHz shows a decrease in peak momentum corresponding to an actuation cutoff frequency of 800 Hz. The model results show that the cutoff frequency is primarily a function of orifice diameter and cavity volume. To further verify model accuracy, experimental testing was performed involving time-dependent, cavity pressure and arc power measurements as a function of orifice diameter, cavity volume, input energy, and electrode gap. The cavity pressure measurements showed that pressure-based efficiency ranges from 20% to 40%. The arc power measurements exposed the deficiency in assuming instantaneous energy deposition and a calorically perfect gas and also showed that arc efficiency was approximately 80%. Additional comparisons between the pressure-based modeling and experimental results show that the model captures the actuator dependence on orifice diameter, cavity volume, and input energy but over-estimates the duration of the jet flow during Stage 2. The likely cause of the disagreement is an inaccurate representation of thermal heat transfer related to convective heat transfer or heat loss to the electrodes.