Theses and Dissertations - Department of Aerospace Engineering and Mechanics

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Multidisciplinary Design Optimization and Analysis Using a Hybrid Augmented Lagrangian Genetic Algorithm
(University of Alabama Libraries, 2020) Benabbou, Adam; Su, Weihua; University of Alabama Tuscaloosa
A major challenge in the aerospace design process is the balance between simulation fidelity and various types of resource constraints such as financial and computational. To develop new data analytical capabilities and improve the reliability of medium fidelity projects using well selected algorithms and methods, a multidisciplinary simulation and optimization tool is developed using MATLAB. In the process, several key areas of research are identified, investigated and methods of analyses are discussed. Primarily, a novel method for implementing a hybrid augmented Lagrangian genetic algorithm is developed and thoroughly tested using a variety of benchmark problems. The algorithm is then implemented to optimize surrogate models for an MDO case study. This test case is a wing model of size comparable to a Cessna 172, flying at cruising speed. However, the methodology presented in this thesis could be extended beyond subsonic aircraft wing optimization and even outside the realm of aerospace engineering. The study allows for a better understanding of the methods used in multidisciplinary design optimization for conceptual design. The proof-of-concept successfully demonstrates the versatility of the MDO framework and opens the project up to several future avenues of research.
Performance of Shark Skin Inspired Manufactured Models for Separation Control
(University of Alabama Libraries, 2020) Parsons, Jacob Chase; Lang, Amy W.; University of Alabama Tuscaloosa
The skin of fast-swimming sharks has been shown to have mechanisms able to reduce flow separation in both laminar and turbulent flows. This study analyzes arrays of bio-inspired microflaps and scales in a separated region generated by an adverse pressure gradient in a water tunnel environment. In the laminar boundary layer case, the microflap model bristles due to vortex interaction. This bristling controls the separation downstream of the model, reducing overall reversing flow by up to 59%. This investigation finds that the height of the protrusion into the boundary layer is an crucial factor in controlling the reversing flow. For the turbulent boundary layer, arrays of manufactured scales are tested in weak and strong adverse pressure gradients, controlled by a rotating cylinder. It has been found that the scales are ineffective at controlling separation in the weaker adverse pressure gradient case and increase the separation. However, in the stronger adverse pressure gradient conditions, the scales are highly effective at controlling separation, reducing the reversing flow by up to 70%. Additionally, the models are able to reattach the flow in extreme separation conditions.
Characterization and Modeling of Dual Phase Thermoplastic Self Healing System for Fiber Reinforced Thermoset Composite Structures
(University of Alabama Libraries, 2020) Jony, Bodiuzzaman; Roy, Samit; University of Alabama Tuscaloosa
The one characteristic that sets biological systems apart from human-engineered systems is its ability to heal itself repeatedly without any external intervention. In the last few decades, drawing inspiration from nature, there has been a tenacious drive towards the design and development of bio-mimetic multifunctional polymers and polymer matrix composites that possess the capability of repeatable self-healing. In this dissertation, bio-mimetic self-healing methods are explored for recover-ing the mechanical and structural performances of damaged ?ber reinforced thermoset polymer composite using thermoplastic healants. Speci?cally, repeatable Mode-I interlaminar fracture healing capabilities of thermoplastic polycaprolactone (PCL) particles and polyurethane shape memory polymer (SMP) ?brils in a thermoset unidirectional carbon/epoxy composite were investigated. During the bio-mimetic healing process, the polyurethane SMP ?brils were used to close the open crack through a thermally-activated contraction, and then the thermoplastic PCL was heated to heal the damage through melt intercalation into the crack. The chemical and thermal properties of the polymer composite and healants and interactions between the brittle epoxy and ductile healants were investigated. Further, repeatable Mode-II interlaminar shear fracture property recovery of unidirectional carbon/epoxy by a blend of the same biphasic healants was experimentally investigated. The shear crack growth phenomenon and mechanism of fracture toughness recovery were thoroughly investigated and contrasted with Mode-I failure. Finally, the real-time in-situ application of self-healing in ?ber-reinforced composite was accomplished by using a macro ?ber composite (MFC) actuator assisted healing. The parameters for generating stimulus (heat) from MFC without damaging it or the composite were calculated. Relative crack growth stability was also investigated during in-situ healing for virgin and healed cases by using R ? curve and crack growth rate phenomenon. For a comprehensive understanding of the healing mechanism and fracture behavior of the polymer composite, analytical and numerical models were generated using a bilinear cohesive law. The critical fracture parameters obtained from the analytical studies were thoroughly veri?ed with experimental results and ?nite element numerical simulations. It is envisioned that this work will provide a solid foundation for the future development and implementation of self-healing polymer composites in real life structural applications.
Life Estimation of SR-FSW Pin Tools for NASA Application
(University of Alabama Libraries, 2021) Hagan, Zachary Matthew; Barkey, Mark E.; University of Alabama Tuscaloosa
Self-reacting friction stir welding (SR-FSW) is one of the processes used in the fabrication of the liquid oxygen and hydrogen tanks that comprise NASA’s Space Launch System’s (SLS) rocket. The SR-FSW tool assembly both clamps and stirs the material by means of rotation and translation through the work piece being welded. One weld tool component of interest is manufactured from MP159, a cobalt-based alloy with excellent mechanical properties at elevated temperatures. Failure of the welding tool during production would result in downtime and require structural mitigation of the rocket body. The repaired weld produced may then exhibit less than desirable mechanical properties. Given the Certified Weld Procedure (CWP) used during production, the lineal inches of weld in a single production “pass”, and the requirement for frequent tool replacement, a weld pin-tool failure during production would result from low cycle fatigue (LCF) damage accumulation. Unfortunately, there is limited data available in the literature for this material’s fatigue behavior at elevated temperatures. This work has generated a statically significant strain life curve of MP159 at 800Â°F with fully reversed loading at a frequency of f = 1 Hz. Subsequent tests have produced data to support an exploratory strain life curve at a frequency of f = 2.4 Hz. Additionally, a combined kinematic and isotropic hardening constitutive model has been calibrated to the materials elevated temperature cyclic response. The constitutive model is used to determine the history-dependent deformation response of the tool at critical geometric locations as a function of loading conditions, boundary conditions and weld pin tool material evolution. Finally, a weld pin tool failure analysis engine has been implemented in MATLAB. This engine estimates the life of the weld pin tool as a function of accumulated damage resulting from any number of loading scenarios, or combined loading scenarios. Failure of the pin tool is predicted by use of Miner’s rule. The failure algorithm has the ability to combines damage from weld start up, steady state operation, and tool “pull out” as well as any combination of these individual operational components. The analysis engine may be used to determine safe operational conditions for the weld tool given weld procedures, or it may be used to redesign welding pin tool geometry to mitigate failures during production, thereby reducing production cost and schedule as well as ensuring the most structurally sound weld possible.
Exploring the Use of an Echo State Network in Modeling Turbulent Jet Behavior
(University of Alabama Libraries, 2021) Sapkota, Pradeep; Baker, John; University of Alabama Tuscaloosa
This work investigates the use of an Echo State Network (ESN) to predict turbulent jet flow behavior. ESNs are a particular class of Recurrent Neural Networks (RNNs) that have been shown to model transient chaotic systems while avoiding some of the difficulties associated with training other types of recurrent neural networks. It is a large, random, fixed recurrent neural network in which each neuron receives a non-linear input signal, and the weights of the input and hidden neurons are fixed randomly. An extensive literature review is performed regarding the history of turbulent jet modeling and the use of an ESN to model turbulent flow analogs. In an initial investigation, a turbulent free jet issuing from a circular tube into a quiescent medium was modeled using an ESN. ESN training was achieved using a validated LES dataset obtained from commercially available CFD software. A separate LES dataset was used to evaluate how well the ESN predicted flow field behavior. A hyperparameter search was undertaken to enhance the ESN's ability to model the turbulent flow field under consideration. The ESN model proved capable of reproducing instantaneous vortical structures and centerline velocity behavior relative to LES model data and previously published experimental data. In a second investigation, two cases of heated turbulent jets discharging from a nozzle to a cold surrounding were studied using an ESN. LES of the jets were carried out in commercial CFD software, and the data obtained from LES were used for training and testing the ESN. Detailed comparisons of the mean velocity profiles and the mean temperature profiles along the streamwise and radial directions were provided, along with turbulence quantities. ESN showed a good agreement with LES simulation and the experiment data. The coherent structure of the jet was investigated by the visualization of the isosurface of the Q criterion. ESN was shown to be efficient in capturing the vortex rings at the vicinity of the nozzle. The ESN also proved capable of capturing mean turbulent kinetic energy distribution for different temperature gradient values. In the third and final investigation, the model problem was a variable density jet originating from a cylindrical tube that passed through a weakly restricted co-flow of low-speed air streams. ESN training and testing were carried out with the help of a validated LES dataset obtained from commercial CFD software. Compared to LES model data and previously published experimental data, the ESN model was able to reproduce turbulent flow field statistics. The ESN model correctly reproduced the profile shapes of radial shear stresses. The vortical evolution for the Helium jet was studied with the ESN model, and the ESN model captured the vortex rings formed at the jet exit and the large-scale structures downstream of the jet. Based on the study, it was concluded that the ESN model has the potential to model turbulent flow fields effectively.
An Experimental Study of Drag Reduction Due to the Roller Bearing Effect Over Grooved Surfaces Inspired by Butterfly Scales
(University of Alabama Libraries, 2021) Gautam, Sashank; Lang, Amy; University of Alabama Tuscaloosa
Monarch butterfly wings are covered in minuscule scales (approximately 100 Âµm in length) which align together in a pattern that resembles roof shingles. Flight tests performed on live butterflies showed that these scales provide a beneficial aerodynamic function. The scales are angled upwards such that transverse cavities form for a flow passing perpendicular to the rows of scales. Flow visualization of butterfly inspired cavities has shown that the entrapped vortex (or vortices) inside each cavity can act as a fluidic bearing to the outer boundary layer flow resulting in reduced surface or skin friction drag. This study conducted experiments on cavity embedded flat plates, mimicking the butterfly scale geometry, documenting this “roller-bearing effect”. The experiments were performed in a tow-tank facility, and DPIV measurements were used to calculate the surface drag based on the measured boundary layer velocity profiles. Initially, a net skin friction or surface drag reduction for a simple rectangular cavity of aspect ratio (AR) 2 was measured. However, as the cavity geometry was varied to better mimic the butterfly scales, the shape of the vortex and the dividing streamline also changed, which in turn affected the surface drag. This study aimed to determine the cavity geometry that results in the highest drag reduction. Experimental models with cavity geometries of AR 2 and 3 and wall inclination angles of 22Â°, 45Â° and 90Â° were tested, where the slanted models were inspired from observed butterfly scale geometries. Each of these models exhibited net surface drag reduction for lower Red range of 4.5 to 8.5, except for the 22Â° cavity with AR 2 which showed an increase in drag for all the Red cases. The Red is defined based on the cavity depth which is kept constant for all the models. The model with a 45Â° cavity wall inclination and AR 2 had the highest surface drag reduction ranging from 18.63% to 26.33% at lower Red range. Additionally, the critical Red region, beyond which the embedded vortex becomes unstable, and fluid begins to enter and eject out of the cavity, mixing with the outer boundary layer flow and thereby eliminating the drag reduction effect, is documented for various cavity geometries and ranged from Red 90 to 120. As was hypothesized, the increase in Red beyond the critical region resulted in an increase in surface drag.
On the Use of Echo State Networks in Various Configurations to Predict the Dynamics of Adversarial Swarms
(University of Alabama Libraries, 2021) Gupta, Soham; Baker, John; University of Alabama Tuscaloosa
Adversarial (competitive) swarms consist of two or more systems (each system consisting of a collection of individuals, interconnected agents) where the goals of each group are conflicting. This work aims to use an Echo State Network to predict the individual behavior of agents in two adversarial swarms and thereby develop an improved understanding of the dynamics of such systems. The current study was divided into three phases. An agent-based Adversarial swarm model was initially developed comprising of two competing swarms, the Attackers, and the Defenders, respectively. The Defender aimed to protect a point of interest in unbounded 2D Euclidean space called the Goal. In contrast, the Attacker’s main task was to intercept the Goal while continually trying to evade the Defenders, which get attracted to it when they are in a certain vicinity of the Goal. The simulation was considered Semi-Hybrid as agent compromise, and goal compromise criteria were modeled to introduce realism as real-world engineering applications. The final system state was studied for all the varied number of agents making up each swarm. The effectiveness of the Semi-Hybrid approach was validated by using Multiscale Entropy, which revealed a greater degree of randomness for the Defenders than Attackers. In the second investigation, two configurations were used to evaluate the use of Echo State Networks for predicting group dynamics for each swarm. Configuration 1 employed a single ESN, i.e., the patio-temporal data for all agents of an Adversarial Swarm model was used input. In configuration 2, two separate ESNs, in parallel, were used to predict Defender and Attacker swarm dynamics. It was concluded that the parallel ESN configuration was more effective in achieving qualitatively similar predictions of the dynamics for the Adversarial Swarms. In the final investigation, an instance of an ESN in a massively parallel framework was trained on individual spatio-temporal data of every agent. The optimal hyperparameters obtained for every individual agent in the framework showed considerable variance that implied every agent in the Adversarial swarm reacted uniquely when a uniform stimulus was applied and thus reaffirmed the concept of individuality of agents in a swarm.
Application of Numerical Methods to the Hamilton-Jacobi-Isaacs Equation in Various Dynamical Systems
(University of Alabama Libraries, 2021) Ledbetter, William Gordon; Sood, Rohan; University of Alabama Tuscaloosa
The field of differential games has broad applicability to topics of economics, engineering, business, and warfare. Given the increasing levels of autonomy implemented in man-made systems in these fields, competition-based analysis may be the best option for understanding behavioral bounds when such systems interact. Differential games are governed by the Hamilton-Jacobi-Isaacs PDE, and many solution techniques are explored before identifying a gap in the existing literature. This dissertation develops a new approach to analyzing differential games based on a saddle-point solution technique. In a 2D system, the standard algorithmic approach produces both a value function interpolation and an approximate control map. Additionally, analysis of the observation error indicates that future analysis should prefer a problem formulation with relative motion. In the Circular Restricted Three-Body Problem, the same algorithms are applied to a system with real-world implications. The value and control interpolations produce a near-optimal trajectory, but the radial basis function approach suffered from high data density and did not exactly recreate the nominal solution. A perturbation analysis indicated that any mid-flight disturbance to the game state is most likely to benefit the pursuer, extending the works of Isaacs to a new domain. Ultimately, the proposed method is demonstrated to be a valuable tool for future differential games research.
Rapid On-Demand Launch Mission Planning for Timely Delivery of Spacecraft Constellations
(University of Alabama Libraries, 2021) Simpson, Christopher Ross; Branam, Richard D.; O'Neill, Charles R.; University of Alabama Tuscaloosa
On-demand launch from an airborne launch vehicle can provide a nanosatellite overhead in under an hour anywhere in the world. On-demand launch can support capabilities already on-orbit and provide tailored or new capabilities rapidly. A military rapid on-demand launch fulfills operational requirements for on-demand space support and reconstitution. Current capabilities in tracking and denial of space-assets limits the effectiveness of constellations already on-orbit to be operationally responsive in military space. Time-sensitive returns using scheduling algorithms for the timely deployment of a nanosatellite constellation can be achieved. Four case studies show the capability to provide persistent coverage of a moving target to provide terminal guidance to a net-enabled weapon, provide reconstitution of a PNT constellation, view persistence, and on-demand coverage of natural phenomena for disaster response. The three novel contributions of this work are creation of a mission planning system to deliver a constellation on-demand to observe a target from anywhere in the world using tactical airborne aircraft, combining existing vehicle models for aircraft, launch vehicle, satellite, and optimization tools to modify on-demand launch scheduling of a constellation for optimal coverage, and applying previously existing coverage quality measurements to measure replenishment and reconstitution value added by on-demand launch. A mission planning system for delivery of multiple satellites from multiple similar air-launched platforms for constellation installation over any selected point constrained for reaction time with optimized coverage quality is delivered.
Game Theory Applications in Astrodynamics and Space Domain Awareness
(University of Alabama Libraries, 2021) Schoenwetter, Luke; Sood, Rohan; University of Alabama Tuscaloosa
As the number of nations possessing space launch capabilities increases, Earth orbit inherently becomes a competitive environment. Furthermore, each competing agent possesses unique objectives that may or may not align with the objectives of other agents. The competitive dynamics presented by this system are well suited for the application of game theory: the study of rational competitors from a mathematical perspective. The presented work combines the disciplines of game theory, optimal control, and astrodynamics to form generic game solution methods. These solution methods are used to obtain optimal open-loop strategies for an interceptor and an evasive target. A game involving an interceptor, a defender, and a ballistic target is also studied. Parameter space analyses are performed across a wide range of initial conditions to identify and visualize trends in the solution spaces. Additionally, a framework for testing strategies in a closed-loop format is developed to evaluate the consequences of sub-optimal actions. The various trends and characteristics found in the solution spaces are discussed, as is the relevancy of the results to modern space security and contingency planning.
Generalized Targeting Parameters for the Guidance of Aerospace Vehicles
(University of Alabama Libraries, 1968) McCraney, Richard Marvin; University of Alabama Tuscaloosa
This thesis presents the development of the equations necessary to compute the targeting parameters for the iterative guidance mode for all types of space missions. A brief introduction to the iterative guidance mode is presented as well as the derivation and explanation of the necessary equations to compute the targeting parameters. Experimental verification of the equations is also included.
Optimized extensions of arbitrary cubatures
(University of Alabama Libraries, 2021) Fister, Matthew Wood; Mulani, Sameer B.; University of Alabama Tuscaloosa
An optimization of cubature (OOC) method is developed to refine computationally intensive numerical integrations by optimally extending an existing cubature. Typically, to increase the accuracy of numerical integrations using methods which cannot be nested, such as Gaussian quadrature, the integrand is evaluated over a larger, disjoint set of abscissas, without using the previous integrand evaluations. The developed OOC method adds any number of abscissas to the existing quadrature and reevaluates all associated weights. To optimize the abscissas and weights, a global optimizationtechnique is used to minimize the sum of squared numerical integration error for a set of training functions, calculated as a function of the optimized weights and abscissas. The training functions must have a known integral over a hypercube domain. The abscissas are constrained to the domain and weights are constrained to be positive. The optimized weights and abscissas are then used to numerically integrate a function of over the domain. Additionally, a method for the optimization of weights, a variant of the OOC method in which only cubature weights are optimized, is presented and compared to traditional methods such as Gauss quadrature and Bayesian quadrature. The OOC method performs well on multivariate integrands, however, the optimization required by the OOC method may add significant computational expense. Therefore, the OOC method is determined to be best suited for computationally expensive, multivariate integrands for which the number of integrand evaluations is severely limited.
Parametric investigation of aerodynamic interaction between two rotors and a flat plate at low Reynolds number
(University of Alabama Libraries, 2021) Chen, Mingtai; Hubner, James P.; University of Alabama Tuscaloosa
This research studies the rotor wake interaction of small rotors on the surface of a flatplate. The generic model is a simplification of the rotor wake and wing interaction that can exist for small-UAV tiltrotors. Most prior rotor-wing interaction research focuses on large tiltrotor configurations. Compared to these counterparts, the disk loading and Reynolds number in this work are significantly smaller by one to two orders of magnitude. The goal is to better understand the aerodynamic interaction between the rotor thrust and plate download force using experimental, analytical, and numerical tools. Throughout this work, the far-field disk loading and Reynolds number ranged between 25 –45 N/m2 and 57,000 – 80,000, respectively. Rotor thrust, download force on the flat plate, and plate surface pressure were measured in single-rotor/plate interaction and dual-rotor/plate interaction, while only rotor thrust was acquired in dual-rotor interaction. The force measurement results show both rotor thrust and download force increase as rotor-plate distance is reduced and rotor disk coverage is increased in single-rotor/plate interaction and dual-rotor/plate interaction. The increase on rotor thrust is advantageous while download force increase is disadvantageous. The increase on download force is greater than the augmented thrust. In dual-rotor interaction, rotor thrust loss increases as rotor-rotor distance is reduced. Measured pressure contours on the plate surface supports the use of the cylindrical control volume in developing analytical models to estimate download force. Also, pressure contours reveal the nonuniformity of the vertical velocity in the downstream wake, which is an assumption and limitation with momentum theory. Lastly, a smoke visualization technique was used to measure the trajectory of the tip vortex in single-rotor/plate interaction and observe the flow pattern of the upstream wake in dual-rotor interaction. The visualization results were employed in the analytical modeling. The experimental results were compared to numerical simulations using a commercial software, RotCFD package. To facilitate the design on small-UAV tiltrotors, where the wake impinges on the wing,empirical equations derived from ground effect, momentum theory and the acquired experimental data to estimate rotor thrust and download force are developed for single-rotor/plate interaction. Furthermore, two wake models—actuator disk theory and smoke visualization, combined with image vortices and momentum theory—are adopted to estimate rotor thrust and download force in rotor-wing interaction. The analytical models capture the rotor thrust and download force trends in rotor-wing interaction and can be extended to arbitrary rotor disk coverage, rotor height, and other rotor wake models in rotor-wing interaction. The analytical models on rotor thrust are independent from rotor diameter to wing chord ratio, while the empirical equations and the analytical models on download force are dependent on this ratio.
Passive drag sail applications for the accelerated deorbit and targeted reentry of spacecraft
(University of Alabama Libraries, 2021) Sweeten, Andrew Michael; Sood, Rohan; University of Alabama Tuscaloosa
In the relatively short time that space has been an asset to humans, the amount of debris occupying the region has become a noticeable concern. Maintaining the usability of space for future generations requires consideration of novel methods to remove debris and otherwise prevent space from becoming further congested. One such proposed method is aerodynamic drag sails to accelerate the natural deorbit process caused by the high-altitude atmosphere. The method, properly implemented, could cause the spacecraft to reenter the atmosphere and burn up without requiring the planning of additional maneuvers, potentially saving time and money while still meeting international requirements. Analysis of the technique requires solving the expected times in orbit and selecting a sail that optimizes cost relative to the spacecraft's orbital lifetime, initially using CubeSats. Atmospheric drag, however, is only one of many forces that may perturb a spacecraft along its trajectory. Solar radiation pressure is another source of perturbing forces acting on large surfaces in the direction of the Sun. After including these forces in high-fidelity deorbit analysis, one can predict where the spacecraft would likely impact the Earth if components do not burn up in Earth's atmosphere. To prioritize the safety of life and property on the surface, legal requirements dictate the location where the spacecraft may impact the surface. Since a passive drag sail does not have active control authority, the sail's initial deployment timing, orientation, and altitude dictate the final reentry point for a given gravitational and atmospheric drag model. Based on specific initial conditions, it is possible to show that a drag sail is an effective and efficient method of safely deorbiting a spacecraft while optimizing cost and conforming to legal requirements.
Development of multi-fidelity aeroelastic optimization framework for flexible wing conceptual design
(University of Alabama Libraries, 2021) Huang, Yanxin; Su, Weihua; University of Alabama Tuscaloosa
This dissertation introduces a framework to carry out the aeroelastic optimization around the steady-state for flexible fixed wings. Such a framework captures geometrical nonlinearities and aerodynamic unsteadiness, which are the challenges in modern aircraft design. This also takes full advantage of structural geometry by integrating multi-fidelity structures with vortex-lattice method (UVLM), allowing effective and accurate multi-stage aircraft design with a variety of aspect ratios. Functions of structural consistency are used to close the gap between multi-stage designs at different fidelity levels. A model update tool is used to ensure structural consistency and allow communications between stages and fidelity levels. In addition, this framework shows its potential to involve more disciplinaries, such as control and stability, by revealing sensitivities with respect to UVLM vorticities and grids.The lower-fidelity aeroelastic model integrates nonlinear strain-based beam elements with UVLM, while the higher-fidelity model integrates nonlinear displacement-based shell elements with UVLM. The resulting aeroelastic models are coupled with a gradient-based method, focusing on overall performance and detailed analysis, respectively. An artificial neural network (ANN) is established for the model update from the lower- to the higher-fidelity structure. It builds a database and performs a statistical approach to address the parameter inequality. Aerodynamic equations are linearized by performing small perturbations and assuming a frozen aerodynamic geometry. The developed UVLM and aeroelastic models are validated by reference models. Analytical aerodynamic sensitivities are verified by the results of finite-difference. In addition, the feasibility of the model update tool is demonstrated by generating a two-dimensional shell and a three-dimensional wing box from original one-dimensional beams. The feasibility of this framework is demonstrated by optimizing a fixed wing under a large upward bending deflection. The impact of different design variables is also observed and discussed. This research illustrates a comprehensive process for an early design of flexible wings. Although the current framework is limited to the incompressible flow and isotropic shell elements, it has the potential to involve more applications with compressibility corrections of UVLM and composite materials.
Atomistic modeling and structure-property relationship of topologically accurate complex nanotube junction architectures
(University of Alabama Libraries, 2020-08) Nakarmi, Sushan; Barkey, Mark; Unnikrishnan, Vinu; University of Alabama Tuscaloosa
Carbon nanotubes have remarkable material properties and are ideal for different space applications including thermal management devices, light-weight mechanical shock absorbers, and fiber-reinforced composites. Nanotube junctions, which are the interconnections of carbon nanotubes, have properties different from the pristine structures and are promising materials for constructing unit blocks with excellent material properties. However, widespread application of the junctions and nanostructures is limited due to the lack of understanding of their mechanical, thermal, and electronic properties. The overall objective of the current research is to provide a computational methodology to construct atomistic models of nanostructures and study their thermal and mechanical properties under different operating conditions. In the first part of the research, the topologically accurate atomistic models of the junctions are created using a novel CAD-based remeshing and optimization strategies. The most energetically stable configurations are chosen to build 3D architectures, thus, providing an economical way to construct complex and larger dimension nanostructures. The created macro-structures can be used directly in the atomistic simulations to study their structure-property relationships. In this dissertation, the thermal and mechanical characterization of pristine nanotubes and complex nanotube multiterminal junctions have been studied using molecular dynamics (MD) simulation. At the nanoscale, the thermal conductivity of nanotube is found to be dependent on size, strain, temperature, and defects. The effects of each of these parameters on the thermal transport of nanostructures have been determined using MD. This is followed by the comparative study of the phonon density of states and phonon dispersion relations of different configurations. The study provides guidelines for creating nanotube heat transfer devices with desired thermal specifications. In addition to being highly conductive, nanotubes and junctions have very high strength and modulus. Although an extensive amount of research is available with the characterization of the pristine nanotubes, there lacks a proper understanding of the mechanical characteristics of the complex structures (multi-terminal junctions and micro-blocks). With the atomistic models of these structures created, the tensile and compressive strengths of such complex architectures have been presented. These computational models will provide the much needed next step for the realization of nanotube junctions for the industrial applications.
Design for a low-cost k-band communication satellite constellation
(University of Alabama Libraries, 2020-12) Strickland, Peyton Daniel; Olcmen, Semih M.; University of Alabama Tuscaloosa
The feasibility of using a low-cost K-band communication satellite constellation in low-Earth orbit to provide continuous global coverage to ground terminal restricted aerospace vehicles was investigated. A phased array K-band transceiver pointing nadir, steerable ±45° in azimuth and elevation, and laser communication units for satellite-to-satellite cross link capability, were assumed for the payload. The figure of merit was the average percent coverage of the entire surface of the globe and the space surrounding the globe, up to 1000 km, with a goal of achieving 100% coverage, continuously. The results indicate that continuous global coverage is not feasible with a heritage phased array K-band transceiver with a range of 2000 km and 72 satellites; however, a hypothetical phased array K-band transceiver with a range of 2975 km was able to provide continuous global communication. The low-cost goal was not realized. The estimated cost of the constellation with the hypothetical transceiver is $4.861 B due to the large command and data handling and power requirements associated with the K-band transceiver. With the enormous costs associated with this project, despite using commercially available products, further analysis of the proposed satellite constellation is not recommended.
Thrust measurement of the busek bht 20 microhall effect thruster
(University of Alabama Libraries, 2020-12) Burleson, Connor; Branam, Richard; University of Alabama Tuscaloosa
The research presented herein documents the process, data analysis, and results of testing a low power micro-hall effect thruster suitable for various mission objectives such as orbital maneuvers, momentum dumping, and precision pointing for microsatellites. The BHT 20 is a low power micro-hall effect thruster capable of generating thrust at the µN thrust level. The research presents high thrust level measurements between 1242 to 3193 µN for thruster conditions between 14.5 to 35.7 W at 413 to 474 µg/s argon flow rates with errors between 3.46 to 11.6%. Also, the discharge voltage and current operating envelope is characterized at the above conditions. Thruster operation was considered suboptimal due to unoptimized high flow rate to thruster power operating conditions. Overall, the research was considered a success by operating the BHT 20 at higher discharge voltages and obtaining thrust measurements at the respective test conditions. Results and recommendations to improve testing are included.
Experimental investigations of shark skin inspired surfaces in air
(University of Alabama Libraries, 2020-12) Devey, Sean Patrick; Hubner, James P.; Lang, Amy W.; University of Alabama Tuscaloosa
Flow separation is detrimental to the performance of a wide array of engineered systems. It is theorized that the shortfin mako shark has evolved a passive flow separation control mechanism – distinct from that of riblet skin-friction reduction – based on the passive flow-actuation of its microscopic scales in regions of incipient separation. Water tunnel studies have supported this theory, demonstrating flow separation control with samples of mako flank skin over a range of experimental conditions. The current study investigates the potential of this mechanism to be applied to systems in air. Characterization of the response of mako skin samples to airflows reveals that natural skin is unsuited to aerodynamic testing. A fabrication methodology for mechanical shark skin facsimiles, or “microflap arrays,” is developed. Prototype microflap arrays are found to be capable of similar passive actuation responses in air to those of mako skin in water. The aerodynamic performance of a NACA 0012 airfoil covered with a microflap array is evaluated at Re = 160000. Microflaps are found to decrease aerodynamic efficiency over a smooth surface, but are successful at passively responding to local flow separation.
Helicopter rotor smoothing with a continuous trailing - edge flap
(University of Alabama Libraries, 2020) Ding, Lan; Shen, Jinwei; University of Alabama Tuscaloosa
Helicopters are prone to high vibration with its elastic blades whirling in a turbulent aerodynamic environment. The high vibration leads to discomfort of crew and passengers and shortens service life of on-board avionics, structures, and mechanical parts. The primary source of helicopter vibration is the main rotor. Since rotor hub loads are the summation of loads on each blade, the vibratory loads, for a rotor with identical blades, consist of harmonics of integer multiples of the rotational speed. A conventional vibration reduction process (only handles blade-number related harmonics) can be implemented. For a rotor with dissimilar blades, either in terms of inertia or aerodynamics, the vibratory rotor hub loads consist of a full spectrum of harmonics related to the rotor speed. In order to reduce the vibration in such a large range of frequency content, a novel approach named rotor smoothing is needed. This dissertation investigates a helicopter rotor smoothing process with a continuous trailing-edge flap (CTEF). The CTEF is a monolithic active blade control design with no mechanical linkages compared to the conventional discrete trailing-edge flap (DTEF). In this design, micro fiber composite (MFC) layers are embedded inside the airfoil to deform the trailing-edge when voltages are applied to the MFC. The CTEF airfoil sectional analysis and design optimization are iipresented in the first part of this dissertation. Several steps are conducted to perform this sectional analysis. First, a computational fluid dynamics (CFD) analysis is developed using OpenFOAM to study the aerodynamics of the CTEF airfoil. Next, a reduced-order structural analysis is used to predict the deflection of the actuated CTEF airfoil. Then, an aero-structural coupling procedure is developed to calculate the CTEF airfoil deflection under both actuation and aerodynamic loads. Finally, the coupling procedure is validated using the static structural bench test and wind tunnel test data from previous studies. The CTEF airfoil displacements are calculated for three different actuation voltages - 0 and ±750 V at different far-field velocities. Predicted deformation as well as aerodynamic coefficients of the baseline and actuated CTEF airfoil are calculated and compared well with the test data. To obtain the optimal CTEF airfoil layouts to maximize its actuation output, a gradient-based optimization procedure is developed. The MFC ply parameters are set as the optimization process variables with aerodynamic coefficients as the object function. The MFC parameters include bender lengths, ply numbers, and core area materials, which are filled in between upper and lower MFC stacks. The optimization with and without aerodynamic loads is conducted and analyzed. Core area shapes designed with second-order and third-order polynomial curvatures are studied. Once the optimal sectional design is obtained, the variational-asymptotical beam sectional analysis (VABS) is used to calculate the beam sectional properties for applying the CTEF airfoil to helicopter blades. To study a helicopter rotor with the CTEF airfoil on the blade, an aeromechanics (elasticity) analysis is developed in the second part of this dissertation. A 4-bladed baseline helicopter rotor model is developed using a multibody dynamics code Dymore. To validate the Dymore rotor models, flight test data are compared with the Dymore predictions. A CTEF airfoil embedded rotor model is then developed using Dymore. A numerical wind tunnel trim is conducted by prescribing a total lift and zero blade flapping. The variation of vibratory rotor loads with different CTEF inputs are presented, and the control authority of the CTEF airfoil is tested. The application of the CTEF airfoil to the rotor smoothing process is promising. To reduce vibration and smooth rotor in operation, a helicopter rotor smoothing process using the CTEF is developed at the last step. Two dissimilar rotor models with unbalanced inertial force and unbalanced aerodynamics are developed. A dissimilar rotor harmonics analysis is conducted and unbalance harmonics introduced by the dissimilarity are identified. A closed-loop regulator is applied to target the identified unbalance harmonics and conventional vibratory harmonics on both dissimilar rotor models. The smoothing processes are shown to be successful for the complete speed range, and the harmonics are compared before and after the smoothing process. The maximum harmonics reduction of the vertical hub force is 60%. To manage more hub loads harmonics with less flap motion inputs, a higher-harmonic-control (HHC) controller is developed and applied to the rotor smoothing. A rotor with identical blades open-loop flap motion sweep is conducted to validate the HHC controller. A rotor smoothing, targeting the full spectrum harmonics on the unbalanced inertia model, is conducted. The reduction rate reaches 40%.

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