R-3 Repository :: Browsing by Author "Spanos, Pol D"

Browsing by Author "Spanos, Pol D"

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A Resistor Network Model for the Determination of Electrical and Thermal Properties of Nanocomposites
(2017-04-19) Higginson, Clayton; Spanos, Pol D
Superior electrical, thermal, and mechanical properties of carbon nanotubes have made them popular candidates for use as fillers in polymer nanocomposites. This thesis presents a numerical model developed to determine the electrical and heat transport properties of these materials via percolation theory. Realistic nanocomposite representative volume elements are generated in three-dimensional space according to user-defined input parameters. A spanning network algorithm is used to search for connections between nanotubes. Interconnected nanotubes are then converted into equivalent resistor networks. The resistor network is then examined using finite element analysis through Kirchoff’s current law for electrical transport, and Fourier’s law for thermal transport. Monte Carlo simulations eliminate statistical variation at each volume fraction of nanotube filler. Several boundary treatment methods are examined to determine which is the most computationally efficient. The model is validated through comparison to experimental data reported in the literature. The presented model is unique in that it can predict both the electrical and thermal conductivity of carbon nanotube based polymer nanocomposites.
Aluminum Nitride Memristors: The Fabrication and Analysis of a Next Generation Processor
(2024-04-21) Attarwala, Ali; Spanos, Pol D; Ghorbel, Fathi; Tang, Ming
This thesis details the experimental development of memristors with an Aluminum Nitride (AlN) insulative layer that can switch its resistance by adjusting its phase. Upon conducting an exhaustive literature review, the opportunity to study the ferroelectric properties of AlN in a resistive switching setting came about. The experimental study starts by detailing the fabrication of memristors utilizing atomic layer deposition (ALD) and electrode deposition. To study the characteristics of the device, relevant AlN memristor samples underwent a full electrical characterization. While there were some interesting results, the current-voltage (I-V) data did not match the expected behavior and results. To further investigate the data, transmission electron microscopy (TEM) analysis was conducted to look inside the device at the nanometer scale. The TEM data highlights the difficulties of memristor fabrication and processing. The experimental process provided insight into the behavior of the ferroelectric properties of AlN suggesting resistive switching applications. However, further exploration of the fabrication and processing of AlN in memristors is required, before any industrial applications are pursued.
Analysis of Hysteretic Systems: Preisach Formalism and Bouc-Wen Modeling
(2017-04-21) Grimmer, Michael L; Spanos, Pol D
The inherently nonlinear phenomenon of hysteresis is notoriously difficult to model. Of notable interest are the inverse models of hysteresis which identify the parameters of a particular model to closely match experimental data. Two major models of hysteresis are the Preisach and Bouc-Wen models. As researchers typically deal with solely one model for their analyses, this thesis initially develops techniques to convert from the Bouc-Wen model to the Preisach model, using first a least squares fit followed by using artificial neural networks. The parameters of each of the two models are investigated in further detail, with emphasis on how each parameter affects the loop and how to arrive at an adequate initial estimate for the identification problem algorithms. The techniques are then evaluated and compared against several sets of experimental data for hysteresis loops supplied by the Air Force Research Lab. Their optimized solutions are compared to assess the flexibility and viability of each model. Generally, it is found that, while both models are successful, the Preisach model is more flexible in fitting different types of experimental loops. Lastly, both experimental loops and theoretical loops subjected to white noise are identified using Transitional Markov Chain Monte Carlo (TMCMC) algorithms via the Preisach model. These results show promise for the TMCMC method being applied on data, particularly when the loop is induced by white noise.
Autonomous Vision Navigation of Spacecraft in Lunar Orbit
(2016-04-22) Bader, Nolan A.; Spanos, Pol D; Woffinden, Dave C
NASA aims to achieve unprecedented navigational reliability for the first manned lunar mission of the Orion spacecraft in 2023. A technique for accomplishing this is to integrate autonomous feature tracking as an added means of improving position and velocity estimation. In this thesis, a template matching algorithm and optical sensor are tested onboard three simulated lunar trajectories using linear covariance techniques under various conditions. A preliminary characterization of the camera gives insight into its ability to determine azimuth and elevation angles to points on the surface of the Moon. A navigation performance analysis shows that an optical camera sensor can aid in decreasing position and velocity errors, particularly in a loss of communication scenario. Furthermore, it is found that camera quality and computational capability are driving factors affecting the performance of such a system.
Minimization of Roll Firings for Optimal Propellant Maneuvers
(2016-04-21) Leach, Parker C; Spanos, Pol D
Attitude control of the International Space Station (ISS) is critical for operations, impacting power, communications, and thermal systems. The station uses gyroscopes and thrusters for attitude control, and reorientations are normally assisted by thrusters on docked vehicles. When the docked vehicles are unavailable, the reduction in control authority in the roll axis results in frequent jet firings and massive fuel consumption. To improve this situation, new guidance and control schemes are desired that provide control with fewer roll firings. Optimal control software was utilized to solve for potential candidates that satisfied desired conditions with the goal of minimizing total propellant. An ISS simulation too was then used to test these solutions for feasibility. After several problem reformulations, multiple candidate solutions minimizing or completely eliminating roll firings were found. Flight implementation would not only save massive amounts of fuel and thus money, but also reduce ISS wear and tear, thereby extending its lifetime.
Monte Carlo Simulations on Resistive Switching Memristor Modeling
(2020-04-23) Ketron, Tyler Wayne; Spanos, Pol D
Promising attributes in data processing such as faster read/write times, longer retention, and superior scalability have put resistive switching memristors in the spotlight of research. This thesis presents a numerical method to determine the variability in switching that different memristors exhibit. Current-voltage relationships and cyclic voltage sweeps are gathered from specific memristor devices with various geometrical configurations. Using the inverse sampling method and Monte Carlo simulations, the variation in switching characteristics for these memristors are characterized by a phenomenological approach. Further, randomness is introduced to assess the effect of geometric parameters in a probabilistic model and trends within the response. The model is validated by comparison with experimental data reported in the literature. The presented model is effective in capturing the variation in memristor responses. This, has been shown to be an important attribute in neuromorphic computing applications, and can aid experimentalists and manufacturers in refining memristor designs.
Peridynamics and Applications
(2017-09-05) Chen, Jingkai; Spanos, Pol D
Peridynamics is a nonlocal mechanics theory using integro-differential equations without spatial derivatives. Unlike the classical continuum mechanics, peridynamics possesses certain advantages when solving problems involving cracks. In the beginning of the thesis, an analytical solution to the vibration problem via fixed horizon peridynamics is developed, including dynamic responses of a bar and of a beam. The analytical solution to the vibration of the bar is derived through a Taylor series expansion approximation. Numerical examples demonstrate the nonlocal dynamic behavior of the bar and its consistency, in the limit, with local behavior. Further, a new nonlocal beam theory is proposed. The proposed nonlocal beam equation is a generalization of the Euler-Bernoulli beam equation. An analytical solution for the beam deformation is derived. The numerical example of the nonlocal beam deformation shows that the fixed horizon peridynamics has boundary conditions related inaccuracy problems. Thus, a new numerical technique to reduce the discrepancy problem is introduced, which is called: Variable Horizon Peridynamics. This method is quite efficient and it does not require a pseudo-layer to be added outside the physical boundary. Next, an efficient algorithm to model the bit-rock interaction process based on the variable horizon peridynamics is developed. This model iterates adaptively with the propagation of the crack and with the penetration of the drill bit. The crack propagation in the rock is captured in this model. The relationship between the penetration rate and other drilling parameters is investigated. Finally, the Navier-Stokes equation is reformulated in a nonlocal sense via the variable horizon peridynamics. It is shown that the reformulated Navier-Stokes equation satisfies Newton’s second law. When the nonlocal parameter reduces to zero, the reformulated Navier-Stokes equation reduces to the classical Navier-Stokes equation. To elucidate the features of the approach, numerical examples of both local and nonlocal Navier-Stokes equations are used.
Response evaluation of nonlinear dynamic systems endowed with fractional-order derivatives under evolutionary stochastic excitation
(2023-12-01) Zhang, Wei; Spanos, Pol D
Natural hazards and excitations often exhibit stochastic characteristics, such as winds, earthquakes, and ocean waves. These load scenarios deserve extensive attention and investigation that account for the uncertain characteristics; otherwise, it may lead to unpredictable damage. In addition, the viscoelasticity phenomenon is prevalent in a variety of engineering materials. When resisted by dynamic loads, the viscoelastic materials exhibit viscous, smoothly varying, and time-dependent deformation, associated with energy dissipation. It can be essential to understand the viscoelastic behavior and its potential influence on structural response, particularly when structural design and response analysis are considered. In this regard, during recent decades it has been shown that the implementation of fractional derivatives allows a more descent description of the viscoelasticity phenomenon. Therefore, in this thesis, the challenge of viscoelastic oscillators subjected to evolutionary stochastic loads is addressed. More specifically, the fractional-order derivative element is introduced to effectively represent the viscoelastic nature of the materials. Further, several analytical and numerical methods are examined. To start, the statistical linearization method is extended for oscillators with fractional derivative elements, where a quite versatile discretization approach is introduced that makes the proposed method applicable to any kind of nonlinearity. Next, the stochastic averaging method is applied on fractional oscillators reducing the dimensionality of the system, and thus accelerates the following computation. Thirdly, the wavelets-galerkin method is adopted to address the evolutionary response statistics of either linear or nonlinear systems. Note that, by accounting unnoticeable overlapping of the basis wavelets functions, the method predicts accurately responses of relatively flexible and/or lightly damped systems. Results in juxtaposition with the pertinent Monte Carlo simulation data demonstrate the reliability and accuracy of the proposed methods of analysis.
Robust Optimal Guidance for Spacecraft Reorientation Maneuvers
(2015-04-23) Svecz, Andrew John; Meade, Andrew J., Jr.; Spanos, Pol D; Padgett, Jamie E; Bhatt, Sagar A; Alaniz, Abran
Spacecraft can be commanded to perform fuel-optimal attitude maneuvers subject to path constraints and specified boundary conditions using techniques from optimal control theory. This thesis presents solutions to two optimal maneuver guidance problems for increasing the robustness of space station Optimal Propellant Maneuvers (OPMs). An optimal maneuver is generated to avoid excess solar heating for use during high solar beta angle conditions. In cases where beta angle is not a factor, ground-based analysis can be reduced by implementing an on-orbit maneuver correction algorithm based on neighboring optimal control theory. This algorithm can be used to find an approximate optimal solution in the presence of uncertainties in the model or initial conditions for the maneuver, and allows on-orbit generation of maneuvers from a reference trajectory even when mass properties are changed due to the docking of visiting vehicles.
Stochastic Dynamics of Oscillating Water Column for Energy Harvesting
(2021-12-22) Kumar, Amitabh; Spanos, Pol D
Future energy sources are expected to have a substantial contribution from renewable sources. This is expected to equal or surpass liquid fuels as the largest source of energy by 2050 [1] [2]. The ocean accounts for nearly 71% of Mother Earth’s total surface area. In combination with other renewable sources (solar, wind, etc.), harvesting energy from oceans can provide the cleanest form of energy to meet global demand. This thesis focuses on analyzing a special kind of energy harvesting device called: Oscillating Water Column (OWC). OWC works on the principle of air pressure oscillations (compression and suction/decompression) in the vertical OWC chamber induced by an oscillating water column due to the random wave actions on the exposed submerged inlet to the OWC structure. The back-and-forth movement of air (due to compression and suction) turns a turbine that rotates the rotor connected by a gear system to the generator. The overall process eventually generates electricity that can be stored and distributed by a grid system. It is noted that irrespective of the direction of the air, the turbine always turns in the same direction due to the design of the blades. These are termed self-rectifying turbines. In the thesis, first, an exhaustive literature survey of the OWC concepts and applications is done. Next, the equations of motion (EOM) for the OWC are formulated based on the unsteady Bernoulli equation and the wave pressure based on the linear potential wave theory. This yields a set of two coupled nonlinear integro-differential equations that describe the OWC system. This set of nonlinear equations does not lend itself to an exact analytical solution. A thorough stochastic dynamics approach using two independent methods is employed to treat these equations. The first method uses the Monte Carlo technique in the time domain. The latter employs a quite comprehensive statistical linearization technique working in the frequency domain. To ensure that the results from the Monte Carlo assessment are reliable, a comparison of the water column height inside the OWC chamber, obtained from the Monte Carlo assessment, is made with that obtained from a pertinent laboratory test. Next, an equivalent linear system that resembles the nonlinear system in a statistical sense is sought to estimate the response using the statistical linearization technique. This set of equations representing the equivalent linear system is then solved iteratively to estimate the dynamical response statistics of the system. Next, the reliability of the solution procedure is assessed by comparing relevant Monte Carlo data versus results obtained from the statistical linearization technique. To evaluate the performance of the statistical linearization technique, requisite computations are done using various random input wave characteristics relating to the JONSWAP ocean waves spectrum. Several parametric studies are also conducted.

Browsing by Author "Spanos, Pol D"

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