Browsing by Author "Spanos, Pol"

Now showing 1 - 5 of 5
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    A NOVEL METHOD FOR ESTIMATING THE EVOLUTIONARY SPECTRA OF NONSTATIONARY RANDOM PROCESSES
    (2022-12-01) Zhang, Hanshu; Spanos, Pol
    Evolutionary spectra, which capture the frequency profile change in time domain, is the key quantity in vibrations. An approach for estimating the evolutionary power and cross spectrum of nonstationary stochastic processes is developed through the determination of the total energy of lightly damped linear systems. In this regard, a novel approach is discussed for appropriately smoothing the total energy of a linear single degree of freedom (SDOF) system, based on the use of Savitzky-Golay moving average filter. Further, a novel approximated energy function is proposed to fit the numerical total energy of the SDOF system. Then, the evolutionary power and cross spectrum can be generated by the approximated energy function. In this thesis, several numerical applications have been used to test the accuracy and reliability of the proposed energy function. In this regard, uni- and multi-variate nonstationary stochastic processes are simulated through the autoregressive (AR) algorithm and spectral representation method. Then, the evolutionary power and cross spectrum of simulated time series are estimated and compared to the target evolutionary power and cross spectrum to assess the reliability of the proposed method. Furthermore, real earthquake data are used to estimate the corresponding evolutionary spectra by the proposed approach, and accuracy of the results is proved by comparing with the evolutionary spectra obtained by generalized harmonic wavelets (GHWs) transform.
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    Analysis of Memristor Hysteretic Systems: A Hybrid Coupled Model with emphasis on Pinching/Degrading Behavior
    (2023-04-20) Palm, Elijah Okoe; Spanos, Pol
    This thesis proposes a novel model that accurately captures the asymmetric hysteretic behavior of memristive devices exhibiting self-crossing hysteresis loops. The proposed model combines the Bouc-Wen Baber-Noori model with other non-linear elements in the resistive switching process. Existing models for memristive behavior, including physical-based memristive models, phenomenological-based models, and models that employ stochastic techniques, are reviewed. The Bouc-Wen model is significant for predicting the current-voltage outputs within memristors, and a new phenomenological model is presented that incorporates the modified Bouc-Wen-Baber-Noori model, a nonlinear geometric-based equation, and bilinear optimization to predict the response of a memristive system. The potency of the proposed model is demonstrated by a dynamic simulation of a memristor system in relation to an integrated circuit system. The proposed model provides reliable predictions of the hysteretic behavior of memristive devices, enabling their efficient use in future computing applications. The thesis provides insights into the development of accurate and efficient models for the hysteretic behavior of memristive devices, which will facilitate their widespread use in various engineering applications.
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    Probabilistic Seismic Hazard Perspectives on Japan's Nuclear Energy Policy: Implications for Energy Demand and Economic Growth
    (James A. Baker III Institute for Public Policy, 2000) Sofiolea, Eleftheria; Sickles, Robin C.; Spanos, Pol; James A. Baker III Institute for Public Policy
    This report reflects an effort to assess the status of seismic risk implications on the nuclear plants providing energy in Japan. In this regard, existing and projected plants along with their power capacity have been identified and cataloged. Further, historical data of seismic events deemed significant for the functionality and safety of the plants are included in terms of the Richter magnitude. Also, documents describing standard procedures for aseismic design of power plants have been perused. The coping of the Japanese industry with the major seismic event of Kobe (January 17, 1995) has been considered. It is believed that the procedures followed in designing and operating nuclear power plants reflect sound engineering practices. Barring an extraordinary seismic event, it is expected that the nuclear plants based energy supply in Japan can be maintained with manageable disruptions. Nevertheless, it is recommended that more focused studies regarding individual plants, especially the older ones, be undertaken in the future, regarding the probability of ‘incapacitating’ seismic events. In this manner, a reasonable, reliable model can be calibrated providing the expected percentage of nuclear power loss in Japan, in any given time period. In view of Japan’s stated policy of heavy reliance on nuclear energy, it is nonetheless prudent to plan for aseismic events that could significantly reduce its electricity generating capacity. Such a shortfall would have substantial impacts on world energy markets, on Japan’s ability to provide clean energy in line with its commitments in the Kyoto Protocols, and on Japan’s economic growth. Under standard growth scenarios, we estimate that seismic events that prevent planned new capacity from being brought on line would reduce growth in total factor productivity by about ½ percent per year. This would dampen Japanese energy demand to a level of 2400 (1013 Btu) instead of a level of 2488.5 (1013 Btu) that we forecast in 2010. The impact on economic growth is due to the increase in CO2 emissions caused by substitute energy sources, particularly imported oil. Such increases would need to be moderated by modifying the aggregate production process, and such a change has implications for technical and efficiency change and thus for growth in total factor productivity.
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    Stochastic Analysis of Whirl in Drillstrings
    (2020-04-21) Popp, Timothy Michael; Spanos, Pol
    Presented in this dissertation are novel models and techniques to analyze the whirling vibration of drillstrings during oil and gas (O&G) operations. Mathematical models are developed to solve for the dynamic whirl response. For this purpose, the drillstring is modeled by a system of nonlinear ordinary differential equations (ODEs), which are derived using established mechanics theory. The ODEs form a lumped parameter representation accounting for the physical system. Improvements are made to these equations, such as asymmetry, Hertzian friction contact, non-Hertzian contact, geometrical misalignment, and coupling of patterns of vibration. The equations are solved using numerical integration, but analytics technique are also applied when possible. Further, stochastic techniques are introduced to solve for drillstring whirl with random process excitation. In this regard, Auto-Regressive-Moving-Average (ARMA) digital filters enable the synthesis of artificial time-histories that are compatible with a target excitation power spectrum. The target spectrum is obtained from downhole torque-on-bit (TOB) measurements, which are common excitation sources during drilling. Through Monte-Carlo simulation, the response of the dynamical system to artificial TOB time-histories is determined in a statistical sense. Furthermore, the drillstring whirl behavior is elucidated through finite element analysis (FEA) and experimental measurements. A transient FEA model is developed using a commercial software package to represent a three-dimensional (3D) whirling system. An experimental testing system is presented for the purpose of acquiring dynamic data during controlled drillstring whirl events. The FEA model and experimental data are compared against the mathematical whirl models. The efficacy of the models to characterize drillstring whirl is determined from this comparison. From a holistic perspective, the endeavor of this work is to enrich the field of drillstring dynamics by introducing novel models and techniques to better understand drillstring whirl in deterministic and stochastic systems. With the inclusion of modeling and experimental measurement the thesis seeks to strengthen the correlation between theory and reality.
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    Stochastic Dynamics in Rotary and Vibration-assisted Drilling
    (2016-04-20) Marquez, Eleazar; Spanos, Pol
    Understanding the complete spectrum of vibration phenomena remains a theme of chronic effort in rotary drilling and vibration-assisted (VAD) technology due to the stochastic nature of bottom-hole assembly (BHA) dynamics, and the limited number of models involving probabilistic approaches. In particular, lateral vibration represents an aggressive and disruptive type of oscillatory pattern given its high frequency content and ability to induce geometrical variations, centrifugal-induced bowing patterns, and severe bore-hole damage. In this study, three improved mathematical representations are proposed with the intent of pragmatically characterizing the manifestation of phenomenological irregularities induced from bit-rock interference, fluid motion along the annulus, and recent VAD technology. In this manner, elucidating the complex physical attributes of a demanding engineering problem surrounding the national economy is achieved. Parameter identification for each dynamic model implies incorporating a finite element technique, where the flexibility of the drill-string and elastic characteristics of the well-bore are accounted for. To address the nature of the nonlinearity, the method of statistical linearization is incorporated to replace the nonlinear dynamical system with a set of linear equations, and thus establish an exact, analytical form of solution. Further, the stochastic nature of the BHA is addressed by imposing stationary/non-stationary excitations at the drill-bit segment and implementing Monte Carlo simulation to approximate the corresponding spectral density function. For this purpose, colored noise is filtered through an auto-regressive scheme to replicate the performance of a polycrystalline diamond compact (PDC) drill-bit.