Browsing by Author "Politis, Nikolaos P."
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Item An approach for efficient analysis of drill-string random vibrations(2002) Politis, Nikolaos P.; Spanos, Pol D.Lateral vibrations are widely recognized as the leading cause of drill-string failures in oil well operations. The finite elements method formulation is used for the mathematical modelling. Due to the erratic nature of the forces at the drill-bit, a stochastic dynamics approach is followed. Both the method of statistical linearization, and the Monte Carlo method are used to analyze the system dynamics. Numerical results pertaining to data obtained by measurement while drilling tools are presented. The significance of the present study, over studies available in the literature, hinges upon the computational efficiency of the adopted piecewise linear model of the drill-string---well formation interaction and the derivation of information on the probability density function, and the power spectrum of the erratic drill-string motion. It is expected that this study will enhance the interest in using stochastic dynamics techniques in drilling system analysis and design.Item Wavelets based time-frequency analysis techniques in structural engineering(2005) Politis, Nikolaos P.; Spanos, Pol D.Future design procedures for civil structures, especially those to be protected from extreme loads, will need to account for temporal evolution of their frequency content. Separate time analysis and frequency analysis by themselves do not fully describe the nature of these nonstationary dynamic loads. In the past few years, significant effort has been devoted to wavelets and time-frequency analysis. The appropriateness of emerging joint-time frequency analysis techniques for structural engineering problems is evaluated. Emphasis is focused on adaptive methods and the wavelet transform is also considered for validation purposes. In particular, the adaptive chirplet decomposition method and the empirical mode decomposition method are investigated. The required level of sophistication of the adaptive analysis is assessed. Mathematical expressions pertaining to time-frequency estimators are derived. Specifically, a transition from individual spectrograms to evolutionary power spectrum is attempted and function-specific decompositions are used for the estimation of the mean instantaneous frequency. Further, the Bootstrap method is employed for the assessment of the accuracy of the statistical estimators for the cases where limited data is available. An alternative approach utilizing function-specific decompositions for the derivation of well defined Hilbert spectra is suggested. Data pertaining to simulated and recorded earthquake signals, nonlinear structural responses due to earthquake excitations, and sea level recordings during the 2004 tsunami in Indian Ocean are used. The significance of the present study, over studies available in the literature for wavelets based structural analysis, hinges upon the consideration of adaptive and nonadaptive methods, the computational efficiency and effectiveness assessment of the suggested time-frequency estimators. It is expected that this study will enhance the interest in using advanced signal processing methods in structural systems analysis and design.