Second Order Statistics and Discrete-Time Detection Modeling for Partially Saturated Processes

The basic problem in underwater detection is formulated under the general assumptions of partially saturated propagation of narrow‐band and acoustic signals. Expressions for the joint probability density function (PDF) of ρ, the short‐time average root‐mean‐square pressure at the receiver are obtained. This joint PDF is a general result reducing to the PDF’s for the fully saturated and the unsaturated cases for limiting values of the appropriate variables. Subsequently, defining detection as occurring whenever ρ exceeds a specified threshold level ρ0 and, using the above results, the upcrossing and downcrossing statistics of the envelope process are studied. Closed form expressions for the probability mass functions (PMF’s) of the interarrival time (time between two successive detections) and holding time (time between an upcrossing and the first subsequent downcrossing) are obtained. Results using our partially saturated detection model reduce, in limiting cases, to results already obtained in the literature for fully saturated and unsaturated propagation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86233/1/Perakis15.pd

Reliability-based structural optimization: A proposed analytical-experimental study

An analytical and experimental study for assessing the potential of reliability-based structural optimization is proposed and described. In the study, competing designs obtained by deterministic and reliability-based optimization are compared. The experimental portion of the study is practical because the structure selected is a modular, actively and passively controlled truss that consists of many identical members, and because the competing designs are compared in terms of their dynamic performance and are not destroyed if failure occurs. The analytical portion of this study is illustrated on a 10-bar truss example. In the illustrative example, it is shown that reliability-based optimization can yield a design that is superior to an alternative design obtained by deterministic optimization. These analytical results provide motivation for the proposed study, which is underway

Closed-form Approximations and Series Representations for Partially Saturated Ocean Acoustic Processes

An approximate, closed‐form expression for the value of the integral encountered in the calculation of the probability density function (PDF) of the envelope of a partially saturated ocean acoustic process is obtained. Furthermore, an expression of this PDF as a series of modified Bessel functions is presented. The results may also be directly applied to the evaluation of the PDF encountered in the structural reliability analysis of rotating machinery components. Numerical applications show that the closed‐form expression is always within 1%2% of the exact result. The required computational effort is substantially lower than that required by direct numerical integration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86232/1/Perakis14.pd

Deep Learning for Energy Time-Series Analysis and Forecasting

Energy time-series analysis describes the process of analyzing past energy observations and possibly external factors so as to predict the future. Different tasks are involved in the general field of energy time-series analysis and forecasting, with electric load demand forecasting, personalized energy consumption forecasting, as well as renewable energy generation forecasting being among the most common ones. Following the exceptional performance of Deep Learning (DL) in a broad area of vision tasks, DL models have successfully been utilized in time-series forecasting tasks. This paper aims to provide insight into various DL methods geared towards improving the performance in energy time-series forecasting tasks, with special emphasis in Greek Energy Market, and equip the reader with the necessary knowledge to apply these methods in practice.Comment: 13 papges, 4 figure

Structural Reliability of a Marine Diesel Engine Propulsion Shafting System (Probabilistic, Analysis).

A method for estimating the reliability of the propulsion shafting system of a diesel ship is presented, involving: (a) probabilistic modeling of the diesel engine and the propeller torsional excitations; (b) determination of the statistical properties of the torsional vibratory stress amplitude at any point on the shaft; (c) analysis of first excursion and fatigue failure modes; and (d) estimation of each shafting segment failure probability. Examples using an existing propulsion shafting system are presented. It is shown that the requirement of a high value for the traditional safety factor does not always ensure that the system is safe. Significant criticals that cannot be identified by the traditional deterministic analysis are revealed when the probabilistic method is applied. Furthermore, the shafting segments most likely to fail are different from the segments with the higher mean stress as determined by the traditional deterministic analysis. The reliability based analysis is more realistic and provides a better measure of structural safety to the designer than the traditional deterministic analysis.Ph.D.Ocean engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160705/1/8520956.pd

Recent Research in Partially Saturated Ocean Acoustic Processes

An approximate, closed-form expression for the value of the integral encountered in the calculation of the probability density function (PDF) of the envelope of a partially saturated ocean acoustic process is obtained. Other possible derivations for specific cases are presented. Furthermore, an expression of this PDF as a series of modified Bessel functions is presented. Numerical applications show that the closed form expression is always within 1-2% of the exact result, and is obtained at a substantially lower computational effort. The results may also be directly applied to the evaluation of the PDF encountered in the structural reliability analysis of marine diesel engine shafting systems.http://deepblue.lib.umich.edu/bitstream/2027.42/91682/1/Publication_No_299.pd

Probabilistic Torsional Vibration Analysis of a Marine Diesel Engine Shafting System: The Level Crossing Problem

A probabilistic approach to the torsional vibration problem of a marine diesel engine shafting system has been developed. In this analysis, the shafting shear stress is found to be a Gaussian, harmonizable cyclostationary process with a harmonic series representation consisting of two complex conjugate components. In this paper, the level crossing problem for this stress process is studied. Two methods for estimating the probability that the stress exceeds a specified threshold at least once over a given time interval are presented. In the first method, a local maximum of the process is approximated by the value of the corresponding envelope at the time of occurrence of this maximum. A Markov-type condition is assumed to hold for the local maxima. The second method assumes that the maximum of the process over a reasonable number of cycles is approximately equal to that of the envelope process. The envelope crossings are assumed to constitute a Poisson process. The two methods are applied to estimate the upcrossing probability in various cases. The results of both approaches are found to be in good agreement with those from Monte Carlo simulation. ©1989 ASMEPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86230/1/Perakis12.pd