The k-distribution method for treating the spectral properties of and absorbing-emitting medium represents an alternative to line-by-line calculations which reduces the number of evaluations of the radiative transport equation from the order of a million to the order of ten without any significant loss of accuracy. For problems where an appropriate reference temperature can be defined, the k-distribution method is formally exact and consists only of a change of variables in the spectral domain. However, when no appropriate reference temperature can be defined such as for strongly non-isothermal media, the method results in errors. These errors are difficult to quantify. There have been several attempts to implement corrections to the k-distribution method to extend its application to inhomogeneous media by modeling the effects of temperature, pressure, and concentration gradient. The Multi-Source Full Spectrum K-Distribution Method (MSFSK) introduced here extends the k-distribution method to non-isothermal media without variations in pressure or concentration. The MSFSK method manages to attain this goal by applying the superposition principle to the original RTE before applying the k-distribution transformation to decompose the problem into a set of sub-problems each of which is able to be solved effectively via the ordinary or modified full spectrum k-distribution method. The concept behind this new Multi-Source Full Spectrum K-Distribution Method is to break up the problem domain into isothermal or nearly isothermal emission zones. For each zone, the heat flux and flux divergence are calculated considering only emission from that zone. The RTE is solved using the full spectrum k-distribution method. The k-distribution for each gas volume is generated using the temperature of the current emission zone as the reference temperature. This process is repeated for each emission zone and the heat flux and flux divergence are summed. This method is applied to a variety of one dimensional slab geometry problems are results are presented. It is shown that the MSFSK method provides very accurate results for the radiative heat flux and flux divergence in these geometries. The effect of different quadrature schemes for performing the spectral integration on solution accuracy. / text

Analysis was performed to quantify and compare the reliability of several different notional shipboard DC distribution system topologies in serving their equipment loads. Further, the relationship between the relative placement of loads and generators within a distribution system and the system’s reliability was investigated, resulting in an algorithmically-derived optimal placement configuration in the system topology found to be the most reliable in the initial analysis. Using Markov models and fault-tree analysis, system reliability indices were derived from distribution system component reliability indices, and these values were compared between competing topologies and equipment configurations. A distribution system based on the breaker-and-a-half topology often used in terrestrial utility substations was found to be superior in terms of reliability to the currently-standard ring bus topology. Expected rates of service interruptions to equipment systems served by the breaker-and-a-half system were reduced overall, in some cases dropping dramatically to less than one expected interruption per 10,000 years. This improvement, however, came at the expense of requiring more circuit breakers in the distribution system’s construction. Within this breaker-and-a-half distribution system, an optimal placement of loads and generators was algorithmically derived, which further improved the reliability of the system. This improvement over the base case was marginal, but the optimized placement configuration was able to reduce the expected interruption rate of the ship’s radar system by over 40%. / text

published_or_final_version / Applied Statistics / Master / Master of Social Sciences

published_or_final_version / abstract / toc / Statistics and Actuarial Science / Master / Master of Philosophy

Αρκετές φορές σε προβλήματα αξιοπιστίας, το ενδιαφέρον μας εστιάζεται στην ουρά κατανομής του χρόνου ζωής. Συνήθως οι ουρές κατανομών ακόμη και των πλέον εύχρηστων κατανομών δεν υπολογίζονται με αναλυτικό τρόπο για αυτό χρησιμοποιούνται φράγματα. Υπολογιστικά εύχρηστα φράγματα είναι τα τύπου Lundberg. Απαραίτητο εργαλείο για τον προσδιορισμό φραγμάτων είναι ο ρυθμός ακαριαίου θανάτου. / -

Silver was evaluated as disinfectant to maintain water quality in water distribution system. It was used to inhibit growth of two opportunistic bacteria in planktonik form and in biofilm formation in Robbins devices with stainless steel and PVC surfaces. The results of this work show that silver is a potential secondary disinfectant to be used in water distribution systems

Mathematical models of water distribution systems (WDS) serve as tools to represent the real systems for many different purposes. Calibration is the process of fine tuning the model parameters so that the real system is well-represented. In practice, calibration is performed considering all information is deterministic. Recent researches have incorporated uncertainties caused by field measurements into the calibration process. Parameter (D-optimality) and predictive (I-optimality) uncertainties have been used as indicators of how well a system is calibrated.This study focuses on a methodology that extends previous work by considering the impact of uncertainty on decisions that are made using the model. A new sampling strategy that would take into account the accuracy needed for different model objectives is proposed.The methodology uses an optimization routine that minimizes square differences between the observed and model calculated head values by adjusting the model parameters. Given uncertainty in measurements, the parameters from this nonlinear regression are imprecise and the model parameter uncertainties are computed using a first order second moment (FOSM) analysis. Parameter uncertainties are then propagated to model prediction uncertainties through a second FOSM analysis. Finally, the prediction uncertainty relationships are embedded in optimization problems to assess the effect of the uncertainties on model-based decisions. Additional data is collected provided that the monetary benefits of reducing uncertainties can be addressed.The proposed procedure is first applied on a small hypothetical network for a system expansion design problem using a steady state model. It is hypothesized that the model accuracy and data required calibrating WDS models with different objectives would require different amount of data. A real-scale network for design and operation problems is studied using the same methodology for comparison. The effect of a common practice, grouping pipes in the system, is also examined in both studies.Results suggest that the cost reductions are related to the convergence of the mean parameter estimates and the reduction of parameter variances. The impact of each factor changes during the calibration process as the parameters become more precise and the design is modified. Identification of the cause of cost changes, however, is not always obvious.

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