Yeou-Koung Tung

NETWORK WITH DEGRADABLE LINKS: CAPACITY ANALYSIS AND DESIGN

Abstract This study developed an approach to model network performance when its link capacities are subject to stochastic degradations, as in the form of day-to-day traffic incidents, which cause travel time variability. We postulate that drivers would select routes to lower their travel time variabilities, just as they would to lower their mean travel times. Over time, commuters learn the routes’ travel time variabilities based on past experiences, factor such variabilities into their route choice considerations, and settle into a long-term equilibrium pattern. We characterize this route choice behavior in the face of uncertain travel times with the notion of probabilistic user equilibrium (PUE). This study then defined and formulated PUE with a reliability approach. We developed a nonlinear mathematical program to study the tradeoff between the maximum flow a network can carry and the extent of satisfying the PUE reliability constraints. As an analytical model, this formulation demonstrates certain interesting properties. The formulation can be used to analyze existing networks or to improve them by link capacity modifications. Numerical studies for a 19-link example are provided to show its performance and properties.

Leak detection in complex series pipelines by using the system frequency response method

This research investigates the applicability of the transient-based frequency-response function (FRF) method for detecting leaks in complex series pipelines. The behaviour of transient waves in these pipelines with internal series junctions indicates that junction reflections modify the system resonant frequencies but have a small effect on the leak-induced information contained within the system frequency responses. The analogous method previously developed for single pipelines is extended to complex series pipe systems by using the analytical transfer matrix method herein and the extended method is validated by two simple numerical experimental cases consisting of 3-series pipes and 10-series pipes, respectively. The applied results indicate that the extended FRF method can be applied to detect single and multiple leaks in complex series pipelines as long as the location and size of the resonant peaks of system frequency responses are accurately determined.

Extended Blockage Detection in Pipelines by Using the System Frequency Response Analysis

This paper proposes a technique for the detection of extended blockages in pressurized water pipelines by using the system frequency responses under transient conditions. The impact of extended blockages on the system frequency responses is illustrated by using the analytical transfer matrix method, and the results indicate that extended blockages cause the resonant frequencies of the system to shift, and the blockage locations and sizes can be determined by analyzing the occurrences of the resonant peaks in the frequency axis. The analytical derivations of the blockage effect on the resonant frequencies are validated through different numerical experiments, and the applied results demonstrate that the extended blockages in pipeline can be detected by the proposed method as long as the system resonant frequencies are accurately determined. Practical implementations of the findings are discussed at the end of this paper.

Investigation of polynomial normal transform

Normal transformation is often used in probabilistic analysis especially when multivariate non-normal random variables are involved. A third-order polynomial normal transformation technique is presented in this paper and its characteristics examined. Four methods based on different statistical information of a random variable are used to determine the polynomial coefficients in this normal transformation technique. The performance of these four methods is investigated by comparing with parametric technique using Rosenblatt transformation that preserves the marginal distribution of a non-normal random variable. From the numerical experiment conducted, this simple technique is found to be quite accurate, and it is less restrictive in its usage for merely requiring the information of the first four statistical moments of a random variable rather than requiring a stronger assumption of the full distribution information in the Rosenblatt transformation. The technique is especially attractive when only samples of the random variables are available.

Probabilistic Analysis of Transient Design for Water Supply Systems

Systematic transient analysis in water supply systems is intended to determine the reliability of water supply and the integrity of hydraulic devices when hydraulic transients (i.e., water hammers) occur. In the analysis, parameters such as wavespeed, pipe diameter, friction, and others, usually considered as known inputs in traditional water hammer models, are actually subject to uncertainties due to natural or human behaviors. The paper considers these uncertainties and investigates their effects on the reliability of water transportation systems and water supply networks under transient conditions with or without protection devices. In addition, the optimization of transient protection devices is also evaluated based on the probabilistic analysis. The results show that the probabilistic analysis method could inspect the influence of the different input uncertainties on the system responses under transient conditions and provide detailed information on the design for water supply systems and protection ...

Essential system response information for transient-based leak detection methods

Many transient-based techniques have been developed for leak detection in piping fluid systems, and they are thought to use leak-induced damping and reflection effects to detect and locate the problem. This work investigates the relative importance of the damping and reflective effects from the leak to the leak detection methods with the importance of both effects tested using four representative methods, which vary significantly in their modes of operation and provide the coverage of the major developments in the transient leak detection field. It was found that all transient-based methods are unable to detect and locate the leak in the absence of the leak-reflected signal, whereas the leak is detected and located even in the absence of leak-induced damping. The results indicate that the information previously thought to be contained in the damping of slow transients was found to be low frequency reflections from the leak.

Sensitivity and uncertainty analysis of a sediment transport model: a global approach

Computerized sediment transport models are frequently employed to quantitatively simulate the movement of sediment materials in rivers. In spite of the deterministic nature of the models, the outputs are subject to uncertainty due to the inherent variability of many input parameters in time and in space, along with the lack of complete understanding of the involved processes. The commonly used first-order method for sensitivity and uncertainty analyses is to approximate a model by linear expansion at a selected point. Conclusions from the first-order method could be of limited use if the model responses drastically vary at different points in parameter space. To obtain the global sensitivity and uncertainty features of a sediment transport model over a larger input parameter space, the Latin hypercubic sampling technique along with regression procedures were employed. For the purpose of illustrating the methodologies, the computer model HEC2-SR was selected in this study. Through an example application, the results about the parameters sensitivity and uncertainty of water surface, bed elevation and sediment discharge were discussed.

Regionalization of unit hydrograph parameters .1. Comparison of regression analysis techniques

Hydrologic regionalization is a useful tool that allows for the transfer of hydrological information from gaged sites to ungaged sites. This study developed regional regression equations that relate the two parameters in Nashs IUH model to the basin characteristics for 42 major watersheds in Taiwan. In the process of developing the regional equations, different regression procedures including the conventional univariate regression, multivariate regression, and seemingly unrelated regression were used. Multivariate regression and seeming unrelated regression were applied because there exists a rather strong correlation between the Nashs IUH parameters. Furthermore, a validation study was conducted to examine the predictability of regional equations derived by different regression procedures. The study indicates that hydrologic regionalization involving several dependent variables should consider their correlations in the process of establishing the regional equations. The consideration of such correlation will enhance the predictability of resulting regional equations as compared with the ones from the conventional univariate regression procedure.

Flood Defense Systems Design by Risk-Based Approaches

Abstract Floods are one of the major natural disasters that threaten many human lives and causes significant economic losses globally every year. The design and performance analysis of a flood defense system is a complex decision-making process involving multiple criteria and uncertainties. This paper presents an overview of a risk-based design procedure and offers possible means to extend from its present form of application to multi-criteria decision-making frameworks.

Determination of optimal unit hydrographs by linear programming

A unit hydrograph (UH) obtained from past storms can be used to predict a direct runoff hydrograph (DRH) based on the effective rainfall hyetograph (ERH) of a new storm. The objective functions in commonly used linear programming (LP) formulations for obtaining an optimal UH are (1) minimizing the sum of absolute deviations (MSAD) and (2) minimizing the largest absolute deviation (MLAD). This paper proposes two alternative LP formulations for obtaining an optimal UH, namely, (1) minimizing the weighted sum of absolute deviations (MWSAD) and (2) minimizing the range of deviations (MRNG). In this paper the predicted DRHs as well as the regenerated DRHs by using the UHs obtained from different LP formulations were compared using a statistical cross-validation technique. The golden section search method was used to determine the optimal weights for the model of MWSAD. The numerical results show that the UH by MRNG is better than that by MLAD in regenerating and predicting DRHs. It is also found that the model MWSAD with a properly selected weighing function would produce a UH that is better in predicting the DRHs than the commonly used MSAD.