System importance measures: A new approach to resilient systems-of-systems

Resilience is the ability to withstand and recover rapidly from disruptions. While this attribute has been the focus of research in several fields, in the case of system-of-systems (SoSs), addressing resilience is particularly interesting and challenging. As infrastructure SoSs, such as power, transportation, and communication networks, grow in complexity and interconnectivity, measuring and improving the resilience of these SoSs is vital in terms of safety and providing uninterrupted services. ^ The characteristics of systems-of-systems make analysis and design of resilience challenging. However, these features also offer opportunities to make SoSs resilient using unconventional methods. In this research, we present a new approach to the process of resilience design. The core idea behind the proposed design process is a set of system importance measures (SIMs) that identify systems crucial to overall resilience. Using the results from the SIMs, we determine appropriate strategies from a list of design principles to improve SoS resilience. The main contribution of this research is the development of an aid to design that provides specific guidance on where and how resources need to be targeted. Based on the needs of an SoS, decision-makers can iterate through the design process to identify a set of practical and effective design improvements. ^ We use two case studies to demonstrate how the SIM-based design process can inform decision-making in the context of SoS resilience. The first case study focuses on a naval warfare SoS and describes how the resilience framework can leverage existing simulation models to support end-to-end design. We proceed through stages of the design approach using an agent-based model (ABM) that enables us to demonstrate how simulation tools and analytical models help determine the necessary inputs for the design process and, subsequently, inform decision-making regarding SoS resilience. ^ The second case study considers the urban transportation network in Boston. This case study focuses on interpreting the results of the resilience framework and on describing how they can be used to guide design choices in large infrastructure networks. We use different resilience maps to highlight the range of design-related information that can be obtained from the framework. ^ Specific advantages of the SIM-based resilience design include: (1) incorporates SoS- specific features within existing risk-based design processes - the SIMs determine the relative importance of different systems based on their impacts on SoS-level performance, and suggestions for resilience improvement draw from design options that leverage SoS- specific characteristics, such as the ability to adapt quickly (such as add new systems or re-task existing ones) and to provide partial recovery of performance in the aftermath of a disruption; (2) allows rapid understanding of different areas of concern within the SoS - the visual nature of the resilience map (a key outcome of the SIM analysis) provides a useful way to summarize the current resilience of the SoS as well as point to key systems of concern; and (3) provides a platform for multiple analysts and decision- makers to study, modify, discuss and documentoptions for SoS

Mitigating environmental impacts using aircraft operations: A systematic overview and a focus on end-around taxiways

Concerns about the aviation\u27s environmental impact have prompted research efforts around the world. Much of this research has focused on changes to future aircraft and engine designs. However, it will take a long time for these changes to be developed and propagate through the operational fleet in sufficient numbers to have a significant impact on overall emission levels. Until then, operational mitigations that reduce the environmental impacts of existing aircraft will be the most effective strategies. Part I of this thesis involved the identification and qualitative assessment of operational changes that could mitigate aviation’s environmental footprint in the near-term. This resulted in the development of a comprehensive list of mitigation strategies that indicated environmental mitigation potential of various operational changes. Part II of this research effort focused on a specific mitigation measure, that is, the use of end-around taxiways at high-traffic airports with parallel runways. An explicit evaluation of the environmental implications of these taxiways at a candidate airport, DFW (Dallas/Fort-Worth International Airport) was conducted. Observed usage patterns indicated that the use of the end-around taxiway varied significantly on a daily basis. Since operations on the taxiway did not follow strict procedures, there existed the potential to develop procedures for the effective use of the end-around taxiway, in terms of surface fuel burn reduction. A focused investigation on a short taxi segment in the southeast quadrant at DFW revealed that that, while the end-around taxiway shows significant promise in terms of fuel burn and emissions reduction, these benefits depend on the manner of operation of the taxiway. Factors such as traffic conditions on adjacent runways, traffic flow direction, arrival time of aircraft, and the specific aircraft type play a major role in extracting environmental benefits from end-around taxiways. For example, the results showed that the average fuel burn for aircraft using the end-around taxiway was less than the average fuel burn of aircraft using a conventional taxiway during a select portion of the day, specifically, between 5pm and 1am. Decision rule-based studies showed that always or never using the end-around taxiway did not bring about environmental benefits. Further, implementation of decision rules based on the results of this research showed promise in terms of fuel burn reduction. Hence, better understanding of the factors that affect endaround taxiway operations and the manner in which they should be used could lead to the development of improved taxiway procedures that maximize the environmental benefits of the EAT

Jet Engine Health Signal Denoising Using Optimally Weighted Recursive Median Filters

The removal of noise and outliers from health signals is an important problem in jet engine health monitoring. Typically, health signals are time series of damage indicators, which can be sensor measurements or features derived from such measurements. Sharp or sudden changes in health signals can represent abrupt faults and long term deterioration in the system is typical of gradual faults. Simple linear filters tend to smooth out the sharp trend shifts in jet engine signals and are also not good for outlier removal. We propose new optimally designed nonlinear weighted recursive median filters for noise removal from typical health signals of jet engines. Signals for abrupt and gradual faults and with transient data are considered. Numerical results are obtained for a jet engine and show that preprocessing of health signals using the proposed filter significantly removes Gaussian noise and outliers and could therefore greatly improve the accuracy of diagnostic systems. [DOI: 10.1115/1.3200907]