In this work, mechanisms governing the generation of unstable vortical disturbances and their spatiotemporal characteristics are examined. Time-harmonic boundary and pressure driven flows are of particular interest. In the inner region near the solid-fluid interface, the vortical components of the particle velocity are taken to behave incompressibly. To accommodate time-dependent channel wall geometries a curvilinear coordinate based pseudo-spectral method is developed. The method allows for the direct numerical solution of the three-dimensional time-dependent Navier-Stokes equation for high streaming Reynolds conditions. The conditions for centrifugal destabilization and transition are examined.

Information search and retrieval from online sources or social forums is often performed with term based boolean queries. Such queries can produce low relevance documents in situations where the user is interested in retrieving information related to a concept, or belonging to a specific domain. In this work an approach for concept-based information retrieval is presented, which exploits word and document distributions derived from topic modeling performed on data from online sources. Documents acquired from the Reddit and Stack Exchange online social forums are used for extracting concepts, and subsequently training and testing a detector that aids in identifying and retrieving documents associated with the concept of interest. The selection of training sets for our concept based detector is aided by pre-partitioning of documents by online users (or crowd) into concept focused sub-forums, such as sub-reddits. Topics derived from a sample of the overall document set are taken to represent concepts. These topics then form the basis for identifying sub-forums that have a strong correspondence with the concept of interest, and documents within are assigned (noisy) binary labels. The applicability of our approach is demonstrated by creating a domain focused detector for Cyber Security content from Reddit data. The cross utility of this detector is demonstrated by successfully retrieving relevant Cyber Security documents from an alternate test online source: Stack Exchange. Document classification results of the proposed approach are compared favorably with classifications performed by human analysts.

The increasing number of persistent attacks on computing systems has inspired considerable research in cyber resilience solutions. Resilient system designers seek objective approaches to aid in the comparison and selection of effective solutions. Decision theoretic techniques such as Markov decision processes can be leveraged for such comparisons and design decisions. Markov decision processes facilitate examination of uncertainty in system dynamics, diversity of responses, and optimization for operational objectives. This paper proposes a system design approach based in decision theory to achieve effective cyber resilience solutions. The prototypical example of a system with network intrusion detection and host reconstitution is used to illustrate this approach and highlight difficulties designers face due to the non-trivial coupling that may arise between response mechanisms.

The control of complex dynamic networks has many applications ranging from the management of electric power grids to the regulation of biological cellular networks. Prior work has focused on understanding the relationship between cascading behavior and network structure, the role of critical nodes, and conditions necessary for sustained cascade propagation. Recent work has begun to examine general approaches for dynamically influencing processes to guide the network or a targeted subnetwork towards a desired state. This paper models the control of an epidemic as a Markov Decision Process and applies a variety of different state-of-the-art planning algorithms to arrive at targeted control strategies. The results reveal the conditions under which certain approaches succeed and fail.

Direct numerical solutions of the three-dimensional time-dependent Navier-Stokes equation are presented for the evolution of three-dimensional finite-amplitude disturbances in the Stokes boundary layer. The basic flow is driven harmonically in time and the disturbances represent a departure from the basic state of the fluid. For fixed value of the fluid viscosity, instability is shown to be a function of the amplitude of oscillation of the basic flow, excitation frequency, and the channel wall geometry. Special attention is given to pressure gradient and surface wave driven flows. Conditions for instability and transition are outlined.

Topic modeling continues to grow as a popular technique for finding hidden patterns, as well as grouping collections of new types of text and non-text data. Recent years have witnessed a growing body of work in developing metrics and techniques for evaluating the quality of topic models and the topics they generate. This is particularly true for text data where significant attention has been given to the semantic interpretability of topics using measures such as coherence. It has been shown however that topic assessments based on coherence metrics do not always align well with human judgment. Other efforts have examined the utility of information-theoretic distance metrics for evaluating topic quality in connection with semantic interpretability. Although there has been progress in evaluating interpretability of topics, the existing intrinsic evaluation metrics do not address some of the other aspects of concern in topic modeling such as: the number of topics to select, the ability to align topics from different models, and assessing the quality of training data. Here we propose an alternative metric for characterizing topic quality that addresses all three aforementioned issues. Our approach is based on clustering topics, and using the silhouette measure, a popular clustering index, for characterizing the quality of topics. We illustrate the utility of this approach in addressing the other topic modeling concerns noted above. Since this metric is not focused on interpretability, we believe it can be applied more broadly to text as well as non-text data. In this paper however we focus on the application of this metric to archival and non-archival text data.

This paper investigates the point source response of a layered medium separating two semi-infinite impedance medium. A detailed numerical method to evaluate Laplace transform of the reflection coefficient for each boundary is presented. A description of the branch integral dominantly contributing to the reflection coefficient in Laplace domain for single and multiple reflections is given. The analysis utilizes the image theory to evaluate the order of the reflection and complex pressure amplitude at each image source location. The impulse response is also evaluated numerically from the Green’s function obtained from the reflection coefficient and complex pressure due to image sources. Fresnel’s plane wave reflection coefficient validates the solution when the height of layer containing the source and the observation position is relatively large. The obtained expression for reflection coefficient for large wave number and observation position is verified numerically.

Conclusion and Future Work Network analysis of inter-airport traffic using FAAs traffic flow management data stream Found daily graph clustering properties to differ from previously reported results (due to limits of those data sets) Quantified temporally complex behavior, which contains a significant non-weekly trend Spectral Analysis Dominant eigenvectors are quasi-stationary Low rank spectral models capture bulk of daily network power Preliminary analysis suggests utility of model in forecasting Correlation analysis suggests alternative approach for network decomposition (future work).

In this paper we seek to characterize traffic in the U.S. air transportation system, and to subsequently develop improved models of traffic demand. We model the air traffic within the U.S. national airspace system as dynamic weighted network. We employ techniques advanced by work in complex networks over the past several years in characterizing the structure and dynamics of the U.S. airport network. We show that the airport network is more dynamic over successive days than has been previously reported. The network has some properties that appear stationary over time, while others exhibit a high degree of variation. We characterize the network and its dynamics using structural measures such as degree distributions and clustering coefficients. We employ spectral analysis to show that dominant eigenvectors of the network are nearly stationary with time. We use this observation to suggest how low dimensional models of traffic demand in the airport network can be fashioned.

In this work an exact formulation and solution for the image source amplitude in a three layer medium is given. To do so a novel generalization of the Sommerfeld integral is employed. Of particular interest in this paper is the time-domain solution for the impulse response between source and observation points in the middle layer. It is shown that global boundary conditions can be accommodated for multiple reflections from media of having infinite extent. A branch integral formulation of inverse Laplace transform of integral powers of the Fresnel reflection coefficient is given for this case. It is shown that only odd-powers of the branch point argument contribute thereby reducing computational effort required to numerically evaluate the impulse response.

• First highly integrated and operationally tested system for the tower environment — Surveillance, Flight Data, Weather, Traffic Constraints, Decision Support, Integrated Displays • Success enabled through early and frequent participation of experienced controllers in designing for user acceptance • Work continuing to refine user interfaces and decision support tools

The Tower Flight Data Manager (TFDM) will serve as the next generation air traffic control tower automation platform for surface and local airspace operations. TFDM provides three primary enhancements over current systems: consolidation of diverse data and information sources into a single platform; electronic data exchange, including flight data entries, within and outside the tower cab; and a suite of decision support capabilities leveraging TFDM's access to external data sources and systems. This paper describes a TFDM prototype system that includes integrated surveillance, flight data, and decision support display components. Enhancements in airport configuration management, runway assignment, taxi routing, sequencing and scheduling, and departure route assurance are expected to yield significant benefits in delay reduction, fuel savings, additional capacity, improved access, enhanced safety, and reduced environmental impact. Data are provided on system performance and air traffic controller acceptance from simulation studies and a preliminary field demonstration at Dallas / Ft. Worth International Airport.

The FAA, carrier airlines and passengers are all familiar with the inefficiencies and costs that result from delays in the air transportation system. A significant portion of the delays are associated with operations on or near the surface of major airports. There continues to be keen interest in improving efficiency of surface operations in order to reduce delay costs. The projected growth of air traffic demand is further fueling this interest. The Tower Flight Data Manager (TFDM) initiative by the FAA is aimed at providing tools in the air traffic control tower that would aid in improving operational efficiency on the surface. This initiative seeks to provide benefits through the consolidation of legacy automation systems/displays, the coherent fusion of information from multiple external systems, and the prediction and planning of flight operations on the surface. The ability to predict and plan operations on the surface is delivered to air traffic controllers through a set of decision support tools. These tools provide decision support in the following functional areas: airport configuration, runway assignment, taxi routing, sequencing & scheduling, departure metering, and departure routing. This paper is focused on describing an implementation of these tools in a prototype of the TFDM system. This prototype system is installed at Dallas/Fort Worth airport and is undergoing operational testing. The paper provides selected results from the application of the decision support tools, as well as discussion of future enhancements to the tools.

The Transformational Satellite Communications System (TSAT) is poised to become the DoD's next generation protected and processed satellite system. This paper describes the network architecture of the TSAT system. The presence of a high capacity IP router on the satellite payload is a key distinguishing feature of the TSAT system. The TSAT payload also provides RF and optical circuit switching capabilities. We describe the physical elements which comprise the TSAT network, and their interconnecting TSAT mechanisms that support fixed and mobile terminals, and the approach TSAT uses to support time-varying link conditions. We discuss key network protocols, communications standards and technologies employed within the TSAT system, with particular focus on routing, quality of service, network services, and network security. This paper also presents the TSAT approach towards supporting circuit services, as well as providing security at physical and link layer.

The transformational satellite communications system (TSAT) is the DoDpsilas next generation geosynchronous satellite communications constellation. The TSAT system features packet and circuit interfaces that support a wide variety of military missions by providing communication services to fixed ground, mobile ground, marine, and airborne terminals. These terminals access a variety of link rates and communications services and allow the TSAT system to interoperate with many different peer networks. The TSAT system features several types of coverage, steerable beams, and adaptive waveforms that can be configured dynamically in real-time. The TSAT system responds to user requests for services ranging from long-term fixed service, to on-demand short-term communications, to critical services under emergency conditions. The TSAT system must also provide network security services at physical, link, and network layers. Finally, the TSAT system must provide highly available services and be able to operate autonomously. In order to achieve these goals of providing a wide range of services in an automated and responsive manner, TSAT employs a sophisticated TSAT mission operations system (TMOS). TMOS provides highly capable, agile and adaptive network planning and management services. This paper discusses the TMOS approach for planning and network management. The first goal of the paper is to explain the approach for reliable configuration of the satellite and terminal network elements. The second goal is to explain the approach for fault and performance management in the TSAT network.

The effort described here is a result of the joint venture between MILSATCOM Systems Wing (MCSW) and the Defense Information Systems Agency (DISA) to demonstrate end to end integration between Transformational Satellite (TSAT) and Defense Information Systems Network(DISN). This paper represents analysis preliminary to the development and execution of technical assessments related to TSAT/DISN peering. The TSAT Global Information Grid (GIG) Border Element (TGBE) is the main gateway between the TSAT and DISN for both circuit and packet traffic. The TSAT network provides satellite communications as part of a global, integrated communications network that enables communications for mobile terrestrial or airborne warfighters. The DISN provides voice, video, transport and IP services with a high-capacity terrestrial network consisting ofrouters and optical switches. The paper will first describe relevant aspects of the TSAT and DISN Architectures in Sections II and III respectively. Then in Section IV, we will describe the TGBE and its role in interconnecting the TSAT Network and the DISN Transport and IP Networks. The paper focuses on establishing a peering relationship between the TGBE and corresponding DISN elements. Key aspects of this peering point will be discussed covering areas such as circuit services, SONET synchronization, routing, QoS, traffic engineering, service level agreements, information assurance, network timing and application layer adaptation.

In this paper we consider the problem of autoconfiguring nodes in an airborne wireless network. The airborne platforms consist of routers as well as one or more hosts. External links from the airborne platform employ directional transmit beams. We expect the nodes to join or leave the network without pre-planning or manual configuration. Additionally we expect the users and applications of this network to discover available services in an automated manner. In this paper we present protocols that allow autoconfiguration of IPv6 addresses and names, as well as the automated discovery of services within the airborne wireless network.

As optical and RF links are deployed on airborne platforms, they will be integrated into networks. These networks will have links with metrics such as capacity, delay, and packet loss rates that may exhibit significant variation with time. To route traffic over such infrastructure, a routing system will need to be adaptive and efficient. Service requirements will differ among different classes of traffic. To address these challenge, we integrate new capabilities into the OSPF framework: continuous collection and distribution of multiple time-varying link metrics. We support multiple traffic classes by evaluating multiple cost functions and running multiple path selection procedures at each node.

This paper compares the performance of the TCP and XCP transport protocols over a lossy, large bandwidth-delay link. The link employs block-based error correction codes ECC, along with interleaving which seems to affect both TCP and XCP bandwidth utilization performance. The results are obtained by numerical simulation of TCP and XCP connections over the channel. Single connection results show that the delay introduced by interleaving significantly affects TCP performance, while XCP performance is somewhat better. However, when large interleavers are employed, both protocols suffer. This is likely due to decorrelation of packet errors introduced by interleaving.

This paper investigates the performance of single and multiple TCP connections sharing a bandwidth-delay link. The link employs block-based error correction codes (ECC), along with interleaving which is first shown to affect the performance of a single TCP connection. We then examine impact of using ECC and interleaving on multiple connections on an ideal two-state fading channel. The results are obtained by numerical simulation of multiple TCP connections over the channel. Single TCP connection results show that the delay introduced by interleaving significantly affects TCP performance on a lossy channel. Without ECC, aggregate bandwidth of the channel is better utilized by multiple instances of TCP than by a single instance scenarios. However, this advantage seems to dissipate when significant ECC is introduced.