Controlled query evaluation with open queries for a decidable relational submodel

Controlled query evaluation for logic-oriented information systems provides a model for the dynamic enforcement of confidentiality policies in scenarios where users are able to reason about a priori knowledge and the answers to previous queries. Previous foundational work assumes that the control mechanism can solve the arising implication problems and deals only with closed queries. In this paper, we overcome these limitations by refining the abstract model for appropriately represented relational databases. We identify a relational submodel where all instances share a fixed infinite Herbrand domain but have finite base relations, and we require finite and domain-independent query results. Then, via suitable syntactic restrictions on the policy and query languages, each occurring implication problem can be equivalently expressed as a universal validity problem within the Bernays-Schönfinkel class, whose (known) decidability in the classical setting is extended to our framework. For refusal and lying, we design and verify evaluation methods for open queries, exploiting controlled query evaluation of appropriate sequences of closed queries, which include answer completeness tests. Additionally, we present alternative evaluation methods that work for lying and the combined approach but at the price of potentially reduced cooperativeness.     

Reliable Routing in Wireless Ad Hoc Networks: The Virtual Routing Protocol

A novel routing protocol for wireless, mobile ad hoc networks is presented. This protocol incorporates features that enhance routing reliability, defined as the ability to provide almost 100% packet delivery rate. The protocol is based on a virtual structure, unrelated to the physical network topology, where mobile nodes are connected by virtual links and are responsible for keeping physical routes to their neighbors in the virtual structure. Routes between pairs of mobiles are set up by using information to translate virtual paths discovered in the virtual structure. Route discovery and maintenance phases of the protocol are based on unicast messages travelling across virtual paths, with sporadic use of flooding protocol. Most flooding is executed in the background using low priority messages. The routing protocol has been evaluated and compared with the Dynamic Source Routing protocol and with the Zone Routing Protocol by means of simulation.

Design of local fuzzy models using evolutionary algorithms

The application of local fuzzy models to determine the remaining life of a unit in a fleet of vehicles is described. Instead of developing individual models based on the track history of each unit or developing a global model based on the collective track history of the fleet, local fuzzy models are used based on clusters of peers—similar units with comparable utilization and performance characteristics. A local fuzzy performance model is created for each cluster of peers. This is combined with an evolutionary framework to maintain the models. A process has been defined to generate a collection of competing models, evaluate their performance in light of the currently available data, refine the best models using evolutionary search, and select the best one after a finite number of iterations. This process is repeated periodically to automatically update and improve the overall model. To illustrate this methodology an asset selection problem has been identified: given a fleet of industrial vehicles (diesel electric locomotives), select the best subset for mission-critical utilization. To this end, the remaining life of each unit in the fleet is predicted. The fleet is then sorted using this prediction and the highest ranked units are selected. A series of experiments using data from locomotive operations was conducted and the results from an initial validation exercise are presented. The approach of constructing local predictive models using fuzzy similarity with neighboring points along appropriate dimensions is not specific to any asset type and may be applied to any problem where the premise of similarity along chosen attribute dimensions implies similarity in predicted future behavior.     

Optical observations of acoustical radiation force effects on individual air bubbles

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 microm to 79 microm was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 microm. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position.     

Effect of Gravity on Film Boiling Heat Transfer and Rewetting Temperature during Quenching

This paper describes the experimental strategy developed to improve the modeling of liquid-vapor flows during the chill down of rocket engines by cryogenic fluid in microgravity. A similarity analysis is performed to determine the relevant dimensionless numbers for the design of an experiment similar to engine flows. A literature review on reduced gravity quenching experiments, and on rewetting temperature and film boiling heat transfer shows the lack of validated models for microgravity. Experimental results obtained with the quenching of a glass tube by FC72 during parabolic flight are presented. Especially the impact of gravity and subcooling on rewetting temperature and film boiling heat transfer is investigated. Results show an increase in rewetting temperature, and a decrease in film boiling heat transfer under reduced gravity in agreement with the literature. The comparison of 0 g flow pattern with corresponding tests on ground points out a behavior at 0 g closest to 1 g upflow than 1 g downflow.