Collective Choice under Dichotomous Preferences

Agents partition deterministic outcomes into good or bad. A direct revelation mechanism selects a lottery over outcomes - also interpreted as time-shares. Under such dichotomous preferences, the probability that the lottery outcome be a good one is a canonical utility representation. The utilitarian mechanism averages over all deterministic outcomes "approved" by the largest number of agents. It is efficient, strategy-proof and treats equally agents and outcomes. We reach the impossibility frontier if we also place the lower bound 1/n on each agent's utility, where n is the number of agents; or if this lower bound is the fraction of good outcomes to feasible outcomes. We conjecture that no ex-ante efficient and strategy-proof mechanism guarantees a strictly positive utility to all agents at all profiles, and prove a weaker version of this conjecture.

Fair Queuing and Other Probabilistic Allocation Methods

A server processes one job per unit of time and randomly schedules the jobs requested by a given set of users; each user may request a different number of jobs. Fair queuing (Shenker 1989) schedules jobs in successive round-robin fashion, where each agent receives one unit in each round until his demand is met and the ordering is random in each round. Fair queuing *, the reverse scheduling of fair queuing, serves first (with uniform probability) one of the users with the largest remaining demand. We characterize fair queuing * by the combination of lower composition--LC--(the scheduling sequence is history independent), demand monotonicity--DM--(increasing my demand cannot result in increased delay) and two equity axioms, equal treatment ex ante--ETEA (two identical demands give the same probability distribution of service) and equal treatment ex post--ETEP (two identical demands must be served in alternating fashion). The set of dual axioms (in which ETEA and ETEP are unchanged) characterizes fair queuing. We also characterize the rich family of methods satisfying LC, DM, and the familiar consistency--CSY--axiom. They work by fixing a standard of comparison (preordering) between a demand of xi units by agent i and one of xj units by agent j. The first job scheduled is drawn from the agents whose demand has the highest standard.

SAS – SST simulations of the flow and heat transfer inside a square ribbed duct with artificial forcing

Scale Resolving Simulations (SRS) are emerging as a promising compromise of cost and accuracy for industrial simulations of flows inside turbine blade cooling systems as they represent a necessary increase of accuracy with respect to Reynolds Averaged Navier Stokes (RANS) in the field. In this paper, several hybrid RANS-LES (Large Eddy Simulation) and SRS approaches are investigated. A Scale Adaptive Simulation (SAS) with spectrally calibrated artificial forcing is used to simulate flow inside a development section of a square duct with eight square equispaced ribs. Energy spectra, two-point correlations as well as other standard metrics are used to assess resolved content qualitatively as well as quantitatively. It is found that unmodified SST-SAS offers a marginal improvement over Unsteady RANS (URANS) for the present type of flow even on a LES-type grid and the solution is essentially steady. The artificial forcing used seems to trigger the resolving capability of the model and the solution is noticeably closer to experimental results while requiring minor extra computational demand. Effects of rotation are examined and it is found that the rotation appears to trigger the resolving mode of the unforced SAS model

A Formal Architecture-Centric Model-Driven Approach for the Automatic Generation of Grid Applications

This paper discusses the concept of model-driven software engineering applied to the Grid application domain. As an extension to this concept, the approach described here, attempts to combine both formal architecture-centric and model-driven paradigms. It is a commonly recognized statement that Grid systems have seldom been designed using formal techniques although from past experience such techniques have shown advantages. This paper advocates a formal engineering approach to Grid system developments in an effort to contribute to the rigorous development of Grids software architectures. This approach addresses quality of service and cross-platform developments by applying the model-driven paradigm to a formal architecture-centric engineering method. This combination benefits from a formal semantic description power in addition to model-based transformations. The result of such a novel combined concept promotes the re-use of design models and facilitates developments in Grid computing.Comment: 11 pages, 9 figures. Proc of the 8th International Conference on Enterprise Information Systems (ICEIS06) Paphos, Cyprus. May 200

Towards investigation of external oil flow from a journal bearing in an epicyclic gearbox

High loads and bearing life requirements make journal bearings the preferred choice for use in high power, planetary gearboxes in jet engines. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each and generate complex kinematic conditions. This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Preliminary modeling considerations are discussed. The findings and conclusions are used to create a three dimensional (3D), two-component computational fluid dynamic (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a non-orbiting representation, rotating about its own axis, with a circumferentially constant, i.e. concentric, lubricating gap. In order to track the phase interface between the oil and the air, the Volume of Fluid (VoF) method is used. External journal bearing oil flow is simulated with a number of different mesh densities. Two different operating temperatures, representing low and high viscosity oil, are used to assess the effect on the external flow field behaviour. In order to achieve the future objective of creating a design tool for routine use, key areas are identified in which further progress is required