Computational Modelling of Wing Downwash Profile with Reynolds-Averaged and Delayed Detached-Eddy Simulations

This paper describes the computational model to predict downwash for a conventional fixed wing configuration at flight scales (ReMAC = 2.26 × 107 ). The lack of resolution in the downwash wake region resulted in an over-dissipation of the turbulent behaviour of airflow in the wing’s wake. This artificially inflates the effectiveness of the horizontal stabilizer where an over-prediction of pitch stiffness was observed. To resolve this over-dissipation, both the Reynolds-Averaged and Delayed Detached-Eddy Simulation methodology were adopted to accurately capture the downwash profile leaving the wing. Comparisons between the estimation of wall shear stresses and viscous wall unit against a ‘first-cut’ simulation are made and discussed. Fundamental features of the downwash profile including the spatial and temporal scales used for the mesh are also presented and detailed in this paper

PID and PID-like controller design by pole assignment within D-stable regions

This paper presents a new PID and PID-like controller design method that permits the designer to control the desired dynamic performance of a closed-loop system by first specifying a set of desired D-stable regions in the complex plane and then running a numerical optimisation algorithm to find the controller parameters such that all the roots of the closed-loop system are within the specified regions. This method can be used for stable and unstable plants with high order degree, for plants with time delay, for controller with more than three design parameters, and for various controller configurations. It also allows a unified treatment of the controller design for both continuous and discrete systems. Examples and comparative simulation results are pro-vided to illustrate its merit

Cost Efficiency of Defense Procurement: What We Can and Can't Learn from French Lessons

During the late 1980s, in the wake of the end of cold war and shrinking defense budget, the Delegation General for Armaments (DGA), the French government agency that is responsible for the contracting and management of all weapon programs, implemented a wide range of restructuring efforts to reform French defense industry. For instance, one notable change was to migrate from cost-plus contracts to fixed-price contracts to control the then prevalent cost overruns. Today, nearly all French weapon procurement contracts are fixed-price based. Research has found that while in France as elsewhere cost overruns still occur, such problem tends to be relatively modest in scope. Specifically, Kapstein and Oudot (2009) document that French cost overrun is normally within the 5-10 percent range as opposed to an average 26 percent overrun in the U.S. Given the French experience has in general been perceived to be successful (OTA Background Paper 1992, Kapstein 2009), what can the U.S. learn from French lessons? Today the U.S. confronts a very similar and difficult cost overrun problem that led DGA in the late 1980s to the reform of the system. We argue that while U.S. can certainly learn useful lessons from the French experience, significant differences nevertheless exist between the two countries in the context of the political and economical environment. These institutional differences indicate that a “copy and paste” approach will not work in U.S. Rather, an individual based assessment of the French experience would make more sense. We aim to address this issue

Azimuthal asymmetry in transverse energy flow in nuclear collisions at high energies

The azimuthal pattern of transverse energy flow in nuclear collisions at RHIC and LHC energies is considered. We show that the probability distribution of the event-by-event azimuthal disbalance in transverse energy flow is essentially sensitive to the presence of the semihard minijet component.Comment: 6 pages, 2 figure

Impulse-driven surface breakdown data : a Weibull statistical analysis

Surface breakdown of oil-immersed solids chosen to insulate high-voltage, pulsed-power systems is a problem that can lead to catastrophic failure. Statistical analysis of the breakdown voltages, or times, associated with such liquid-solid interfaces can reveal useful information to aid system designers in the selection of solid materials. Described in this paper are the results of a Weibull statistical analysis, applied to both breakdown-voltage data and time-to-breakdown data generated in gaps consisting of five different solid polymers immersed in mineral oil. Values of the location parameter γ provide an estimate of the applied voltage below which breakdown will not occur, and under uniform-field conditions, γ varied from 192 kV (480 kV/cm) for polypropylene to zero for ultra-high molecular weight polyethylene. Longer times to breakdown were measured for UHMWPE when compared with the other materials. However, high values of the shape parameter β reported in the present paper suggest greater sensitivity to an increase in applied voltage – that is, the probability of breakdown increases more sharply with increasing applied voltage for UHMWPE compared to the other materials. Analysing peak-applied-voltage data, only PP consistently reflected a low value of β across the different sets of test conditions. In general, longer mean times to breakdown were found for solid materials of εr more closely matched to that of the surrounding mineral oi

Dependence of energy loss of jets on the initial thermodynamic state of deconfined matter at RHIC

The dependence of the radiative energy loss of fast partons on the initial thermodynamic parameters is studied for deconfined matter to be expected at RHIC. We demonstrate that the specific QCD radiation pattern with a quadratic dependence of the energy loss on the propagated distance leads to a strong increase of the energy loss with increasing initial entropy of deconfined matter supposed its life-time is less than the average time to pass through the medium. This is in contrast to a parameterization with constant energy loss per unit length of propagation. For a sufficiently high initial temperature a two-regime behavior of the energy loss as a function of the initial parton momentum occurs. The angular structure of the energy loss of hard jets with respect to the initial temperature is also discussed for RHIC conditions.Comment: 10 pages with fig

Photon tagged correlations in heavy ion collisions

A detailed study of various two-particle correlation functions involving photons and neutral pions is presented in proton-proton and lead-lead collisions at the LHC energy. The aim is to use these correlation functions to quantify the effect of the medium (in lead-lead collisions) on the jet decay properties. The calculations are carried out at the leading order in QCD but the next-to-leading order corrections are also discussed. The competition between different production mechanisms makes the connection between the jet energy loss spectrum and the gamma-pi correlations somewhat indirect while the gamma-gamma correlations have a clearer relation to the jet fragmentation properties.Comment: 32 pages, 19 figures. Minor changes, published versio

High transverse momentum suppression and surface effects in Cu+Cu and Au+Au collisions within the PQM model

We study parton suppression effects in heavy-ion collisions within the Parton Quenching Model (PQM). After a brief summary of the main features of the model, we present comparisons of calculations for the nuclear modification and the away-side suppression factor to data in Au+Au and Cu+Cu collisions at 200 GeV. We discuss properties of light hadron probes and their sensitivity to the medium density within the PQM Monte Carlo framework.Comment: Comments: 6 pages, 8 figures. To appear in the proceedings of Hot Quarks 2006: Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions, Villasimius, Italy, 15-20 May 200

Limiting absorption principle for the dissipative Helmholtz equation

Adapting Mourre's commutator method to the dissipative setting, we prove a limiting absorption principle for a class of abstract dissipative operators. A consequence is the resolvent estimates for the high frequency Helmholtz equation when trapped trajectories meet the set where the imaginary part of the potential is non-zero. We also give the resolvent estimates in Besov spaces