Statistical Methods for Planning Diesel Engine Overhauls in the U. S. Coast Guard

The United States Coast Guard has recently investigated new strategies to maintain cutter propulsion diesel engines. Reliability centered maintenance with statistical methods may allow the time between costly scheduled overhauls to be increased. One indicator of engine aging is the number of failures experienced with increasing operating hours. The purpose of this paper is to investigate the failure-time relationship of the ALCO 251 marine diesel propulsion engine operated on Reliance class cutters. This analysis used exponential, Weibull, and three-part composite Weibull failure density functions to model engine casualty data dating back to 1978. The data does not indicate the source of the engine failure, but every failure had a significant operational impact. Results indicate an increasing failure rate as the engine ages to the 24,000 hour overhaul time. The evidence indicates a constant failure useful life region, but the increasing failure rate from the Weibull models suggests that the periodic over-hauls do not prevent wearout failures. As a result, the Coast Guard should consider refining diesel engine overhaul policy in order to prevent increasing age-related failures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86221/1/Perakis3.pd

Femtosecond Coherent Control of Spin with Light in (Ga,Mn)As ferromagnets

Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near--ultraviolet ($\sim$3eV), coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the \{111\} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. We predict nonthermal magnetization control by tuning the laser frequency and polarization direction. Our mechanism explains recent ultrafast pump--probe experiments.Comment: 4 pages, 3 figures, published in Physical Review Letter

The Impact of Demand Uncertainty on Consumer Subsidies for Green Technology Adoption

This paper studies government subsidies for green technology adoption while considering the manufacturing industry’s response. Government subsidies offered directly to consumers impact the supplier’s production and pricing decisions. Our analysis expands the current understanding of the price-setting newsvendor model, incorporating the external influence from the government, who is now an additional player in the system. We quantify how demand uncertainty impacts the various players (government, industry, and consumers) when designing policies. We further show that, for convex demand functions, an increase in demand uncertainty leads to higher production quantities and lower prices, resulting in lower profits for the supplier. With this in mind, one could expect consumer surplus to increase with uncertainty. In fact, we show that this is not always the case and that the uncertainty impact on consumer surplus depends on the trade-off between lower prices and the possibility of underserving customers with high valuations. We also show that when policy makers such as governments ignore demand uncertainty when designing consumer subsidies, they can significantly miss the desired adoption target level. From a coordination perspective, we demonstrate that the decentralized decisions are also optimal for a central planner managing jointly the supplier and the government. As a result, subsidies provide a coordination mechanism

Pricing with Limited Knowledge of Demand

How should a firm price a new product for which little is known about demand? We propose a pricing rule that can be used if the firm can estimate (even roughly) the maximum price it can charge and still expect to sell some units, and the firm need not know in advance the quantity it will sell. The rule is simple: Set price as though the demand curve were linear. We show that if the true demand curve is one of many commonly used demand functions, or even a more complex function, and if marginal cost is known and constant, the firm can expect its profit to be close to what it would earn if it knew the true demand curve. We derive analytical performance bounds for a variety of demand functions, calculate expected profit performance for randomly generated demand curves, and evaluate the welfare implications of our pricing rule

Effect of conduction electron interactions on Anderson impurities

The effect of conduction electron interactions for an Anderson impurity is investigated in one dimension using a scaling approach. The flow diagrams are obtained by solving the renormalization group equations numerically. It is found that the Anderson impurity case is different from its counterpart -- the Kondo impurity case even in the local moment region. The Kondo temperature for an Anderson impurity shows nonmonotonous behavior, increasing for weak interactions but decreasing for strong interactions. The implication of the study to other related impurity models is also discussed.Comment: 10 pages, revtex, 4 figures (the postscript file is included), to appear in Phys. Rev. B (Rapid Commun.

Size-dependent Correlation Effects in Ultrafast Optical Dynamics of Metal Nanoparticles

We study the role of collective surface excitations in the electron relaxation in small metal particles. We show that the dynamically screened electron-electron interaction in a nanoparticle contains a size-dependent correction induced by the surface. This leads to new channels of quasiparticle scattering accompanied by the emission of surface collective excitations. We calculate the energy and temperature dependence of the corresponding rates, which depend strongly on the nanoparticle size. We show that the surface-plasmon-mediated scattering rate of a conduction electron increases with energy, in contrast to that mediated by a bulk plasmon. In noble-metal particles, we find that the dipole collective excitations (surface plasmons) mediate a resonant scattering of d-holes to the conduction band. We study the role of the latter effect in the ultrafast optical dynamics of small nanoparticles and show that, with decreasing nanoparticle size, it leads to a drastic change in the differential absorption lineshape and a strong frequency dependence of the relaxation near the surface plasmon resonance. The experimental implications of our results in ultrafast pump-probe spectroscopy are also discussed.Comment: 29 pages including 6 figure