Flow Induced Spring Coefficients of Labyrinth Seals for Application in Rotor Dynamics

Flow induced aerodynamic spring coefficients of labyrinth seals are discussed and the restoring force in the deflection plane of the rotor and the lateral force acting perpendicularly to it are also considered. The effects of operational conditions on the spring characteristics of these components are examined, such as differential pressure, speed, inlet flow conditions, and the geometry of the labyrinth seals. Estimation formulas for the lateral forces due to shaft rotation and inlet swirl, which are developed through experiments, are presented. The utilization of the investigations is explained and results of stability calculations, especially for high pressure centrifugal compressors, are added. Suggestions are made concerning the avoidance of exciting forces in labyrinths

Real Estate Booms and Banking Busts: An International Perspective

Real estate cycles and banking cycles may occur independently but they are correlated in a remarkable number of instances ranging over a wide variety of institutional arrangements, in both advanced industrial nations and emerging economies. During the recent Asian financial crisis, the most seriously affected countries first experienced a collapse in property prices and a weakening of the banking systems before experiencing their exchange rate crises. Countries where banks play a more dominant role in real estate markets and hold a greater percentage of assets are the most severely affected during such a crisis. In this paper, the authors develop an explanation of how real estate cycles and banking crises are related and why they occur. The authors first discuss how real estate prices are determined and why they are so vulnerable to deviations from long-run equilibrium prices, paying special attention to the role of the banking system in determining prices. Increases in the price of real estate may increase the economic value of bank capital to the extent that banks own real estate. This then increases the value of loans collateralized by real estate and may lead to a decline in the perceived risk of real estate lending. For these reasons, an increase in real estate prices may increase the supply of credit to the real estate industry which is then likely to lead to further increases in real estate prices. The opposite is also true. A decline in the price of real estate will decrease bank capital by reducing the value of the bank's own real estate assets as well as reduce the value of loans collateralized by real estate. This may lead to defaults, thus further reducing capital. A decline in the price of real estate is also likely to increase the perceived risk in real estate lending. All of these factors reduce the supply of credit to the real estate industry. Supervisors and regulators may also react to the resulting weakening of bank capital positions by increasing capital requirements and instituting stricter rules for classifying and provisioning against real estate assets, leading to even further decline in prices and supply of credit to the real estate industry. In order to explain how real estate cycles begin, the authors turn to a model of land prices developed by Mark Carey that details the role of optimists in the process. They then bring in the role of non-financial variables as well as of banks and then turn to the part played by "disaster myopia" -- the tendency over time to underestimate the probability of low-frequency shocks -- in determining cycles. Other factors that contribute to cycles are inadequate data and weak analysis by bank managers as well as "perverse incentives" -- one result of "disaster myopia" that occurs when lenders believe that they can accept higher loan-to-value rations, weaker commitments or guarantees and looser loan covenants without increasing their risk of loss. Using this framework of the interactions between the real estate market and bank behavior, the authors interpret recent examples of real estate booms linked to banking crises in Sweden, the United States, Japan and Thailand. They then discuss measures that can be taken to limit the amplitude of real estate cycles and ways to insulate the banking system from real estate cycles. They believe that the heart of the problem is the structure of the real estate market and that cycles can be avoided by taking measures that counter: The bias towards optimism; Excessive leverage; Disaster myopia; Inadequate data and weak analysis; Perverse incentives. The authors detail their recommendations for avoiding these problems in the future. These include the development of an options market for commercial real estate, greater reliance on equity financing, changes in supervisory policy that allow the identification of vulnerable banks before they become weak banks, better publication of information relevant to the valuation of commercial real estate projects, and refraining from providing full protection to all bank creditors, especially sophisticated creditors such as corporations, banks, and institutional investors.

SGR J1550–5418 Bursts Detected with the Fermi Gamma-Ray Burst Monitor during its Most Prolific Activity

We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550–5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550–5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties. We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J1550–5418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of –0.92, close to the OTTB index of –1. We show that there is an anti-correlation between the Comptonized E_(peak) and the burst fluence and average flux. For the BB+BB fits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the high-temperature BB has a much smaller area and shows an anti-correlation between emission area and temperature. We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models

White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope. II. Full Snapshot Survey Results

Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low mass stellar and substellar companions. Of the 72 targets which remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of years. No intermediate projected separations of a few to several AU are found among these pairs. However, a few double M dwarfs (within triples) are spatially resolved in this range, empirically demonstrating that such separations were readily detectable among the binaries with white dwarfs. A straightforward and testable prediction emerges: all spatially unresolved, low mass stellar and substellar companions to white dwarfs should be in short period orbits. This result has implications for substellar companion and planetary orbital evolution during the post-main sequence lifetime of their stellar hosts.Comment: Accepted to ApJ Supplement Series, emulateapj format, 14 figures, 8 table

Agent Causation: Before and After the Ontological Turn

Imagine Ludwig has a cup of tea for breakfast. He\ud pours it; he eats his egg until it seems to him that the tea\ud should have the right temperature; he moves his hand to\ud the cup, puts his fingers at the handle, and then, careful\ud not to spill anything, he does something with his arm;\ud namely, he raises it, and if all goes well he then drinks the\ud tea without burning his lips.\ud The rising of Ludwig"s arm surely has a cause. But\ud what is the cause? Defenders of agent causation, such as\ud Thomas Reid (1788), Richard Taylor (1966), Roderick\ud Chisholm (1976a), and many more recent authors (see\ud Swinburne 1997, ch. 5; Thorp 1980; Meixner 1999; Clarke\ud 1996; O'Connor 2000) have argued that the rising of\ud Ludwig"s arm is caused by Ludwig himself. Some events\ud are caused, not by other events, but by concrete things, by\ud substances, more specifically by intentional agents

A World of Fields

Trope ontology is exposed and confronted with the question where one trope ends and another begins. It is argued that tropes do not have determinate boundaries, it is arbitrary how tropes are carved up. An ontology, which I call field ontology, is proposed which takes this into account. The material world consists of a certain number of fields, each of which is extended over all of space. It is shown how field ontology can also tackle the problem of determin-able properties and the problem of completeness of things