Numerical Simulations of Radiatively-Driven Dusty Winds

[abridged] Radiation pressure on dust grains may be an important mechanism in driving winds in a wide variety of astrophysical systems. However, the efficiency of the coupling between the radiation field and the dusty gas is poorly understood in environments characterized by high optical depths. We present a series of idealized numerical experiments, performed with the radiation-hydrodynamic code ORION, in which we study the dynamics of such winds and quantify their properties. We find that, after wind acceleration begins, radiation Rayleigh-Taylor instability forces the gas into a configuration that reduces the rate of momentum transfer from the radiation field to the gas by a factor ~ 10 - 100 compared to an estimate based on the optical depth at the base of the atmosphere; instead, the rate of momentum transfer from a driving radiation field of luminosity L to the gas is roughly L/c multiplied by one plus half the optical depth evaluated using the photospheric temperature, which is far smaller than the optical depth one would obtain using the interior temperature. When we apply our results to conditions appropriate to ULIRGs and star clusters, we find that the asymptotic wind momentum flux from such objects should not significantly exceed that carried by the direct radiation field, L/c. This result constrains the expected mass loss rates from systems that exceed the Eddington limit to be of order the so-called "single-scattering" limit, and not significantly higher. We present an approximate fitting formula for the rate of momentum transfer from radiation to dusty gas through which it passes, which is suitable for implementation in sub-grid models of galaxy formation. Finally, we provide a first map of the column density distribution of gas in a radiatively-driven wind as a function of velocity, and velocity dispersion.Comment: 19 pages, 17 figures, MNRAS in press; some additional discussion compared to previous version, no changes in conclusion

Introducing IPv6 Tokenised Interface Identifiers into the Linux Kernel

IPv6 Stateless Address Autoconfiguration (SLAAC) enables network administrators to deploy devices in a network and have those devices automatically generate global addresses without any administrative intervention, and without the need for any stateful configuration service such as DHCPv6. However, certain services --- such as HTTP, SMTP and IMAP --- may better benefit from having "well known" identifiers that do not depend on the physical hardware address of the server's network interface card. Tokenised addresses offer facility for administrators to specify the bottom 64 bits of an IPv6 address for a node whilst allowing the top 64 bits (the network prefix) to be automatically configured from router advertisements. This report documents the approach taken and experience gained from introducing tokenised interface identifiers into the Linux 2.6.11 kernel, as shipped with Redhat Fedora Core 4. This proof of concept work demonstrates the relative ease of introducing this useful utility for network node deployment, and further motivates wider deployment of the semi-automatic configuration approach

Will electricity deregulation push inflation lower?

Deregulation of electricity generation will offer consumers many advantages, including dramatically lower energy costs. From a macroeconomic viewpoint, electricity purchases are interesting because they are a major component of consumers’ budgets (and thus of the CPI) and a large factor of production for many companies. This raises the possibility that electricity deregulation could create a substantial shock to the overall price trend, comparable to other recent energy shocks. The benefits to consumers and producers identified in this article strongly support legislative efforts to increase competition in one of the last strongholds of regulated profits.Electric utilities ; Inflation (Finance)

The social potency of affect: identification and power in the immanent structuring of practice

We address the centrality of affect in structuring social practices, including those of organizing and managing. Social practices, it is argued, are contingent upon actors’ affectively charged involvement in immanent, yet indeterminate social relations. To understand this generative involvement, we commend a temporally-sensitive, critically-oriented theoretical framework, grounded in an affect-based ontology of practice. We demonstrate the relevance and credibility of this proposal through an analysis of the interactions of Board members in a UK consulting company

Wake states and frequency selection of a streamwise oscillating cylinder

This paper presents the results of an in-depth study of the flow past a streamwise oscillating cylinder, examining the impact of varying the amplitude and frequency of the oscillation, and the Reynolds number of the incoming flow. These findings are presented in a framework that shows that the relationship between the frequency of vortex shedding fs and the amplitude of oscillation A* is governed by two primary factors: the first is a reduction of fs proportional to a series in A*2 over a wide range of driving frequencies and Reynolds numbers; the second is nonlinear synchronization when this adjusted fs is in the vicinity of N = (1 - fs/fd)-1, where N is an integer. Typically, the influence of higher-order terms is small, and truncation to the first term of the series (A*2) well represents the overall trend of vortex shedding frequency as a function of amplitude. However, discontinuous steps are overlaid on this trend due to the nonlinear synchronization. When fs is normalized by the Strouhal frequency fSt (the frequency of vortex shedding from an unperturbed cylinder), the rate at which fs/fSt decreases with amplitude, at least for fd/fSt = 1, shows a linear dependence on the Reynolds number. For a fixed Re = 175, the truncated series shows that the rate of decrease of fs/fSt with amplitude varies as (2 - fd/fSt)-1/2 for 1 < or egal fd/fSt < or egal 2, but is essentially independent of fd/fSt for fd/fSt < 1. These trends of the rate of decrease of fs with respect to amplitude are also used to predict the amplitudes of oscillation around which synchronization occurs. These predicted amplitudes are shown to fall in regions of the parameter space where synchronized modes occur. Further, for the case of varying fd/fSt, a very reasonable prediction of the amplitude of oscillation required for the onset of synchronization to the mode where fs = 0.5fd is given. In a similar manner, amplitudes at which fs = 0 are calculated, predicting where the natural vortex shedding is completely supplanted by the forcing. These amplitudes are found to coincide approximately with those at which the onset of a symmetric vortex shedding mode is observed. This result is interpreted as meaning that the symmetric shedding mode occurs when the dynamics crosses over from being dominated by the vortex shedding to being dominated by the forcing