22,303 research outputs found
The June 2012 transit of Venus. Framework for interpretation of observations
Ground based observers have on 5/6th June 2012 the last opportunity of the
century to watch the passage of Venus across the solar disk from Earth. Venus
transits have traditionally provided unique insight into the Venus atmosphere
through the refraction halo that appears at the planet outer terminator near
ingress/egress. Much more recently, Venus transits have attracted renewed
interest because the technique of transits is being successfully applied to the
characterization of extrasolar planet atmospheres. The current work
investigates theoretically the interaction of sunlight and the Venus atmosphere
through the full range of transit phases, as observed from Earth and from a
remote distance. Our model predictions quantify the relevant atmospheric
phenomena, thereby assisting the observers of the event in the interpretation
of measurements and the extrapolation to the exoplanet case. Our approach
relies on the numerical integration of the radiative transfer equation, and
includes refraction, multiple scattering, atmospheric extinction and solar limb
darkening, as well as an up to date description of the Venus atmosphere. We
produce synthetic images of the planet terminator during ingress/egress that
demonstrate the evolving shape, brightness and chromaticity of the halo.
Guidelines are offered for the investigation of the planet upper haze from
vertically-unresolved photometric measurements. In this respect, the comparison
with measurements from the 2004 transit appears encouraging. We also show
integrated lightcurves of the Venus/Sun system at various phases during transit
and calculate the respective Venus-Sun integrated transmission spectra. The
comparison of the model predictions to those for a Venus-like planet free of
haze and clouds (and therefore a closer terrestrial analogue) complements the
discussion and sets the conclusions into a broader perspective.Comment: 14 pages; 14 figures; Submitted on 02/06/2012; A&A, accepted for
publication on 30/08/201
Accommodating repair actions into gas turbine prognostics
Elements of gas turbine degradation, such as compressor
fouling, are recoverable through maintenance actions like
compressor washing. These actions increase the usable engine
life and optimise the performance of the gas turbine.
However, these maintenance actions are performed by a separate
organization to those undertaking fleet management operations,
leading to significant uncertainty in the maintenance
state of the asset. The uncertainty surrounding maintenance
actions impacts prognostic efficacy. In this paper, we adopt
Bayesian on-line change point detection to detect the compressor
washing events. Then, the event detection information
is used as an input to a prognostic algorithm, advising an
update to the estimation of remaining useful life. To illustrate
the capability of the approach, we demonstrated our on-line
Bayesian change detection algorithms on synthetic and real
aircraft engine service data, in order to identify the compressor
washing events for a gas turbine and thus provide demonstrably
improved prognosis
Vapor Deposited Tungsten for Application as a Thermionic Emitter Material
Purity and resistance to grain growth of vapor deposited tungsten tubing for use as thermionic emitte
Fluid flow and heat convection studies for actively cooled airframes
This report details progress made on the jet impingement - liquid crystal - digital imaging experiment. With the design phase complete, the experiment is currently in the construction phase. In order to reach this phase two design related issues were resolved. The first issue was to determine NASP leading edge active cooling design parameters. Meetings were arranged with personnel at SAIC International, Torrance, CA in order to obtain recent publications that characterized expected leading edge heat fluxes as well as other details of NASP operating conditions. The information in these publications was used to estimate minimum and maximum jet Reynolds numbers needed to accomplish the required leading edge cooling, and to determine the parameters of the experiment. The details of this analysis are shown in Appendix A. One of the concerns for the NASP design is that of thermal stress due to large surface temperature gradients. Using a series of circular jets to cool the leading edge will cause a non-uniform temperature distribution and potentially large thermal stresses. Therefore it was decided to explore the feasibility of using a slot jet to cool the leading edge. The literature contains many investigations into circular jet heat transfer but few investigations of slot jet heat transfer. The first experiments will be done on circular jets impinging on a fiat plate and results compared to previously published data to establish the accuracy of the method. Subsequent experiments will be slot jets impinging on full scale models of the NASP leading edge. Table 1 shows the range of parameters to be explored. Next a preliminary design of the experiment was done. Previous papers which used a similar experimental technique were studied and elements of those experiments adapted to the jet impingement study. Trade-off studies were conducted to determine which design was the least expensive, easy to construct, and easy to use. Once the final design was settled, vendors were contacted to verify that equipment could be obtained to meet our specifications. Much of the equipment required to complete the construction of the experiment has been ordered or received. The material status list is shown in Appendix B
Whistling Rufus / words by Kerry Mills
Cover: a drawing of an African American man playing guitar and couples dancing in the background; Publisher: F. A. Mills (New York)https://egrove.olemiss.edu/sharris_a/1066/thumbnail.jp
Impecunious Davis / words by Kerry Mills
Cover: caricature of an African American male reclining against bales of cotton set on a city dock; description reads characteristic two-step march, polka and cake-walk [all music is missing, only the title page remains]; Publisher: F. A. Mills (New York)https://egrove.olemiss.edu/sharris_a/1048/thumbnail.jp
Rastus on Parade / words by Kerry Mills
Cover: description reads characteristic two-step march for piano; Publisher: F. A. Mills (New York)https://egrove.olemiss.edu/sharris_a/1023/thumbnail.jp
Kerry Mills Barn Dance / words by Kerry Mills
Cover: drawing of Caucasian couples dancing in a barn; Publisher: F. A. Mills (New York)https://egrove.olemiss.edu/sharris_b/1090/thumbnail.jp
Pre-conditioned backward Monte Carlo solutions to radiative transport in planetary atmospheres
CONTEXT. The interpretation of polarised radiation emerging from a planetary atmosphere must rely on solutions to the vector radiative transport equation (VRTE). Monte Carlo integration of the VRTE is a valuable approach for its flexible treatment of complex viewing and/or illumination geometries, and it can intuitively incorporate elaborate physics.
AIMS. We present a novel pre-conditioned backward Monte Carlo (PBMC) algorithm for solving the VRTE and apply it to planetary atmospheres irradiated from above. As classical BMC methods, our PBMC algorithm builds the solution by simulating the photon trajectories from the detector towards the radiation source, i.e. in the reverse order of the actual photon displacements. METHODS. We show that the neglect of polarisation in the sampling of photon propagation directions in classical BMC algorithms leads to unstable and biased solutions for conservative, optically-thick, strongly polarising media such as Rayleigh atmospheres. The numerical difficulty is avoided by pre-conditioning the scattering matrix with information from the scattering matrices of prior (in the BMC integration order) photon collisions. Pre-conditioning introduces a sense of history in the photon polarisation states through the simulated trajectories. RESULTS. The PBMC algorithm is robust, and its accuracy is extensively demonstrated via comparisons with examples drawn from the literature for scattering in diverse media. Since the convergence rate for MC integration is independent of the integral’s dimension, the scheme is a valuable option for estimating the disk-integrated signal of stellar radiation reflected from planets. Such a tool is relevant in the prospective investigation of exoplanetary phase curves. We lay out two frameworks for disk integration and, as an application, explore the impact of atmospheric stratification on planetary phase curves for large star-planet-observer phase angles. By construction, backward integration provides a better control than forward integration over the planet region contributing to the solution, and this presents a clear advantage when estimating the disk-integrated signal at moderate and large phase angles
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