4,969 research outputs found
Application of laminar flow control to supersonic transport configurations
The feasibility and impact of implementing a laminar flow control system on a supersonic transport configuration were investigated. A hybrid laminar flow control scheme consisting of suction controlled and natural laminar flow was developed for a double-delta type wing planform. The required suction flow rates were determined from boundary layer stability analyses using representative wing pressure distributions. A preliminary design of structural modifications needed to accommodate suction through a perforated titanium skin was carried out together with the ducting and systems needed to collect, compress and discharge the suction air. The benefits of reduced aerodynamic drag were weighed against the weight, volume and power requirement penalties of suction system installation in a mission performance and sizing program to assess the net benefits. The study showed a feasibility of achieving significant laminarization of the wing surface by use of a hybrid scheme, leading to an 8.2 percent reduction in the cruise drag. This resulted in an 8.5 percent reduction in the maximum takeoff weight and a 12 percent reduction in the fuel burn after the inclusion of the LFC system installation penalties. Several research needs were identified for a resolution of aerodynamics, structural and systems issues before these potential benefits could be realized in a practical system
Stability and control of compressible flows over a surface with concave-conves curvature
The active control of spatially unstable disturbances in a laminar, two-dimensional, compressible boundary layer over a curved surface is numerically simulated. The control is effected by localized time-periodic surface heating. We consider two similar surfaces of different heights with concave-convex curvature. In one, the height is sufficiently large so that the favorable pressure gradient is sufficient to stabilize a particular disturbance. In the other case the pressure gradient induced by the curvature is destabilizing. It is shown that by using active control that the disturbance can be stabilized. The results demonstrate that the curvature induced mean pressure gradient significantly enhances the receptivity of the flow localized time-periodic surface heating and that this is a potentially viable mechanism in air
THE INFLUENCE OF DIET COMPOSITION, PLANT DEFENSIVE CHEMICALS, AND WINTER SEVERITY ON THE NUTRITIONAL CONDITION OF A FREERANGING, GENERALIST HERBIVORE
Herbivory requires animals to manage intake of toxic phytochemicals. Detoxification and excretion of these chemicals prevents toxicity, but is energetically expensive. I investigated the relationship between investment in detoxification and nutritional condition for moose on Isle Royale National Park (Alces alces) during winter, using urinary indices from urine samples collected in snow. The ratio of urinary urea nitrogen:creatinine is an indicator of nutritional condition, and the ratio of glucuronic acid:creatinine is an indicator of investment in detoxification. Nutritional condition declined with greater investment in detoxification. An alternative means of managing defensive chemical intake is to diversify the diet. Microhistological analysis of fecal pellets determined diet composition. Diet diversity was weakly associated with improved nutritional condition. However, the strongest predictors of nutritional condition were winter severity and proportion of balsam fir in the diet (a dominant food for moose in this ecosystem)
Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9342058
We report on two new quiescent {\it XMM-Newton} observations (in addition to
the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling
neutron star crust in the low-mass X-ray binary 1RXS J180408.9342058. Its
crust was heated during the 4.5 month accretion outburst of the source.
From our quiescent observations, fitting the spectra with a neutron star
atmosphere model, we found that the crust had cooled from 100 eV to
73 eV from 8 days to 479 days after the end of its outburst.
However, during the most recent observation, taken 860 days after the end
of the outburst, we found that the crust appeared not to have cooled further.
This suggested that the crust had returned to thermal equilibrium with the
neutron star core. We model the quiescent thermal evolution with the
theoretical crustal cooling code NSCool and find that the source requires a
shallow heat source, in addition to the standard deep crustal heating
processes, contributing 0.9 MeV per accreted nucleon during outburst to
explain its observed temperature decay. Our high quality {\it XMM-Newton} data
required an additional hard component to adequately fit the spectra. This
slightly complicates our interpretation of the quiescent data of 1RXS
J180408.9342058. The origin of this component is not fully understood.Comment: Accepted for publication by MNRA
Emissivity measurements of reflective surfaces at near-millimeter wavelengths
We have developed an instrument for directly measuring the emissivity of reflective surfaces at near-millimeter wavelengths. The thermal emission of a test sample is compared with that of a reference surface, allowing the emissivity of the sample to be determined without heating. The emissivity of the reference surface is determined by one’s heating the reference surface and measuring the increase in emission. The instrument has an absolute accuracy of Δe = 5 x 10^-4 and can reproducibly measure a difference in emissivity as small as Δe = 10^-4 between flat reflective samples. We have used the instrument to measure the emissivity of metal films evaporated on glass and carbon fiber-reinforced plastic composite surfaces. We measure an emissivity of (2.15 ± 0.4) x 10^-3 for gold evaporated on glass and (2.65 ± 0.5) x 10^-3 for aluminum evaporated on carbon fiber-reinforced plastic composite
Dynamic behavior of an unsteady trubulent boundary layer
Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
Efficiency of Socialist Cooperative Farming: Appearance and Reality
Understanding the nature of the world food system has been a major objective of IIASA's Food and Agriculture Program (FAP), since it began in 1977.
Scholars from different nations have worked together in the FAP. The interactions of scholars from many different nations and perspectives sometimes reveal preconceptions regarding systems one is not fully familiar with. And often this provides interesting questions for fresh analysis.
This paper on efficiency of socialist cooperative agriculture is one such example of such analysis. The questions posed here were raised in discussions when a group of FAP scholars from different nations visited a cooperative farm in Hungary. In this paper the authors have advanced some hypotheses to explore the reality behind the apparent comparative inefficiency of socialist cooperative farming
From Questions to Effective Answers: On the Utility of Knowledge-Driven Querying Systems for Life Sciences Data
We compare two distinct approaches for querying data in the context of the
life sciences. The first approach utilizes conventional databases to store the
data and intuitive form-based interfaces to facilitate easy querying of the
data. These interfaces could be seen as implementing a set of "pre-canned"
queries commonly used by the life science researchers that we study. The second
approach is based on semantic Web technologies and is knowledge (model) driven.
It utilizes a large OWL ontology and same datasets as before but associated as
RDF instances of the ontology concepts. An intuitive interface is provided that
allows the formulation of RDF triples-based queries. Both these approaches are
being used in parallel by a team of cell biologists in their daily research
activities, with the objective of gradually replacing the conventional approach
with the knowledge-driven one. This provides us with a valuable opportunity to
compare and qualitatively evaluate the two approaches. We describe several
benefits of the knowledge-driven approach in comparison to the traditional way
of accessing data, and highlight a few limitations as well. We believe that our
analysis not only explicitly highlights the specific benefits and limitations
of semantic Web technologies in our context but also contributes toward
effective ways of translating a question in a researcher's mind into precise
computational queries with the intent of obtaining effective answers from the
data. While researchers often assume the benefits of semantic Web technologies,
we explicitly illustrate these in practice
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