12,607 research outputs found
The characteristics of the flow field over the mid-upper fuselage of Lancaster P. A. 474
This note describes a series of tests conducted to determine the
characteristics of the flow field over the mid-upper fuselage of.
Lancaster P. A, 474.
The range of the tests was to include a determination of the distributions
of total head, static pressure and velocity together with
the flow directional characteristics in the pitching plane for a
number of aircraft flight configurations as listed in paragraph 1. 2.
Curves are presented in Figs. 9, 20 - 25, showing the flow directional
characteristics and the distributions of static pressure and velocity
in the region of investigation
Mixing and reaction studies of hydrazine and nitrogen tetroxide using photographic and spectral techniques
Mixing and reaction studies of hydrazine and nitrogen tetroxide using photographic and spectral technique
Application of Pade Approximants to Determination of alpha_s(M_Z^2) from Hadronic Event Shape Observables in e+e- Annihilation
We have applied Pade approximants to perturbative QCD calculations of event
shape observables in e+e- --> hadrons. We used the exact O(alpha_s^2)
prediction and the [0/1] Pade approximant to estimate the O(alpha_s^3) term for
15 observables, and in each case determined alpha_s(M_Z^2) from comparison with
hadronic Z^0 decay data from the SLD experiment. We found the scatter among the
alpha_s(M_Z^2) values to be significantly reduced compared with the standard
O(alpha_s^2) determination, implying that the Pade method provides at least a
partial approximation of higher-order perturbative contributions to event shape
observables.Comment: 15 pages, 1 EPS figure, Submitted to Physics Letters
The proto--neutron--star dynamo -- viability and impediments
We study convective motions taken from hydrodynamic simulations of rotating
proto--neutron stars (PNSs) with respect to their ability to excite a dynamo
instability which may be responsible for the giant neutron star magnetic
fields. Since it is impossible to simulate the magnetic field evolution
employing the actual magnetic Reynolds numbers (\Rm) resulting from the
hydrodynamic simulations, (smallest) critical \Rms and the corresponding
field geometries are derived on the kinematic level by rescaling the velocity
amplitudes. It turns out that the actual values of \Rm are by many orders of
magnitude larger than the critical values found. A dynamo might therefore start
to act vigorously very soon after the onset of convection. But as in general
dynamo growth rates are non--monotonous functions of \Rm the later fate of
the magnetic field is uncertain. Hence, no reliable statements on the existence
and efficiency of PNS dynamos can be drawn without considering the interplay of
magnetic field and convection from the beginning. Likewise, in so far as
convection inside the PNS is regarded to be essential in re--launching the
supernova explosion, a revision of its role in this respect could turn out to
be necessary.Comment: 7 pages, 4 figures, accepted by Astronomy & Astrophysic
Electromechanical systems with transient high power response operating from a resonant AC link
The combination of an inherently robust asynchronous (induction) electrical machine with the rapid control of energy provided by a high frequency resonant AC link enables the efficient management of higher power levels with greater versatility. This could have a variety of applications from launch vehicles to all-electric automobiles. These types of systems utilize a machine which is operated by independent control of both the voltage and frequency. This is made possible by using an indirect field-oriented control method which allows instantaneous torque control in all four operating quadrants. Incorporating the AC link allows the converter in these systems to switch at the zero crossing of every half cycle of the AC waveform. This zero loss switching of the link allows rapid energy variations to be achieved without the usual frequency proportional switching loss. Several field-oriented control systems were developed by LeRC and General Dynamics Space Systems Division under contract to NASA. A description of a single motor, electromechanical actuation system is presented. Then, focus is on a conceptual design for an AC electric vehicle. This design incorporates an induction motor/generator together with a flywheel for peak energy storage. System operation and implications along with the associated circuitry are addressed. Such a system would greatly improve all-electric vehicle ranges over the Federal Urban Driving Cycle (FUD)
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Testing the models: NIR imaging and spectroscopy of the benchmark T-dwarf binary Eps Indi B
The relative roles of metallicity and surface gravity on the near-infrared
spectra of late-T brown dwarfs are not yet fully understood, and evolutionary
models still need to be calibrated in order to provide accurate estimates of
brown dwarf physical parameters from measured spectra. The T-type brown dwarfs
Eps Indi Ba and Bb forming the tightly bound binary Eps Indi B, which orbits
the K4V star Eps Indi A, are nowadays the only such benchmark T dwarfs for
which all important physical parameters such as metallicity, age and mass are
(or soon will be) known. We present spatially resolved VLT/NACO images and low
resolution spectra of Eps Indi B in the J, H and K near-infrared bands. The
spectral types of Eps Indi Ba and Bb are determined by direct comparison of the
flux-calibrated JHK spectra with T dwarf standard template spectra and also by
NIR spectral indices. Eps Indi Bb is confirmed as a T6 while the spectral type
of Eps Indi Ba is T1.5 so somewhat later than the previously reported T1.
Constrained values for surface gravity and effective temperature are derived by
comparison with model spectra. The evolutionary models predict masses around
about 53 M_J for Eps Indi Ba and about 34 M_J for Eps Indi Bb, slightly higher
than previously reported values. The suppressed J-band and enhanced K-band flux
of Eps Indi Ba indicates that a noticeable cloud layer is still present in a
T1.5 dwarf while no clouds are needed to model the spectrum of Eps Indi Bb.Comment: 7 pages, 5 figures, accepted by Ap
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Evolutionarily stable sexual allocation by both stressed and unstressed potentially simultaneous hermaphrodites within the same population.
Factors influencing allocation of resources to male and female offspring continue to be of great interest to evolutionary biologists. A simultaneous hermaphrodite is capable of functioning in both male and female mode at the same time, and such a life-history strategy is adopted by most flowering plants and by many sessile aquatic animals. In this paper, we focus on hermaphrodites that nourish post-zygotic stages, e.g. flowering plants and internally fertilising invertebrates, and consider how their sex allocation should respond to an environmental stress that reduces prospects of survival but does not affect all individuals equally, rather acting only on a subset of the population. Whereas dissemination of pollen and sperm can begin at sexual maturation, release of seeds and larvae is delayed by embryonic development. We find that the evolutionarily stable strategy for allocation between male and female functions will be critically dependent on the effect of stress on the trade-off between the costs of male and female reproduction, (i.e. of sperm and embryos). Thus, we identify evaluation of this factor as an important challenge to empiricists interested in the effects of stress on sex allocation. When only a small fraction of the population is stressed, we predict that stressed individuals will allocate their resources entirely to male function and unstressed individuals will increase their allocation to female function. Conversely, when the fraction of stress-affected individuals is high, stressed individuals should respond to this stressor by increasing investment in sperm and unstressed individuals should invest solely in embryos. A further prediction of the model is that we would not expect to find populations in the natural world where both stressed and unstressed individuals are both hermaphrodite
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