704 research outputs found
Tertiary flow injection thrust vectoring system Patent
Tertiary flow injection system for thrust vectoring of propulsive nozzle flo
Supersonic through-flow fan assessment
A study was conducted to assess the performance potential of a supersonic through-flow fan engine for supersonic cruise aircraft. It included a mean-line analysis of fans designed to operate with in-flow velocities ranging from subsonic to high supersonic speeds. The fan performance generated was used to estimate the performance of supersonic fan engines designed for four applications: a Mach 2.3 supersonic transport, a Mach 2.5 fighter, a Mach 3.5 cruise missile, and a Mach 5.0 cruise vehicle. For each application an engine was conceptualized, fan performance and engine performance calculated, weight estimates made, engine installed in a hypothetical vehicle, and mission analysis was conducted
Had the planet mars not existed: Kepler's equant model and its physical consequences
We examine the equant model for the motion of planets, which has been the
starting point of Kepler's investigations before he modified it because of Mars
observations. We show that, up to first order in eccentricity, this model
implies for each orbit a velocity which satisfies Kepler's second law and
Hamilton's hodograph, and a centripetal acceleration with an inverse square
dependence on the distance to the sun. If this dependence is assumed to be
universal, Kepler's third law follows immediately. This elementary execice in
kinematics for undergraduates emphasizes the proximity of the equant model
coming from Ancient Greece with our present knowledge. It adds to its
historical interest a didactical relevance concerning, in particular, the
discussion of the Aristotelian or Newtonian conception of motion
Probing Mass Segregation in NGC 6397
In this study, we present a detailed study of mass segregation in the
globular clister NGC 6397. First, we carry out a photometric analysis of
projected ESO-VLT data (between 1 and 10 arcmin from the cluster centre),
presenting the luminosity function corrected by completeness. The luminosity
function shows a higher density of bright stars near the central region of the
data, with respect to the outer region. We calculate a deprojected model
(covering the whole cluster) estimating a total number of stars of 193000 +-
19000. The shapes of the surface brightness and density-number profiles versus
the radial coordinate r (instead of the projected coordinate R) lead to a
decreasing luminosity for an average star, and thus of mass, up to 1 arcmin,
quantifying the mass segregation. The deprojected model does not show evidence
of mass segregation outside this region
Mode identification of Pulsating White Dwarfs using the HST
We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV
stars G226-29 and G185-32, and for the pulsating DBV star PG1351+489 with the
Hubble Space Telescope Faint Object Spectrograph, to compare the ultraviolet to
the optical pulsation amplitude and determine the pulsation indices. We find
that for essentially all observed pulsation modes, the amplitude rises to the
ultraviolet as the theoretical models predict for l=1 non-radial g-modes. We do
not find any pulsation mode visible only in the ultraviolet, nor any modes
whose phase flips by 180 degrees; in the ultraviolet, as would be expected if
high l pulsations were excited. We find one periodicity in the light curve of
G185-32, at 141 s, which does not fit theoretical models for the change of
amplitude with wavelength of g-mode pulsations.Comment: Accepted for publication in the Astrophysical Journal, Aug 200
Computational fluid dynamics study of the variable-pitch split-blade fan concept
A computational fluid dynamics study was conducted to evaluate the feasibility of the variable-pitch split-blade supersonic fan concept. This fan configuration was conceived as a means to enable a supersonic fan to switch from the supersonic through-flow type of operation at high speeds to a conventional fan with subsonic inflow and outflow at low speeds. During this off-design, low-speed mode of operation, the fan would operate with a substantial static pressure rise across the blade row like a conventional transonic fan; the front (variable-pitch) blade would be aligned with the incoming flow, and the aft blade would remain fixed in the position set by the supersonic design conditions. Because of these geometrical features, this low speed configuration would inherently have a large amount of turning and, thereby, would have the potential for a large total pressure increase in a single stage. Such a high-turning blade configuration is prone to flow separation; it was hoped that the channeling of the flow between the blades would act like a slotted wing and help alleviate this problem. A total of 20 blade configurations representing various supersonic and transonic configurations were evaluated using a Navier Stokes CFD program called ADAPTNS because of its adaptive grid features. The flow fields generated by this computational procedure were processed by another data reduction program which calculated average flow properties and simulated fan performance. These results were employed to make quantitative comparisons and evaluations of blade performance. The supersonic split-blade configurations generated performance comparable to a single-blade supersonic, through-flow fan configuration. Simulated rotor total pressure ratios of the order of 2.5 or better were achieved for Mach 2.0 inflow conditions. The corresponding fan efficiencies were approximately 75 percent or better. The transonic split-blade configurations having large amounts of turning were able to generate large amounts of total turning and achieve simulated total pressure ratios of 3.0 or better with subsonic inflow conditions. These configurations had large losses and low fan efficiencies in the 70's percent. They had large separated regions and low velocity wakes. Additional turning and diffusion of this flow in a subsequent stator row would probably be very inefficient. The high total pressure ratios indicated by the rotor performance would be substantially reduced by the stators, and the stage efficiency would be substantially lower. Such performance leaves this dual-mode fan concept less attractive than originally postulated
New SX Phe variables in the globular cluster NGC 288
We report the discovery of two new variable stars in the metal-poor globular
cluster NGC 288, found by means of time-series CCD photometry. We classified
the new variables as SX Phoenicis due to their characteristic fundamental mode
periods (1.02 +- 0.01 and 0.69 +- 0.01 hours), and refine the period estimates
for other six known variables. SX Phe stars are known to follow a well-defined
Period-Luminosity (P-L) relation and, thus, can be used for determining
distances; they are more numerous than RR Lyraes in NGC~288. We obtain the P-L
relation for the fundamental mode M_V = (-2.59 +- 0.18) log P_0(d) + (-0.34 +-
0.24) and for the first-overtone mode M_V = (-2.59 +- 0.18) log P_1(d) + (0.50
+- 0.25). Multi-chromatic isochrone fits to our UBV color-magnitude diagrams,
based on the Dartmouth Stellar Evolution Database, provide = -1.3 +-
0.1, E(B-V) = 0.02 +- 0.01 and absolute distance modulus (m-M)0 = 14.72 +- 0.01
for NGC 288.Comment: 8 pages, 9 figures, 3 table
A comparative analysis of the observed white dwarf cooling sequence from globular clusters
We report our study of features at the observed red end of the white dwarf
cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae
and M\,4. We use deep colour-magnitude diagrams constructed from archival
Hubble Space Telescope (ACS) to systematically investigate the blue turn at
faint magnitudes and the age determinations for each cluster. We find that the
age difference between NGC\,6397 and 47\,Tuc is 1.98\,Gyr,
consistent with the picture that metal-rich halo clusters were formed later
than metal-poor halo clusters. We self-consistently include the effect of
metallicity on the progenitor age and the initial-to-final mass relation. In
contrast with previous investigations that invoked a single white dwarf mass
for each cluster, the data shows a spread of white dwarf masses that better
reproduce the shape and location of the blue turn. This effect alone, however,
does not completely reproduce the observational data - the blue turn retains
some mystery. In this context, we discuss several other potential problems in
the models. These include possible partial mixing of H and He in the atmosphere
of white dwarf stars, the lack of a good physical description of the
collision-induced absorption process and uncertainties in the opacities at low
temperatures. The latter are already known to be significant in the description
of the cool main sequence. Additionally, we find that the present day local
mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc
presents a bottom-heavy profile.Comment: Accepted for publication in MNRAS (16 pages, 19 figures
Evidence from K2 for rapid rotation in the descendant of an intermediate-mass star
Using patterns in the oscillation frequencies of a white dwarf observed by
K2, we have measured the fastest rotation rate, 1.13(02) hr, of any isolated
pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy
from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is
a 13,590(340) K, 0.87(03) solar-mass white dwarf. This is the highest mass
measured for any pulsating white dwarf with known rotation, suggesting a
possible link between high mass and fast rotation. If it is the product of
single-star evolution, its progenitor was a roughly 4.0 solar-mass
main-sequence B star; we know very little about the angular momentum evolution
of such intermediate-mass stars. We explore the possibility that this rapidly
rotating white dwarf is the byproduct of a binary merger, which we conclude is
unlikely given the pulsation periods observed.Comment: 5 pages, 4 figure, 1 table; accepted for publication in The
Astrophysical Journal Letter
Like-charge attraction through hydrodynamic interaction
We demonstrate that the attractive interaction measured between like-charged
colloidal spheres near a wall can be accounted for by a nonequilibrium
hydrodynamic effect. We present both analytical results and Brownian dynamics
simulations which quantitatively capture the one-wall experiments of Larsen and
Grier (Nature 385, p. 230, 1997).Comment: 10 pages, 4 figure
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