6,950 research outputs found
Dynamic stability study for sounding rockets Final report
Joint rotation and compliance, body and fin flexibility, and aerodynamic characteristics effect on roll resonance of sounding rocket
Collisions of boosted black holes: perturbation theory prediction of gravitational radiation
We consider general relativistic Cauchy data representing two nonspinning,
equal-mass black holes boosted toward each other. When the black holes are
close enough to each other and their momentum is sufficiently high, an
encompassing apparent horizon is present so the system can be viewed as a
single, perturbed black hole. We employ gauge-invariant perturbation theory,
and integrate the Zerilli equation to analyze these time-asymmetric data sets
and compute gravitational wave forms and emitted energies. When coupled with a
simple Newtonian analysis of the infall trajectory, we find striking agreement
between the perturbation calculation of emitted energies and the results of
fully general relativistic numerical simulations of time-symmetric initial
data.Comment: 5 pages (RevTex 3.0 with 3 uuencoded figures), CRSR-107
Understanding initial data for black hole collisions
Numerical relativity, applied to collisions of black holes, starts with
initial data for black holes already in each other's strong field. The initial
hypersurface data typically used for computation is based on mathematical
simplifying prescriptions, such as conformal flatness of the 3-geometry and
longitudinality of the extrinsic curvature. In the case of head on collisions
of equal mass holes, there is evidence that such prescriptions work reasonably
well, but it is not clear why, or whether this success is more generally valid.
Here we study these questions by considering the ``particle limit'' for head on
collisions of nonspinning holes. Einstein's equations are linearized in the
mass of the small hole, and described by a single gauge invariant spacetime
function psi, for each multipole. The resulting equations have been solved by
numerical evolution for collisions starting from various initial separations,
and the evolution is studied on a sequence of hypersurfaces. In particular, we
extract hypersurface data, that is psi and its time derivative, on surfaces of
constant background Schwarzschild time. These evolved data can then be compared
with ``prescribed'' data, evolved data can be replaced by prescribed data on
any hypersurface, and evolved further forward in time, a gauge invariant
measure of deviation from conformal flatness can be evaluated, etc. The main
findings of this study are: (i) For holes of unequal mass the use of prescribed
data on late hypersurfaces is not successful. (ii) The failure is likely due to
the inability of the prescribed data to represent the near field of the smaller
hole. (iii) The discrepancy in the extrinsic curvature is more important than
in the 3-geometry. (iv) The use of the more general conformally flat
longitudinal data does not notably improve this picture.Comment: 20 pages, REVTEX, 26 PS figures include
Black Hole Data via a Kerr-Schild Approach
We present a new approach for setting initial Cauchy data for multiple black
hole spacetimes. The method is based upon adopting an initially Kerr-Schild
form of the metric. In the case of non-spinning holes, the constraint equations
take a simple hierarchical form which is amenable to direct numerical
integration. The feasibility of this approach is demonstrated by solving
analytically the problem of initial data in a perturbed Schwarzschild geometry.Comment: 13 pages, RevTeX forma
Design and analysis of a fuel-efficient single-engine, turboprop-powered, business airplane
The speed, range, payload, and fuel efficiency of a general aviation airplane powered by one turboprop engine was determined and compared to a twin engine turboprop aircraft. An airplane configuration was developed which can carry six people for a noreserve range of 2,408 km at a cruise speed above 154 m/s, and a cruise altitude of about 9,144 m. The cruise speed is comparable to that of the fastest of the current twin turboprop powered airplanes. It is found that the airplane has a cruise specific range greater than all twin turboprop engine airplanes flying in its speed range and most twin piston engine airplanes flying at considerably slower cruise airspeeds
Bounds on Integrals of the Wigner Function
The integral of the Wigner function over a subregion of the phase-space of a
quantum system may be less than zero or greater than one. It is shown that for
systems with one degree of freedom, the problem of determining the best
possible upper and lower bounds on such an integral, over all possible states,
reduces to the problem of finding the greatest and least eigenvalues of an
hermitian operator corresponding to the subregion. The problem is solved
exactly in the case of an arbitrary elliptical region. These bounds provide
checks on experimentally measured quasiprobability distributions.Comment: 10 pages, 1 PostScript figure, Latex file; revised following
referees' comments; to appear in Physical Review Letter
Experimental Bounds on Masses and Fluxes of Nontopological Solitons
We have re-analyzed the results of various experiments which were not
originally interested as searches for the Q-ball or the Fermi-ball. Based on
these analyses, in addition to the available data on Q-balls, we obtained
rather stringent bounds on flux, mass and typical energy scale of Q-balls as
well as Fermi-balls. In case these nontopological solitons are the main
component of the dark matter of the Galaxy, we found that only such solitons
with very large quantum numbers are allowed. We also estimate how sensitive
future experiments will be in the search for Q-balls and Fermi-balls.Comment: 19 pages, 7 eps figures, RevTeX, psfig.st
Genetic Covariance Structure of Reading, Intelligence and Memory in Children
This study investigates the genetic relationship among reading performance, IQ, verbal and visuospatial working memory (WM) and short-term memory (STM) in a sample of 112, 9-year-old twin pairs and their older siblings. The relationship between reading performance and the other traits was explained by a common genetic factor for reading performance, IQ, WM and STM and a genetic factor that only influenced reading performance and verbal memory. Genetic variation explained 83% of the variation in reading performance; most of this genetic variance was explained by variation in IQ and memory performance. We hypothesize, based on these results, that children with reading problems possibly can be divided into three groups: (1) children low in IQ and with reading problems; (2) children with average IQ but a STM deficit and with reading problems; (3) children with low IQ and STM deficits; this group may experience more reading problems than the other two
Variational quantum Monte Carlo study of two-dimensional Wigner crystals: exchange, correlation, and magnetic field effects
The two-dimensional Wigner crystals are studied with the variational quantum
Monte Carlo method. The close relationship between the ground-state
wavefunction and the collective excitations in the system is illustrated, and
used to guide the construction of the ground-state wavefunction of the strongly
correlated solid. Exchange, correlation, and magnetic field effects all give
rise to distinct physical phenomena. In the absence of any external magnetic
field, interesting spin-orderings are observed in the ground-state of the
electron crystal in various two-dimensional lattices. In particular,
two-dimensional bipartite lattices are shown not to lead necessarily to an
antiferromagnetic ground-state. In the quantum Hall effect regime, a strong
magnetic field introduces new energy and length scales. The magnetic field
quenches the kinetic energy and poses constraints on how the electrons may
correlate with each other. Care is taken to ensure the appropriate
translational properties of the wavefunction when the system is in a uniform
magnetic field. We have examined the exchange, intra-Landau-level correlation
as well as Landau-level-mixing effects with various variational wavefunctions.
We also determine their dependences on the experimental parameters such as the
carrier effective mass at a modulation-doped semiconductor heterojunction. Our
results, when combined with some recent calculations for the energy of the
fractional quantum Hall liquid including Landau-level-mixing, show
quantitatively that in going from -doping to -doping in
heterojunction systems, the crossover filling factor from the fractional
quantum Hall liquid to the Wigner crystal changes from filling factor to . This lends strong support to the claim that theComment: LaTex file, 14 figures available from [email protected]
Co-evolution of density and topology in a simple model of city formation
We study the influence that population density and the road network have on
each others' growth and evolution. We use a simple model of formation and
evolution of city roads which reproduces the most important empirical features
of street networks in cities. Within this framework, we explicitely introduce
the topology of the road network and analyze how it evolves and interact with
the evolution of population density. We show that accessibility issues -pushing
individuals to get closer to high centrality nodes- lead to high density
regions and the appearance of densely populated centers. In particular, this
model reproduces the empirical fact that the density profile decreases
exponentially from a core district. In this simplified model, the size of the
core district depends on the relative importance of transportation and rent
costs.Comment: 13 pages, 13 figure
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