27,850 research outputs found
In-flight transition measurement on a 10 deg cone at Mach numbers from 0.5 to 2.0
Boundary layer transition measurements were made in flight on a 10 deg transition cone tested previously in 23 wind tunnels. The cone was mounted on the nose of an F-15 aircraft and flown at Mach numbers room 0.5 to 2.0 and altitudes from 1500 meters (5000 feet) to 15,000 meters (50,000 feet), overlapping the Mach number/Reynolds number envelope of the wind tunnel tests. Transition was detected using a traversing pitot probe in contact with the surface. Data were obtained near zero cone incidence and adiabatic wall temperature. Transition Reynolds number was found to be a function of Mach number and of the ratio of wall temperature to adiabatic all temperature. Microphones mounted flush with the cone surface measured free-stream disturbances imposed on the laminar boundary layer and identified Tollmien-Schlichting waves as the probable cause of transition. Transition Reynolds number also correlated with the disturbance levels as measured by the cone surface microphones under a laminar boundary layer as well as the free-stream impact
Cohomology for Anyone
Crystallography has proven a rich source of ideas over several centuries.
Among the many ways of looking at space groups, N. David Mermin has pioneered
the Fourier-space approach. Recently, we have supplemented this approach with
methods borrowed from algebraic topology. We now show what topology, which
studies global properties of manifolds, has to do with crystallography. No
mathematics is assumed beyond what the typical physics or crystallography
student will have seen of group theory; in particular, the reader need not have
any prior exposure to topology or to cohomology of groups.Comment: 21 pages + figures, bibliography, Mathematica code homology.
A randomized controlled pilot trial of classroom-based mindfulness meditation compared to an active control condition in sixth-grade children
The current study is a pilot trial to examine the effects of a nonelective, classroom-based, teacher-implemented, mindfulness meditation intervention on standard clinical measures of mental health and affect in middle school children. A total of 101 healthy sixth-grade students (55 boys, 46 girls) were randomized to either an Asian history course with daily mindfulness meditation practice (intervention group) or an African history course with a matched experiential activity (active control group). Self-reported measures included the Youth Self Report (YSR), a modified Spielberger State-Trait Anxiety Inventory, and the Cognitive and Affective Mindfulness Measure –Revised. Both groups decreased significantly on clinical syndrome subscales and affect but did not differ in the extent of their improvements. Meditators were significantly less likely to develop suicidal ideation or thoughts of self-harm than controls. These results suggest that mindfulness training may yield both unique and non-specific benefits that are shared by other novel activities
Scattering length of Andreev reflection from quantized vortices in He-
Andreev reflection of thermal quasiparticles from quantized vortices is an
important technique to visualize quantum turbulence in low temperature
He-. We revisit a problem of Andreev reflection from the isolated,
rectilinear vortex line. For quasiparticle excitations whose impact parameters,
defined as distances of the closest approach to the vortex core, do not exceed
some arbitrary value, , we calculate exactly the reflected fraction of the
total flux of excitations incident upon the vortex in the direction orthogonal
to the vortex line. We then define and calculate exactly, as a function of ,
the scattering length, that is the scattering cross-section per unit length of
the vortex line. We also define and calculate the scattering lengths for the
flux of energy carried by thermal excitations, and for the net energy flux
resulting from a (small) temperature gradient, and analyze the dependence of
these scattering lengths on temperature.Comment: 8 pages, 4 figure
Cross-sections of Andreev scattering by quantized vortex rings in 3He-B
We studied numerically the Andreev scattering cross-sections of
three-dimensional isolated quantized vortex rings in superfluid 3He-B at
ultra-low temperatures. We calculated the dependence of the cross-section on
the ring's size and on the angle between the beam of incident thermal
quasiparticle excitations and the direction of the ring's motion. We also
introduced, and investigated numerically, the cross-section averaged over all
possible orientations of the vortex ring; such a cross-section may be
particularly relevant for the analysis of experimental data. We also analyzed
the role of screening effects for Andreev reflection of quasiparticles by
systems of vortex rings. Using the results obtained for isolated rings we found
that the screening factor for a system of unlinked rings depends strongly on
the average radius of the vortex ring, and that the screening effects increase
with decreasing the rings' size.Comment: 11 pages, 8 figures ; submitted to Physical Review
Visualizing Pure Quantum Turbulence in Superfluid He: Andreev Reflection and its Spectral Properties
Superfluid He-B in the zero-temperature limit offers a unique means of
studying quantum turbulence by the Andreev reflection of quasiparticle
excitations by the vortex flow fields. We validate the experimental
visualization of turbulence in He-B by showing the relation between the
vortex-line density and the Andreev reflectance of the vortex tangle in the
first simulations of the Andreev reflectance by a realistic 3D vortex tangle,
and comparing the results with the first experimental measurements able to
probe quantum turbulence on length scales smaller than the inter-vortex
separation.Comment: 5 pages, 4 figures, and Supplemental Material (2 pages, 2 figures
Trapped ghosts: a new class of wormholes
We construct examples of static, spherically symmetric wormhole solutions in
general relativity with a minimally coupled scalar field whose kinetic
energy is negative in a restricted region of space near the throat (of
arbitrary size) and positive far from it. Thus in such configurations a "ghost"
is trapped in the strong-field region, which may in principle explain why no
ghosts are observed under usual conditions. Some properties of general wormhole
models with the field are revealed: it is shown that (i) trapped-ghost
wormholes are only possible with nonzero potentials ; (ii) in twice
asymptotically flat wormholes, a nontrivial potential has an
alternate sign, and (iii) a twice asymptotically flat wormhole which is
mirror-symmetric with respect to its throat has necessarily a zero
Schwarzschild mass at both asymptotics.Comment: 4.2 pages, 4 figures. Version to appear in CQ
A new correlator in quantum spin chains
We propose a new correlator in one-dimensional quantum spin chains, the
Emptiness Formation Probability (EFP). This is a natural generalization
of the Emptiness Formation Probability (EFP), which is the probability that the
first spins of the chain are all aligned downwards. In the EFP we let
the spins in question be separated by sites. The usual EFP corresponds to
the special case when , and taking allows us to quantify non-local
correlations. We express the EFP for the anisotropic XY model in a
transverse magnetic field, a system with both critical and non-critical
regimes, in terms of a Toeplitz determinant. For the isotropic XY model we find
that the magnetic field induces an interesting length scale.Comment: 6 pages, 1 figur
Pearson's random walk in the space of the CMB phases: evidence for parity asymmetry
The temperature fluctuations of the Cosmic Microwave Background (CMB) are
supposed to be distributed randomly in both magnitude and phase, following to
the simplest model of inflation. In this paper, we look at the odd and even
multipoles of the spherical harmonic decomposition of the CMB, and the
different characteristics of these, giving rise to a parity asymmetry. We
compare the even and odd multipoles in the CMB power spectrum, and also the
even and odd mean angles. We find for the multipoles of the power spectrum,
that there is power excess in odd multipoles, compared to even ones, meaning
that we have a parity asymmetry. Further, for the phases, we present a random
walk for the mean angles, and find a significant separation for even/odd mean
angles, especially so for galactic coordinates. This is further tested and
confirmed with a directional parity test, comparing the parity asymmetry in
galactic and ecliptic coordinates.Comment: Accepted for publication in Phys. Rev. D, 10 pages, 10 figures, 1
table. Some typographical errors corrected, and further references adde
IRAS versus POTENT Density Fields on Large Scales: Biasing and Omega
The galaxy density field as extracted from the IRAS 1.2 Jy redshift survey is
compared to the mass density field as reconstructed by the POTENT method from
the Mark III catalog of peculiar velocities. The reconstruction is done with
Gaussian smoothing of radius 12 h^{-1}Mpc, and the comparison is carried out
within volumes of effective radii 31-46 h^{-1}Mpc, containing approximately
10-26 independent samples. Random and systematic errors are estimated from
multiple realizations of mock catalogs drawn from a simulation that mimics the
observed density field in the local universe. The relationship between the two
density fields is found to be consistent with gravitational instability theory
in the mildly nonlinear regime and a linear biasing relation between galaxies
and mass. We measure beta = Omega^{0.6}/b_I = 0.89 \pm 0.12 within a volume of
effective radius 40 h^{-1}Mpc, where b_I is the IRAS galaxy biasing parameter
at 12 h^{-1}Mpc. This result is only weakly dependent on the comparison volume,
suggesting that cosmic scatter is no greater than \pm 0.1. These data are thus
consistent with Omega=1 and b_I\approx 1. If b_I>0.75, as theoretical models of
biasing indicate, then Omega>0.33 at 95% confidence. A comparison with other
estimates of beta suggests scale-dependence in the biasing relation for IRAS
galaxies.Comment: 35 pages including 10 figures, AAS Latex, Submitted to The
Astrophysical Journa
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