5,530 research outputs found
On Conditional Statistics in Scalar Turbulence: Theory vs. Experiment
We consider turbulent advection of a scalar field T(\B.r), passive or
active, and focus on the statistics of gradient fields conditioned on scalar
differences across a scale . In particular we focus on two
conditional averages and
. We find exact relations between
these averages, and with the help of the fusion rules we propose a general
representation for these objects in terms of the probability density function
of . These results offer a new way to analyze
experimental data that is presented in this paper. The main question that we
ask is whether the conditional average is linear in . We show that there exists a dimensionless
parameter which governs the deviation from linearity. The data analysis
indicates that this parameter is very small for passive scalar advection, and
is generally a decreasing function of the Rayleigh number for the convection
data.Comment: Phys. Rev. E, Submitted. REVTeX, 10 pages, 5 figs. (not included) PS
Source of the paper with figure available at
http://lvov.weizmann.ac.il/onlinelist.html#unpub
Dynamics of Scalar Fields in the Background of Rotating Black Holes
A numerical study of the evolution of a massless scalar field in the
background of rotating black holes is presented. First, solutions to the wave
equation are obtained for slowly rotating black holes. In this approximation,
the background geometry is treated as a perturbed Schwarzschild spacetime with
the angular momentum per unit mass playing the role of a perturbative
parameter. To first order in the angular momentum of the black hole, the scalar
wave equation yields two coupled one-dimensional evolution equations for a
function representing the scalar field in the Schwarzschild background and a
second field that accounts for the rotation. Solutions to the wave equation are
also obtained for rapidly rotating black holes. In this case, the wave equation
does not admit complete separation of variables and yields a two-dimensional
evolution equation. The study shows that, for rotating black holes, the late
time dynamics of a massless scalar field exhibit the same power-law behavior as
in the case of a Schwarzschild background independently of the angular momentum
of the black hole.Comment: 14 pages, RevTex, 6 Figure
The Constraints and Spectra of a Deformed Quantum Mechanics
We examine a deformed quantum mechanics in which the commutator between
coordinates and momenta is a function of momenta. The Jacobi identity
constraint on a two-parameter class of such modified commutation relations
(MCR's) shows that they encode an intrinsic maximum momentum; a sub-class of
which also imply a minimum position uncertainty. Maximum momentum causes the
bound state spectrum of the one-dimensional harmonic oscillator to terminate at
finite energy, whereby classical characteristics are observed for the studied
cases. We then use a semi-classical analysis to discuss general concave
potentials in one dimension and isotropic power-law potentials in higher
dimensions. Among other conclusions, we find that in a subset of the studied
MCR's, the leading order energy shifts of bound states are of opposite sign
compared to those obtained using string-theory motivated MCR's, and thus these
two cases are more easily distinguishable in potential experiments.Comment: 30 pages inclusive of 7 figure
Passive Scalar: Scaling Exponents and Realizability
An isotropic passive scalar field advected by a rapidly-varying velocity
field is studied. The tail of the probability distribution for
the difference in across an inertial-range distance is found
to be Gaussian. Scaling exponents of moments of increase as
or faster at large order , if a mean dissipation conditioned on is
a nondecreasing function of . The computed numerically
under the so-called linear ansatz is found to be realizable. Some classes of
gentle modifications of the linear ansatz are not realizable.Comment: Substantially revised to conform with published version. Revtex (4
pages) with 2 postscript figures. Send email to [email protected]
Cauchy-characteristic Evolution of Einstein-Klein-Gordon Systems: The Black Hole Regime
The Cauchy+characteristic matching (CCM) problem for the scalar wave equation
is investigated in the background geometry of a Schwarzschild black hole.
Previously reported work developed the CCM framework for the coupled
Einstein-Klein-Gordon system of equations, assuming a regular center of
symmetry. Here, the time evolution after the formation of a black hole is
pursued, using a CCM formulation of the governing equations perturbed around
the Schwarzschild background. An extension of the matching scheme allows for
arbitrary matching boundary motion across the coordinate grid. As a proof of
concept, the late time behavior of the dynamics of the scalar field is
explored. The power-law tails in both the time-like and null infinity limits
are verified.Comment: To appear in Phys. Rev. D, 9 pages, revtex, 5 figures available at
http://www.astro.psu.edu/users/nr/preprints.htm
Wave Propagation in Gravitational Systems: Late Time Behavior
It is well-known that the dominant late time behavior of waves propagating on
a Schwarzschild spacetime is a power-law tail; tails for other spacetimes have
also been studied. This paper presents a systematic treatment of the tail
phenomenon for a broad class of models via a Green's function formalism and
establishes the following. (i) The tail is governed by a cut of the frequency
Green's function along the ~Im~ axis,
generalizing the Schwarzschild result. (ii) The dependence of the cut
is determined by the asymptotic but not the local structure of space. In
particular it is independent of the presence of a horizon, and has the same
form for the case of a star as well. (iii) Depending on the spatial
asymptotics, the late time decay is not necessarily a power law in time. The
Schwarzschild case with a power-law tail is exceptional among the class of the
potentials having a logarithmic spatial dependence. (iv) Both the amplitude and
the time dependence of the tail for a broad class of models are obtained
analytically. (v) The analytical results are in perfect agreement with
numerical calculations
Opportunistic Uses of the Traditional School Day Through Student Examination of Fitbit Activity Tracker Data
In large part due to the highly prescribed nature of the typical school day for children, efforts to design new interactions with technology have often focused on less-structured after-school clubs and other out-of-school environments. We argue that while the school day imposes serious restrictions, school routines can and should be opportunistically leveraged by designers and by youth. Specifically, wearable activity tracking devices open some new avenues for opportunistic collection of and reflection on data from the school day. To demonstrate this, we present two cases from an elementary statistics classroom unit we designed that intentionally integrated wearable activity trackers and childcreated data visualizations. The first case involves a group of students comparing favored recess activities to determine which was more physically demanding. The second case is of a student who took advantage of her knowledge of teachers’ school day routines to test the reliability of a Fitbit activity tracker against a commercial mobile app
High-Order Contamination in the Tail of Gravitational Collapse
It is well known that the late-time behaviour of gravitational collapse is
{\it dominated} by an inverse power-law decaying tail. We calculate {\it
higher-order corrections} to this power-law behaviour in a spherically
symmetric gravitational collapse. The dominant ``contamination'' is shown to
die off at late times as . This decay rate is much {\it
slower} than has been considered so far. It implies, for instance, that an
`exact' (numerical) determination of the power index to within
requires extremely long integration times of order . We show that the
leading order fingerprint of the black-hole electric {\it charge} is of order
.Comment: 12 pages, 2 figure
Wave Propagation in Gravitational Systems: Completeness of Quasinormal Modes
The dynamics of relativistic stars and black holes are often studied in terms
of the quasinormal modes (QNM's) of the Klein-Gordon (KG) equation with
different effective potentials . In this paper we present a systematic
study of the relation between the structure of the QNM's of the KG equation and
the form of . In particular, we determine the requirements on in
order for the QNM's to form complete sets, and discuss in what sense they form
complete sets. Among other implications, this study opens up the possibility of
using QNM expansions to analyse the behavior of waves in relativistic systems,
even for systems whose QNM's do {\it not} form a complete set. For such
systems, we show that a complete set of QNM's can often be obtained by
introducing an infinitesimal change in the effective potential
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