234 research outputs found
Shock wave structure in highly rarefied flows
The Boltzmann equation is written in terms of two functions associated
with the gain and loss of a certain type of molecule due to collisions.
Its integral form is then applied to the problem of normal shock structure,
and an iteration technique is used to determine the solution. The first
approximation to the velocity distribution function of the Chapman-Enskog
sequence, which leads to the Navier-Stokes equations, is used to initiate
the iteration scheme. Expressions for the distribution function and the
flow parameters pertinent to the first iteration are derived and show that
the B-G-K model results can be obtained as a special case. This model is
found to be valid in the continuum regime only, and is consequently limited
to the study of strong shocks. In the present treatment the iteration is
carried out on the distribution function and the analysis indicates that
the method is equally valid for variations in both Mach and Knudsen numbers.
Finally, the results of the first approximation are simplified, and expressed
in a form suitable for numerical computation, and the range of their validity
is discussed. The method should be equally suitable for other flow problems
of linear or nonlinear nature
A non-equilibrium kinetic description of shock wave structure
A formulation for the shock wave structure is devised by viewing
the transition as a phenomenon in which non-equilibrium effects play
an important role. The essence of the method is the approximation
of Boltzmann's equation by a simpler kinetic model. Initially, the
distribution function in Boltzmann's collision integral is expressed
in terms of a function of deviation from local equilibrium. Then,
by suitably transforming the complete collision term, the molecular
velocities after collision are eliminated. At this stage the
formulation of the method is specialized to hard sphere molecules and
the problem of deriving a model equation thus reduces to one of
assigning an expression for the deviation function. In the first
instance, this function is chosen to be zero and an exploratory model is
obtained which, when its variable collision frequency is replaced by
its mean value, reduces identically to the Bhatnagar-Gross-Krook model.
However, it is found that the exploratory model provides a somewhat
crude representation of Boltzmann's equation and is shown to imply a
Prandtl number very nearly equal to unity. A more accurate model is
then derived by choosing for the deviation function the first order term
of Chapman-Enskog’s sequence, leading to the Navier-Stokes equations.
Here, the specific form of Boltzmann's collision term is represented more
accurately than hitherto and the model is found to possess all the known
features of the Boltzmann equation. It is shown that this model contains
a description of a gas in non-equilibrium state
Solar system constraints on the Dvali-Gabadadze-Porrati braneworld theory of gravity
A number of proposals have been put forward to account for the observed
accelerating expansion of the Universe through modifications of gravity. One
specific scenario, Dvali-Gabadadze-Porrati (DGP) gravity, gives rise to a
potentially observable anomaly in the solar system: all planets would exhibit a
common anomalous precession, dw/dt, in excess of the prediction of General
Relativity. We have used the Planetary Ephemeris Program (PEP) along with
planetary radar and radio tracking data to set a constraint of |dw/dt| < 0.02
arcseconds per century on the presence of any such common precession. This
sensitivity falls short of that needed to detect the estimated universal
precession of |dw/dt| = 5e-4 arcseconds per century expected in the DGP
scenario. We discuss the fact that ranging data between objects that orbit in a
common plane cannot constrain the DGP scenario. It is only through the relative
inclinations of the planetary orbital planes that solar system ranging data
have sensitivity to the DGP-like effect of universal precession. In addition,
we illustrate the importance of performing a numerical evaluation of the
sensitivity of the data set and model to any perturbative precession.Comment: 9 pages, 2 figures, accepted for publication in Phys. Rev.
APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections
A next-generation lunar laser ranging apparatus using the 3.5 m telescope at
the Apache Point Observatory in southern New Mexico has begun science
operation. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation)
has achieved one-millimeter range precision to the moon which should lead to
approximately one-order-of-magnitude improvements in the precision of several
tests of fundamental properties of gravity. We briefly motivate the scientific
goals, and then give a detailed discussion of the APOLLO instrumentation.Comment: 37 pages; 10 figures; 1 table: accepted for publication in PAS
An absolute calibration system for millimeter-accuracy APOLLO measurements
Lunar laser ranging provides a number of leading experimental tests of
gravitation -- important in our quest to unify General Relativity and the
Standard Model of physics. The Apache Point Observatory Lunar Laser-ranging
Operation (APOLLO) has for years achieved median range precision at the ~2 mm
level. Yet residuals in model-measurement comparisons are an order-of-magnitude
larger, raising the question of whether the ranging data are not nearly as
accurate as they are precise, or if the models are incomplete or
ill-conditioned. This paper describes a new absolute calibration system (ACS)
intended both as a tool for exposing and eliminating sources of systematic
error, and also as a means to directly calibrate ranging data in-situ. The
system consists of a high-repetition-rate (80 MHz) laser emitting short (< 10
ps) pulses that are locked to a cesium clock. In essence, the ACS delivers
photons to the APOLLO detector at exquisitely well-defined time intervals as a
"truth" input against which APOLLO's timing performance may be judged and
corrected. Preliminary analysis indicates no inaccuracies in APOLLO data beyond
the ~3 mm level, suggesting that historical APOLLO data are of high quality and
motivating continued work on model capabilities. The ACS provides the means to
deliver APOLLO data both accurate and precise below the 2 mm level.Comment: 21 pages, 10 figures, submitted to Classical and Quantum Gravit
A map of OMC-1 in CO 9-8
The distribution of 12C16O J=9-8 (1.037 THz) emission has been mapped in
OMC-1 at 35 points with 84" resolution. This is the first map of this source in
this transition and only the second velocity-resolved ground-based observation
of a line in the terahertz frequency band. There is emission present at all
points in the map, a region roughly 4' by 6' in size, with peak antenna
temperature dropping only near the edges. Away from the Orion KL outflow, the
velocity structure suggests that most of the emission comes from the OMC-1
photon-dominated region, with a typical linewidthof 3-6 km/s. Large velocity
gradient modeling of the emission in J=9-8 and six lower transitions suggests
that the lines originate in regions with temperatures around 120 K and
densities of at least 10^(3.5) cm^(-3) near theta^(1) C Ori and at the Orion
Bar, and from 70 K gas at around 10^(4) cm^(-3) southeast and west of the bar.
These observations are among the first made with the 0.8 m Smithsonian
Astrophysical Observatory Receiver Lab Telescope, a new instrument designed to
observe at frequencies above 1 THz from an extremely high and dry site in
northern Chile.Comment: Minor changes to references, text to match ApJ versio
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