559 research outputs found
Tests of Spheres with Reference to Reynolds Number, Turbulence, and Surface Roughness
The behavior of the Reynolds Number of the sphere is explained (in known manner) with the aid of the boundary-layer theory. Rear spindles may falsify, under certain conditions, the supercritical sphere drag, while suspension wires in the space behind the sphere leave no traceable influence. The critical Reynolds Number of the sphere was arrived at by an unconventional method; that is, by determining the critical wind speed at which the static pressure at the back of the sphere is the same as that of the undisturbed flow. The method makes it possible to interpret the critical Reynolds Number with only one test station
Studying Turbulence using Doppler-broadened lines: Velocity Coordinate Spectrum
We discuss a new technique for studying astrophysical turbulence that
utilizes the statistics of Doppler-broadened spectral lines. The technique
relates the power Velocity Coordinate Spectrum (VCS), i.e. the spectrum of
fluctuations measured along the velocity axis in Position-Position-Velocity
(PPV) data cubes available from observations, to the underlying power spectra
of the velocity/density fluctuations. Unlike the standard spatial spectra, that
are function of angular wavenumber, the VCS is a function of the velocity wave
number k_v ~ 1/v. We show that absorption affects the VCS to a higher degree
for small k_v and obtain the criteria for disregarding the absorption effects
for turbulence studies at large k_v. We consider the retrieval of turbulence
spectra from observations for high and low spatial resolution observations and
find that the VCS allows one to study turbulence even when the emitting
turbulent volume is not spatially resolved. This opens interesting prospects
for using the technique for extragalactic research. We show that, while thermal
broadening interferes with the turbulence studies using the VCS, it is possible
to separate thermal and non-thermal contributions. This allows a new way of
determining the temperature of the interstellar gas using emission and
absorption spectral lines.Comment: 27 page, 3 figures, content extended and presentation reorganized to
correspond to the version accepted to Ap
Out-Of-Focus Holography at the Green Bank Telescope
We describe phase-retrieval holography measurements of the 100-m diameter
Green Bank Telescope using astronomical sources and an astronomical receiver
operating at a wavelength of 7 mm. We use the technique with parameterization
of the aperture in terms of Zernike polynomials and employing a large defocus,
as described by Nikolic, Hills & Richer (2006). Individual measurements take
around 25 minutes and from the resulting beam maps (which have peak signal to
noise ratios of 200:1) we show that it is possible to produce low-resolution
maps of the wavefront errors with accuracy around a hundredth of a wavelength.
Using such measurements over a wide range of elevations, we have calculated a
model for the wavefront-errors due to the uncompensated gravitational
deformation of the telescope. This model produces a significant improvement at
low elevations, where these errors are expected to be the largest; after
applying the model, the aperture efficiency is largely independent of
elevation. We have also demonstrated that the technique can be used to measure
and largely correct for thermal deformations of the antenna, which often exceed
the uncompensated gravitational deformations during daytime observing.
We conclude that the aberrations induced by gravity and thermal effects are
large-scale and the technique used here is particularly suitable for measuring
such deformations in large millimetre wave radio telescopes.Comment: 10 pages, 7 figures (accepted by Astronomy & Astrophysics
PyCOOL - a Cosmological Object-Oriented Lattice code written in Python
There are a number of different phenomena in the early universe that have to
be studied numerically with lattice simulations. This paper presents a graphics
processing unit (GPU) accelerated Python program called PyCOOL that solves the
evolution of scalar fields in a lattice with very precise symplectic
integrators. The program has been written with the intention to hit a sweet
spot of speed, accuracy and user friendliness. This has been achieved by using
the Python language with the PyCUDA interface to make a program that is easy to
adapt to different scalar field models. In this paper we derive the symplectic
dynamics that govern the evolution of the system and then present the
implementation of the program in Python and PyCUDA. The functionality of the
program is tested in a chaotic inflation preheating model, a single field
oscillon case and in a supersymmetric curvaton model which leads to Q-ball
production. We have also compared the performance of a consumer graphics card
to a professional Tesla compute card in these simulations. We find that the
program is not only accurate but also very fast. To further increase the
usefulness of the program we have equipped it with numerous post-processing
functions that provide useful information about the cosmological model. These
include various spectra and statistics of the fields. The program can be
additionally used to calculate the generated curvature perturbation. The
program is publicly available under GNU General Public License at
https://github.com/jtksai/PyCOOL . Some additional information can be found
from http://www.physics.utu.fi/tiedostot/theory/particlecosmology/pycool/ .Comment: 23 pages, 12 figures; some typos correcte
Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence
(Modified) The scaling of velocity fluctuation, dv, as a function of spatial
scale L in molecular clouds can be measured from size-linewidth relations,
principal component analysis, or line centroid variation. Differing values of
the power law index of the scaling relation dv = L^(g3D) in 3D are given by
these different methods: the first two give g3D=0.5, while line centroid
analysis gives g3D=0. This discrepancy has previously not been fully
appreciated, as the variation of projected velocity line centroid fluctuations
(dv_{lc} = L^(g2D)) is indeed described, in 2D, by g2D=0.5. However, if
projection smoothing is accounted for, this implies that g3D=0. We suggest that
a resolution of this discrepancy can be achieved by accounting for the effect
of density inhomogeneity on the observed g2D obtained from velocity line
centroid analysis. Numerical simulations of compressible turbulence are used to
show that the effect of density inhomogeneity statistically reverses the effect
of projection smoothing in the case of driven turbulence so that velocity line
centroid analysis does indeed predict that g2D=g3D=0.5. Using our numerical
results we can restore consistency between line centroid analysis, principal
component analysis and size-linewidth relations, and we derive g3D=0.5,
corresponding to shock-dominated (Burgers) turbulence. We find that this
consistency requires that molecular clouds are continually driven on large
scales or are only recently formed.Comment: 28 pages total, 20 figures, accepted for publication in Ap
MYRIAD: A new N-body code for simulations of Star Clusters
We present a new C++ code for collisional N-body simulations of star
clusters. The code uses the Hermite fourth-order scheme with block time steps,
for advancing the particles in time, while the forces and neighboring particles
are computed using the GRAPE-6 board. Special treatment is used for close
encounters, binary and multiple sub-systems that either form dynamically or
exist in the initial configuration. The structure of the code is modular and
allows the appropriate treatment of more physical phenomena, such as stellar
and binary evolution, stellar collisions and evolution of close black-hole
binaries. Moreover, it can be easily modified so that the part of the code that
uses GRAPE-6, could be replaced by another module that uses other
accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate
choice of the free parameters give a good accuracy and speed for simulations of
star clusters up to and beyond core collapse. Simulations of Plummer models
consisting of equal-mass stars reached core collapse at t~17 half-mass
relaxation times, which compares very well with existing results, while the
cumulative relative error in the energy remained below 0.001. Also, comparisons
with published results of other codes for the time of core collapse for
different initial conditions, show excellent agreement. Simulations of King
models with an initial mass-function, similar to those found in the literature,
reached core collapse at t~0.17, which is slightly smaller than the expected
result from previous works. Finally, the code accuracy becomes comparable and
even better than the accuracy of existing codes, when a number of close binary
systems is dynamically created in a simulation. This is due to the high
accuracy of the method that is used for close binary and multiple sub-systems.Comment: 24 pages, 29 figures, accepted for publication to Astronomy &
Astrophysic
Power Balance in Aerodynamic Flows
A control volume analysis of the compressible viscous flow about an aircraft is performed,including integrated propulsors and flow control systems. In contrast to most past analyses which have focused on forces and momentum flow, in particular thrust and drag, the present analysis
focuses on mechanical power and kinetic energy flow. The result is a clear identification and quantification of all the power sources, power sinks, and their interactions which are present in any aerodynamic flow. The formulation does not require any separate definitions of thrust and drag, and hence it is especially useful for analysis and optimization of aerodynamic configurations which have tightly integrated propulsion and boundary layer control systems
Small Scale Structure at High Redshift: II. Physical Properties of the CIV Absorbing Clouds
Keck HIRES spectra were obtained of the separate images of three
gravitationally lensed QSOs (UM 673, Q1104-1804, and Q1422+2309). We studied
the velocity and column density differences in CIV doublets in each QSO. Unlike
the low ionization gas clouds typical of the interstellar gas in the Galaxy or
damped Ly alpha galaxies, the spatial density distribution of CIV absorbing gas
clouds turns out to be mostly featureless on scales up to a few hundred
parsecs, with column density differences rising to 50 percent or more over
separations beyond a few kpc. Similarly, velocity shear becomes detectable only
over distances larger than a few hundred pc, rising to 70 km/s at a few kpc.
The energy transmitted to the gas is substantially less than in present day
star-forming regions, and the gas is less turbulent on a given spatial scale
than, e.g., local HII regions. The quiescence of CIV clouds, taken with their
probable low density, imply that these objects are not internal to galaxies.
The CIV absorbers could be gas expelled recently to large radii and raining
back onto its parent galaxy, or pre-enriched gas from an earlier (population
III) episode of star formation, falling into the nearest mass concentration.
However, while the metals in the gas may have been formed at higher redshifts,
the residual turbulence in the clouds and the minimum coherence length measured
here imply that the gas was stirred more recently, possibly by star formation
events recurring on a timescale on the order of 10-100 Million years (abstract
abbreviated).Comment: latex file plus 15 postscript figures (45 pages in total); to be
published in the ApJ, June 20, 2001 issu
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