4,472 research outputs found
Measurements in the turbulent boundary layer at constant pressure in subsonic and supersonic flow. Part 1: Mean flow
Experiments were carried out to test the accuracy of laser Doppler instrumentation for measurement of Reynolds stresses in turbulent boundary layers in supersonic flow. Two facilities were used to study flow at constant pressure. In one facility, data were obtained on a flat plate at M sub e = 0.1, with Re theta up to 8,000. In the other, data were obtained on an adiabatic nozzle wall at M sub e = 0.6, 0.8, 1.0, 1.3, and 2.2, with Re theta = 23,000 and 40,000. The mean flow as observed using Pitot tube, Preston tube, and floating element instrumentation is described. Emphasis is on the use of similarity laws with Van Driest scaling and on the inference of the shearing stress profile and the normal velocity component from the equations of mean motion. The experimental data are tabulated
Detecting Pulsars with Interstellar Scintillation in Variance Images
Pulsars are the only cosmic radio sources known to be sufficiently compact to
show diffractive interstellar scintillations. Images of the variance of radio
signals in both time and frequency can be used to detect pulsars in large-scale
continuum surveys using the next generation of synthesis radio telescopes. This
technique allows a search over the full field of view while avoiding the need
for expensive pixel-by-pixel high time resolution searches. We investigate the
sensitivity of detecting pulsars in variance images. We show that variance
images are most sensitive to pulsars whose scintillation time-scales and
bandwidths are close to the subintegration time and channel bandwidth.
Therefore, in order to maximise the detection of pulsars for a given radio
continuum survey, it is essential to retain a high time and frequency
resolution, allowing us to make variance images sensitive to pulsars with
different scintillation properties. We demonstrate the technique with
Murchision Widefield Array data and show that variance images can indeed lead
to the detection of pulsars by distinguishing them from other radio sources.Comment: 8 papes, 9 figures, accepted for publication in MNRA
Experiments in free shear flows: Status and needs for the future
Experiments in free turbulent flows are recommended with the primary concern placed on classical flows in order to augment understanding and for model building. Five classes of experiments dealing with classical free turbulent flows are outlined and proposed as being of particular significance for the near future. These classes include the following: (1) Experiments clarifying the effect of density variation owing to use of different gases, with and without the additional effect of density variation due to high Mach number or other effects; (2) experiments clarifying the role and importance of various parameters which determine the behavior of the near field as well as the condictions under which any of these parameters can be neglected; (3) experiments determining the cumulative effect of initial conditions in terms of distance to fully established flow; (4) experiments for cases where two layers of distinctly different initial turbulence structure flow side by side at the same mean speed; and (5) experiment using contemporary experimental techniques to study structure in free turbulent shear flows in order to compliment and support contemporary work on boundary layers
Particle decay in inflationary cosmology
We investigate the relaxation and decay of a particle during inflation by
implementing the dynamical renormalization group. This investigation allows us
to give a meaningful definition for the decay rate in an expanding universe. As
a prelude to a more general scenario, the method is applied here to study the
decay of a particle in de Sitter inflation via a trilinear coupling to massless
conformally coupled particles, both for wavelengths much larger and much
smaller than the Hubble radius. For superhorizon modes we find that the decay
is of the form eta^{Gamma1} with eta being conformal time and we give an
explicit expression for Gamma1 to leading order in the coupling which has a
noteworthy interpretation in terms of the Hawking temperature of de Sitter
space-time. We show that if the mass M of the decaying field is << H then the
decay rate during inflation is enhanced over the Minkowski spacetime result by
a factor 2H/[pi M]. For wavelengths much smaller than the Hubble radius we find
that the decay law is e^{-alpha/[k H C(eta)} with C(eta) the scale factor and
alpha determined by the strength of the trilinear coupling. This result
suggests a suppression of power for long wavelength modes upon horizon
crossing. In all cases we find a substantial enhancement in the decay law as
compared to Minkowski space-time. These results suggest potential implications
for the spectrum of scalar density fluctuations as well as non-gaussianities.Comment: 19 pages, 1 .eps figure. Improved version to appear in Phys. Rev.
Far Noise Field of Air Jets and Jet Engines
An experimental investigation was conducted to study and compare the acoustic radiation of air jets and jet engines. A number of different nozzle-exit shapes were studied with air jets to determine the effect of exit shape on noise generation. Circular, square, rectangular, and elliptical convergent nozzles and convergent-divergent and plug nozzles were investigated. The spectral distributions of the sound power for the engine and the air jet were in good agreement for the case where the engine data were not greatly affected by reflection or jet interference effects. Such power spectra for a subsonic or slightly choked engine or air jet show that the peaks of the spectra occur at a Strouhal number of 0.3
Full-Scale Investigation of Several Jet-Engine Noise-Reduction Nozzles
A number of noise-suppression nozzles were tested on full-scale engines. In general, these nozzles achieved noise reduction by the mixing interference of adjacent jets, that is, by using multiple-slot-nozzles. Several of the nozzles achieved reductions in sound power of approximately 5 decibels (nearly 70 percent) with small thrust losses (approx. 1 percent). The maximum sound-pressure level was reduced by as much as 18 decibels in particular frequency bands. Some of the nozzles showed considerable spatial asymmetry; that is, the sound field was not rotationally symmetrical. A method of calculating the limiting frequency effected by such nozzles is presented. Furthermore data are shown that appear to indicate that further reductions in sound power will not be easily achieved from nozzles using mixing interference as a means of noise suppressio
Large-Scale Magnetic Fields, Dark Energy and QCD
Cosmological magnetic fields are being observed with ever increasing
correlation lengths, possibly reaching the size of superclusters, therefore
disfavouring the conventional picture of generation through primordial seeds
later amplified by galaxy-bound dynamo mechanisms. In this paper we put forward
a fundamentally different approach that links such large-scale magnetic fields
to the cosmological vacuum energy. In our scenario the dark energy is due to
the Veneziano ghost (which solves the problem in QCD). The Veneziano
ghost couples through the triangle anomaly to the electromagnetic field with a
constant which is unambiguously fixed in the standard model. While this
interaction does not produce any physical effects in Minkowski space, it
triggers the generation of a magnetic field in an expanding universe at every
epoch. The induced energy of the magnetic field is thus proportional to
cosmological vacuum energy: , hence acting as a source for the magnetic energy
. The corresponding numerical estimate leads to a magnitude in the
nG range. There are two unique and distinctive predictions of our proposal: an
uninterrupted active generation of Hubble size correlated magnetic fields
throughout the evolution of the universe; the presence of parity violation on
the enormous scales , which apparently has been already observed in CMB.
These predictions are entirely rooted into the standard model of particle
physics.Comment: jhep style, 22 pages, v2 with updated estimates and extended
discussion on parity violation, v3 as published (references updated
The quantum inflaton, primordial perturbations and CMB fluctuations
We compute the primordial scalar, vector and tensor metric perturbations
arising from quantum field inflation. Quantum field inflation takes into
account the nonperturbative quantum dynamics of the inflaton consistently
coupled to the dynamics of the (classical) cosmological metric. For chaotic
inflation, the quantum treatment avoids the unnatural requirements of an
initial state with all the energy in the zero mode. For new inflation it allows
a consistent treatment of the explosive particle production due to spinodal
instabilities. Quantum field inflation (under conditions that are the quantum
analog of slow roll) leads, upon evolution, to the formation of a condensate
starting a regime of effective classical inflation. We compute the primordial
perturbations taking the dominant quantum effects into account. The results for
the scalar, vector and tensor primordial perturbations are expressed in terms
of the classical inflation results. For a N-component field in a O(N) symmetric
model, adiabatic fluctuations dominate while isocurvature or entropy
fluctuations are negligible. The results agree with the current WMAP
observations and predict corrections to the power spectrum in classical
inflation.Such corrections are estimated to be of the order of m^2/[N H^2]
where m is the inflaton mass and H the Hubble constant at horizon crossing.
This turns to be about 4% for the cosmologically relevant scales. This quantum
field treatment of inflation provides the foundations to the classical
inflation and permits to compute quantum corrections to it.Comment: 23 pages, no figures. Improved version to appear in Phys. Rev.
Ordering dynamics of blue phases entails kinetic stabilization of amorphous networks
The cubic blue phases of liquid crystals are fascinating and technologically
promising examples of hierarchically structured soft materials, comprising
ordered networks of defect lines (disclinations) within a liquid crystalline
matrix. We present the first large-scale simulations of their domain growth,
starting from a blue phase nucleus within a supercooled isotropic or
cholesteric background. The nucleated phase is thermodynamically stable; one
expects its slow orderly growth, creating a bulk cubic. Instead, we find that
the strong propensity to form disclinations drives the rapid disorderly growth
of a metastable amorphous defect network. During this process the original
nucleus is destroyed; re-emergence of the stable phase may therefore require a
second nucleation step. Our findings suggest that blue phases exhibit
hierarchical behavior in their ordering dynamics, to match that in their
structure.Comment: 11 pages, 5 figures, 2 supplementary figures, 2 supplementary tables,
accepted by PNA
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