159,303 research outputs found
The effect of helicopter configuration on the fluid dynamics of brownout
Brown’s Vorticity Transport Model, coupled to an additional particle transport model, is used to simulate
the development of the dust cloud that can form around a helicopter when operating in dusty or desert
conditions. The flow field around a tandem rotor configuration is simulated during the final stages of
landing. The time-averaged flow field around the helicopter is characterised by the existence of two
stationary points immediately adjacent to the ground plane. Almost all entrainment of dust into the flow
takes place forward of the rearmost stationary point; the dust initially remains in a thin, sheet-like layer
above the ground. As the dust sheet approaches the forward stationary point, the layer thickens and
forms a characteristic wedge-shaped ‘separation zone’. The amount of sand that is subsequently drawn
up away from the ground then appears to be critically dependent on the strength and position relative to
the separation zone of strong regions of recirculation. VTM simulations suggest that, for a tandem rotor
helicopter at least, the sudden growth of the dust cloud that is responsible for the onset of brownout may
be due to a change in mode within the flow field surrounding the aircraft. At higher advance ratios the
flow is dominated by a strong ground vortex that is created by the rear rotor. The forward extent of the
resultant dust cloud is limited though by the absence of any strong recirculation within the flow below
the front rotor of the system. At lower forward speed the ground vortex of the rear rotor is replaced
by a strong vortex that lies just below the leading edge of the front rotor. This vortex is responsible for
drawing a significant amount of dust out of the surface layer of entrained particulates to form a dense
wall of dust some distance upstream of the helicopter. A study of the effect of blade twist on the strength
and shape of the dust cloud formed in the flow surrounding helicopters with tandem rotors suggests that
systems with smaller blade twist but the same disc loading might produce denser dust clouds than those
with high blade twist
Entanglement and Relativity
In this paper we survey, in an elementary fashion, some of the questions that
arise when one considers how entanglement and relativity are related via the
notion of non-locality. We begin by reviewing the role of entangled states in
Bell inequality violation and question whether the associated notions of
non-locality lead to problems with relativity. The use of entanglement and
wavefunction collapse in Einstein's famous incompleteness argument is then
considered, before we go on to see how the issue of non-locality is transformed
if one considers quantum mechanics without collapse to be a complete theory, as
in the Everett interpretation. The opportunity is taken to consider whether
teleportation and dense coding might constitute a source of non-locality within
the Everett interpretation.Comment: 18 pages, uses amsmath, amsfonts, natbib and fancyheadings packages.
Typos corrected and additional referenc
Phase modulator Patent
Phase modulator with tuned variable length electrical lines including coupling and varactor diode circuit
Satellite magnetic modeling of north African hot spots
The primary objectives of the MAGSAT mission was to measure the intensity and direction of magnetization of the Earth's crust. A significant effort was directed to the large crustal anomalies first delineated by the POGO mission. The MAGSAT data are capable of spatial resolution of the crustal field to 250 km wavelength with reliability limits to less than 1 nT in the mean. The difficulties of dealing with less than the most robust of the MAGSAT anomalies is that often there is no more than the magnetic fields themselves to constrain geophysical models of the interior, and no independent means of assessing the quality of the crustal anomaly data in interpreting the subsurface are available
Static tests of excess ground attenuation at Wallops Flight Center
An extensive experimental measurement program which evaluated the attenuation of sound for close to horizontal propagation over the ground was designed to replicate, under static conditions, results of the flight measurements carried out earlier by NASA at the same site (Wallops Flight Center). The program consisted of a total of 41 measurement runs of attenuation, in excess of spreading and air absorption losses, for one third octave bands over a frequency range of 50 to 4000 Hz. Each run consisted of measurements at 10 locations up to 675 m, from a source located at nominal elevations of 2.5, or 10 m over either a grassy surface or an adjacent asphalt concrete runway surface. The tests provided a total of over 8100 measurements of attenuation under conditions of low wind speed averaging about 1 m/s and, for most of the tests, a slightly positive temperature gradient, averaging about 0.3 C/m from 1.2 to 7 m. The results of the measurements are expected to provide useful experimental background for the further development of prediction models of near grazing incidence sound propagation losses
- …