2,830 research outputs found
The leaking mode problem in atmospheric acoustic-gravity wave propagation
The problem of predicting the transient acoustic pressure pulse at long horizontal distances from large explosions in the atmosphere is examined. Account is taken of poles off the real axis and of branch line integrals in the general integral governing the transient waveform. Perturbation techniques are described for the computation of the imaginary ordinate of the poles and numerical studies are described for a model atmosphere terminated by a halfspace with c = 478 m/sec above 125 km. For frequencies less than 0.0125 rad/sec, the GR sub 1 mode, for example, is found to have a frequency dependent amplitude decay of the order of 0.0001 nepers/km. Examples of numerically synthesized transient waveforms are exhibited with and without the inclusion of leaking modes. The inclusion of leaking modes results in waveforms with a more marked beginning rather than a low frequency oscillating precursor of gradually increasing amplitude. Also, the revised computations indicate that waveforms invariably begin with a pressure rise, a result supported by other theoretical considerations and by experimental data
Experimental Research on 4-Duct Tandem VTOL Aircraft Configurations
This paper presents a brief summary of several wind-tunnel investigations conducted at the Langley Research Center of the NASA to study the aerodynamic and stability and control characteristics of several VTOL aircraft configurations powered by four tilting ducted propellers arranged in tandem pairs. Specifically the two rear ducts could be mounted close alongside the upper rear portion of the fuselage with small wing panels attached to the outboard side of the ducts or could be mounted outboard on the tips of a small wing located high on the rear portion of the fuselage. The two front ducts were always mounted close inboard on the forward part of the fuselage and could be mounted either in a high or low position on the fuselage. The results of the investigation indicated that aircraft of this type could have acceptable aerodynamic and static longitudinal and lateral stability and control characteristics in both transition and normal cruise flight except for the possible qualification that the lateral force due to sideslip is abnormally high and might cause the aircraft to be too sensitive to side gusts
New Solutions of the Inflationary Flow Equations
The inflationary flow equations are a frequently used method of surveying the
space of inflationary models. In these applications the infinite hierarchy of
differential equations is truncated in a way which has been shown to be
equivalent to restricting the set of models considered to those characterized
by polynomial inflaton potentials. This paper explores a different method of
solving the flow equations, which does not truncate the hierarchy and in
consequence covers a much wider class of models while retaining the practical
usability of the standard approach.Comment: References added, and a couple of comment
Slow roll in simple non-canonical inflation
We consider inflation using a class of non-canonical Lagrangians for which
the modification to the kinetic term depends on the field, but not its
derivatives. We generalize the standard Hubble slow roll expansion to the
non-canonical case and derive expressions for observables in terms of the
generalized slow roll parameters. We apply the general results to the
illustrative case of ``Slinky'' inflation, which has a simple, exactly
solvable, non-canonical representation. However, when transformed into a
canonical basis, Slinky inflation consists of a field oscillating on a
multi-valued potential. We calculate the power spectrum of curvature
perturbations for Slinky inflation directly in the non-canonical basis, and
show that the spectrum is approximately a power law on large scales, with a
``blue'' power spectrum. On small scales, the power spectrum exhibits strong
oscillatory behavior. This is an example of a model in which the widely used
solution of Garriga and Mukhanov gives the wrong answer for the power spectrum.Comment: 9 pages, LaTeX, four figures. (V2: minor changes to text. Version
submitted to JCAP.
A Hamilton-Jacobi approach to non-slow-roll inflation
I describe a general approach to characterizing cosmological inflation
outside the standard slow-roll approximation, based on the Hamilton-Jacobi
formulation of scalar field dynamics. The basic idea is to view the equation of
state of the scalar field matter as the fundamental dynamical variable, as
opposed to the field value or the expansion rate. I discuss how to formulate
the equations of motion for scalar and tensor fluctuations in situations where
the assumption of slow roll is not valid. I apply the general results to the
simple case of inflation from an ``inverted'' polynomial potential, and to the
more complicated case of hybrid inflation.Comment: 21 pages, RevTeX (minor revisions to match published version
Completing Natural Inflation
If the inflaton is a pseudo-scalar axion, the axion shift symmetry can
protect the flatness of its potential from too large radiative corrections.
This possibility, known as natural inflation, requires an axion scale which is
greater than the (reduced) Planck scale. It is unclear whether such a high
value is compatible with an effective field theoretical description, and if the
global axionic symmetry survives quantum gravity effects. We propose a
mechanism which provides an effective large axion scale, although the original
one is sub-Planckian. The mechanism is based on the presence of two axions,
with a potential provided by two anomalous gauge groups. The effective large
axion scale is due to an almost exact symmetry between the couplings of the
axions to the anomalous groups. We also comment on a possible implementation in
heterotic string theory.Comment: 9 pages, 1 figur
Ribbons on the CBR Sky: A Powerful Test of a Baryon Symmetric Universe
If the Universe consists of domains of matter and antimatter, annihilations
at domain interfaces leave a distinctive imprint on the Cosmic Background
Radiation (CBR) sky. The signature is anisotropies in the form of long, thin
ribbons of width , separated by angle where L is the characteristic domain size, and
y-distortion parameter . Such a pattern could potentially be
detected by the high-resolution CBR anisotropy experiments planned for the next
decade, and such experiments may finally settle the question of whether or not
our Hubble volume is baryon symmetric.Comment: LaTeX, 10 pages, 4 figures in epsf. Revised version corrects a couple
of relevant mistake
Investigation of shock waves in explosive blasts using fibre optic pressure sensors
The published version of this article may be accessed at the link below. Copyright @ IOP Publishing, 2006.We describe miniature all-optical pressure sensors, fabricated by wafer etching techniques, less than 1 mm(2) in overall cross-section with rise times in the mu s regime and pressure ranges typically 900 kPa (9 bar). Their performance is suitable for experimental studies of the pressure-time history for test models exposed to shocks initiated by an explosive charge. The small size and fast response of the sensors promises higher quality data than has been previously available from conventional electrical sensors, with potential improvements to numerical models of blast effects. Results from blast tests are presented in which up to six sensors were multiplexed, embedded within test models in a range of orientations relative to the shock front.Support from the UK Engineering&Physical
Sciences Research Council and Dstl Fort Halstead through the MoD Joint Grants Scheme are acknowledged. WN MacPherson is supported by an EPSRC Advanced Research Fellowship
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