111,642 research outputs found
Heat transfer in aerospace propulsion
Presented is an overview of heat transfer related research in support of aerospace propulsion, particularly as seen from the perspective of the NASA Lewis Research Center. Aerospace propulsion is defined to cover the full spectrum from conventional aircraft power plants through the Aerospace Plane to space propulsion. The conventional subsonic/supersonic aircraft arena, whether commercial or military, relies on the turbine engine. A key characteristic of turbine engines is that they involve fundamentally unsteady flows which must be properly treated. Space propulsion is characterized by very demanding performance requirements which frequently push systems to their limits and demand tailored designs. The hypersonic flight propulsion systems are subject to severe heat loads and the engine and airframe are truly one entity. The impact of the special demands of each of these aerospace propulsion systems on heat transfer is explored
Sensitive Absorption Imaging of Single Atoms in Front of a Mirror
In this paper we show that the sensitivity of absorption imaging of ultracold
atoms can be significantly improved by imaging in a standing-wave
configuration. We present simulations of single-atom absorption imaging both
for a travelling-wave and a standing-wave imaging setup, based on a scattering
approach to calculate the optical density of a single atom. We find that the
optical density of a single atom is determined only by the numerical aperture
of the imaging system. We determine optimum imaging parameters, taking all
relevant sources of noise into account. For reflective imaging we find an
improvement of 1.7 in the maximum signal-to-noise ratio can be achieved. This
is particularly useful for imaging in the vicinity of an atom chip, where a
reflective surface is naturally present
The Rényi Redundancy of Generalized Huffman Codes
Huffman's algorithm gives optimal codes, as measured by average codeword length, and the redundancy can be measured as the difference between the average codeword length and Shannon's entropy. If the objective function is replaced by an exponentially weighted average, then a simple modification of Huffman's algorithm gives optimal codes. The redundancy can now be measured as the difference between this new average and A. Renyi's (1961) generalization of Shannon's entropy. By decreasing some of the codeword lengths in a Shannon code, the upper bound on the redundancy given in the standard proof of the noiseless source coding theorem is improved. The lower bound is improved by randomizing between codeword lengths, allowing linear programming techniques to be used on an integer programming problem. These bounds are shown to be asymptotically equal. The results are generalized to the Renyi case and are related to R.G. Gallager's (1978) bound on the redundancy of Huffman codes
Asymptotic Weight Enumerators of Randomly Punctured, Expurgated, and Shortened Code Ensembles
In this paper, we examine the effect of random
puncturing, expurgating, and shortening on the asymptotic
weight enumerator of certain linear code ensembles. We begin
by discussing the actions of the three alteration methods on
individual codes. We derive expressions for the average resulting
code weight enumerator under each alteration. We then extend
these results to the spectral shape of linear code ensembles
whose original spectral shape is known, and demonstrate our
findings on two specific code ensembles: the Shannon ensemble
and the regular (j, k) Gallager ensemble
Evidence for Stellar Streaming in the Cores of Elliptical Galaxies: A Kinematic Signature of Mergers?
We present evidence for non-Gaussian velocity fields within the cores of
luminous elliptical galaxies. This evidence is based upon high signal-to-noise,
medium-resolution spectroscopy of the cores of early-type members of the Virgo
and Coma clusters obtained with the WIYN 3.5-m telescope. The Virgo data were
acquired using an integral-field unit (DensePak) allowing the velocity field to
be sampled over a variety of spatial scales. The Coma data were obtained
through single, 2-arcsec diameter fibers. The cross-correlation profiles of
luminous ellipticals show considerable structure, often having several features
with amplitudes as high as 10% that of the cross-correlation peak itself. This
structure is most obvious within a radius of 1.5 arcsec (at Virgo), or < 100
pc, and is nearly undetectable when the data are binned over R < 15 arcsec, or
< 1 kpc. Similar features are found in the single-fiber spectra of the luminous
ellipticals in the Coma Cluster suggesting they are ubiquitous to giant
ellipticals. Interesting, only the most luminous elliptical galaxies show this
phenomena; the central regions of lower luminosity ellipticals have regular,
Gaussian-like profiles. We interpret this kinematic structure as ``stellar
streaming'' and suggest that this phenomena could be a relic signature of the
merger history of luminous elliptical galaxies.Comment: Latex, 5 pages, 2 figure
Native and Non-Native Speaker Judgements on the Quality of Synthesized Speech
The difference between native speakers' and non-native speak- ers' naturalness judgements of synthetic speech is investigated. Similar/difference judgements are analysed via a multidimen- sional scaling analysis and compared to Mean opinion scores. It is shown that although the two groups generally behave in a similar manner the variance of non-native speaker judgements is generally higher. While both groups of subject can clearly distinguish natural speech from the best synthetic examples, the groups' responses to different artefacts present in the synthetic speech can vary
The Elementary Particles as Quantum Knots in Electroweak Theory
We explore a knot model of the elementary particles that is compatible with
electroweak physics. The knots are quantized and their kinematic states are
labelled by , irreducible representations of , where j =
N/2, m = w/2, m' = (r+1)/2 and (N,w,r) designate respectively the number of
crossings, the writhe, and the rotation of the knot. The knot quantum numbers
(N,w,r) are related to the standard isotopic spin quantum numbers
by , where is the hypercharge. In this
model the elementary fermions are low lying states of the quantum trefoil (N=3)
and the gauge bosons are ditrefoils (N=6). The fermionic knots interact by the
emission and absorption of bosonic knots. In this framework we have explored a
slightly modified standard electroweak Lagrangian with a slightly modified
gauge group which agrees closely but not entirely with standard electroweak
theory.Comment: 29 pages; LaTex fil
Acoustic Oscillations in the Early Universe and Today
During its first ~100,000 years, the universe was a fully ionized plasma with
a tight coupling by Thompson scattering between the photons and matter. The
trade--off between gravitational collapse and photon pressure causes acoustic
oscillations in this primordial fluid. These oscillations will leave
predictable imprints in the spectra of the cosmic microwave background and the
present day matter-density distribution. Recently, the BOOMERANG and MAXIMA
teams announced the detection of these acoustic oscillations in the cosmic
microwave background (observed at redshift ~1000). Here, we compare these CMB
detections with the corresponding acoustic oscillations in the matter-density
power spectrum (observed at redshift ~0.1). These consistent results, from two
different cosmological epochs, provide further support for our standard Hot Big
Bang model of the universe.Comment: To appear in the journal Science. 6 pages, 1 color figur
Deviations from ozone photostationary state during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign: Use of measurements and photochemical modeling to assess potential causes
Nitric oxide (NO) and nitrogen dioxide (NO2) were monitored at the University of New Hampshire Atmospheric Observing Station at Thompson Farm (TF) during the ICARTT campaign of summer 2004. Simultaneous measurement of ozone (O3), temperature, and the photolysis rate of NO2 (jNO2) allow for assessment of the O3 photostationary state (Leighton ratio, Φ). Leighton ratios that are significantly greater than unity indicate that peroxy radicals (PO2), halogen monoxides, nitrate radicals, or some unidentified species convert NO to NO2 in excess of the reaction between NO and O3. Deviations from photostationary state occurred regularly at TF (1.0 ≤ Φ ≤ 5.9), particularly during times of low NOx (NOx = NO + NO2). Such deviations were not controlled by dynamics, as indicated by regressions between Φ and several meteorological parameters. Correlation with jNO2 was moderate, indicating that sunlight probably controls nonlinear processes that affect Φ values. Formation of PO2 likely is dominated by oxidation of biogenic hydrocarbons, particularly isoprene, the emission of which is driven by photosynthetically active radiation. Halogen atoms are believed to form via photolysis of halogenated methane compounds. Nitrate radicals are believed to be insignificant. Higher Φ values are associated with lower mixing ratios of isoprene and chloroiodomethane and lower ratios of NOx to total active nitrogen, indicating that photochemical aging may very well lead to increased Φ values. PO2 levels calculated using a zero‐dimensional model constrained by measurements from TF can account for 71% of the observed deviations on average. The remainder is assumed to be associated with halogen atoms, most likely iodine, with necessary mixing ratios up to 0.6 or 1.2 pptv, for chlorine and iodine, respectively
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