58,929 research outputs found
Development of an analytical technique for the optimization of jet engine and duct acoustic liners
A special integral representation of the external solutions of the Helmholtz equation is described. The analytical technique developed for the generation of the optimum acoustic admittance for an arbitrary axisymmetric body is also presented along with some numerical procedures and some preliminary results for a straight duct
Development of an analytical technique for the optimization of jet engine and duct acoustic liners
A new method was developed for the calculation of optimum constant admittance solutions for the minimization of the sound radiated from an arbitrary axisymmetric body. This method utilizes both the integral equation technique used in the calculation of the optimum non-constant admittance liners and the independent solution generated as a by product of these calculations. The results generated by both these methods are presented for three duct geometries: (1) a straight duct; (2) the QCSEE inlet; and (3) the QCSEE inlet less its centerbody
Age differences in encoding-related alpha power reflect sentence comprehension difficulties
When sentence processing taxes verbal working memory, comprehension difficulties arise. This is specifically the case when processing resources decline with advancing adult age. Such decline likely affects the encoding of sentences into working memory, which constitutes the basis for successful comprehension. To assess age differences in encoding-related electrophysiological activity, we recorded the electroencephalogram from three age groups (24, 43, and 65 years). Using an auditory sentence comprehension task, age differences in encoding-related oscillatory power were examined with respect to the accuracy of the given response. That is, the difference in oscillatory power between correctly and incorrectly encoded sentences, yielding subsequent memory effects (SME), was compared across age groups. Across age groups, we observed an age-related SME inversion in the alpha band from a power decrease in younger adults to a power increase in older adults. We suggest that this SME inversion underlies age-related comprehension difficulties. With alpha being commonly linked to inhibitory processes, this shift may reflect a change in the cortical inhibition–disinhibition balance. A cortical disinhibition may imply enriched sentence encoding in younger adults. In contrast, resource limitations in older adults may necessitate an increase in cortical inhibition during sentence encoding to avoid an information overload. Overall, our findings tentatively suggest that age-related comprehension difficulties are associated with alterations to the electrophysiological dynamics subserving general higher cognitive functions
Configuration management and automatic control of an augmentor wing aircraft with vectored thrust
An advanced structure for automatic flight control logic for powered-lift aircraft operating in terminal areas is under investigation at Ames Research Center. This structure is based on acceleration control; acceleration commands are constructed as the sum of acceleration on the reference trajectory and a corrective feedback acceleration to regulate path tracking errors. The central element of the structure, termed a Trimmap, uses a model of the aircraft aerodynamic and engine forces to calculate the control settings required to generate the acceleration commands. This report describes the design criteria for the Trimmap and derives a Trimmap for Ames experimental augmentor wing jet STOL research aircraft
Nuclear Reactions Rates Governing the Nucleosynthesis of Ti44
Large excesses of Ca44 in certain presolar graphite and silicon carbide
grains give strong evidence for Ti44 production in supernovae. Furthermore,
recent detection of the Ti44 gamma-line from the Cas A SNR by CGRO/COMPTEL
shows that radioactive Ti44 is produced in supernovae. These make the Ti44
abundance an observable diagnostic of supernovae. Through use of a nuclear
reaction network, we have systematically varied reaction rates and groups of
reaction rates to experimentally identify those that govern Ti44 abundance in
core-collapse supernova nucleosynthesis. We survey the nuclear-rate dependence
by repeated calculations of the identical adiabatic expansion, with peak
temperature and density chosen to be 5.5xE9 K and 1E7 g/cc, respectively, to
approximate the conditions in detailed supernova models. We find that, for
equal total numbers of neutrons and protons (eta=0), Ti44 production is most
sensitive to the following reaction rates: Ti44(alpha,p)V47,
alpha(2alpha,gamma)C12, Ti44(alpha,gamma)Cr48, V45(p,gamma)Cr46. We tabulate
the most sensitive reactions in order of their importance to the Ti44
production near the standard values of currently accepted cross-sections, at
both reduced reaction rate (0.01X) and at increased reaction rate (100X)
relative to their standard values. Although most reactions retain their
importance for eta > 0, that of V45(p,gamma)Cr46 drops rapidly for eta >=
0.0004. Other reactions assume greater significance at greater neutron excess:
C12(alpha,gamma)O16, Ca40(alpha,gamma)Ti44, Al27(alpha,n)P30, Si30(alpha,n)S33.
Because many of these rates are unknown experimentally, our results suggest the
most important targets for future cross section measurements governing the
value of this observable abundance.Comment: 37 pages, LaTex, 17 figures, 8 table
AVIRIS data characteristics and their effects on spectral discrimination of rocks exposed in the Drum Mountains, Utah: Results of a preliminary study
Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data collected over a geologically diverse field site and over a nearby calibration site were analyzed and interpreted in efforts to document radiometric and geometric characteristics of AVIRIS, quantify and correct for detrimental sensor phenomena, and evaluate the utility of AVIRIS data for discriminating rock types and identifying their constituent mineralogy. AVIRIS data acquired for these studies exhibit a variety of detrimental artifacts and have lower signal-to-noise ratios than expected in the longer wavelength bands. Artifacts are both inherent in the image data and introduced during ground processing, but most may be corrected by appropriate processing techniques. Poor signal-to-noise characteristics of this AVIRIS data set limited the usefulness of the data for lithologic discrimination and mineral identification. Various data calibration techniques, based on field-acquired spectral measurements, were applied to the AVIRIS data. Major absorption features of hydroxyl-bearing minerals were resolved in the spectra of the calibrated AVIRIS data, and the presence of hydroxyl-bearing minerals at the corresponding ground locations was confirmed by field data
Acoustical evaluation of the NASA Langley full-scale wind tunnel
Determining types of acoustical measurements suitable for test section of NASA Langley wind tunne
Integrability of the critical point of the Kagom\'e three-state Potts mode
The vicinity of the critical point of the three-state Potts model on a
Kagom\'e lattice is studied by mean of Random Matrix Theory. Strong evidence
that the critical point is integrable is given.Comment: 1 LaTex file + 3 eps files 7 page
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