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  • Other Sources  (9)
  • 1
    Publication Date: 2013-08-31
    Description: Axial segmentation of acoustically absorbing liners in rectangular, circular or annual duct configurations is a very useful concept for obtaining higher noise attenuation with respect to the bandwidth of absorption as well as the maximum attenuation. As a consequence, advanced liner concepts are proposed which induce a modal energy transfer in both cross-sectional directions to further reduce the noise radiated from turbofan engines. However, these advanced liner concepts require three-dimensional geometries which are difficult to treat theoretically. A very simple three-dimensional problem is investigated analytically. The results show a strong dependence on the positioning of the liner for some incident source modes while the effect of three-dimensional segmentation appears to be negligible over the frequency range considered.
    Keywords: ACOUSTICS
    Type: NASA-TM-80118
    Format: application/pdf
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  • 2
    Publication Date: 2006-02-14
    Description: Contamination sensitive optical devices were to be evaluated under thermal vacuum conditions. Test specifications called for an extremely clean chamber environment, ambient temperature chamber walls, and optical surface temperatures at approximately - 30 C. Chamber preparation included the replacement of diffusion pumps with cryopumps and the cleaning of the chamber walls to reduce contamination levels. Chamber cleaning using vacuum bake was tried and abandoned as time consuming and ineffective. Chemical cleaning and cleanliness verification methods were developed. Gas chromatograph analysis techniques were used extensively throughout the cleaning process to verify cleanliness levels prior to evacuating the chamber. A chamber cleanliness verification test using a thermoelectric quartz crystal microbalance and witness samples was performed to demonstrate that an appropriate cleanliness level was achieved.
    Keywords: GROUND SUPPORT SYSTEMS AND FACILITIES (SPACE)
    Type: NASA. Goddard Space Flight Center 13th Space Simulation Conf.; p 343-352
    Format: text
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  • 3
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    In:  Other Sources
    Publication Date: 2011-08-24
    Description: The potential energy surface of the C4H7+ cation has been investigated with ab initio quantum chemical theory. Extended basis set calculations, including electronic correlation, show that cyclobutyl and cyclopropylcarbinyl cation are equally stable isomers. The saddle point connecting these isomers lies 0.6 kcal/mol above the minima. The global C4H7+ minimum corresponds to the 1-methylallyl cation, which is 9.0 kcal/mol more stable than the cyclobutyl and the cyclopropylcarbinyl cation and 9.5 kcal/mol below the 2-methylallyl cation. These results are in excellent agreement with experimental data.
    Keywords: Exobiology
    Type: Journal of the American Chemical Society (ISSN 0002-7863); Volume 110; 22; 7325-8
    Format: text
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  • 4
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    In:  Other Sources
    Publication Date: 2011-08-24
    Description: No abstract available
    Keywords: Exobiology
    Type: Journal of the American Chemical Society (ISSN 0002-7863); Volume 111; 1527-8
    Format: text
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  • 5
    Publication Date: 2019-07-12
    Description: The rate coefficient for the ion-molecule reaction NH3(+) + H2 yields NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He yields NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.
    Keywords: ATOMIC AND MOLECULAR PHYSICS
    Type: Journal of Chemical Physics (ISSN 0021-9606); 94; 12:00 PM
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  • 6
    Publication Date: 2019-07-13
    Description: Sound attenuation in a rectangular acoustically lined duct containing uniform mean flow is analytically investigated using the generalized Wiener-Hopf technique. Uniqueness of the solution is enforced for lined sections of the finite axial extent by imposing edge conditions at the liner interface. Possible edge conditions are considered, including the Kutta condition, and the causal solution corresponding to edge conditions is considered the best choice. Solution methods such as the mode matching and singularity methods imply differing edge conditions, and results show that power attenuation is insensitive to the imposed edge conditions, although significant differences are observed for the reflection coefficient. The amplitude of the exponentially increasing instability mode in the lined section must be set to zero as a first approximation to the nonlinear situation, and results indicate that measurements of the reflection factor can be used to make a more definite decision about physically appropriate edge conditions.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 81-2017 , Aeroacoustics Conference; Oct. 5-7, 1981; Palo Alto, CA
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  • 7
    Publication Date: 2019-07-13
    Description: No abstract available
    Keywords: Exobiology
    Type: Angewandte Chemie (International ed. in English) (ISSN 0570-0833); 29; 2; 183-5
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  • 8
    Publication Date: 2019-07-12
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 237; 1057-106
    Format: text
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  • 9
    Publication Date: 2019-06-28
    Description: A pressure box test fixture was designed and fabricated to evaluate the effects of internal pressure, biaxial tension loads, curvature, and damage on the fracture response of composite fuselage structure. Previous work in composite fuselage tension damage tolerance, performed during NASA contract NAS1-17740, evaluated the above effects on unstiffened panels only. This work extends the tension damage tolerance testing to curved stiffened fuselage crown structure that contains longitudinal stringers and circumferential frame elements. The pressure box fixture was designed to apply internal pressure up to 20 psi, and axial tension loads up to 5000 lb/in, either separately or simultaneously. A NASTRAN finite element model of the pressure box fixture and composite stiffened panel was used to help design the test fixture, and was compared to a finite element model of a full composite stiffened fuselage shell. This was done to ensure that the test panel was loaded in a similar way to a panel in the full fuselage shell, and that the fixture and its attachment plates did not adversely affect the panel.
    Keywords: COMPOSITE MATERIALS
    Type: NASA. Langley Research Center, Third NASA Advanced Composites Technology Conference, Volume 1, Part 2; p 789-805
    Format: application/pdf
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