16,934 research outputs found
Comparison of inlet suppressor data with approximate theory based on cutoff ratio
Inlet suppressor far-field directivity suppression was quantitatively compared with that predicted using an approximate linear design and evaluation method based upon mode cutoff ratio. The experimental data was obtained using a series of cylindrical point-reacting inlet liners on a YF102 engine. The theoretical prediction program is based upon simplified sound propagation concepts derived from exact calculations. These indicate that all of the controlling phenomenon can be approximately correlated with mode cutoff ratio which itself is intimately related to the angles of propagation within the duct. The theory-data comparisons are intended to point out possible deficiencies in the approximate theory which may be corrected. After all theoretical refinements are made, then empirical corrections can be applied
Acoustic performance of inlet suppressors on an engine generating a single mode
Three single degree of freedom liners with different open area ratio face sheets were designed for a single spinning mode in order to evaluate an inlet suppressor design method based on mode cutoff ratio. This mode was generated by placing 41 rods in front of the 28 blade fan of a JT15D turbofan engine. At the liner design this near cutoff mode has a theoretical maximum attenuation of nearly 200 dB per L/D. The data show even higher attenuations at the design condition than predicted by the theory for dissipation of a single mode within the liner. This additional attenuation is large for high open area ratios and should be accounted for in the theory. The data show the additional attenuation to be inversely proportional to acoustic resistance. It was thought that the additional attenuation could be caused by reflection and modal scattering at the hard to soft wall interface. A reflection model was developed, and then modified to fit the data. This model was checked against independent (multiple pure tone) data with good agreement
Experimental evaluation of a spinning-mode acoustic-treatment design concept for aircraft inlets
An aircraft-inlet noise suppressor method based on mode cutoff ratio was qualitatively checked by testing a series of liners on a YF-102 turbofan engine. Far-field directivity of the blade passing frequency was used extensively to evaluate the results. The trends and observations of the test data lend much qualitative support to the design method. The best of the BPF liners attained a suppression at design frequency of 19 dB per unit length-diameter ratio. The best multiple-pure-tone linear attained a remarkable suppression of 65.6 bB per unit length-diameter ratio
Preliminary analysis of space mission applications for electromagnetic launchers
The technical and economic feasibility of using electromagnetically launched EML payloads propelled from the Earth's surface to LEO, GEO, lunar orbit, or to interplanetary space was assessed. Analyses of the designs of rail accelerators and coaxial magnetic accelerators show that each is capable of launching to space payloads of 800 KG or more. A hybrid launcher in which EML is used for the first 2 KM/sec followed by chemical rocket stages was also tested. A cost estimates study shows that one to two EML launches per day are needed to break even, compared to a four-stage rocket. Development models are discussed for: (1) Earth orbital missions; (2) lunar base supply mission; (3) solar system escape mission; (4) Earth escape missions; (5) suborbital missions; (6) electromagnetic boost missions; and (7) space-based missions. Safety factors, environmental impacts, and EML systems analysis are discussed. Alternate systems examined include electrothermal thrustors, an EML rocket gun; an EML theta gun, and Soviet electromagnetic accelerators
Analysis of nuclear waste disposal in space, phase 3. Volume 2: Technical report
The options, reference definitions and/or requirements currently envisioned for the total nuclear waste disposal in space mission are summarized. The waste form evaluation and selection process is documented along with the physical characteristics of the iron nickel-base cermet matrix chosen for disposal of commercial and defense wastes. Safety aspects of radioisotope thermal generators, the general purpose heat source, and the Lewis Research Center concept for space disposal are assessed as well as the on-pad catastrophic accident environments for the uprated space shuttle and the heavy lift launch vehicle. The radionuclides that contribute most to long-term risk of terrestrial disposal were determined and the effects of resuspension of fallout particles from an accidental release of waste material were studied. Health effects are considered. Payload breakup and rescue technology are discussed as well as expected requirements for licensing, supporting research and technology, and safety testing
Analysis of nuclear waste disposal in space, phase 3. Volume 1: Executive summary of technical report
The objectives, approach, assumptions, and limitations of a study of nuclear waste disposal in space are discussed with emphasis on the following: (1) payload characterization; (2) safety assessment; (3) health effects assessment; (4) long-term risk assessment; and (5) program planning support to NASA and DOE. Conclusions are presented for each task
Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3
The electronic structure of the corundum-type transition-metal oxides V2O3
and Ti2O3 is studied by means of the augmented spherical wave method, based on
density-functional theory and the local density approximation. Comparing the
results for the vanadate and the titanate allows us to understand the peculiar
shape of the metal 3d a_{1g} density of states, which is present in both
compounds. The a_{1g} states are subject to pronounced bonding-antibonding
splitting due to metal-metal overlap along the c-axis of the corundum
structure. However, the corresponding partial density of states is strongly
asymmetric with considerably more weight on the high energy branch. We argue
that this asymmetry is due to an unexpected broadening of the bonding a_{1g}
states, which is caused by hybridization with the e_g^{pi} bands. In contrast,
the antibonding a_{1g} states display no such hybridization and form a sharp
peak. Our results shed new light on the role of the a_{1g} orbitals for the
metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi}
hybridization, an interpretation in terms of molecular orbital singlet states
on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Stochastic modelling of intermittent scrape-off layer plasma fluctuations
Single-point measurements of fluctuations in the scrape-off layer of
magnetized plasmas are generally found to be dominated by large-amplitude
bursts which are associated with radial motion of blob-like structures. A
stochastic model for these fluctuations is presented, with the plasma density
given by a random sequence of bursts with a fixed wave form. Under very general
conditions, this model predicts a parabolic relation between the skewness and
kurtosis moments of the plasma fluctuations. In the case of exponentially
distributed burst amplitudes and waiting times, the probability density
function for the fluctuation amplitudes is shown to be a Gamma distribution
with the scale parameter given by the average burst amplitude and the shape
parameter given by the ratio of the burst duration and waiting times.Comment: 11 pages, 1 figur
3-D Photoionization Structure and Distances of Planetary Nebulae III. NGC 6781
Continuing our series of papers on the three-dimensional (3-D) structures of
and accurate distances to Planetary Nebulae (PNe), we present our study of the
planetary nebula NGC6781. For this object we construct a 3-D photoionization
model and, using the constraints provided by observational data from the
literature we determine the detailed 3-D structure of the nebula, the physical
parameters of the ionizing source and the first precise distance. The procedure
consists in simultaneously fitting all the observed emission line morphologies,
integrated intensities and the 2-D density map from the [SII] line ratios to
the parameters generated by the model, and in an iterative way obtain the best
fit for the central star parameters and the distance to NGC6781, obtaining
values of 950+-143pc and 385 Lsun for the distance and luminosity of the
central star respectively. Using theoretical evolutionary tracks of
intermediate and low mass stars, we derive the mass of the central star of
NGC6781 and its progenitor to be 0.60+-0.03 Msun and 1.5+-0.5 Msun
respectively.Comment: 16 pp, 6 figues, 2 tables, submitted to the Ap
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