2,834 research outputs found
Multiple pure tone noise generated by fans at supersonic tip speeds
The existence of clusters of pure tones at integral multiples of shaft speed has been noted for supersonic-tip-speed operation of fans and compressors. A continuing program to explore this phenomenon, often called combination-tone noise, has been in effect for several years. This paper reviews the research program, which involves a wide range of engines, compressor rigs, and special apparatus. Elements of the aerodynamics of the blade-associated shock waves are outlined and causes of blade-to-blade shock inequalities, responsible for the multiple tones, are described
Method of fan sound mode structure determination
A method for the determination of fan sound mode structure in the Inlet of turbofan engines using in-duct acoustic pressure measurements is presented. The method is based on the simultaneous solution of a set of equations whose unknowns are modal amplitude and phase. A computer program for the solution of the equation set was developed. An additional computer program was developed which calculates microphone locations the use of which results in an equation set that does not give rise to numerical instabilities. In addition to the development of a method for determination of coherent modal structure, experimental and analytical approaches are developed for the determination of the amplitude frequency spectrum of randomly generated sound models for use in narrow annulus ducts. Two approaches are defined: one based on the use of cross-spectral techniques and the other based on the use of an array of microphones
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Association of Alfvén waves and proton cyclotron waves with electrostatic bipolar pulses: magnetic hole events observed by Polar
International audienceTwo magnetic hole events observed by Polar on 20 May 1996 when it was in the polar cap/polar cusp boundary layer are studied. Low-frequency waves, consisting of nonlinear Alfvén waves and large amplitude (±14nT peak-to-peak) obliquely propagating proton cyclotron waves (with frequency f~0.6 to 0.7 fcp), accompanied by electric bipolar pulses (electron holes) and electron heating have been observed located within magnetic holes. It is shown that low-frequency waves can provide free energy to drive some high frequency instabilities which saturate by trapping electrons, thus, leading to the generation of electron holes
Implications of the B20 Crystal Structure for the Magneto-electronic Structure of MnSi
Due to increased interest in the unusual magnetic and transport behavior of
MnSi and its possible relation to its crystal structure (B20) which has unusual
coordination and lacks inversion symmetry, we provide a detailed analysis of
the electronic and magnetic structure of MnSi. The non-symmorphic P2_13
spacegroup leads to unusual fourfold degenerate states at the zone corner R
point, as well as ``sticking'' of pairs of bands throughout the entire
Brillouin zone surface. The resulting Fermi surface acquires unusual features
as a result of the band sticking. For the ferromagnetic system (neglecting the
long wavelength spin spiral) with the observed moment of 0.4 \mu_B/Mn, one of
the fourfold levels at R in the minority bands falls at the Fermi energy (E_F),
and a threefold majority level at k=0 also falls at E_F. The band sticking and
presence of bands with vanishing velocity at E_F imply an unusually large phase
space for long wavelength, low energy interband transitions that will be
important for understanding the unusual resistivity and far infrared optical
behavior.Comment: Nine two-column pages with eight figures include
Men, Muscles, and Body Image: Comparisons of Competitive Bodybuilders, Weight Trainers, and Athletically Active Controls
Objectives: To investigate body image and psychosocial adjustment among competitive bodybuilders, non-competitive weight trainers, and athletically active men. Methods: Participants were 40 men in each of the three groups who were assessed on body composition and multiple facets of body image evaluation, investment and anxiety, eating attitudes, and social self esteem. Results: Relative to the other two groups, competitive bodybuilders had greater body mass due to fat-free body mass. Although groups did not differ in their situational body image discomfort, competitive bodybuilders and weight trainers had a more positive global appearance evaluation and were more psychologically invested in their physical appearance. Compared with active controls, men in both weightlifting groups were more satisfied with their upper torso and muscle tone. Competitive bodybuilders reported more mid torso satisfaction than the other two groups. Competitive bodybuilders also wished to be significantly heavier than controls did and reported higher social self esteem but greater eating disturbance. Conclusions: The findings suggest that competitive bodybuilders as a group are not more muscle dysmorphic\u27\u27 than either non-competitive weight trainers or physically active men who do not train with weights
Low temperature resistivity in a nearly half-metallic ferromagnet
We consider electron transport in a nearly half-metallic ferromagnet, in
which the minority spin electrons close to the band edge at the Fermi energy
are Anderson-localized due to disorder. For the case of spin-flip scattering of
the conduction electrons due to the absorption and emission of magnons, the
Boltzmann equation is exactly soluble to the linear order. From this solution
we calculate the temperature dependence of the resistivity due to single magnon
processes at sufficiently low temperature, namely , where is
the Anderson localization length and is the magnon stiffness. And depending
on the details of the minority spin density of states at the Fermi level, we
find a or scaling behavior for resistivity. Relevance to the
doped perovskite manganite systems is discussed
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
Doping dependence and anisotropy of minority electron mobility in molecular beam epitaxy-grown p type GaInP
The article of record as published may be found at http://dx.doi.org/10.1063/1.4902316Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300 K vary from~2000 to 400 cm2/V s and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction.USDOEAC05-06OR23100DEAC36-08GO28308This work was supported at the Naval Postgraduate School in part by National Science Foundation Grant No. DMR-0804527 and in part by the NPS Energy Academic Group with funding from the Navy Energy Coordination Office. T.C. acknowledges support from the Department of Energy, Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Control No. DE-AC05-06OR23100. TRPL work at NREL was supported by the Department of Energy Office of Science, Basic Energy Sciences under DEAC36-08GO28308
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