10,059 research outputs found
Analysis of noise measured from a propeller in a wake
In this experimental study, the acoustic characteristics of a propeller operating in a wake were studied. The propeller performance and noise were measured from two 0.25 scale propellers operating in an open jet anechoic flow environment with and without a wake. One propeller had NACA 16 series sections; the other, ARA-D. Wake thicknesses of 1 and 3 propeller chords were generated by an airfoil which spanned the full diameter of the propeller. The airfoil wake profiles were measured. Noise measurements were made in and out of the flow. The propellers were operated at 40, 83, and 100 inf of thrust. The acoustic data are analyzed, and the effects on the overall sound pressure level (OASPL) and scaled A weighted sound level L sub A with propeller thrust, wake thickness, and observer location are presented. The analysis showed that, generally, the wake increased the overall noise (OASPL) produced by the propeller; increased the harmonic content of the noise, thus the scaled L sub a; and produced an azimuthal dependence. With few exceptions, both propellers generally produced the same trends in delta OASPL and delta L sub a with thrust and wake thickness
Noise response of cavities of varying dimensions at subsonic speeds
An expression for the Strouhal number of lengthwise cavity oscillations is obtained which includes the effect of length-to-depth ratio. This expression, which agrees well with the experimental data, is also used to predict the Mach number at which cavity acoustic response is maximum. Interaction between lengthwise and depthwise modes is seen to occur at Mach numbers from 0.1 to 0.5. Cavity shape is shown to affect the noise spectra in generating either a broadband or narrowband signal
Decoupling the coupled DGLAP evolution equations: an analytic solution to pQCD
Using Laplace transform techniques, along with newly-developed accurate
numerical inverse Laplace transform algorithms, we decouple the solutions for
the singlet structure function and of the two
leading-order coupled singlet DGLAP equations, allowing us to write fully
decoupled solutions: F_s(x,Q^2)={\cal F}_s(F_{s0}(x), G_0(x)), G(x,Q^2)={\cal
G}(F_{s0}(x), G_0(x)). Here and are known
functions---found using the DGLAP splitting functions---of the functions
and , the chosen
starting functions at the virtuality . As a proof of method, we compare
our numerical results from the above equations with the published MSTW LO gluon
and singlet distributions, starting from their initial values at . Our method completely decouples the two LO distributions, at the same
time guaranteeing that both distributions satisfy the singlet coupled DGLAP
equations. It furnishes us with a new tool for readily obtaining the effects of
the starting functions (independently) on the gluon and singlet structure
functions, as functions of both and . In addition, it can also be
used for non-singlet distributions, thus allowing one to solve analytically for
individual quark and gluon distributions values at a given and , with
typical numerical accuracies of about 1 part in , rather than having to
evolve numerically coupled integral-differential equations on a two-dimensional
grid in , as is currently done.Comment: 6 pages, 2 figure
Measurements of farfield sound generation from a flow-excited cavity
Results of 1/3-octave-band spectral measurements of internal pressures and the external acoustic field of a tangentially blown rectangular cavity are compared. Proposed mechanisms for sound generation are reviewed, and spectra and directivity plots of cavity noise are presented. Directivity plots show a slightly modified monopole pattern. Frequencies of cavity response are calculated using existing predictions and are compared with those obtained experimentally. The effect of modifying the upstream boundary layer on the noise was investigated, and its effectiveness was found to be a function of cavity geometry and flow velocity
The Structure and C=C Vibrational Frequencies of the all- trans Polyenes C2nH2n+2(n=2-15), C2nH2n(Me)2(n=2-13), and C2nH2n(tert-Butyl)2(n=2-5): Computational Results
Carbon-carbon bond lengths and C=C vibrational frequencies are reported for the linear, all-trans unsubstituted C2nH2n+2 (n=2-15), methyl capped C2nH2nMe2 (n=2-13), and tert-butyl capped C2nH2n(tert-butyl)2 (n=2-5) polyenes (C2h) calculated at the B3LYP/6-311++G(d,p) level. The C=C/C-C bond length alternation remains evident at this level for the unsubstituted and methyl capped polyenes as the chain length increases; the center-most difference in the length of the C-C/C=C bonds is ~0.06 AÌŠ for C30H32 and C26H26Me2. The Ag, in-phase, harmonic C=C Raman frequency for the unsubstituted polyenes decreases from 1699.2 cm-1 (n = 2) to 1528.9 cm-1 (n=15); the anharmonic frequency decreases from 1651.5 cm-1 (n = 2) to 1547.7 cm-1 (n = 8). The harmonic C=C frequency for the methyl capped polyenes decreases from 1717.9 cm-1 (n = 2) to 1539.6 cm- 1 (n= 13), and the anharmonic C=C frequency decreases from 1675.0 cm-1 (n = 2) to 1562.8 cm-1 (n = 7)
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