1,613 research outputs found
Effects of high subsonic flow on sound propagation in a variable-area duct
The propagation of sound in a converging-diverging duct containing a quasi-one-dimensional steady flow with a high subsonic throat Mach number was studied. The behavior of linearized acoustic theory at the throat of the duct was shown to be singular. This singularity implies that linearized acoustic theory is invalid. The explicit singular behavior was determined and used to sketch the development (by the method of matched asymptotic expansions) of a nonlinear theory for sound propagation in a sonic throat region
Nonlinear theory of shocked sound propagation in a nearly choked duct flow
The development of shocks in the sound field propagating through a nearly choked duct flow is analyzed by extending a quasi-one dimensional theory. The theory is applied to the case in which sound is introduced into the flow by an acoustic source located in the vicinity of a near-sonic throat. Analytical solutions for the field are obtained which illustrate the essential features of the nonlinear interaction between sound and flow. Numerical results are presented covering ranges of variation of source strength, throat Mach number, and frequency. It is found that the development of shocks leads to appreciable attenuation of acoustic power transmitted upstream through the near-sonic flow. It is possible, for example, that the power loss in the fundamental harmonic can be as much as 90% of that introduced at the source
Quantum Hydrodynamic Model for the enhanced moments of Inertia of molecules in Helium Nanodroplets: Application to SF
The increase in moment of inertia of SF in helium nanodroplets is
calculated using the quantum hydrodynamic approach. This required an extension
of the numerical solution to the hydrodynamic equation to three explicit
dimensions. Based upon an expansion of the density in terms of the lowest four
Octahedral spherical harmonics, the predicted increase in moment of inertia is
, compared to an experimentally determined value of , i.e., 55% of the observed value. The difference is likely in at
least part due to lack of convergence with respect to the angular expansion,
but at present we do not have access to the full densities from which a higher
order expansion can be determined. The present results contradict those of Kwon
et al., J. Chem. Phys. {\bf 113}, 6469 (2000), who predicted that the
hydrodynamic theory predicted less than 10% of the observed increase in moment
of inertia.Comment: 10 pages, including 1 figur
Spin-orbit coupling for tidally evolving super-Earths
We investigate the spin behavior of close-in rocky planets and the
implications for their orbital evolution. Considering that the planet rotation
evolves under simultaneous actions of the torque due to the equatorial
deformation and the tidal torque, both raised by the central star, we analyze
the possibility of temporary captures in spin-orbit resonances. The results of
the numerical simulations of the exact equations of motions indicate that,
whenever the planet rotation is trapped in a resonant motion, the orbital decay
and the eccentricity damping are faster than the ones in which the rotation
follows the so-called pseudo-synchronization. Analytical results obtained
through the averaged equations of the spin-orbit problem show a good agreement
with the numerical simulations. We apply the analysis to the cases of the
recently discovered hot super-Earths Kepler-10 b, GJ 3634 b and 55 Cnc e. The
simulated dynamical history of these systems indicates the possibility of
capture in several spin-orbit resonances; particularly, GJ 3634 b and 55 Cnc e
can currently evolve under a non-synchronous resonant motion for suitable
values of the parameters. Moreover, 55 Cnc e may avoid a chaotic rotation
behavior by evolving towards synchronization through successive temporary
resonant trappings.Comment: Accepted for publication in MNRA
The Effects of Ram-pressure Stripping and Supernova Winds on the Tidal Stirring of Disky Dwarfs: Enhanced Transformation into Dwarf Spheroidals
A conclusive model for the formation of dwarf spheroidal (dSph) galaxies
still remains elusive. Owing to their proximity to the massive spirals Milky
Way (MW) and M31, various environmental processes have been invoked to explain
their origin. In this context, the tidal stirring model postulates that
interactions with MW-sized hosts can transform rotationally supported dwarfs,
resembling present-day dwarf irregular (dIrr) galaxies, into systems with the
kinematic and structural properties of dSphs. Using N-body+SPH simulations, we
investigate the dependence of this transformation mechanism on the gas
fraction, fgas, in the disk of the progenitor dwarf. Our numerical experiments
incorporate for the first time the combined effects of radiative cooling,
ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV
background. For a given orbit inside the primary galaxy, rotationally supported
dwarfs with gas fractions akin to those of observed dIrrs (fgas >= 0.5),
demonstrate a substantially enhanced likelihood and efficiency of
transformation into dSphs relative to their collisionless (fgas = 0)
counterparts. We argue that the combination of ram-pressure stripping and SN
winds causes the gas-rich dwarfs to respond more impulsively to tides,
augmenting their transformation. When fgas >= 0.5, disky dwarfs on previously
unfavorable low-eccentricity or large-pericenter orbits are still able to
transform. On the widest orbits, the transformation is incomplete; the dwarfs
retain significant rotational support, a relatively flat shape, and some gas,
naturally resembling transition-type systems. We conclude that tidal stirring
constitutes a prevalent evolutionary mechanism for shaping the structure of
dwarf galaxies within the currently favored CDM cosmological paradigm.Comment: Accepted for publication in ApJ Letters, 8 pages, 2 figures, LaTeX
(uses emulateapj.cls
A sub-Doppler resolution double resonance molecular beam infrared spectrometer operating at chemically relevant energies (~2 eV)
A molecular beam spectrometer capable of achieving sub-Doppler resolution at 2 eV (~18 000 cm^–1) of vibrational excitation is described and its performance demonstrated using the CH stretch chromophore of HCN. Two high finesse resonant power-buildup cavities are used to excite the molecules using a sequential double resonance technique. A v = 0-->2 transition is first saturated using a 1.5 µm color center laser, whereupon a fraction of the molecules is further excited to the v = 6 level using an amplitude modulated Ti:Al2O3 laser. The energy absorbed by the molecules is detected downstream of both excitation points by a cryogenically cooled bolometer using phase sensitive detection. A resolution of approximately 15 MHz (i.e., three parts in 10^8) is demonstrated by recording a rotational line in the v = 6 manifold of HCN. Scan speeds of up to several cm^–1/h were obtained, with signal-to-noise ratios in excess of 100. The high signal-to-noise ratio and a dynamic range of 6×10^4 means that future experiments to study statistical intramolecular vibrational energy redistribution in small molecules and unimolecular isomerizations can be attempted. We would also like to point out that, with improved metrology in laser wavelengths, this instrument can also be used to provide improved secondary frequency standards based upon the rovibrational spectra of molecules
Properties and beneficial uses of (bio)chars, with special attention to products from sewage sludge pyrolysis
Long-term dynamics of Methone, Anthe and Pallene
We numerically investigate the long-term dynamics of the Saturn's small
satellites Methone (S/2004 S1), Anthe (S/2007 S4) and Pallene (S/2004 S2). In
our numerical integrations, these satellites are disturbed by non-spherical
shape of Saturn and the six nearest regular satellites. The stability of the
small bodies is studied here by analyzing long-term evolution of their orbital
elements.
We show that long-term evolution of Pallene is dictated by a quasi secular
resonance involving the ascending nodes () and longitudes of
pericentric distances () of Mimas (subscript 1) and Pallene (subscript
2), which critical argument is . Long-term
orbital evolution of Methone and Anthe are probably chaotic since: i) their
orbits randomly cross the orbit of Mimas in time scales of thousands years);
ii) numerical simulations involving both small satellites are strongly affected
by small changes in the initial conditions.Comment: 9 pages; 4 figures. Submitted to Proceedings IAU Symposium No. S263,
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