169,006 research outputs found
Nonclassical acoustics
A statistical approach to sound propagation is considered in situations where, due to the presence of large gradients of properties of the medium, the classical (deterministic) treatment of wave motion is inadequate. Mathematical methods for wave motions not restricted to small wavelengths (analogous to known methods of quantum mechanics) are used to formulate a wave theory of sound in nonuniform flows. Nonlinear transport equations for field probabilities are derived for the limiting case of noninteracting sound waves and it is postulated that such transport equations, appropriately generalized, may be used to predict the statistical behavior of sound in arbitrary flows
Effects of friction and heat conduction on sound propagation in ducts
The theory of sound propagation is examined in a viscous, heat-conducting fluid, initially at rest and in a uniform state, and contained in a rigid, impermeable duct with isothermal walls. Topics covered include: (1) theoretical formulation of the small amplitude fluctuating motions of a viscous, heat-conducting and compressible fluid; (2) sound propagation in a two dimensional duct; and (3) perturbation study of the inplane modes
Long-lived Magnetic-Tension-Driven Modes in a Molecular Cloud
We calculate and analyze the longevity of magnetohydrodynamic (MHD) wave
modes that occur in the plane of a magnetic thin sheet. Initial turbulent
conditions applied to a magnetically subcritical cloud are shown to lead to
relatively rapid energy decay if ambipolar diffusion is introduced at a level
corresponding to partial ionization primarily by cosmic rays. However, in the
flux-freezing limit, as may be applicable to photoionized molecular cloud
envelopes, the turbulence persists at "nonlinear" levels in comparison with the
isothermal sound speed \cs, with one-dimensional rms material motions in the
range of \approx 2\,\cs -5\,\cs for cloud sizes in the range of \approx
2\,\pc - 16\,\pc. These fluctuations persist indefinitely, maintaining a
significant portion of the initial turbulent kinetic energy. We find the
analytic explanation for these persistent fluctuations. They are
magnetic-tension-driven modes associated with the interaction of the sheet with
the external magnetic field. The phase speed of such modes is quite large,
allowing residual motions to persist without dissipation in the flux-freezing
limit, even as they are nonlinear with respect to the sound speed. We speculate
that long-lived large-scale MHD modes such as these may provide the key to
understanding observed supersonic motions in molecular clouds.Comment: Accepted by The Astrophysical Journal, 6 pages, 5 figures. Animations
and a 3D pdf file are available at http://www.astro.uwo.ca/~basu/pb.ht
Discussion of the De Generatione Sonorum, a treatise on sound and phonetics by Robert Grosseteste
Here I am proposing a translation and discussion of the De Generatione Sonorum, one of the short scientific treatises written by Robert Grosseteste. The subject is the sound and the phonetics. In this treatise we find a discussion of oscillations and elasticity of materials and the description of some motions and their combinations to
Correlating Infall with Deuterium Fractionation in Dense Cores
We present a survey of HCO+ (3-2) observations pointed towards dense cores
with previous measurements of N(N2D+)/N(N2H+). Of the 26 cores in this survey,
five show the spectroscopic signature of outward motion, nine exhibit neither
inward nor outward motion, eleven appear to be infalling, and one is not
detected. We compare the degree of deuterium fractionation with infall
velocities calculated from the HCO+ spectra and find that those cores with
[D]/[H] > 0.1 are more likely to have the signature of inward motions than
cores with smaller [D]/[H] ratios. Infall motions are also much more common in
cores with masses exceeding their thermal Jeans masses. The fastest infall
velocity measured belongs to one of the two protostellar cores in our survey,
L1521F, and the observed motions are typically on the order of the sound speed.Comment: Accepted to Ap
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