DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

volume scattering function (VSF) The ratio
of the scattered intensity [W sr −1 ] to the incident
irradiance [W m −2 ] per unit volume [m 3 ],
given in [m −1 sr −1 ]; the integral of the volume
scattering function over all directions and all
final wavelengths is the scattering coefficient;
the VSF can be written as the product of the
phase function [sr −1 ] and the scattering coefficient
[m −1 ].
volumetric water content (or simply water
content) (θ) The ratio of liquid water volume to
the total soil or rock volume: θ =Vw/Vs, where
Vw is the volume of water in the sample, and Vs
is the total volume of soil or rock in the sample.
The theoretical range of the volumetric water
content is from 0 (completely dry) to saturation
(porosity or φ), but the range in natural systems
is generally much less than this.
von Mises criterion The condition by which
a polycrystal can deform coherently to large
strains by dislocation slip. von Mises criterion
points out that five independent slip systems
are needed in order to deform a polycrystalline
material by crystallographic slip. This is
because arbitrary shape change must be possible
to satisfy the displacement compatibility at
grain boundaries.
vortex defect of the vacuum See cosmic
string.
vortex street The series of vortices shed systematically
downstream from a body in a rapid
fluid flow (sufficiently high Reynolds number).
For a cylindrically symmetrical body, a pair of
symmetrically placed opposite vortices (with a
sign which can be deduced from the streamflow)
form behind the body on either side of the down-
© 2001 by CRC Press LLC
vorton
stream direction. Small disturbances then lead
to a shedding of one of the two vortices, and
subsequently alternate shedding of vortices of
opposite sign. The trail of vortices behind the
body is the vortex street.
vorticity A vector measure, ω, of local rotation
in a fluid flow:
ω = ∇ × v .
vorton Equilibrium configurations of conducting
cosmic string loops, where the string
tension, tending to make the loop collapse, is
balanced by the centrifugal repulsion due to rotation.
Symmetry reasons (Lorentz invariance
along the string world sheet) forbid rotating configurations
for Goto–Nambu strings and, therefore,
vortons can only arise in theories where
cosmic strings have a microscopic internal structure,
like currents flowing along the core.
The existence of vorton equilibrium configurations
would prevent the loops from decaying
away in the form of radiation. They would then
evolve like ordinary (eventually charged) nonrelativistic
matter very early on in the history of
the universe, eventually disrupting some of the
predictions of standard cosmology.
This possible incompatibility is known as the
vorton excess problem. Many mechanisms aiming
at diluting vorton overdensity are currently
being studied. If successful, vortons could stop
being a problem and would become one more
interesting candidate for the non-baryonic dark
matter that combined observations and theoretical
models tend to require. See CHUMP,
conducting string, cosmic string, Goto–Nambu
string.
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