3,392 research outputs found
Stationary field-aligned MHD flows at astropauses and in astrotails. Principles of a counterflow configuration between a stellar wind and its interstellar medium wind
A stellar wind passing through the reverse shock is deflected into the
astrospheric tail and leaves the stellar system either as a sub-Alfvenic or as
a super-Alfvenic tail flow. An example is our own heliosphere and its
heliotail. We present an analytical method of calculating stationary,
incompressible, and field-aligned plasma flows in the astrotail of a star. We
present a recipe for constructing an astrosphere with the help of only a few
parameters, like the inner Alfven Mach number and the outer Alfven Mach number,
the magnetic field strength within and outside the stellar wind cavity, and the
distribution of singular points of the magnetic field within these flows.
Within the framework of a one-fluid approximation, it is possible to obtain
solutions of the MHD equations for stationary flows from corresponding static
MHD equilibria, by using noncanonical mappings of the canonical variables. The
canonical variables are the Euler potentials of the magnetic field of
magnetohydrostatic equilibria. Thus we start from static equilibria determined
by the distribution of magnetic neutral points, and assume that the Alfven Mach
number for the corresponding stationary equilibria is finite. The topological
structure determines the geometrical structure of the interstellar gas -
stellar wind interface. Additional boundary conditions like the outer magnetic
field and the jump of the magnetic field across the astropause allow
determination of the noncanonical transformations. This delivers the strength
of the magnetic field at every point in the astrotail region beyond the reverse
shock. The mathematical technique for describing such a scenario is applied to
astrospheres in general, but is also relevant for the heliosphere. It shows the
restrictions of the outer and the inner magnetic field strength in comparison
with the corresponding Alfven Mach numbers in the case of subalfvenic flows.Comment: 19 pages, 17 figures, accepted for publication in A&
Oscillations of rotating trapped Bose-Einstein condensates
The tensor-virial method is applied for a study of oscillation modes of
uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is
supported by an vortex array. The second order virial equations are derived in
the hydrodynamic regime for an arbitrary external harmonic trapping potential
assuming that the condensate is a superfluid at zero temperature. The
axisymmetric equilibrium shape of the condensate is determined as a function of
the deformation of the trap; its domain of stability is bounded by the
constraint on the rotation rate (measured in units of the trap
frequency .) The oscillations of the axisymmetric condensate are
stable with respect to the transverse-shear, toroidal and quasi-radial modes of
oscillations, corresponding to the , surface
deformations. In non-axisymmetric traps, the equilibrium constrains the
(dimensionless) deformation in the plane orthogonal to the rotation to the
domain with . The second harmonic oscillation modes
in non-axisymmetric traps separate into two classes which have even or odd
parity with respect to the direction of the rotation axis. Numerical solutions
show that these modes are stable in the parameter domain where equilibrium
figures exist.Comment: 16 pages, including 4 figures, uses Revtex; v2 includes a treatment
of modes in unisotropic traps; PRA in pres
Spectral methods in fluid dynamics
Fundamental aspects of spectral methods are introduced. Recent developments in spectral methods are reviewed with an emphasis on collocation techniques. Their applications to both compressible and incompressible flows, to viscous as well as inviscid flows, and also to chemically reacting flows are surveyed. The key role that these methods play in the simulation of stability, transition, and turbulence is brought out. A perspective is provided on some of the obstacles that prohibit a wider use of these methods, and how these obstacles are being overcome
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