54,186 research outputs found
Active control of primary mirror of an orbiting telescope with thermal excitation
The generalization is presented that was made to model a layered structure of a kind that represents a light-weighted mirror. This theory is presented along with the strategy for error suppression. The results of a variety of error-suppression studies are also presented. The computer programs for all parts of this study are included
Cosmological structure formation from soft topological defects
Some models have extremely low-mass pseudo-Goldstone bosons that can lead to vacuum phase transitions at late times, after the decoupling of the microwave background.. This can generate structure formation at redshifts z greater than or approx 10 on mass scales as large as M approx 10 to the 18th solar masses. Such low energy transitions can lead to large but phenomenologically acceptable density inhomogeneities in soft topological defects (e.g., domain walls) with minimal variations in the microwave anisotropy, as small as delta Y/T less than or approx 10 to the minus 6 power. This mechanism is independent of the existence of hot, cold, or baryonic dark matter. It is a novel alternative to both cosmic string and to inflationary quantum fluctuations as the origin of structure in the Universe
Symmetry group analysis of an ideal plastic flow
In this paper, we study the Lie point symmetry group of a system describing
an ideal plastic plane flow in two dimensions in order to find analytical
solutions. The infinitesimal generators that span the Lie algebra for this
system are obtained. We completely classify the subalgebras of up to
codimension two in conjugacy classes under the action of the symmetry group.
Based on invariant forms, we use Ansatzes to compute symmetry reductions in
such a way that the obtained solutions cover simultaneously many invariant and
partially invariant solutions. We calculate solutions of the algebraic,
trigonometric, inverse trigonometric and elliptic type. Some solutions
depending on one or two arbitrary functions of one variable have also been
found. In some cases, the shape of a potentially feasible extrusion die
corresponding to the solution is deduced. These tools could be used to thin,
curve, undulate or shape a ring in an ideal plastic material
Commensurate Dy magnetic ordering associated with incommensurate lattice distortion in orthorhombic DyMnO3
Synchrotron x-ray diffraction and resonant magnetic scattering experiments on
single crystal DyMnO3 have been carried out between 4 and 40 K. Below TN(Dy) =
5K, the Dy magnetic moments order in a commensurate structure with propagation
vector 0.5 b*. Simultaneous with the Dy magnetic ordering, an incommensurate
lattice modulation with propagation vector 0.905 b* evolves while the original
Mn induced modulation is suppressed and shifts from 0.78 b* to 0.81 b*. This
points to a strong interference of Mn and Dy induced structural distortions in
DyMnO3 besides a magnetic coupling between the Mn and Dy magnetic moments.Comment: submitted to Phys. Rev. B Rapid Communication
Opportunities for use of exact statistical equations
Exact structure function equations are an efficient means of obtaining
asymptotic laws such as inertial range laws, as well as all measurable effects
of inhomogeneity and anisotropy that cause deviations from such laws. "Exact"
means that the equations are obtained from the Navier-Stokes equation or other
hydrodynamic equations without any approximation. A pragmatic definition of
local homogeneity lies within the exact equations because terms that explicitly
depend on the rate of change of measurement location appear within the exact
equations; an analogous statement is true for local stationarity. An exact
definition of averaging operations is required for the exact equations. Careful
derivations of several inertial range laws have appeared in the literature
recently in the form of theorems. These theorems give the relationships of the
energy dissipation rate to the structure function of acceleration increment
multiplied by velocity increment and to both the trace of and the components of
the third-order velocity structure functions. These laws are efficiently
derived from the exact velocity structure function equations. In some respects,
the results obtained herein differ from the previous theorems. The
acceleration-velocity structure function is useful for obtaining the energy
dissipation rate in particle tracking experiments provided that the effects of
inhomogeneity are estimated by means of displacing the measurement location.Comment: accepted by Journal of Turbulenc
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