44,789 research outputs found
On the structure of kinematic systems with complete symmetry
This paper provides a new perspective on the structure of kinematic systems with complete symmetry. These systems naturally occur as models for mechanical systems with symmetry, for example flying or submersible robots. The configuration space of such systems is a homogeneous space of the symmetry Lie group, and it is well known that their kinematics can be lifted to equivariant kinematics on the symmetry group thus allowing global state observer constructions. We provide explicitly checkable sufficient differential-algebraic conditions on the symmetry that will lead to a lifted system in the form of standard left or right invariant kinematics on the symmetry group. Previously known conditions for one of these two cases required finding a velocity lift map with particular properties for which there was no general construction known.This work was partially supported by the Australian Research Council
through the ARC Discovery Project DP160100783 “Sensing a complex
world: Infinite dimensional observer theory for robots”
A geometric description of the intermediate behaviour for spatially homogeneous models
A new approach is suggested for the study of geometric symmetries in general
relativity, leading to an invariant characterization of the evolutionary
behaviour for a class of Spatially Homogeneous (SH) vacuum and orthogonal
law perfect fluid models. Exploiting the 1+3 orthonormal frame
formalism, we express the kinematical quantities of a generic symmetry using
expansion-normalized variables. In this way, a specific symmetry assumption
lead to geometric constraints that are combined with the associated
integrability conditions, coming from the existence of the symmetry and the
induced expansion-normalized form of the Einstein's Field Equations (EFE), to
give a close set of compatibility equations. By specializing to the case of a
\emph{Kinematic Conformal Symmetry} (KCS), which is regarded as the direct
generalization of the concept of self-similarity, we give the complete set of
consistency equations for the whole SH dynamical state space. An interesting
aspect of the analysis of the consistency equations is that, \emph{at least}
for class A models which are Locally Rotationally Symmetric or lying within the
invariant subset satisfying , a proper KCS \emph{always
exists} and reduces to a self-similarity of the first or second kind at the
asymptotic regimes, providing a way for the ``geometrization'' of the
intermediate epoch of SH models.Comment: Latex, 15 pages, no figures (uses iopart style/class files); added
one reference and minor corrections; (v3) improved and extended discussion;
minor corrections and several new references are added; to appear in Class.
Quantum Gra
Rotating Globular Clusters
Internal rotation is considered to play a major role in the dynamics of some
globular clusters. However, in only few cases it has been studied by
quantitative application of realistic and physically justified global models.
Here we present a dynamical analysis of the photometry and three-dimensional
kinematics of omega Cen, 47 Tuc, and M15, by means of a recently introduced
family of self-consistent axisymmetric rotating models. The three clusters,
characterized by different relaxation conditions, show evidence of differential
rotation and deviations from sphericity. The combination of line-of-sight
velocities and proper motions allows us to determine their internal dynamics,
predict their morphology, and estimate their dynamical distance. The
well-relaxed cluster 47 Tuc is very well interpreted by our model; internal
rotation is found to explain the observed morphology. For M15, we provide a
global model in good agreement with the data, including the central behavior of
the rotation profile and the shape of the ellipticity profile. For the
partially relaxed cluster omega Cen, the selected model reproduces the complex
three-dimensional kinematics; in particular the observed anisotropy profile,
characterized by a transition from isotropy, to weakly-radial anisotropy, and
then to tangential anisotropy in the outer parts. The discrepancy found for the
steep central gradient in the observed line-of-sight velocity dispersion
profile and for the ellipticity profile is ascribed to the condition of only
partial relaxation of this cluster and the interplay between rotation and
radial anisotropy.Comment: 19 pages, 14 figures, accepted for publication in the Astrophysical
Journa
A numerically efficient finite element hydroelastic analysis
A finite element hydroelastic analysis formulation is developed on the basis of Toupin's complementary variational principle. Emphasis is placed on the special case of an incompressible fluid model which is applicable to propellant tank hydroelastic analysis. A concise fluid inertia representation results from the assumption of incompressibility and the hydroelastic equations reduce to a simplified form associated with the structure alone. The efficiency of the incompressible hydroelastic formulation in unhanced for both fluid and structure by introduction of harmonic reduction as an alternative to Guyan reduction. The theoretical developments are implemented in the NASTRAN Program and the technique is verified and demonstrated as an efficient and accurate approach with a series of illustrative problems including the 1/8 scale space shuttle external tank
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