71,054 research outputs found
Energy storage apparatus
A high efficiency, flywheel type energy storage device which comprises an electronically commutated d.c. motor/generator unit having a massive flywheel rotor magnetically suspended around a ring shaped stator is presented. During periods of low energy demand, the storage devices were operated as a motor, and the flywheel motor was brought up to operating speed. Energy was drawn from the device functioning as a generator as the flywheel rotor rotated during high energy demand periods
Planetary Stability Zones in Hierarchical Triple Star Systems
A symplectic integrator algorithm suitable for hierarchical triple systems is
formulated and tested. The positions of the stars are followed in hierarchical
Jacobi coordinates, whilst the planets are referenced purely to their primary.
The algorithm is fast, accurate and easily generalised to incorporate
collisions. There are five distinct cases -- circumtriple orbits, circumbinary
orbits and circumstellar orbits around each of the stars in the hierarchical
triple -- which require a different formulation of the symplectic integration
algorithm. As an application, a survey of the stability zones for planets in
hierarchical triples is presented, with the case of a single planet orbiting
the inner binary considered in detail. Fits to the inner and outer edges of the
stability zone are computed. Considering the hierarchical triple as two
decoupled binary systems, the earlier work of Holman & Wiegert on binaries is
shown to be applicable to triples, except in the cases of high eccentricities
and close or massive stars. Application to triple stars with good data in the
multiple star catalogue suggests that more than 50 per cent are unable to
support circumbinary planets, as the stable zone is almost non-existent.Comment: 16 pages, MNRAS, in pres
Numerical Studies of Three-dimensional Breakdown in Trailing Vortex Wakes
Finite element, three dimensional relaxation methods are used to calculate the development of vortex wakes behind aircraft for a considerable downstream distance. The inclusion of a self-induction term in the solution, dependent upon local curvature and vortex core radius, permits calculation of finite lifetimes for systems for which infinite life would be predicted two dimensionally. The associated computer program is described together with single-pair, twin-pair, and multiple-pair studies carried out using it. It is found, in single-pair studies, that there is a lower limit to the wavelengths at which the Crow-type of instability can occur. Below this limit, self-induction effects cause the plane of the disturbance waves to rotate counter to the vortex direction. Self induction in two dimensionally generated twin spiral waves causes an increase in axial length which becomes more marked with decreasing initial wavelength. The time taken for vortex convergence toward the center plane is correspondingly increased. The limited parametric twin-pair study performed suggests that time-to-converge increases with increasing flap span. Limited studies of Boeing 747 configurations show correct qualitative response to removal of the outer flap and to gear deployment, as compared with wind tunnel and flight test experience
Prenatal programming of neuroendocrine reproductive function
It is now well recognized that the gestational environment can have long-lasting effects not only on the life span and health span of an individual but also, through potential epigenetic changes, on future generations. This article reviews the “prenatal programming” of the neuroendocrine systems that regulate reproduction, with a specific focus on the lessons learned using ovine models. The review examines the critical roles played by steroids in normal reproductive development before considering the effects of prenatal exposure to exogenous steroid hormones including androgens and estrogens, the effects of maternal nutrition and stress during gestation, and the effects of exogenous chemicals such as alcohol and environment chemicals. In so doing, it becomes evident that, to maximize fitness, the regulation of reproduction has evolved to be responsive to many different internal and external cues and that the GnRH neurosecretory system expresses a degree of plasticity throughout life. During fetal life, however, the system is particularly sensitive to change and at this time, the GnRH neurosecretory system can be “shaped” both to achieve normal sexually differentiated function but also in ways that may adversely affect or even prevent “normal function”. The exact mechanisms through which these programmed changes are brought about remain largely uncharacterized but are likely to differ depending on the factor, the timing of exposure to that factor, and the species. It would appear, however, that some afferent systems to the GnRH neurons such as kisspeptin, may be critical in this regard as it would appear to be sensitive to a wide variety of factors that can program reproductive function. Finally, it has been noted that the prenatal programming of neuroendocrine reproductive function can be associated with epigenetic changes, which would suggest that in addition to direct effects on the exposed offspring, prenatal programming could have transgenerational effects on reproductive potential
Johnson-Kendall-Roberts theory applied to living cells
Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion
energies of soft slightly deformable material. Little is known about the
validity of this theory on complex systems such as living cells. We have
addressed this problem using a depletion controlled cell adhesion and measured
the force necessary to separate the cells with a micropipette technique. We
show that the cytoskeleton can provide the cells with a 3D structure that is
sufficiently elastic and has a sufficiently low deformability for JKR theory to
be valid. When the cytoskeleton is disrupted, JKR theory is no longer
applicable
Rules for transition rates in nonequilibrium steady states
Just as transition rates in a canonical ensemble must respect the principle
of detailed balance, constraints exist on transition rates in driven steady
states. I derive those constraints, by maximum information-entropy inference,
and apply them to the steady states of driven diffusion and a sheared lattice
fluid. The resulting ensemble can potentially explain nonequilibrium phase
behaviour and, for steady shear, gives rise to stress-mediated long-range
interactions.Comment: 4 pages. To appear in Physical Review Letter
Diffusion and rheology in a model of glassy materials
We study self-diffusion within a simple hopping model for glassy materials.
(The model is Bouchaud's model of glasses [J.-P. Bouchaud, J. Physique I 2,
1705 (1992)], as extended to describe rheological properties [P. Sollich, F.
Lequeux, P. Hebraud and M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)].) We
investigate the breakdown, near the glass transition, of the (generalized)
Stokes-Einstein relation between self-diffusion of a tracer particle and the
(frequency-dependent) viscosity of the system as a whole. This stems from the
presence of a broad distribution of relaxation times of which different moments
control diffusion and rheology. We also investigate the effect of flow
(oscillatory shear) on self-diffusion and show that this causes a finite
diffusivity in the temperature regime below the glass transition (where this
was previously zero). At higher temperatures the diffusivity is enhanced by a
power law frequency dependence that also characterises the rheological
response. The relevance of these findings to soft glassy materials (foams,
emulsions etc.) as well as to conventional glass-forming liquids is discussed.Comment: 39 page (double spaced), 2 figure
A New Superintegrable Hamiltonian
We identify a new superintegrable Hamiltonian in 3 degrees of freedom,
obtained as a reduction of pure Keplerian motion in 6 dimensions. The new
Hamiltonian is a generalization of the Keplerian one, and has the familiar 1/r
potential with three barrier terms preventing the particle crossing the
principal planes. In 3 degrees of freedom, there are 5 functionally independent
integrals of motion, and all bound, classical trajectories are closed and
strictly periodic. The generalisation of the Laplace-Runge-Lenz vector is
identified and shown to provide functionally independent isolating integrals.
They are quartic in the momenta and do not arise from separability of the
Hamilton-Jacobi equation. A formulation of the system in action-angle variables
is presented.Comment: 11 pages, 4 figures, submitted to The Journal of Mathematical Physic
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