1,721,031 research outputs found
Properties of self-gravitating quasi-stationary states
Full text linkInitially far out-of-equilibrium, self-gravitating systems form quasi-stationary states (QSS) through a collisionless relaxation dynamics. These may arise from a bottom-up aggregation of structures or in a top-down frame; their quasi-equilibrium properties are well described by the Jeans equation and are not universal. These QSS depend on initial conditions. To understand the origin of such dependence, we present the results of numerical experiments of initially cold and spherical systems characterized by various choices of the spectrum of initial density fluctuations. The amplitude of such fluctuations determines whether the system relaxes in a top-down or bottom-up manner. We find that statistical properties of the resulting QSS mainly depend upon the amount of energy exchanged during the formation process. In particular, in the violent top-down collapses the energy exchange is large and the QSS show an inner core with an almost flat density profile and a quasi Maxwell-Boltzmann (isotropic) velocity distribution, while their outer regions display a density profile (r) / r (0) with radially elongated orbits. We show analytically that = 4, in agreement with numerical experiments. In the less violent bottom-up dynamics, the energy exchange is much smaller, the orbits are less elongated, and 0 (r) 4, where the density profile is well fitted by the Navarro-Frenk-White behavior. Such a dynamical evolution is shown by both nonuniform spherical isolated systems and by halos extracted from cosmological simulations. We consider the relation of these results with the core-cusp problem and conclude that this can be solved naturally if galaxies form through a monolithic collapse
Gravitational force distribution in fractal structures
No full textWe study the (Newtonian) gravitational force distribution arising from a fractal set of sources. We show that, in the case of real structures in finite samples, an important role is played by morphological properties and finite-size effects. For dimensions smaller than d - 1 (being d the space dimension) the convergence of the net gravitational force is assured by the fast decaying of the density, while for fractal dimension D > d-1 the morphological properties of the structure determine the eventual convergence of the force as a function of distance. We clarify the role played by the cut-offs of the distribution. Some cosmological implications are discussed
Interazione pelle-ossatura nelle strutture sismo-resistenti di acciaio.
No full textGiornate Italiane della Costruzione in Acciaio C.T.A., Perugia, 23-25 October 198
Interazione pelle-ossatura nelle strutture sismo-resistenti di acciaio.
No full textGiornate Italiane della Costruzione in Acciaio C.T.A., Perugia, 23-25 October 198
Forme e applicazioni tecniche del calcestruzzo armato: La Città dello Sport di Tor Vergata in Roma (un progetto dell’Arch. Ing. Santiago Calatrava)
No full textHuman Locomotion under Reduced Gravity Conditions: Biomechanical and Neurophysiological Considerations
Full text linkReduced gravity offers unique opportunities to study motor behavior. This paper aims at providing a review on current issues of the known tools and techniques used for hypogravity simulation and their effects on human locomotion. Walking and running rely on the limb oscillatory mechanics, and one way to change its dynamic properties is to modify the level of gravity. Gravity has a strong effect on the optimal rate of limb oscillations, optimal walking speed, and muscle activity patterns, and gait transitions occur smoothly and at slower speeds at lower gravity levels. Altered center of mass movements and interplay between stance and swing leg dynamics may challenge new forms of locomotion in a heterogravity environment. Furthermore, observations in the lack of gravity effects help to reveal the intrinsic properties of locomotor pattern generators and make evident facilitation of nonvoluntary limb stepping. In view of that, space neurosciences research has participated in the development of new technologies that can be used as an effective tool for gait rehabilitation
Oscillopsia in labyrinthine defective patients: Comparison of objective and subjective measures
Full text linkObjective: To compare the oscillopsia sensation in vestibular defective patients, using a specific handicap questionnaire and a specific Visual Analog Scale, with objective measure of the vertical vestibulo-ocular reflex efficiency in the pitch plane, using the computerized Dynamic Visual Acuity (DVA) test and Gaze Stabilization Test (GST).Design: Controlled retrospective study.Setting: Day hospital in ENT Rehabilitation Unit.Subjects: Sixty-five subjects: 35 controls (12 men and 23 women; mean age, 50.77 +/- 13.39 years) and 30 patients with chronic dizziness: 18 with unilateral vestibular hypofunction (7 men and 11 women; mean age, 55.50 +/- 12.72 years) and 12 with bilateral hypofunction (7 men and 5 women; mean age, 57.25 +/- 9.18 years).Main measures: Computerize vertical DVA and GST; subjective Visual Analog Scale, Oscillopsia Score questionnaire.Results: Instrumental tests had different means between subject groups; vertical DVA results and subjective measures were significantly correlated.Conclusions: Vertical DVA and GST test in up and down direction are able to separate healthy and vestibular patients. Moreover, the DVA test in down direction differentiates patients with unilateral vestibular hypofunction and with bilateral vestibular hypofunction. These results show that vertical DVA test can be used for the assessment of the visual field instability referred to as disabling. (C) 2010 Elsevier Inc. All rights reserved
Gravitational dynamics of an infinite shuffled lattice: Particle coarse-graining, nonlinear clustering, and the continuum limit
No full textWe study the evolution under their self-gravity of particles evolving from infinite "shuffled lattice" initial conditions. We focus here specifically on the comparison between the evolution of such a system and that of "daughter" coarse-grained particle distributions. These are sparser (i.e., lower density) particle distributions, defined by a simple coarse-graining procedure, which share the same large-scale mass fluctuations. We consider both the case that such coarse-grainings are performed (i) on the initial conditions, and (ii) at a finite time with a specific additional prescription. In numerical simulations we observe that, to a first approximation, these coarse-grainings represent well the evolution of the two-point correlation properties over a significant range of scales. We note, in particular, that the form of the two-point correlation function in the original system, when it is evolving in the asymptotic "self-similar" regime, may be reproduced well in a daughter coarse-grained system in which the dynamics are still dominated by two-body (nearest neighbor) interactions. This provides a simple physical description of the origin of the form of part of the asymptotic nonlinear correlation function. Using analytical results on the early time evolution of these systems, however, we show that small observed differences between the evolved system and its coarse-grainings at the initial time will in fact diverge as the ratio of the coarse-graining scale to the original interparticle distance increases. The second coarse-graining studied, performed at a finite time in a specified manner, circumvents this problem. It also makes it more physically transparent why gravitational dynamics from these initial conditions tends toward a self-similar evolution. We finally discuss the precise definition of a limit in which a continuum (specifically Vlasov-type) description of the observed linear and nonlinear evolution should be applicable. This requires the introduction of an additional intrinsic length scale (e.g., a physical smoothing in the force at small scales), which is kept fixed as the particle density diverges. In this limit the different coarse-grainings are equivalent and leave the evolution of the "mother" system invariant
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