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We study the influence of stationary axisymmetric spacetimes on Casimir energy. We consider a massive scalar field and analyze its dependence on the apparatus orientation with respect to the dragging direction associated with such spaces. We show that, for an apparatus orientation not considered before in the literature, the Casimir energy can change its sign, producing a repulsive force. As applications, we analyze two specific metrics: one associated with a linear motion of a cylinder and a circular equatorial motion around a gravitational source described by Kerr geometry., 13 pages, 3 figures, matches the version accepted in Eur. Phys. J. C

Using the Reduced Relativistic Gas (RRG) model, we analytically determine the matter power spectrum for Warm Dark Matter (WDM) on small scales, $k>1\ h\text{/Mpc}$. The RRG is a simplified model for the ideal relativistic gas, but very accurate in the cosmological context. In another work, we have shown that, for typical allowed masses for dark matter particles, $m>5\ \text{keV}$, the higher order multipoles, $\ell>2$, in the Einstein-Boltzmann system of equations are negligible on scales $k, 23 pages, 9 figures, matches the accepted version in JCAP

The Reduced Relativistic Gas (RRG) is a simplified version of the ideal relativistic gas, which assumes that all particles have the same momentum magnitude. Although this is a very idealized situation, the resulting model preserves the phenomenology of Maxwell-Boltzmann distribution and, in some situations, can be described as a perfect fluid, without introducing large errors in both cosmological background and first-order perturbations. The perfect fluid description of RRG model was already used to study the warmness of dark matter, massive neutrinos and interaction of baryons and photons before recombination, showing very good agreement with previous works based on the full Einstein-Boltzmann system of equations. In order to understand these results and construct a more general and formal framework for RRG, we develop a theoretical description of first-order cosmological perturbations of RRG, based on a distribution function which encodes the simplifying assumption that all particles have the same momentum magnitude. The full set of Einstein-Boltzmann equations for RRG distribution are derived and quantities beyond the perfect fluid approximation are studied. Using RRG to describe warm dark matter, we show that, for particles with $m \sim \text{keV}$, the perfect fluid approximation is valid on scales $k < 10\, \text{h}/\text{Mpc}$, for most of the universe evolution. We also determine initial conditions for RRG in the early universe and study the evolution of potential in a toy model of universe composed only by RRG., 22 pages, 5 figures

Within the standard paradigm, dark energy is taken as a homogeneous fluid that drives the accelerated expansion of the universe and does not contribute to the mass of collapsed objects such as galaxies and galaxy clusters. The abundance of galaxy clusters -- measured through a variety of channels -- has been extensively used to constrain the normalization of the power spectrum: it is an important probe as it allows us to test if the standard $\Lambda$CDM model can indeed accurately describe the evolution of structures across billions of years. It is then quite significant that the Planck satellite has detected, via the Sunyaev-Zel'dovich effect, less clusters than expected according to the primary CMB anisotropies. One of the simplest generalizations that could reconcile these observations is to consider models in which dark energy is allowed to cluster, i.e., allowing its sound speed to vary. In this case, however, the standard methods to compute the abundance of galaxy clusters need to be adapted to account for the contributions of dark energy. In particular, we examine the case of clustering dark energy -- a dark energy fluid with negligible sound speed -- with a redshift-dependent equation of state. We carefully study how the halo mass function is modified in this scenario, highlighting corrections that have not been considered before in the literature. We address modifications in the growth function, collapse threshold, virialization densities and also changes in the comoving scale of collapse and mass function normalization. Our results show that clustering dark energy can impact halo abundances at the level of 10\%--30\%, depending on the halo mass, and that cluster counts are modified by about 30\% at a redshift of unity., Comment: 18 pages and 9 figures, v2: references added and some points clarified, matches the accepted version in JCAP

The possibility of dark matter being a dissipative component represents an option for the standard view where cold dark matter (CDM) particles behave on large scales as an ideal fluid. By including a physical mechanism in the dark matter description like viscosity, we construct a more realistic model for the Universe. Also, the known small scale pathologies of the standard CDM model either disappear or become less severe. We study clustering properties of a $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$-like model in which dark matter is described as a bulk viscous fluid. The linear power spectrum, the nonlinear spherical "top hat" collapse, and the mass functions are presented. We use the analysis with such structure formation tools in order to place an upper bound on the magnitude of the dark matter's viscosity.

We show that in clustering dark energy models the growth index of linear matter perturbations, $\gamma$, can be much lower than in $\Lambda$CDM or smooth quintessence models and present a strong variation with redshift. We find that the impact of dark energy perturbations on $\gamma$ is enhanced if the dark energy equation of state has a large and rapid decay at low redshift. We study four different models with these features and show that we may have $0.33, Comment: 7 pages, 6 figures, minor changes, matches the accepted version in Phys. Rev. D

In the framework of the spherical collapse model we study the influence of shear and rotation terms for dark matter fluid in clustering dark energy models. We evaluate, for different equations of state, the effects of these terms on the linear overdensity threshold parameter, $\delta_{\rm c}$, and on the virial overdensity, $\Delta_{\rm V}$. The evaluation of their effects on $\delta_{\rm c}$ allows us to infer the modifications occurring on the mass function. Due to ambiguities in the definition of the halo mass in the case of clustering dark energy, we consider two different situations: the first is the classical one where the mass is of the dark matter halo only, while the second one is given by the sum of the mass of dark matter and dark energy. As previously found, the spherical collapse model becomes mass dependant and the two additional terms oppose to the collapse of the perturbations, especially on galactic scales, with respect to the spherical non-rotating model, while on clusters scales the effects of shear and rotation become negligible. The values for $\delta_{\rm c}$ and $\Delta_{\rm V}$ are higher than the standard spherical model. Regarding the effects of the additional non-linear terms on the mass function, we evaluate the number density of halos. As expected, major differences appear at high masses and redshifts. In particular, quintessence (phantom) models predict more (less) objects with respect to the $\Lambda$CDM model and the mass correction due to the contribution of the dark energy component has negligible effects on the overall number of structures., Comment: 12 pages, 8 figures, accepted for publication in MNRAS

Steel structures of large machines, such as ship unloaders, are normally subjected to both cyclic and impulsive loadings of very large magnitude, and therefore they are prone to damage and often need some repair. This paper outlines the mechanisms of buckling and fracture caused by dynamic loadings, and focuses on the main aspects of the repair and strengthening of large and slender thin-walled box girders of the portal frames of large machines. The increase of edge stresses due to shear lag was found to be one of the major problems. The effectiveness of the adopted solution is discussed briefly. In addition, a is given of the combined theoretical-experimental-numerical procedure which was used to model the structural-mechanical system and the dynamic loading, in order then to assess the structural behaviour, to forecast fatigue cracks and to estimate the ulterior service life of two similar large and compliant structures of ship unloaders that had been in duty for a period of 10 years.

We study the impact of Early Dark Energy fluctuations in the linear and non-linear regimes of structure formation. In these models the energy density of dark energy is non-negligible at high redshifts and the fluctuations in the dark energy component can have the same order of magnitude of dark matter fluctuations. Since two basic approximations usually taken in the standard scenario of quintessence models, that both dark energy density during the matter dominated period and dark energy fluctuations on small scales are negligible, are not valid in such models, we first study approximate analytical solutions for dark matter and dark energy perturbations in the linear regime. This study is helpful to find consistent initial conditions for the system of equations and to analytically understand the effects of Early Dark Energy and its fluctuations, which are also verified numerically. In the linear regime we compute the matter growth and variation of the gravitational potential associated with the Integrated Sachs-Wolf effect, showing that these observables present important modifications due to Early Dark Energy fluctuations, though making them more similar to $\Lambda$CDM model. We also make use of the Spherical Collapse model to study the influence of Early Dark Energy fluctuations in the nonlinear regime of structure formation, especially on $\delta_c$ parameter, and their contribution to the halo mass, which we show can be of the order of 10%. We finally compute how the number density of halos is modified in comparison to $\Lambda$CDM model and address the problem of how to correct the mass function in order to take into account the contribution of clustered dark energy. We conclude that the inhomogeneous Early Dark Energy models are more similar to $\Lambda$CDM model than its homogeneous counterparts., Comment: 26 pages and 12 figures, improved discussion, matches the accepted by JCAP

Cable-stayed bridges under wind loading exhibit dynamic behaviors that depend on the aeroelastic forces and coupling among vibration modes. In particular, the new long-span cable-stayed designs can lead to behaviors highly dependent on the coupling between distinct modal forms of vibration. To broaden the applications of existing analytical models and to examine their possible shortcomings when facing new design concepts, this paper presents a finite-element modal formulation to deal with cable-stayed bridges under laminar wind flow. In this numerical model aeroelastic forces are described as functions of the experimentally obtained flutter derivatives, and coupling among various vibration modes can be easily considered. The derived equations of motions are solved for a numerical example and the results are compared with those obtained from an analytical method in which instability is rationally considered to occur in one particular mode.

Made available in DSpace on 2022-04-28T20:48:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-01-05 The abundance and distribution of collapsed objects such as galaxy clusters will become an important tool to investigate the nature of dark energy and dark matter. Number counts of very massive objects are sensitive not only to the equation of state of dark energy, which parametrizes the smooth component of its pressure, but also to the sound speed of dark energy, which determines the amount of pressure in inhomogeneous and collapsed structures. Since the evolution of these structures must be followed well into the nonlinear regime, and a fully relativistic framework for this regime does not exist yet, we compare two approximate schemes: the widely used spherical collapse model and the pseudo-Newtonian approach. We show that both approximation schemes convey identical equations for the density contrast, when the pressure perturbation of dark energy is parametrized in terms of an effective sound speed. We also make a comparison of these approximate approaches to general relativity in the linearized regime, which lends some support to the approximations. © 2009 The American Physical Society. Instituto de Física Universidade de São Paulo, CP 66318, 05315-970, São Paulo Instituto de Física Teórica Universidade Estadual Paulista, R. Pamplona 145, 01405-900, São Paulo Instituto de Física Teórica Universidade Estadual Paulista, R. Pamplona 145, 01405-900, São Paulo

All models of dynamical dark energy possess fluctuations, which affect the number of galaxy clusters in the Universe. We have studied the impact of dark energy clustering on the number of clusters using a generalization of the spherical collapse model and the Press-Schechter formalism. Our statistical analysis is performed in a 7-parameter space using the Fisher matrix method, for several hypothetical Sunyaev-Zel'dovich and weak lensing (shear maps) surveys. In some scenarios, the impact of these fluctuations is large enough that their effect could already be detected by existing instruments such as the South Pole Telescope, when its data is combined with WMAP and SDSS. Future observations could go much further and probe the nature of dark energy by distinguishing between different models on the basis of their perturbations, not only their expansion histories., Comment: 5 pages, 4 figures

Wave-basin test results from a reduced scale model for an offshore compliant tower for deep water is used to assess fluid-structure interaction effects and to calibrate finite-element models. The experimental model complies with the theoretical conditions of hydroelastic structural similitude and the fluid in which it is immersed is plain water. This latter feature allows for observation of rigid plus elastic body motions of the model both in still water and under waves. The techniques employed for physical modeling and experimental analysis are briefly outlined, dynamic characteristics and responses obtained from tests are reported and comparisons are made with their theoretical-numerical counterpart.

We discuss the intriguing possibility that dark energy may change its equation of state in situations where large dark energy fluctuations are present. We show indications of this dynamical mutation in some generic models of dark energy., 4 pages, 2 figures. New important references and comments added

We derive, in the large scale limit, analytical solutions for a dark energy model described by the Born-Infeld field plus a perfect fluid, both for homogeneous background and first order perturbations. These analytical results are compared with numerical solutions in a model with radiation, pressureless matter and the field. We investigate the non-adiabatic perturbed pressure associated with this attractor and whether it corresponds to isocurvature contribution.

We study non-linear structure formation in the presence of dark energy. The influence of dark energy on the growth of large-scale cosmological structures is exerted both through its background effect on the expansion rate, and through its perturbations as well. In order to compute the rate of formation of massive objects we employ the Spherical Collapse formalism, which we generalize to include fluids with pressure. We show that the resulting non-linear evolution equations are identical to the ones obtained in the Pseudo-Newtonian approach to cosmological perturbations, in the regime where an equation of state serves to describe both the background pressure relative to density, and the pressure perturbations relative to the density perturbations as well. We then consider a wide range of constant and time-dependent equations of state (including phantom models) parametrized in a standard way, and study their impact on the non-linear growth of structure. The main effect is the formation of dark energy structure associated with the dark matter halo: non-phantom equations of state induce the formation of a dark energy halo, damping the growth of structures; phantom models, on the other hand, generate dark energy voids, enhancing structure growth. Finally, we employ the Press-Schechter formalism to compute how dark energy affects the number of massive objects as a function of redshift., 21 pages, 8 figures. Matches published version, with caption of Fig. 6 corrected

A hydroelastic reduced scale model of a jacket-type offshore structure was designed and constructed according to the principles of physical similitude. The techniques employed for physical modeling, construction and experimental analysis are outlined briefly and some results from free and forced vibration tests are reported. Along with these test results, some comments are made on the damping-factor components obtained, effects of mass variation and changes on the vibration responses due to the loss of structural integrity.