509 research outputs found
On the interaction between two Kerr black holes
The double-Kerr solution is generated using both a Backlund transformation
and the Belinskii-Zakharov inverse-scattering technique. We build a dictionary
between the parametrisations naturally obtained in the two methods and show
their equivalence. We then focus on the asymptotically flat double-Kerr system
obeying the axis condition which is Z_2^\phi invariant; for this system there
is an exact formula for the force between the two black holes, in terms of
their physical quantities and the coordinate distance. We then show that 1) the
angular velocity of the two black holes decreases from the usual Kerr value at
infinite distance to zero in the touching limit; 2) the extremal limit of the
two black holes is given by |J|=cM^2, where c depends on the distance and
varies from one to infinity as the distance decreases; 3) for sufficiently
large angular momentum the temperature of the black holes attains a maximum at
a certain finite coordinate distance. All of these results are interpreted in
terms of the dragging effects of the system.Comment: 19 pages, 4 figures. v2: changed statement about thermodynamical
equilibrium in section 3; minor changes; added references. v3: added
references to previous relevant work; removed one equation (see note added);
other minor corrections; final version to be published in JHE
Thermodynamical description of stationary, asymptotically flat solutions with conical singularities
We examine the thermodynamical properties of a number of asymptotically flat,
stationary (but not static) solutions having conical singularities, with both
connected and non-connected event horizons, using the thermodynamical
description recently proposed in arXiv:0912.3386 [gr-qc]. The examples
considered are the double-Kerr solution, the black ring rotating in either S^2
or S^1 and the black Saturn, where the balance condition is not imposed for the
latter two solutions. We show that not only the Bekenstein-Hawking area law is
recovered from the thermodynamical description but also the thermodynamical
angular momentum is the ADM angular momentum. We also analyse the
thermodynamical stability and show that, for all these solutions, either the
isothermal moment of inertia or the specific heat at constant angular momentum
is negative, at any point in parameter space. Therefore, all these solutions
are thermodynamically unstable in the grand canonical ensemble.Comment: 19 pages, 12 figure
Estudo paramétrico de coeficientes de comportamento de pórticos metálicos com ligações semi-rígidas
Nesta comunicação são apresentados alguns resultados relativos ao estudo paramétrico de coeficientes de comportamento de pórticos metálicos. Os modelos numéricos usados incluem a análise geometricamente não-linear dos pórticos e as relações constitutivas não-lineares das ligações. No estudo são consideradas ligações viga-coluna semi-rígidas caracterizadas por relações bi-lineares momento-rotação definidas pela sua rigidez inicial e pós-elástica, resistência e capacidade de deformação. São considerados três níveis de performance ou estados limites, Estado Limite de Serviço (SLS), Estado Limite de Acumulação de Danos (DLS) e Estado Limite Último (ULS)
Investigação, inserção profissional e espírito empresarial
As universidades encontram na sua relação com o meio envolvente o seu maior desafio
estratégico, não sendo desprezível, nem independente deste, os esforços dirigidos à
investigação, pura ou aplicada, às saídas profissionais e ao desenvolvimento do espírito
empresarial entre os estudantes, em todos os estudantes e não só naqueles que frequentam
cursos de MBA
Behaviour factors of MR steel frames with semi-rigid connections: a parametric study
In this paper same results are presented, related to the parametric study of the q-factor of steel moment resistant frames. The study considers semi-rigid beam-to-column joints with standard behaviour, characterized by their stiffness, resistance and deformation capacity. Three performance levels or limit states are considered, Serviceability Limit State (SLS), Damageability Limit State (DLS) and Ultimate Limit State (ULS)
A Double Myers-Perry Black Hole in Five Dimensions
Using the inverse scattering method we construct a six-parameter family of
exact, stationary, asymptotically flat solutions of the 4+1 dimensional vacuum
Einstein equations, with U(1)^2 rotational symmetry. It describes the
superposition of two Myers-Perry black holes, each with a single angular
momentum parameter, both in the same plane. The black holes live in a
background geometry which is the Euclidean C-metric with an extra flat time
direction. This background possesses conical singularities in two adjacent
compact regions, each corresponding to a set of fixed points of one of the U(1)
actions in the Cartan sub-algebra of SO(4). We discuss several aspects of the
black holes geometry, including the conical singularities arising from force
imbalance, and the torsion singularity arising from torque imbalance. The
double Myers-Perry solution presented herein is considerably simpler than the
four dimensional double Kerr solution and might be of interest in studying
spin-spin interactions in five dimensional general relativity.Comment: 23 pages, 7 figures, LaTeX. v2: minor changes, references added;
version published in JHE
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