27,222 research outputs found
Non-axisymmetric relativistic Bondi-Hoyle accretion onto a Kerr black hole
In our program of studying numerically the so-called Bondi-Hoyle accretion in
the fully relativistic regime, we present here first results concerning the
evolution of matter accreting supersonically onto a rotating (Kerr) black hole.
These computations generalize previous results where the non-rotating
(Schwarzschild) case was extensively considered. We parametrize our initial
data by the asymptotic conditions for the fluid and explore the dependence of
the solution on the angular momentum of the black hole. Towards quantifying the
robustness of our numerical results, we use two different geometrical
foliations of the black hole spacetime, the standard form of the Kerr metric in
Boyer-Lindquist coordinates as well as its Kerr-Schild form, which is free of
coordinate singularities at the black hole horizon. We demonstrate some
important advantages of using such horizon adapted coordinate systems.
Our numerical study indicates that regardless of the value of the black hole
spin the final accretion pattern is always stable, leading to constant
accretion rates of mass and momentum. The flow is characterized by a strong
tail shock, which, unlike the Schwarzschild case, is increasingly wrapped
around the central black hole as the hole angular momentum increases. The
rotation induced asymmetry in the pressure field implies that besides the well
known drag, the black hole will experience also a lift normal to the flow
direction. This situation exhibits some analogies with the Magnus effect of
classical fluid dynamics.Comment: 33 pages, 20 figures, submited to MNRA
Analysis and optimization of energy usage in Supermarkets
The thesis performed in this research is focused on a particular type of energy system, energy systems in supermarkets. As supermarkets are high-energy using buildings, their energy system optimization has been investigated in recent years, with the main focus in the refrigeration system, which can take up to 50% of the total energy of the supermarket. However, the complexity and interconnections of the different systems increase the difficulty of the task.
The aim of this work is to contribute in SuperSmart project, an EU project which main objective is to reduce the impact of the supermarket sector overall Europe, through the development of an ecolabel criteria. To simulate the energy use in supermarkets, CyberMart software is bring forward. This tool is used both to determine the parameters which have a higher impact in the supermarket energy system and perform energy representation based on those parameters. Finally, the design of the most energy efficient store is also presented.
According to CyberMart, some of the most determinant parameters in the supermarket energy system are refrigeration capacities, plug in cabinets used, lights power, heating system technologies used and whether the cabinets are covered or not. Using some of these parameters plus other important characteristics from the store, two energy representations are performed. The linear energy representation provides the increase or decrease of kWh per each parameter, enabling supermarkets owners to compare different parameters within the global system.
These representations, which distinguish between heat recovery and floating condensing technologies, conclude that the most important parameters in the global system are the temperature inside at winter and the refrigeration capacity. However, some unreasonable events appear, like the decline of electricity demand when the height of the building increases or the drop of heat demand with the rise of opening hours. These facts occur due to the high complexity of the global system, implying different connections between the sub-systems within CyberMart.
Concerning the most energy efficient store located in Stockholm, the obtained results show the most energy efficient supermarket is composed by CO2 refrigeration and heating systems, and a R410_A air conditioning system. The optimal electricity use of each establishment size from large to small area is 382, 394, 390 and 281 kWh/m2*year respectively, with the highest values obtained in Supermarkets and Discount stores, due to their higher rate of refrigeration power per store area
Dynamics of thick discs around Schwarzschild-de Sitter black holes
We consider the effects of a cosmological constant on the dynamics of
constant angular momentum discs orbiting Schwarzschild-de Sitter black holes.
The motivation behind this study is to investigate whether the presence of a
radial force contrasting the black hole's gravitational attraction can
influence the occurrence of the runaway instability, a robust feature of the
dynamics of constant angular momentum tori in Schwarzschild and Kerr
spacetimes. In addition to the inner cusp near the black hole horizon through
which matter can accrete onto the black hole, in fact, a positive cosmological
constant introduces also an outer cusp through which matter can leave the torus
without accreting onto the black hole. To assess the impact of this outflow on
the development of the instability we have performed time-dependent and
axisymmetric hydrodynamical simulations of equilibrium initial configurations
in a sequence of background spacetimes of Schwarzschild-de Sitter black holes
with increasing masses. The simulations have been performed with an unrealistic
value for the cosmological constant which, however, yields sufficiently small
discs to be resolved accurately on numerical grids and thus provides a first
qualitative picture of the dynamics. The calculations, carried out for a wide
range of initial conditions, show that the mass-loss from the outer cusp can
have a considerable impact on the instability, with the latter being rapidly
suppressed if the outflow is large enough.Comment: 12 pages; A&A, in pres
Local characteristic algorithms for relativistic hydrodynamics
Numerical schemes for the general relativistic hydrodynamic equations are
discussed. The use of conservative algorithms based upon the characteristic
structure of those equations, developed during the last decade building on
ideas first applied in Newtonian hydrodynamics, provides a robust methodology
to obtain stable and accurate solutions even in the presence of
discontinuities. The knowledge of the wave structure of the above system is
essential in the construction of the so-called linearized Riemann solvers, a
class of numerical schemes specifically designed to solve nonlinear hyperbolic
systems of conservation laws. In the last part of the review some astrophysical
applications of such schemes, using the coupled system of the
(characteristic) Einstein and hydrodynamic equations, are also briefly
presented.Comment: 20 pages, 4 figures, To appear in the proceedings of the workshop
"The conformal structure of space-time", J. Frauendiener, H. Friedrich, eds,
Springer Lecture Notes in Physic
Non-Geometric Vacua of the Heterotic String and Little String Theories
We study a class of 6d non-geometric vacua of the
heterotic string which can be understood as
fibrations of genus-two curves over a complex one-dimensional base. The 6d
theories living on the defects that arise when the
genus-two fiber degenerates at a point of the base are analyzed by dualizing to
F-theory on elliptic K3-fibered non-compact Calabi-Yau threefolds. We consider
all possible degenerations of genus-two curves and systematically attempt to
resolve the singularities of the dual threefolds. As in the analogous
non-geometric vacua of the heterotic string, we find that many
of the resulting dual threefolds contain singularities which do not admit a
crepant resolution. When the singularities can be resolved crepantly, we
determine the emerging effective theories which turn out to be little string
theories at a generic point on their tensor branch. We also observe a form of
duality in which theories living on distinct defects are the same.Comment: 39 pages, 3 figures, and 6 table
Long life communication satellites: Electric power supply during the eclipse period
The electric batteries, essentially nickel-cadmium for French satellites such as D1 A, D1 C, D1 D, D2 B, D5 A, D5 B, etc. and the batteries for such satellites as Symphonie, ANS, INTASAT, ESRO 4, and COS-B are discussed. The experience obtained led to the development of long lifetime batteries for communication satellites. Real simulation tests showed a lifetime of four years and accelerated lifetime tests of twelve years. These batteries will be applied in OTS, METEOSAT, and Marots. At the same time, new batteries are being developed, based on nickel-hydrogen or on silver-hydrogen, which should provide longer lifetime and better reliability
Higgsino mass matrix ansatz for MSSM
An ansatz, Det. for the Higgsino mass matrix in string
orbifold trinification is suggested toward the minimal supersymmetric standard
model(MSSM). Small instanton solutions effective around the GUT scale can
fulfil this condition. An argument that the couplings contain a moduli field is
given for a dynamical realization of this Higgsino mass matrix ansatz.Comment: 15 pages of LaTeX fil
Unveiling vertical state downscaling: identity and/or the economy?
State rescaling may take a variety of shapes although scant research has been carried out into the mechanisms and economic incentives that underpin rescaling processes. Recent literature in economics, economic sociology and political economy has identified at least two broad rescaling mechanisms, namely the development of regional identity - operating at the cultural level and proxing preference heterogeneity-, and the heterogeneity in levels of economic development, which influence the extent of regional redistribution. This paper empirically examines the mechanisms of vertical state rescaling by drawing upon empirical evidence from Catalonia and the Basque Country, to explore the evolution of sub-state identity and the rise of inter-territorial fiscal grievances - weakening intraregional economic solidarity. Findings corroborate the idea that the combination of widening sub-national identity raises the costs of managing heterogeneous spatial identities and strengthens support for vertical state downscaling. Similarly, ending regional fiscal solidarity it is found to increase the average income of Catalonia by 37% and even 17% in the Basque Country. However, the effect of regional identity exceeds that of regional redistribution in explaining state rescaling support in the magnitude of one to seven. These findings speak to the debate on the formation of Europe, in that they reveal limits to regional redistribution and highlight the importance of a common spatial identity
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