402 research outputs found
Asymptotically Non-Static Kerr-deSitter Spacetime With No Event Horizon
We present our derivations for Kerr-deSitter metric in a proper comoving
coordinate system.It asymptotically approaches to the deSitter metric in
Robertson-walker form.This has been done by considring the stationary
axially-symmetric spacetime in which motion of particle is integrable.That is
the Hamilton-Jacobi and Klein-Gordon equations are separable.In this form it is
asymptotically consistent with comoving frame.Comment: Title changed,revised arguments,results unchanged
Cosmological constant influence on cosmic string spacetime
We investigate the line element of spacetime around a linear cosmic string in
the presence of a cosmological constant. We obtain the metric and argue that it
should be discarded because of asymptotic considerations. Then a time dependent
and consistent form of the metric is obtained and its properties are discussed.Comment: 3 page
Can an odd number of fermions be created due to chiral anomaly?
We describe a possibility of creation of an odd number of fractionally
charged fermions in 1+1 dimensional Abelian Higgs model. We point out that for
1+1 dimensions this process does not violate any symmetries of the theory, nor
makes it mathematically inconsistent. We construct the proper definition of the
fermionic determinant in this model and underline its non-trivial features that
are of importance for realistic 3+1 dimensional models with fermion number
violation.Comment: 12 pages revtex, 2 figure
Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field
Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science
Energy-momentum Distribution in Static and Non-static Cosmic String Space-times
We elaborate the problem of energy-momentum in general relativity by
energy-momentum prescriptions theory. In this regard, we calculate
M\oller,Landau-Lifshitz, Papapetrou, Einstein, Bergman, Tolman, and Weinberg's
energy-momentum complexes in static and nonstatic cosmic string space-times. We
obtain strong coincidences between the results. These coincidences can be
considered as an extension of Virbhadra's viewpoint that different
energy-momentum prescriptions may provide some basis to define a unique
quantity. In addition, our results disagree with Lessner's belief about
M\oller's prescription and support the Virbhadra's conclusion about the power
of Einstein's prescription.Comment: LaTeX, 5 page: added reference
Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results
The North Tabriz Fault is a major seismogenic fault in NW Iran. The last damaging earthquakes on this fault
occurred in 1721, rupturing the southeastern fault segment, and in 1780, rupturing the northwestern one. The
understanding of the seismic behavior of this fault is critical for assessing the hazard in Tabriz, one of the major
cities of Iran; the city suffered major damage in both the 1721 and 1780 events. Our study area is located on the
northwestern fault segment, west of the city of Tabriz. We performed geomorphic and trenching investigations,
which allowed us to recognize evidence for repeated faulting events since the Late Pleistocene. From the trenches,
we found evidence for at least four events during the past 3.6 ka, the most recent one being the 1780 earthquake.
On the basis of different approaches, horizontal slip per event and slip rates are found in the ranges of 4 ± 0.5 m
and 3.1-6.4 mm/yr, respectively. We also attempted an estimate of the average recurrence intervals which appears
to be in the range 350-1430 years, with a mean recurrence interval of 821 ± 176 years. On the basis of these results,
the northwestern segment of the North Tabriz Fault does not appear to present a major seismic potential for
the near future, however, not enough is known about the southeastern segment of the fault to make a comparable
conclusion
Experimental and numerical analyses of magnesium alloy hot workability
AbstractDue to their hexagonal crystal structure, magnesium alloys have relatively low workability at room temperature. In this study, the hot workability behavior of cast-extruded AZ31B magnesium alloy is studied through hot compression testing, numerical modeling and microstructural analyses. Hot deformation tests are performed at temperatures of 250 °C to 400 °C under strain rates of 0.01 to 1.0 s−1. Transmission electron microscopy is used to reveal the presence of dynamic recrystallization (DRX), dynamic recovery (DRY), cracks and shear bands. To predict plastic instabilities during hot compression tests of AZ31B magnesium alloy, the authors use Johnson–Cook damage model in a 3D finite element simulation. The optimal hot workability of magnesium alloy is found at a temperature (T) of 400 °C and strain rate (ε˙) of 0.01 s−1. Stability is found at a lower strain rate, and instability is found at a higher strain rate
On the characteristic connection of gwistor space
We give a brief presentation of gwistor space, which is a new concept from
G_2 geometry. Then we compute the characteristic torsion T^c of the gwistor
space of an oriented Riemannian 4-manifold with constant sectional curvature k
and deduce the condition under which T^c is \nabla^c-parallel; this allows for
the classification of the G_2 structure with torsion and the characteristic
holonomy according to known references. The case with the Einstein base
manifold is envisaged.Comment: Many changes since first version, including title; Central European
Journal of Mathematics, 201
Numerical modelling of a fast pyrolysis process in a bubbling fluidized bed reactor
In this study, the Eulerian-Granular approach is applied to simulate a fast pyrolysis bubbling fluidized bed reactor. Fast pyrolysis converts biomass to bio-products through thermochemical conversion in absence of oxygen. The aim of this study is to employ a numerical framework for simulation of the fast pyrolysis process and extend this to more complex reactor geometries. The framework first needs to be validated and this was accomplished by modelling a lab-scale pyrolysis fluidized bed reactor in 2-D and comparing with published data. A multi-phase CFD model has been employed to obtain clearer insights into the physical phenomena associated with flow dynamics and heat transfer, and by extension the impact on reaction rates. Biomass thermally decomposes to solid, condensable and non-condensable and therefore a multi-fluid model is used. A simplified reaction model is sued where the many components are grouped into a solid reacting phase, condensable/non-condensable phase, and non-reacting solid phase (the heat carrier). The biomass decomposition is simplified to four reaction mechanisms based on the thermal decomposition of cellulose. A time-splitting method is used for coupling of multi-fluid model and reaction rates. A good agreement is witnessed in the products yield between the CFD simulation and the experiment
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