879 research outputs found
A numerical model for predicting bubble formation in a 3D fluidized bed
AbstractFluidized bed systems have the potential to be widely used in the power generation, mineral processing and chemical industries. One factor limiting their increased use is the lack of adequate design techniques for scaling such systems. A model has been developed for simulating gasâsolid fluidized bed plant. The model uses a multiphase EulerianâEulerian technique to predict the transient behaviour of fluidized bed systems. The commercial CFD code CFX is used as the computational framework for solving the discretized equations. To overcome the problem of accurate geometrical representation experienced in previous models a body fitted grid system is employed. The model is used to predict isothermal flow in a three-dimensional bubbling fluidized bed. Predictions of the three-dimensional model show bubble formation with gas bubbles or voids preferentially moving along the centre of the bed. Predicted behaviour is qualitatively consistent with experimental observations
Non-Singularity of the Exact Two-Dimensional String Black Hole
We study the global structure of the exact two-dimensional space-time which
emerges from string theory. Previous work has shown that in the semi-classical
limit, this is a black hole similar to the Schwarzschild solution. However, we
find that in the exact case, a new Euclidean region appears "between" the
singularity and black hole interior. However the boundary between the
Lorentzian and Euclidean regions is a coordinate singularity, which turns out
to be a surface of time reflection symmetry in an extended space-time. Thus
strings having fallen through the black hole horizon would eventually emerge
through another one into a new asymptotically flat region. The maximally
extended space-time consists of an infinite number of universes connected by
wormholes. There are no singularities present in this geometry. We also
calculate the mass and temperature associated with the space-time.Comment: 9 pages, latex, DAMTP R93/
Aspects of Superembeddings
Some aspects of the geometry of superembeddings and its application to
supersymmetric extended objects are discussed. In particular, the embeddings of
(3|16) and (6|16) dimensional superspaces into (11|32) dimensional superspace,
corresponding to supermembranes and superfivebranes in eleven dimensions, are
treated in some detail.Comment: 13 pages, Latex, Contribution to Supersymmetry and Quantum Field
Theory, International Seminar dedicated to the memory of D. V. Volkov
(Kharkov, 1997), some clarifications are mad
Primordial Black Holes: Observational Characteristics of The Final Evaporation
Many early universe theories predict the creation of Primordial Black Holes
(PBHs). PBHs could have masses ranging from the Planck mass to 10^5 solar
masses or higher depending on the size of the universe at formation. A Black
Hole (BH) has a Hawking temperature which is inversely proportional to its
mass. Hence a sufficiently small BH will quasi-thermally radiate particles at
an ever-increasing rate as emission lowers its mass and raises its temperature.
The final moments of this evaporation phase should be explosive and its
description is dependent on the particle physics model. In this work we
investigate the final few seconds of BH evaporation, using the Standard Model
and incorporating the most recent Large Hadron Collider (LHC) results, and
provide a new parameterization for the instantaneous emission spectrum. We
calculate for the first time energy-dependent PBH burst light curves in the
GeV/TeV energy range. Moreover, we explore PBH burst search methods and
potential observational PBH burst signatures. We have found a unique signature
in the PBH burst light curves that may be detectable by GeV/TeV gamma-ray
observatories such as the High Altitude Water Cerenkov (HAWC) observatory. The
implications of beyond the Standard Model theories on the PBH burst
observational characteristics are also discussed, including potential
sensitivity of the instantaneous photon detection rate to a squark threshold in
the 5 -10 TeV range.Comment: Accepted to Astroparticle Physics Journal (71 Pages, 22 Figures
Network development in biological gels: role in lymphatic vessel development
In this paper, we present a model that explains the prepatterning of lymphatic vessel morphology in collagen gels. This model is derived using the theory of two phase rubber material due to Flory and coworkers and it consists of two coupled fourth order partial differential equations describing the evolution of the collagen volume fraction, and the evolution of the proton concentration in a collagen implant; as described in experiments of Boardman and Swartz (Circ. Res. 92, 801â808, 2003). Using linear stability analysis, we find that above a critical level of proton concentration, spatial patterns form due to small perturbations in the initially uniform steady state. Using a long wavelength reduction, we can reduce the two coupled partial differential equations to one fourth order equation that is very similar to the CahnâHilliard equation; however, it has more complex nonlinearities and degeneracies. We present the results of numerical simulations and discuss the biological implications of our model
Dielectric-branes in Non-supersymmetric SO(3)-invariant Perturbation of Three-dimensional N=8 Yang-Mills Theory
We study non-supersymmetric SO(3)-invariant deformations of d=3, N=8 super
Yang-Mills theory and their type IIA string theory dual. By adding both gaugino
mass and scalar mass, dielectric D4-brane potential coincides with D5-brane
potential in type IIB theory. We find the region of parameter space where the
non-supersymmetric vacuum is described by stable dielectric NS5-branes. By
considering the generalized action for NS5-branes in the presence of D4-flux,
we also analyze the properties of dielectric NS5-branes.Comment: latex, 18 pages, 8 figures, typos are corrected, the version to
appear in Phys. Rev.
On Some New Black String Solutions in Three Dimensions
We derive several new solutions in three-dimensional stringy gravity. The
solutions are obtained with the help of string duality transformations. They
represent stationary configurations with horizons, and are surrounded by
(quasi) topologically massive Abelian gauge hair, in addition to the dilaton
and the Kalb-Ramond axion. Our analysis suggests that there exists a more
general family, where our solutions are special limits. Finally, we use the
generating technique recently proposed by Garfinkle to construct a traveling
wave on the extremal variant of one of our solutions.Comment: revtex, 38 pages including 3 figure
An action for the exact string black hole
A local action is constructed describing the exact string black hole
discovered by Dijkgraaf, Verlinde and Verlinde in 1992. It turns out to be a
special 2D Maxwell-dilaton gravity theory, linear in curvature and field
strength. Two constants of motion exist: mass M>1, determined by the level k,
and U(1)-charge Q>0, determined by the value of the dilaton at the origin. ADM
mass, Hawking temperature T_H \propto \sqrt{1-1/M} and Bekenstein-Hawking
entropy are derived and studied in detail. Winding/momentum mode duality
implies the existence of a similar action, arising from a branch ambiguity,
which describes the exact string naked singularity. In the strong coupling
limit the solution dual to AdS_2 is found to be the 5D Schwarzschild black
hole. Some applications to black hole thermodynamics and 2D string theory are
discussed and generalizations - supersymmetric extension, coupling to matter
and critical collapse, quantization - are pointed out.Comment: 41 pages, 2 eps figures, dedicated to Wolfgang Kummer on occasion of
his Emeritierung; v2: added ref; v3: extended discussion in sections 3.2, 3.3
and at the end of 5.3 by adding 2 pages of clarifying text; updated refs;
corrected typo
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
Potassium acetate solution as a promising option to osmotic distillation for sour cherry (Prunus cerasus L.) juice concentration
Different osmotic agents (OA), such as potassium acetate (CH3COOK), potassium carbonate (K2CO3) and ammonium nitrate (NH4NO3), have been examined as alternatives to the traditionally used calcium chloride (CaCl2) for osmotic distillation concentrating of clarified and pre-concentrated sour cherry (Prunus cerasus L.) juice. Comparison of the process performances based on the permeate fluxes has been carried out. Regarding the permeate flux results, simplified estimation of the overall mass transfer coefficient of the most effective osmotic agent and the reference (CaCl2) solution has been also performed. Furthermore, analytical methods such as total antioxidant activity (TAA) and total polyphenolic content (TPC) using spectrophotometric assays have been also carried out to evaluate the effect of the osmotic distillation on the valuable compounds content of concentrated sour cherry juice. CH3COOK was found to be the most effective, resulted more than 25% higher permeate flux during the sour cherry juice concentration. K2CO3 and NH4NO3 were less effective. The simplified mass transfer estimation showed that the CH3COOK is more effective only at near saturated concentrations compared to the CaCl2. Regarding the TAA and TPC contents, a significant loss was found in case of all OAs during the concentration procedures
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