381 research outputs found
Nucleation of reaction-diffusion waves on curved surfaces
We study reaction-diffusion waves on curved two-dimensional surfaces, and
determine the influence of curvature upon the nucleation and propagation of
spatially localized waves in an excitable medium modelled by the generic
FitzHugh-Nagumo model. We show that the stability of propagating wave segments
depends crucially on the curvature of the surface. As they propagate, they may
shrink to the uniform steady state, or expand, depending on whether they are
smaller or larger, respectively, than a critical nucleus. This critical nucleus
for wave propagation is modified by the curvature acting like an effective
space-dependent local spatial coupling, similar to diffusion, thus extending
the regime of propagating excitation waves beyond the excitation threshold of
flat surfaces. In particular, a negative gradient of Gaussian curvature
, as on the outside of a torus surface (positive ), when the
wave segment symmetrically extends into the inside (negative ), allows
for stable propagation of localized wave segments remaining unchanged in size
and shape, or oscillating periodically in size
Analyzing critical propagation in a reaction-diffusion-advection model using unstable slow waves
The effect of advection on the critical minimal speed of traveling waves is
studied. Previous theoretical studies estimated the effect on the velocity of
stable fast waves and predicted the existence of a critical advection strength
below which propagating waves are not supported anymore. In this paper, the
critical advection strength is calculated taking into account the unstable slow
wave solution. Thereby, theoretical results predict, that advection can induce
stable wave propagation in the non-excitable parameter regime, if the advection
strength exceeds a critical value. In addition, an analytical expression for
the advection-velocity relation of the unstable slow wave is derived.
Predictions are confirmed numerically in a two-variable reaction-diffusion
model.Comment: 11 pages, 8 figure
Concepts and Actors in Organic Livestock Husbandry in Bolivia
Traditional smallholder livestock production is expected to correspond widely with principles of organic livestock farming. Though, the real magnitude of livestock under organic and alike management is unknown. From stakeholder analysis and structured interviews with key persons in Bolivia it is deduced that similarities are widely given, whereas it is questioned whether a formal individual certification approach for livestock products will match the farmer interests and consumer demands
Analysis of thermal evolution in textile fabrics using advanced microstructure simulation techniques
Nowadays, membrane structures represent a modern construction element to be used as roof material in modern buildings or as design element in combination with traditional architecture. Membranes are mostly used in an outdoor environment. Therefore they are exposed to wind, radiation (solar and infrared), rain and snow. Specific membranes are three-dimensional fabrics which can be used as energy absorber or as insulation of membrane roofs. The applicability as energy absorber becomes important if the three-dimensional fabrics are designed as a porous flow channel streamed by air and convectively heated up. The transferred energy may be stored in a latent heat storage system.
Due to their porous structure, textile fabrics have a large heat-exchanging surface. If they are handled as homogenized porous structures, the heat transfer processes can not be described in a correct way. Therefore a microstructure model locally resolving all filaments of the three-dimensional fabrics has been formulated. By using an advanced meshing tool, a simulation technique has been developed taking into account the local
heat conduction properties of the different materials.
To analyse the heat transfer processes inside the three-dimensional fabrics, numerical simulations have been performed using the phase-field solver (Pace3D) of the Karlsruhe Institute of Technology and the commercial CFD-Solver StarCCM+. For a better understanding of the thermal behaviour of the fabrics, different thermal loads including thermal conduction in the microstructure (filaments) and convection by the surrounding air have been computed. The results show that the advanced simulation techniques allow to analyse the rate of conductive and convective heat transfer in three-dimensional fabrics. The results of the applied computational methods are compared
Channeling 5-min photospheric oscillations into the solar outer atmosphere through small-scale vertical magnetic flux tubes
We report two-dimensional MHD simulations which demonstrate that photospheric
5-min oscillations can leak into the chromosphere inside small-scale vertical
magnetic flux tubes. The results of our numerical experiments are compatible
with those inferred from simultaneous spectropolarimetric observations of the
photosphere and chromosphere obtained with the Tenerife Infrared Polarimeter
(TIP) at 10830 A. We conclude that the efficiency of energy exchange by
radiation in the solar photosphere can lead to a significant reduction of the
cut-off frequency and may allow for the propagation of the 5 minutes waves
vertically into the chromosphere.Comment: accepted by ApJ
Проект установки отмывки электролитического порошка титана от фторидных солей электролита производительностью 30 кг/час
Выпускная квалификационная работа в форме дипломного проекта 118 с., 24 рис., 23 табл., 46 источников, 1 приложение.
Ключевые слова: электролиз, катодный осадок, отмывка.
Цель работы – спроектировать цех для проведения процесса отмывки титанового порошка.
Основные конструктивные, технологические и технико-эксплуатационные характеристики: Объем и габаритные размеры основного аппарата составляют: V = 0,25 м3, D = 0,8 м, H = 1,8 м.
Степень внедрения: проект находится на стадии разработки.
Область применения: Химическая технология редких металлов.
Экономическая эффективность/значимость работы: экономический эффект – снижение себестоимости титанового порошка и увеличение прибыли.Die Abschlußqualifikationsarbeit in Form vom Diplomprojekt 118 mit., 24 Abb., 23 Tabellen, 46 Quellen, 1 Anlage.
Die Stichwörter: die Elektrolyse, die Kathodenablagerung, das Abwaschen.
Das Ziel der Arbeit – die Abteilung für die Verwirklichung des Prozesses des Abwaschens des Titanpulvers zu entwerfen.
Die Haupt- konstruktiven, technologischen und techniko-Betriebscharakteristiken: der Umfang und die Ladeumfänge des Hauptapparates bilden: V = 0,25 м3, D = 0,8 m, H = 1,8 m
Die Stufe der Einführung: das Projekt befindet sich auf dem Stadium der Entwicklung.
Das Gebiet der Anwendung: die Chemische Technologie der seltenen Metalle.
Die Wirtschaftseffektivität/Bedeutsamkeit der Arbeit: der Nutzeffekt – die Selbstkostenverringerung des Titanpulvers und die Vergrößerung des Gewinnes
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 figure
de-Broglie Wave-Front Engineering
We propose a simple method for the deterministic generation of an arbitrary
continuous quantum state of the center-of-mass of an atom. The method's spatial
resolution gradually increases with the interaction time with no apparent
fundamental limitations. Such de-Broglie Wave-Front Engineering of the atomic
density can find applications in Atom Lithography, and we discuss possible
implementations of our scheme in atomic beam experiments.Comment: The figures' quality was improved, the text remains intact. 5 pages,
3 figures; submitted to PR
Quantum synthesis of arbitrary unitary operators
Nature provides us with a restricted set of microscopic interactions. The
question is whether we can synthesize out of these fundamental interactions an
arbitrary unitary operator. In this paper we present a constructive algorithm
for realization of any unitary operator which acts on a (truncated) Hilbert
space of a single bosonic mode. In particular, we consider a physical
implementation of unitary transformations acting on 1-dimensional vibrational
states of a trapped ion. As an example we present an algorithm which realizes
the discrete Fourier transform.Comment: 6 RevTeX pages with 3 figures, submitted to Phys.Rev.A, see also
http://nic.savba.sk/sav/inst/fyzi/qo
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