33,517 research outputs found
Multiscale Fractal Descriptors Applied to Nanoscale Images
This work proposes the application of fractal descriptors to the analysis of
nanoscale materials under different experimental conditions. We obtain
descriptors for images from the sample applying a multiscale transform to the
calculation of fractal dimension of a surface map of such image. Particularly,
we have used the}Bouligand-Minkowski fractal dimension. We applied these
descriptors to discriminate between two titanium oxide films prepared under
different experimental conditions. Results demonstrate the discrimination power
of proposed descriptors in such kind of application
Characterization of nanostructured material images using fractal descriptors
This work presents a methodology to the morphology analysis and
characterization of nanostructured material images acquired from FEG-SEM (Field
Emission Gun-Scanning Electron Microscopy) technique. The metrics were
extracted from the image texture (mathematical surface) by the volumetric
fractal descriptors, a methodology based on the Bouligand-Minkowski fractal
dimension, which considers the properties of the Minkowski dilation of the
surface points. An experiment with galvanostatic anodic titanium oxide samples
prepared in oxalyc acid solution using different conditions of applied current,
oxalyc acid concentration and solution temperature was performed. The results
demonstrate that the approach is capable of characterizing complex morphology
characteristics such as those present in the anodic titanium oxide.Comment: 8 pages, 5 figures, accepted for publication Physica
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Food System Transformation: Integrating a Political-Economy and Social-Ecological Approach to Regime Shifts.
Sustainably achieving the goal of global food security is one of the greatest challenges of the 21st century. The current food system is failing to meet the needs of people, and at the same time, is having far-reaching impacts on the environment and undermining human well-being in other important ways. It is increasingly apparent that a deep transformation in the way we produce and consume food is needed in order to ensure a more just and sustainable future. This paper uses the concept of regime shifts to understand key drivers and innovations underlying past disruptions in the food system and to explore how they may help us think about desirable future changes and how we might leverage them. We combine two perspectives on regime shifts-one derived from natural sciences and the other from social sciences-to propose an interpretation of food regimes that draws on innovation theory. We use this conceptualization to discuss three examples of innovations that we argue helped enable critical regime shifts in the global food system in the past: the Haber-Bosch process of nitrogen fixation, the rise of the supermarket, and the call for more transparency in the food system to reconnect consumers with their food. This paper concludes with an exploration of why this combination of conceptual understandings is important across the Global North/ Global South divide, and proposes a new sustainability regime where transformative change is spearheaded by a variety of social-ecological innovations
Network Mutual Information and Synchronization under Time Transformations
We investigate the effect of general time transformations on the phase
synchronization (PS) phenomenon and the mutual information rate (MIR) between
pairs of nodes in dynamical networks. We demonstrate two important results
concerning the invariance of both PS and the MIR. Under time transformations PS
can neither be introduced nor destroyed and the MIR cannot be raised from zero.
On the other hand, for proper time transformations the timing between the
cycles of the coupled oscillators can be largely improved. Finally, we discuss
the relevance of our findings for communication in dynamical networks.Comment: 15 p
Solving fluid dynamics problems with Matlab
MATLAB (short for Matrix Laboratory) was created by Cleve Moler and Jack Little in the
1970’s. It is a programming language for technical computing. Its environment is easy to work
with, the syntax is very simple and intuitive, it has powerful toolboxes to treat many different
problems in engineering, and it allows us to produce fantastic graphics as the programme
runs. It also allows us to create a graphical interface (via graphical user interfaces - GUIs) that
gives our programme a look that is very close to professional software.
Because of many of the mentioned features, a MATLAB code can be very compact, allowing
anyone to have "the big picture" of any code without have to look at all its details. Another
great advantage of Matlab is that, if the code is written in a vectorized form, the code can
run much faster than if it was written in the traditional form (’a la C/fortran’). The fact that
MATLAB allows us to use a powerful toolbox for sparse matrices, is also a great advantage
since, many traditional linear algebra operations can be highly improved, allowing the codes
to run much faster than it would run with the traditional linear algebra functions.
In our work we have made extensive use of MATLAB to do ’proof of concept’ studies,
especially when developing new algorithms and techniques for solving systems of coupled
nonlinear partial differential equations, such as those which arise in fluid dynamics. This
includes, for instance, codes for investigating instabilities in lid-driven cavities, Boppana and
Gajjar (2010), instabilities in flow past circular cylinders, Boppana and Gajjar (2011), and
transonic flow past aerofoils Pereira and Gajjar (2010). In some cases MATLAB is used in
its own right for solving small problems, but the fact that MATLAB is an interpreted language
means that for increasing problem sizes, the MATLAB version of the code can be much slower
than equivalent versions in other languages especially when one is dealing with very large
sparse matrices. On the other hand the beauty of MATLAB is that much of the hard work
is buried in the simple syntax and hidden from the user. An example of this is the use of
the backslash operator for solving linear systems. Whether the system is sparse or full, the
manner in which the equations are solved is hidden from the user and this greatly facilitates
code development. In the equivalent fortran versions of the code the replacement for the ’\’
operation requires considerable work and the code translation process is no longer a trivial
exercise.
In this chapter we will discuss the use of hybrid spectral methods to solve two and
three-dimensional problems using MATLAB. There is an excellent book by Trefethen (2000)
which discusses the application of spectral methods using MATLAB to solve ordinary and partial differential equations, and which provides the foundation for the techniques described
below.
To motivate the ideas we first consider the solution of a model elliptic equation of the form
a(x, y)ψxx + b(x, y)ψyy + c(x, y)ψx + e(x, y)ψy + f (x, y)ψ = g(x, y),
say with Dirichlet boundary conditions in a rectangular domain. To obtain a numerical
solution to this problem the first step is to choose an appropriate method and discretization.
In our work we have used a combination of spectral methods in one or two dimensions and
high order finite difference methods in another dimension. The main reasons for this choice
are that a hybrid approach combines the accuracy of spectral methods together with flexibility
in comparison to using spectral methods on their own. One restriction with the use of spectral
methods is that one needs to use somewhat simplified geometries. A hybrid approach gives
more flexibility in this respect. Another important reason stems from consideration of the
type of matrix patterns which arise. With finite differences in say the x direction and spectral
collocation in the y direction, the coefficient matrix with the unknowns ordered in terms of
increasing y values for a fixed x value, has a particular sparsity pattern dependent on the order
of the finite-differencing used. Second order finite-differencing leads to a a block tridiagonal
matrix whilst with fourth-order finite differences, the matrix is block-pentadiagonal of the
form:
AqΨq−2 + BqΨq−1 + CqΨq + DqΨq+1 + EqΨq+2 = Rq, q = 0, 1, ...,M. (1)
Here Ψq is the vector of unknowns at location x = xq, M+1 is typically the number of points
in the x direction and the block matrices are of size N +1 by N +1 where N +1 spectral points
are used.
UsingMATLAB it is not too difficult to generate a short code to solve the above discrete system
and the book by Trefethen (2000) gives plenty of such examples. The problem becomes more
challenging when N and M become large, as for example in some fluid flow applications
where large N,M values are needed to resolve regions of the flow where the solution changes
very rapidly. When using a large number of points the sparse matrix facilities of MATLAB
come into their own. The whole coefficient matrix does not need to be stored and by declaring
this as a sparse matrix, only the non-zero entries of the block matrices are calculated. This
avoids having to store a very large and sparse matrix which can quickly lead to memory
problems.
Increasing the order of the scheme leads to increased bandwidths. This sparsity pattern can
be exploited for 2nd, 4th or even 6th finite-differencing with a direct solver. However, with
spectral methods in two directions, unless the differential operator involved has a special
form, it is not immediately possible to utilize the sparse nature of the matrix. Whilst this
does not pose any intrinsic difficulties if one is coding in MATLAB, with increased number of
points the solution phase can become very memory intensive and requires a lot of processor
time. The use of the hybrid approach in our work is motivated in part by the observation that
the sparse matrix structure can be exploited to write efficient solvers, which not only work
well with MATLAB, but can be coded directly in other languages. MATLAB provides for an
excellent environment in which one can test and develop solvers of this type.
The above techniques have been successfully applied to investigate a whole range of different
flow problems governed by the Navier-Stokes and related equations. In the first example we
consider the onset of instability in the lid-driven cavity flow. MATLAB was used to generate
results on coarse grids and do preliminary eigenvalue computations. For very fine grids, the computations were performed in Fortran 95. The problem is described in detail in Boppana
and Gajjar (2010).
The second problem concerns the onset of instability in the flow past a row of circular
cylinders. Again the same technqiues have been used but for a more complicated geometry.
This problem is described in detail in Boppana and Gajjar (2011).
The third problem we discuss concerns the inviscid transonic flows past thin airfoils. Here
the governing equations are nonlinear and of mixed type and the flow can contain shock
wave discontinuities for certain parameter values. The full details are given in Pereira and
Gajjar (2010). The same methods as described above are used except now type differencing
needs to be incorporated to allow for the different flow behaviours in regions of subsonic
and supersonic flow. The method is fast and very robust and we are able to compute steady
flows with strong shocks. The code was written in MATLAB, using vectorization when
possible, and, in order to produce a good interface with the user, we used GUIs (graphical
user interfaces) from MATLAB. The result was a fast and accurate code, with the extra bonus
of a very good interface with the user, without a lot of effort in terms of programming. The fact
that graphical results can be shown immediately, saves us a lot of work, both on the analysis
of the results and on its presentation
Sinorhizobium Meliloti, A Bacterium Lacking The Autoinducer-2 (AI-2) Synthase, Responds To AI-2 Supplied By Other Bacteria
Many bacterial species respond to the quorum-sensing signal autoinducer-2 (AI-2) by regulating different niche-specific genes. Here, we show that Sinorhizobium meliloti, a plant symbiont lacking the gene for the AI-2 synthase, while not capable of producing AI-2 can nonetheless respond to AI-2 produced by other species. We demonstrate that S. meliloti has a periplasmic binding protein that binds AI-2. The crystal structure of this protein (here named SmlsrB) with its ligand reveals that it binds (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the identical AI-2 isomer recognized by LsrB of Salmonella typhimurium. The gene encoding SmlsrB is in an operon with orthologues of the lsr genes required for AI-2 internalization in enteric bacteria. Accordingly, S. meliloti internalizes exogenous AI-2, and mutants in this operon are defective in AI-2 internalization. S. meliloti does not gain a metabolic benefit from internalizing AI-2, suggesting that AI-2 functions as a signal in S. meliloti. Furthermore, S. meliloti can completely eliminate the AI-2 secreted by Erwinia carotovora, a plant pathogen shown to use AI-2 to regulate virulence. Our findings suggest that S. meliloti is capable of \u27eavesdropping\u27 on the AI-2 signalling of other species and interfering with AI-2-regulated behaviours such as virulence
The universal character of Zwanziger's horizon function in Euclidean Yang-Mills theories
In light of the recently established BRST invariant formulation of the
Gribov-Zwanziger theory, we show that Zwanziger's horizon function displays a
universal character. More precisely, the correlation functions of local BRST
invariant operators evaluated with the Yang-Mills action supplemented with a
BRST invariant version of the Zwanziger's horizon function and quantized in an
arbitrary class of covariant, color invariant and renormalizable gauges which
reduce to the Landau gauge when all gauge parameters are set to zero, have a
unique, gauge parameters independent result, corresponding to that of the
Landau gauge when the restriction to the Gribov region in the latter
gauge is imposed. As such, thanks to the BRST invariance, the cut-off at the
Gribov region acquires a gauge independent meaning in the class of the
physical correlators.Comment: 14 pages. v2: version accepted by Phys.Lett.
Extratos de plantas aromáticas no controle do oídio em plantas de pimentão.
Objetivou-se com o trabalho avaliar a eficácia de extratos aquosos de plantas aromáticas no controle do oídio em pimentão.Resumo
Using the Sound Card as a Timer
Experiments in mechanics can often be timed by the sounds they produce. In
such cases, digital audio recordings provide a simple way of measuring time
intervals with an accuracy comparable to that of photogate timers. We
illustrate this with an experiment in the physics of sports: to measure the
speed of a hard-kicked soccer ball.Comment: 3 pages, 4 figures, Late
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