5,842 research outputs found
Topological characterization of antireflective and hydrophobic rough surfaces: are random process theory and fractal modeling applicable?
The random process theory (RPT) has been widely applied to predict the joint
probability distribution functions (PDFs) of asperity heights and curvatures of
rough surfaces. A check of the predictions of RPT against the actual statistics
of numerically generated random fractal surfaces and of real rough surfaces has
been only partially undertaken. The present experimental and numerical study
provides a deep critical comparison on this matter, providing some insight into
the capabilities and limitations in applying RPT and fractal modeling to
antireflective and hydrophobic rough surfaces, two important types of textured
surfaces. A multi-resolution experimental campaign by using a confocal
profilometer with different lenses is carried out and a comprehensive software
for the statistical description of rough surfaces is developed. It is found
that the topology of the analyzed textured surfaces cannot be fully described
according to RPT and fractal modeling. The following complexities emerge: (i)
the presence of cut-offs or bi-fractality in the power-law power-spectral
density (PSD) functions; (ii) a more pronounced shift of the PSD by changing
resolution as compared to what expected from fractal modeling; (iii) inaccuracy
of the RPT in describing the joint PDFs of asperity heights and curvatures of
textured surfaces; (iv) lack of resolution-invariance of joint PDFs of textured
surfaces in case of special surface treatments, not accounted by fractal
modeling.Comment: 21 pages, 13 figure
Modeling the variability of shapes of a human placenta
While it is well-understood what a normal human placenta should look like, a
deviation from the norm can take many possible shapes. In this paper we propose
a mechanism for this variability based on the change in the structure of the
vascular tree
Software dependability techniques validated via fault injection experiments
The present paper proposes a C/C++ source-to-source compiler able to increase the dependability properties of a given application. The adopted strategy is based on two main techniques: variable duplication/triplication and control flow checking. The validation of these techniques is based on the emulation of fault appearance by software fault injection. The chosen test case is a client-server application in charge of calculating and drawing a Mandelbrot fracta
Chaotic Time Series Analysis in Economics: Balance and Perspectives
To show that a mathematical model exhibits chaotic behaviour does not prove that chaos is also present in the corresponding data. To convincingly show that a system behaves chaotically, chaos has to be identified directly from the data. From an empirical point of view, it is difficult to distinguish between fluctuations provoked by random shocks and endogenous fluctuations determined by the nonlinear nature of the relation between economic aggregates. For this purpose, chaos tests test are developed to investigate the basic features of chaotic phenomena: nonlinearity, fractal attractor, and sensitivity to initial conditions. The aim of the paper is not to review the large body of work concerning nonlinear time series analysis in economics, about which much has been written, but rather to focus on the new techniques developed to detect chaotic behaviours in the data. More specifically, our attention will be devoted to reviewing the results reached by the application of these techniques to economic and financial time series and to understand why chaos theory, after a period of growing interest, appears now not to be such an interesting and promising research area.Economic dynamics, nonlinearity, tests for chaos, chaos
Contrasting Views of Complexity and Their Implications For Network-Centric Infrastructures
There exists a widely recognized need to better understand
and manage complex “systems of systems,” ranging from
biology, ecology, and medicine to network-centric technologies.
This is motivating the search for universal laws of highly evolved
systems and driving demand for new mathematics and methods
that are consistent, integrative, and predictive. However, the theoretical
frameworks available today are not merely fragmented
but sometimes contradictory and incompatible. We argue that
complexity arises in highly evolved biological and technological
systems primarily to provide mechanisms to create robustness.
However, this complexity itself can be a source of new fragility,
leading to “robust yet fragile” tradeoffs in system design. We
focus on the role of robustness and architecture in networked
infrastructures, and we highlight recent advances in the theory
of distributed control driven by network technologies. This view
of complexity in highly organized technological and biological systems
is fundamentally different from the dominant perspective in
the mainstream sciences, which downplays function, constraints,
and tradeoffs, and tends to minimize the role of organization and
design
Fractal Analysis of the Urbanization At the Outskirts of the City: Models, Measurement and Explanation
The application of fractal geometry at the analysis of the urban development patterns has been widely investigated during the last two decades, providing further evidence that cities are complex and emergent structures . The fractal dimension of an urban area is an extremely useful indicator of the urban spatial structure and it’s transformation through time. Various models and algorithms are used to calculate the fractal dimension, such as the Box-Counting Method and the Radial Analysis . The paper concentrates on the urbanization at the edges of the city, the outskirts of the metropolitan areas, which can be considered the examples par excellence of complex, fractal-like urban structures, revealing at the same time dynamic processes of growth and transformation. The basic models of fractal analysis are presented, with the focus on the interpretation of the calculated values and the evaluation of the observed urban patterns: Fractal dimensions can be used as indicators of urban sprawl, of the degree of fragmentation of the urban landscape and of the transition from monocentric to polycentric structures. A further exploration of the above notions is based on the application of the models on an area at the outskirts of Thessaloniki, Greece. The calculation of the fractal dimensions is based on the logarithmic expression of the relationships N(l) = al-D, Ã(R) = bRD, where N presents the number of cells that are developed, l the size of the grid that is used for the calculation, D the fractal dimension, R the distance from and appropriately defined center and a,b are constants. The results are displayed by the presentation of the scatter diagrams and the linear regression between the calculated values. The important changes observed during the last decades at the urban spatial structure of the area under investigation are being evaluated by the quantitative methods provided by the fractal analysis. Attention is given to the divergence of the values across space and scale and through time. As a conclusion the need for further investigation of the models is being noted. The correlation of the fractal analysis with other methods and indices used to quantify the configuration and composition of the urban areas, and also with more general socioeconomic data, should be regarded as a promising field of further research.
Aerospace medicine and biology: A continuing bibliography with indexes (supplement 324)
This bibliography lists 200 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during May, 1989. Subject coverage includes: aerospace medicine and psychology, life support systems and controlled environments, safety equipment, exobiology and extraterrestrial life, and flight crew behavior and performance
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