6,945 research outputs found
Extraordinary absorption of decorated undoped graphene
We theoretically study absorption by an undoped graphene layer decorated with
arrays of small particles. We discuss periodic and random arrays within a
common formalism, which predicts a maximum absorption of for suspended
graphene in both cases. The limits of weak and strong scatterers are
investigated and an unusual dependence on particle-graphene separation is found
and explained in terms of the effective number of contributing evanescent
diffraction orders of the array. Our results can be important to boost
absorption by single layer graphene due to its simple setup with potential
applications to light harvesting and photodetection based on energy (F\"orster)
rather than charge transfer.Comment: 5 pages, 3 figure
A dependent Lindeberg central limit theorem for cluster functionals on stationary random fields
In this paper, we provide a central limit theorem for the finite-dimensional
marginal distributions of empirical processes
whose index set is a family of cluster functionals valued on
blocks of values of a stationary random field. The practicality and
applicability of the result depends mainly on the usual Lindeberg condition and
a sequence which summarizes the dependence between the blocks of the
random field values. Finally, as application, we use the previous result in
order to show the Gaussian asymptotic behavior of the iso-extremogram estimator
introduced in this paper.Comment: 14 page
Biodiversidad de Diplura (Hexapoda: Entognatha) en MĂ©xico
ResumenSe hace una descripciĂłn general de la morfologĂa de los Diplura, dando informaciĂłn sobre su reproducciĂłn, asĂ como de sus hĂĄbitats frecuentes y su importancia desde diversos aspectos. Se muestra la distribuciĂłn de 6 familias en 23 estados de MĂ©xico. Se hace una evaluaciĂłn de la diversidad y se proporciona el listado (ApĂ©ndice) de las 57 especies que se conocen del paĂs, señalando los estados donde han sido citados.AbstractA general description of the morphology of the Diplura is done, information about their reproduction and habitats as well as their common environments and their importance in different aspects is provided. Distribution of 6 families in 23 states of Mexico is shown. An assessment of their diversity and a list of the 57 species known in the country and the states where they have been cited are provided
Analytic estimates and topological properties of the weak stability boundary
The weak stability boundary (WSB) is the transition region of the phase space where the change from gravitational escape to ballistic capture occurs. Studies on this complicated region of chaotic motion aim to investigate its unique, fuel saving properties to enlarge the frontiers of low energy transfers. This âfuzzy stabilityâ region is characterized by highly sensitive motion, and any analysis of it has been carried out almost exclusively using numerical methods. On the contrary this paper presents, for the planar circular restricted 3 body problem (PCR3BP), 1) an analytic definition of the WSB which is coherent with the known algorithmic definitions; 2) a precise description of the topology of the WSB; 3) analytic estimates on the âstable regionâ (nearby the smaller primary) whose boundary is, by definition, the WSB
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Imposing virtual origins on the velocity components in direct numerical simulations
The relative wall-normal displacement of the origin perceived by different components of near-wall turbulence is known to produce a change in drag. This effect is produced for instance by drag-reducing surfaces of small texture size like riblets and superhydrophobic surfaces. To facilitate the research on how these displacements alter near-wall turbulence, this paper studies different strategies to model such displacement effect through manipulated boundary conditions. Previous research has considered the effect of offsetting the virtual origins perceived by the tangential components of the velocity from the reference, boundary plane, where the wall-normal velocity was set to zero. These virtual origins are typically characterised by slip-length coefficients in Robin, slip-like boundary conditions. In this paper, we extend this idea and explore several techniques to define and implement virtual origins for all three velocity components on direct numerical simulations (DNSs) of channel flows, with special emphasis on the wall-normal velocity. The aim of this work is to provide a suitable foundation to extend the existing understanding on how these virtual origins affect the near-wall turbulence, and ultimately aid in the formulation of simplified models that capture the effect of complex surfaces on the overlying flow and on drag, without the need to resolve fully the turbulence and the surface texture. From the techniques tested, Robin boundary conditions for all three velocities are found to be the most satisfactory method to impose virtual origins, relating the velocity components to their respective wall-normal gradients linearly. Our results suggest that the effect of virtual origins on the flow, and hence the change in drag that they produce, can be reduced to an offset between the virtual origin perceived by the mean flow and that perceived by the overlying turbulence, and that turbulence remains otherwise smooth-wall-like, as proposed by Luchini (1996). The origin for turbulence, however, would not be set by the spanwise virtual origin alone, but by a combination of the spanwise and wall-normal origins. These observations suggest the need for an extension of Luchiniâs virtual-origin theory to predict the change in drag, accounting for the wall-normal transpiration when its effect is not negligible
Turbulent drag reduction by anisotropic permeable substrates-analysis and direct numerical simulations
We explore the ability of anisotropic permeable substrates to reduce
turbulent skin-friction, studying the influence that these substrates have on
the overlying turbulence. For this, we perform DNSs of channel flows bounded by
permeable substrates. The results confirm theoretical predictions, and the
resulting drag curves are similar to those of riblets. For small
permeabilities, the drag reduction is proportional to the difference between
the streamwise and spanwise permeabilities. This linear regime breaks down for
a critical value of the wall-normal permeability, beyond which the performance
begins to degrade. We observe that the degradation is associated with the
appearance of spanwise-coherent structures, attributed to a
Kelvin-Helmholtz-like instability of the mean flow. This feature is common to a
variety of obstructed flows, and linear stability analysis can be used to
predict it. For large permeabilities, these structures become prevalent in the
flow, outweighing the drag-reducing effect of slip and eventually leading to an
increase of drag. For the substrate configurations considered, the largest drag
reduction observed is at a friction Reynolds number
Learning CHARME models with neural networks
In this paper, we consider a model called CHARME (Conditional Heteroscedastic
Autoregressive Mixture of Experts), a class of generalized mixture of nonlinear
nonparametric AR-ARCH time series. Under certain Lipschitz-type conditions on
the autoregressive and volatility functions, we prove that this model is
stationary, ergodic and -weakly dependent. These conditions are much
weaker than those presented in the literature that treats this model. Moreover,
this result forms the theoretical basis for deriving an asymptotic theory of
the underlying (non)parametric estimation, which we present for this model. As
an application, from the universal approximation property of neural networks
(NN), we develop a learning theory for the NN-based autoregressive functions of
the model, where the strong consistency and asymptotic normality of the
considered estimator of the NN weights and biases are guaranteed under weak
conditions.Comment: 31 pages, 4 figure
Initial data sets for the Schwarzschild spacetime
A characterisation of initial data sets for the Schwarzschild spacetime is
provided. This characterisation is obtained by performing a 3+1 decomposition
of a certain invariant characterisation of the Schwarzschild spacetime given in
terms of concomitants of the Weyl tensor. This procedure renders a set of
necessary conditions --which can be written in terms of the electric and
magnetic parts of the Weyl tensor and their concomitants-- for an initial data
set to be a Schwarzschild initial data set. Our approach also provides a
formula for a static Killing initial data set candidate --a KID candidate.
Sufficient conditions for an initial data set to be a Schwarzschild initial
data set are obtained by supplementing the necessary conditions with the
requirement that the initial data set possesses a stationary Killing initial
data set of the form given by our KID candidate. Thus, we obtain an algorithmic
procedure of checking whether a given initial data set is Schwarzschildean or
not.Comment: 16 page
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Turbulent Drag Reduction Using Anisotropic Permeable Substrates.
The behaviour of turbulent flow over anisotropic permeable substrates is studied using linear stability analysis and direct numerical simulations (DNS). The flow within the permeable substrate is modelled using the Brinkman equation, which is solved analytically to obtain the boundary conditions at the substrate-channel interface for both the DNS and the stability analysis. The DNS results show that the drag-reducing effect of the permeable substrate, caused by preferential streamwise slip, can be offset by the wall-normal permeability of the substrate. The latter is associated with the presence of large spanwise structures, typically associated to a Kelvin-Helmholtz-like instability. Linear stability analysis is used as a predictive tool to capture the onset of these drag-increasing Kelvin-Helmholtz rollers. It is shown that the appearance of these rollers is essentially driven by the wall-normal permeability Ky+ . When realistic permeable substrates are considered, the transpiration at the substrate-channel interface is wavelength-dependent. For substrates with low Ky+ , the wavelength-dependent transpiration inhibits the formation of large spanwise structures at the characteristic scales of the Kelvin-Helmholtz-like instability, thereby reducing the negative impact of wall-normal permeability
Intraosseous mandibular hemangioma : a case report and review of the literature
Intraosseous vascular lesions are rare conditions, comprising only 0.5% to 1% of all intraosseous tumors. They mainly occur in the second decade of life especially in women. The most common locations are the vertebral column and skull; nevertheless, the mandible is a quite rare location. According to the World Health Organization, hemangiomas are benign vasoformative neoplasms of endothelial origin. However, the origin of central hemangioma is debatable. Some authors believe that it is a true neoplasm, whereas others state it is a hamartomatous lesion. Clinically, the patient may be completely symptom-free or may present discomfort, pulsatile bleeding, bluish discoloration, mobile teeth, derangement of the arch form or accelerated dental exfoliation. Most frequently radiographic finding is a multilocular radiolucent image with honeycombs or soap bubble appearance. Differential diagnosis includes neoplasms such as ameloblastoma, cystic lesions such as residual cyst, keratocyst and fibro-osseous lesions such as fibrous dysplasia. There are some therapeutic alternatives, although wide surgical excision remains as the gold standard. We now present a case report of a 51-year-old woman diagnosed in a mandibular hemangioma. Clinical, radiological and histological features of this unusual tumor are described
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