27,667 research outputs found
Transit Lightcurve Signatures of Artificial Objects
The forthcoming space missions, able to detect Earth-like planets by the
transit method, will a fortiori also be able to detect the transit of
artificial planet-size objects. Multiple artificial objects would produce
lightcurves easily distinguishable from natural transits. If only one
artificial object transits, detecting its artificial nature becomes more
difficult. We discuss the case of three different objects (triangle, 2-screen,
louver-like 6-screen) and show that they have a transit lightcurve
distinguishable from the transit of natural planets, either spherical or
oblate, although an ambiguity with the transit of a ringed planet exists in
some cases. We show that transits, especially in the case of multiple
artificial objects, could be used for the emission of attention-getting
signals, with a sky coverage comparable to the laser pulse method. The large
number of expected planets (several hundreds) to be discovered by the transit
method by next space missions will allow to test these ideas.Comment: Accepted for publication in ApJ. Manuscript: 17 pages, 8 figure
On the thermodynamic framework of generalized coupled thermoelastic-viscoplastic-damage modeling
A complete potential based framework using internal state variables is put forth for the derivation of reversible and irreversible constitutive equations. In this framework, the existence of the total (integrated) form of either the (Helmholtz) free energy or the (Gibbs) complementary free energy are assumed a priori. Two options for describing the flow and evolutionary equations are described, wherein option one (the fully coupled form) is shown to be over restrictive while the second option (the decoupled form) provides significant flexibility. As a consequence of the decoupled form, a new operator, i.e., the Compliance operator, is defined which provides a link between the assumed Gibb's and complementary dissipation potential and ensures a number of desirable numerical features, for example the symmetry of the resulting consistent tangent stiffness matrix. An important conclusion reached, is that although many theories in the literature do not conform to the general potential framework outlined, it is still possible in some cases, by slight modifications of the used forms, to restore the complete potential structure
Explicit robust schemes for implementation of a class of principal value-based constitutive models: Theoretical development
The issue of developing effective and robust schemes to implement a class of the Ogden-type hyperelastic constitutive models is addressed. To this end, explicit forms for the corresponding material tangent stiffness tensors are developed, and these are valid for the entire deformation range; i.e., with both distinct as well as repeated principal-stretch values. Throughout the analysis the various implications of the underlying property of separability of the strain-energy functions are exploited, thus leading to compact final forms of the tensor expressions. In particular, this facilitated the treatment of complex cases of uncoupled volumetric/deviatoric formulations for incompressible materials. The forms derived are also amenable for use with symbolic-manipulation packages for systematic code generation
Electron transfer through a multiterminal quantum ring: magnetic forces and elastic scattering effects
We study electron transport through a semiconductor quantum ring with one
input and two output terminals for an elastic scatterer present within one of
the arms of the ring. We demonstrate that the scatterer not only introduces
asymmetry in the transport probability to the two output leads but also reduces
the visibility of the Aharonov-Bohm conductance oscillations. This reduction
occurs in spite of the phase coherence of the elastic scattering and is due to
interruption of the electron circulation around the ring by the potential
defect. The results are in a qualitative agreement with a recent experiment by
Strambini et al. [Phys. Rev. B {\bf 79}, 195443 (2009)]. We also indicate that
the magnetic symmetry of the sum of conductance of both the output leads as
obtained in the experiment can be understood as resulting from the invariance
of backscattering to the input lead with respect to the magnetic field
orientation.Comment: submitted to PR
Anomalous exponents at the onset of an instability
Critical exponents are calculated exactly at the onset of an instability,
using asymptotic expansiontechniques. When the unstable mode is subject to
multiplicative noise whose spectrum at zero frequency vanishes, we show that
the critical behavior can be anomalous, i.e. the mode amplitude X scales with
departure from onset \mu as with an exponent
different from its deterministic value. This behavior is observed in a direct
numerical simulation of the dynamo instability and our results provide a
possible explanation to recent experimental observations
Investigating a simple model of cutaneous wound healing angiogenesis
A simple model of wound healing angiogenesis is presented, and investigated using numerical and asymptotic techniques. The model captures many key qualitative features of the wound healing angiogenic response, such as the propagation of a structural unit into the wound centre. A detailed perturbative study is pursued, and is shown to capture all features of the model. This enables one to show that the level of the angiogenic response predicted by the model is governed to a good approximation by a small number of parameter groupings. Further investigation leads to predictions concerning how one should select between potential optimal means of stimulating cell proliferation in order to increase the level of the angiogenic response
A mathematical model for the capillary endothelial cell-extracellular matrix interactions in wound-healing angiogenesis
Angiogenesis, the process by which new blood capillaries grow into a tissue from surrounding parent vessels, is a key event in dermal wound healing, malignant-tumour growth, and other pathologic conditions. In wound healing, new capillaries deliver vital metabolites such as amino acids and oxygen to the cells in the wound which are involved in a complex sequence of repair processes. The key cellular constituents of these new capillaries are endothelial cells: their interactions with soluble biochemical and insoluble extracellular matrix (ECM) proteins have been well documented recently, although the biological mechanisms underlying wound-healing angiogenesis are incompletely understood. Considerable recent research, including some continuum mathematical models, have focused on the interactions between endothelial cells and soluble regulators (such as growth factors). In this work, a similar modelling framework is used to investigate the roles of the insoluble ECM substrate, of which collagen is the predominant macromolecular protein. Our model consists of a partial differential equation for the endothelial-cell density (as a function of position and time) coupled to an ordinary differential equation for the ECM density. The ECM is assumed to regulate cell movement (both random and directed) and proliferation, whereas the cells synthesize and degrade the ECM. Analysis and numerical solutions of these equations highlights the roles of these processes in wound-healing angiogenesis. A nonstandard approximation analysis yields insight into the travel ling-wave structure of the system. The model is extended to two spatial dimensions (parallel and perpendicular to the plane of the skin), for which numerical simulations are presented. The model predicts that ECM-mediated random motility and cell proliferation are key processes which drive angiogenesis and that the details of the functional dependence of these processes on the ECM density, together with the rate of ECM remodelling, determine the qualitative nature of the angiogenic response. These predictions are experimentally testable, and they may lead towards a greater understanding of the biological mechanisms involved in wound-healing angiogenesis
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