4,658 research outputs found
A solution of a problem of Sophus Lie: Normal forms of two-dimensional metrics admitting two projective vector fields
No full textWe give a complete list of normal forms for the two-dimensional metrics that admit a transitive Lie pseudogroup of geodesic-preserving transformations and we show that these normal forms are mutually non-isometric. This solves a problem posed by Sophus Lie. © 2007 Springer-Verlag
Response of Pedunculate Oak (Quercus robur L.) to Adverse Environmental Conditions in Genetic and Dendrochronological Studies
No full textPedunculate oak (Quercus robur L.) is widely distributed across Europe and serves critical ecological, economic, and recreational functions. Investigating its responses to stressors such as drought, extreme temperatures, pests, and pathogens provides valuable insights into its capacity to adapt to climate change. Genetic and dendrochronological studies offer complementary perspectives on this adaptability. Tree-ring analysis (dendrochronology) reveals how Q. robur has historically responded to environmental stressors, linking growth patterns to specific conditions such as drought or temperature extremes. By examining tree-ring width, density, and dynamics, researchers can identify periods of growth suppression or enhancement and predict forest responses to future climatic events. Genetic studies further complement this by uncovering adaptive genetic diversity and inheritance patterns. Identifying genetic markers associated with stress tolerance enables forest managers to prioritize the conservation of populations with higher adaptive potential. These insights can guide reforestation efforts and support the development of climate-resilient oak populations. By integrating genetic and dendrochronological data, researchers gain a holistic understanding of Q. robur’s mechanisms of resilience. This knowledge is vital for adaptive forest management and sustainable planning in the face of environmental challenges, ultimately helping to ensure the long-term viability of oak populations and their ecosystems. The topics covered in this review are very broad. We tried to include the most relevant, important, and significant studies, but focused mainly on the relatively recent Eastern European studies because they include the most of the species’ area. However, although more than 270 published works have been cited in this review, we have, of course, missed some published studies. We apologize in advance to authors of those relevant works that have not been cited
Utaennaja ljubov' M.N. Murav'eva
No full textL'immagine letteraria e pubblica del poeta e pedagogo M. N: Murav'ev (1757-1807) generalmenet evoca la serenità dell'amore coniugale e paterno e anche la sua lirica erotica è generalmente considerata un esercizio letterario più che l'espressione di un sentimento autentico. L'articolo presenta una breve serie di poesie inedite dedicate a una fanciulla di nome Aleksandrina e alcune lettere inedite che rivelano l'identità della destinataria.M. N. Muravyov’s image is generally associated with quiet family life and even his love poems are regarded more as literary exercises than expressions of his genuine feelings. The article presents a short series of unpublished poems dedicated to a young girl named Alexandrina and some unpublished letters that reveal the real identity of their addressee
Microstructure-induced giant elastic nonlinearity of threshold origin: Mechanism and experimental demonstration
No full textMicrostructure-induced giant elastic nonlinearity of threshold origin: Mechanism and experimental demonstration V. Yu. Zaitsev, A. Dyskin, E. Pasternak and L. Matveev EPL, 86 (2009) 44005 Please visit the new website www.epljournal.org Europhysics Letters (EPL) has a new online home at www.epljournal.org Take a look for the latest journal news and information on: • reading the latest articles, free! • receiving free e-mail alerts • submitting your work to EP
Identification of clouds and aurorae in optical data images
No full textIn this paper we present an automatic image recognition technique used to identify clouds and aurorae in digital images, taken with a CCD all-sky imager. The image recognition algorithm uses image segmentation to generate a binary block object image. Object analysis is then performed on the binary block image, the results of which are used to assess whether clouds, aurorae and stars are present in the original image. The need for such an algorithm arises because the optical study of particle precipitation into the Earth's atmosphere by the Ionosphere and Radio Propagation Group at Lancaster generates vast data-sets, over 25 000 images/year, making manual classification of all the images impractical
Thermoacoustic Instabilities in the Rijke Tube: Experiments and Modeling
No full textThermoacoustic instability can appear in thermal devices when unsteady heat release is coupled with pressure perturbations. This effect results in excitation of eigen acoustic modes of the system. These instabilities are important in various technical applications, for instance, in rocket motors and thermoacoustic engines.
A Rijke tube, representing a resonator with a mean flow and a concentrated heat source, is a convenient system for studying the fundamental physics of thermoacoustic instabilities. At certain values of the main system parameters, a loud sound is generated through a process similar to that in real-world devices prone to thermoacoustic instability. Rijke devices have been extensively employed for research purposes. The current work is intended to overcome the serious deficiencies of previous investigations with regard to estimating experimental errors and the influence of parameter variation on the results. Also, part of the objective here is to account for temperature field non-uniformity and to interpret nonlinear phenomena. The major goals of this study are to deliver accurate experimental results for the transition to instability and the scope and nature of the excited regimes, and to develop a theory that explains and predicts the effects observed.
An electrically heated, horizontally oriented, Rijke tube is used for the experimental study of transition to instability. The stability boundary is quantified as a function of major system parameters with measured uncertainties for the data collected. Hysteresis in the stability boundary is observed for certain operating regimes of the Rijke tube.
An innovative theory is developed for modeling the Rijke oscillations. First, linear theory, incorporating thermal analysis that accurately determines the properties of the modes responsible for the transition to instability, is used to predict the stability boundary. Then, a nonlinear extension of the theory is derived by introducing a hypothesis for a special form of the nonlinear heat transfer function. This nonlinear modeling is shown to predict the hysteresis phenomenon and the limit cycles observed during the tests.
A new, reduced-order modeling approach for combustion instabilities in systems with vortex shedding is derived using the developed analytical framework. A hypothesis for the vortex detachment criterion is introduced, and a kicked oscillator model is applied to produce nonlinear results characteristic for unstable combustion systems.
The experimental system and the mathematical model, developed in this work for the Rijke tube, are recommended for preliminary design and analysis of real-world thermal devices, where thermoacoustic instability is a concern.</p
High-Q Factor Dual-Layer Anapole Metamaterial
No full textSimulation results and an experimental study are presented for a dual-layer metamaterial exhibiting a resonance with Q-factor of 4000 in the mm-wave frequency range, achieved by combining resonances from two complementary planar metamaterials. The initial layer, which is the original metamaterial, is composed of metallic unit-cells shaped like epsilon letters and is based on a high Q-factor planar toroidal metamaterial [1]. This metamaterial shows significant localization of electromagnetic energy in the near field region rather than in the radiation zone due to the destructive interference of toroidal and electric dipole moments with identical radiation patterns in the far zone. The second layer, which complements the first one, consists of a metallic layer with cutouts in the epsilon letter shape. Individually, the original metamaterial demonstrates a high Q-factor resonance and confines the electric field of the incident wave. Conversely, the inverted structure does not respond to the incident wave but is instead stimulated by near-field interaction with the original metamaterial. The low losses observed can be attributed to the interaction between the two most significant multipoles excited within the metamaterials. In the case of the original metamaterial, this interaction involves destructive interference between toroidal and electric dipole moments, leading to the formation of an anapole. On the other hand, the properties of the inverted metamaterial stem from the interaction between toroidal dipole and magnetic quadrupole moments. Both metamaterials are fabricated as free-standing to avoid dielectric losses from the substrate, which can significantly diminish the resonance’s Q-factor. The simulated metamaterial (Fig.1a) is excited with the wave polarized along the wires of the metamolecules of the original type, allowing for the support of dual loops of currents along the voids, leading to toroidal excitation
Free-standing sub-THz metamaterial with the high Q-factor isolated electric dipole response
No full textAchieving high-Q resonances in the terahertz frequency range is crucial for applications such as sensors, filters, and emitters. One effective strategy for achieving such resonances involves the use of metamaterials. However, the performance of terahertz (THz) metamaterials is often limited by radiation losses in their meta-atoms and in substrates. In this paper, we present both simulation and experimental results of a dual-layer free-standing metamaterial, exhibiting Q-factor of 3570 in the sub-terahertz range, achieved by integrating two complementary planar metamaterial layers that induce an isolated dominant electric dipole moment. This unique metamaterial displays the potential for advances in antennas, quantum computing, THz sensors, offering efficient energy confinement and low losses
Testing the randomness in the sky-distribution of gamma-ray bursts
No full textWe have studied the complete randomness of the angular distribution of gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE). Because GRBs seem to be a mixture of objects of different physical nature, we divided the BATSE sample into five subsamples (short1, short2, intermediate, long1, long2) based on their durations and peak fluxes, and we studied the angular distributions separately. We used three methods, Voronoi tesselation, minimal spanning tree and multifractal spectra, to search for non-randomness in the subsamples. To investigate the eventual non-randomness in the subsamples, we defined 13 test variables (nine from the Voronoi tesselation, three from the minimal spanning tree and one from the multifractal spectrum). Assuming that the point patterns obtained from the BATSE subsamples are fully random, we made Monte Carlo simulations taking into account the BATSE's sky-exposure function. The Monte Carlo simulations enabled us to test the null hypothesis (i.e. that the angular distributions are fully random). We tested the randomness using a binomial test and by introducing squared Euclidean distances in the parameter space of the test variables. We concluded that the short1 and short2 groups deviate significantly (99.90 and 99.98 per cent, respectively) from the full randomness in the distribution of the squared Euclidean distances; however, this is not the case for the long samples. For the intermediate group, the squared Euclidean distances also give a significant deviation (98.51 per cent)
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