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Each piece of data is valuable for unearthing the earliest history of tetrapod origin. Despite frequently incomplete preservation, each skeletal element provides important information on the morphology, phylogeny and faunistic diversity of early tetrapodomorphs. We describe here new and earlier collected material from the fossil vertebrate site Yam-Tesovo on the Oredezh River (Leningrad Region, northwestern Russia) in the deposits of the Yam-Tesovo Formation within the Amata Regional Stage (?lowermost Frasnian, Upper Devonian). Upon similarity of their dermal ornamentation, two mandibular fragments are suggested to belong to the new tetrapodomorph taxon Rubrognathuskuleshovi n. gen. et sp. This species demonstrates a general ‘elpistostegalian' morphological pattern with some early tetrapod characters. The new taxon is characterised by an almost closed intercoronoid fossa, a prearticular that is strongly convex in section and bearing small teeth along its dorsal margin, low vertical coronoid laminae and coronoid fangs that enter the coronoid tooth row. The mandibular canal runs partly in open groove or opens to the surface by a row of large pores. The dermal ornament consists of a network of small ridges forming tubercles in the nodes. The postfrontal assigned to Tetrapodomorpha shows a ‘tetrapod-like' pits-and-ridges sculpturing and a supraorbital ridge characteristic of early tetrapods as well as ‘elpistostegalians'. Its long posterolateral bone margin demonstrates a lateral projection similar to that in Tiktaalik and unknown in other ‘elpistostegalians' and early tetrapods. An unusually flattened vomer is doubtfully related to the tetrapodomorph genus Livoniana Ahlberg, Lukševičs & Mark-Kurik, 2000, based upon characteristic multiple tooth rows. Teeth in rows decrease labially and show no clearly enlarged fang pairs. New finds of the last two decades present the earliest records of some tetrapod characters in non-limbed tetrapodomorphs. This challenges previous hypotheses on the origin of tetrapods.
Stochastic roughness is a widespread feature of natural surfaces and is an inherent byproduct of most fabrication techniques. In view of the rapid development of microfluidics, the important question is how this inevitable problem affects the low-Reynolds-number flows that are common for micro-devices. Moreover, one could potentially turn the flaw into a virtue and control the flow properties by means of specially ‘tuned’ random roughness. In this paper we investigate theoretically the statistics of fluctuations in fluid velocity produced by the waviness irregularities at the surface of a no-slip wall. Particular emphasis is laid on the issue of the universality of our findings.
This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625, 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7, 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1, 5 (2017)].
We investigate theoretically the near-wall region in elastic turbulence of a dilute polymer solution in the limit of large Weissenberg number. As has been established experimentally, elastic turbulence possesses a boundary layer where the fluid velocity field can be approximated by a steady shear flow with relatively small fluctuations on the top of it. Assuming that at the bottom of the boundary layer the dissolved polymers can be considered as passive objects, we examine analytically and numerically the statistics of the polymer conformation, which is highly non-uniform in the wall-normal direction. Next, imposing the condition that the passive regime terminates at the border of the boundary layer, we obtain an estimate for the ratio of the mean flow to the magnitude of the flow fluctuations. This ratio is determined by the polymer concentration, the radius of gyration of polymers and their length in the fully extended state. The results of our asymptotic analysis reproduce the qualitative features of elastic turbulence at finite Weissenberg numbers.
We analyse velocity fluctuations inside coherent vortices generated as a result of the inverse cascade in the two-dimensional (2-D) turbulence in a finite box. As we demonstrated in Kolokolov & Lebedev (Phys. Rev. E, vol. 93, 2016, 033104), the universal velocity profile, established in Laurie et al. (Phys. Rev. Lett., vol. 113, 2014, 254503), corresponds to the passive regime of the flow fluctuations. This property enables one to calculate correlation functions of the velocity fluctuations in the universal region. We present the results of the calculations that demonstrate a non-trivial scaling of the structure function. In addition the calculations reveal strong anisotropy of the structure function.
This study aimed to assess the results of endomyocardial biopsy from the right ventricle to establish the possible cause for drug-refractory arrhythmias in children.
Materials and methods
We enrolled 19 consecutive young patients with drug-refractory arrhythmia, from 2010 to 2013, who underwent endomyocardial biopsy. Inclusion criteria were as follows: age <18 years with a structurally normal heart or mild changes in a structure of the heart initially diagnosed as arrhythmia-induced cardiomyopathy. Overall, 86 biopsies were performed in 19 patients. Histopathological analysis, immunohistochemistry, and polymerase chain reaction were used for the interpretation of the endomyocardial biopsy.
The mean age of the patient population was 14.1±2.9 year (range from 7 to 17 years). All these patients had a history of drug-refractory arrhythmia for >5 months (mean 30 months). Patients underwent a complete history investigation, physical examination, laboratory studies, echocardiography, electrocardiography, treadmill test, and Holter monitoring before endomyocardial biopsy; two patients with arrhythmogenic right ventricular dysplasia had implantable cardioverter defibrillator implantation and further appropriate successful device shocks. Myocarditis was diagnosed based on histopathological and immunohistological analyses in nine (47.4%) patients. Polymerase chain reaction was positive for viral genome in four of them; five patients had active myocarditis. Radiofrequency ablation was performed in 17 patients; five out of six (83%) endomyocardial biopsy-proved myocarditis patients had successful radiofrequency ablation. No significant complication was reported during ablation and endomyocardial biopsy.
Approximately half of the children with drug-refractory arrhythmia had unsuspected myocarditis according to the results of the endomyocardial biopsy.
An incomplete carcass of an extinct bison, Bison ex gr. priscus, was discovered in 2012 in the mouth of the Rauchua River (69°30′N, 166°49′E), Chukotka. The carcass included the rump with two hind limbs, ribs, and large flap of hide from the abdomen and sides, several vertebrae, bones of the forelimbs and anterior autopodia, stomach with its contents, and wool. The limb bones are relatively gracile, which is unusual in bison, and a SEM study of the hair microstructure suggests higher insulating capacity than in extant members of the genus. Additionally, mitochondrial DNA analyses indicate that the Rauchua bison belonged to a distinct and previously unidentified lineage of steppe bison. Two radiocarbon dates suggest a Holocene age for the bison: a traditional 14C date provided an estimate of 8030 ± 70 14C yr ВР (SPb-743) and an AMS radiocarbon date provided an age of 9497 ± 92 14C yr BP (AA101271). These dates make this the youngest known bison from Chukotka. Analysis of stomach contents revealed a diet of herbaceous plants (meadow grasses and sedges) and shrubs, suggesting that the early Holocene vegetation near the mouth of the Rauchua River was similar to that of the present day: tundra-associated vegetation with undersized plants.
A gas-filled cylindrical liner z-pinch configuration has been used to drive convergent radiative shock waves into different gases at velocities of 20–50 km s−1. On application of the 1.4 MA, 240 ns rise-time current pulse produced by the Magpie generator at Imperial College London, a series of cylindrically convergent shock waves are sequentially launched into the gas-fill from the inner wall of the liner. This occurs without any bulk motion of the liner wall itself. The timing and trajectories of the shocks are used as a diagnostic tool for understanding the response of the liner z-pinch wall to a large pulsed current. This analysis provides useful data on the liner resistivity, and a means to test equation of state (EOS) and material strength models within MHD simulation codes. In addition to providing information on liner response, the convergent shocks are interesting to study in their own right. The shocks are strong enough for radiation transport to influence the shock wave structure. In particular, we see evidence for both radiative preheating of material ahead of the shockwaves and radiative cooling instabilities in the shocked gas. Some preliminary results from initial gas-filled liner experiments with an applied axial magnetic field are also discussed.
Asphalt concrete is the most common material for highway and motorway construction. The quality of asphalt is determined, to a large extent, by properties of asphalt binder. Fillers, which are mineral powders from carbonate rocks and aggregates fines, such as limestone and dolomite, are often used in the composition of bitumen mastics affecting the performance of asphalt.
This article explores the feasibility of using the fines of aluminosilicate sedimentary rocks as fillers. These materials are composed of clay minerals, which change their properties upon the contact with water. Normally, the use of such fillers is restricted because of poor water resistance and swelling of asphalt concrete. In order to improve the performance of these fillers, the thermal modification at moderate temperatures of 500–600 °C has been proposed. Such treatment provides sufficient structural stability of obtained materials and results in the reduction of water absorption of asphalt, improved water resistance (up to 2.5 times) and also, in reduced swelling (up to 9 times).
It has been demonstrated that improvement in the filler performance can be achieved by a heat treatment. Such treatment induces changes in the mineral composition and converts the structure of clay minerals into the frame structure of zeolite, as confirmed by X-ray diffraction and infrared spectroscopy. Due to thermal treatment, there is a change in the acid-base properties of the surface of the filler, which is reflected in the profiles of the main adsorption centers. As a result, due to chemisorption, the modified aluminosilicate fillers are able to interact with bitumen. The application of new filler materials in asphalt concrete enables to enhance the performance.
4H-SiC p+n photodiodes based on ultrathin-junctions have been fabricated with distinct processes for the p+-region creation: either with Aluminium conventional ion implantation, or with Boron Plasma Ion Immersion Implantation. Spectral sensitivity measurements were performed at several temperatures from room temperature up to 340°C, with incident wavelengths ranging from 200 to 400 nm. Both responses are characterized by a stability between 200 and 270 nm, and a important increase with temperature between 270 and 380 nm. This fact has to be related to the two different kinds of optical absorption phenomena in SiC with respect to the wavelength, which are direct and indirect (phonon assisted) transitions. When decreasing the temperature, we noticed a hysteresis effect, which could be due to charge trapping by temperature activated defects. After strong proton and electron irradiations, the diodes showed a stability of the response below 270 nm, making them suitable for use in harsh environments. Simulation was performed at room temperature, with a good correlation between simulated and experimental room temperature curves.
Properties of radiatively cooled supersonic plasma jets formed by ablation of thin Al
foils driven by 1.4 MA, 250 ns current pulse are presented. The jets are highly collimated
with half-opening angles of ~2°. Measurements of the flow velocity (~60
km/s) and plasma temperature (~15 eV) in the jet with Thomson scattering diagnostic
give internal Mach number of M ~ 3, suggesting additional collimation of the jet by
toroidal magnetic fields.
In the context of high energy density laboratory astrophysics, we aim to produce and
study a rotating plasma relevant to accretion discs physics. We devised an experimental
setup based on a modified cylindrical wire array and we studied it numerically with the
three-dimensional, resistive magneto-hydrodynamic code GORGON. The simulations show that a
rotating plasma cylinder is formed, with typical rotation velocity ~35 km/s and
Mach number ~5. In addition, the plasma ring is differentially rotating and
strongly radiatively cooled. The introduction of external magnetic fields is
The formation of supersonic, radiatively cooled plasma jets with applications to
laboratory astrophysics has been an active area of research on the MAGPIE generator. One
of the ways of producing astrophysically-relevant jets in the laboratory is by using the
ablation of plasma from a radial foil Z-pinch. In this configuration a ~1.4 MA, 250
ns current pulse is introduced into an aluminium disk with a thickness of 15
μm. The ablated plasma from the foil converges on the axis, producing a
steady and collimated jet with a typical axial velocity of ~100 km/s. The setup
allows for the addition of argon above the foil for jet-ambient interaction studies. The
interaction is characterised by the formation of several shock features, which are
presented and discussed from experimental data and numerical simulations.
Collimated outflows (jets) are ubiquitous in the universe, appearing around sources as diverse as protostars and extragalactic supermassive black holes. Jets are thought to be magnetically collimated, and launched from a magnetized accretion disk surrounding a compact gravitating object. We have developed the first laboratory experiment to address time-dependent, episodic phenomena relevant to the poorly understood jet acceleration and collimation region (Ciardi et al., 2009). The experiments were performed on the MAGPIE pulsed power facility (1.5 MA, 250 ns) at Imperial College. The experimental results show the periodic ejections of magnetic bubbles naturally evolving into a heterogeneous jet propagating inside a channel made of self-collimated magnetic cavities. The results provide a unique view of the possible transition from a relatively steady-state jet launching to the observed highly structured outflows.
With the aim to model jets produced by conical wire arrays on the MAGPIE generator, and to strengthen the link between laboratory and astrophysical jets, we performed three-dimensional magneto-hydro-dynamic numerical simulations using the code GORGON and successfully reproduced the experiments. We found that a minimum resolution of ~100 μm is required to retrieve the unstable character of the jet. Moreover, arrays with less wires produce more unstable jets with stronger magnetic fields around them.
The paper deals with the variational principles for evaluation of the spectral radii of transfer and weighted shift operators associated with a dynamical system. These variational principles have been the matter of numerous investigations and the principal results have been achieved in the situation when the dynamical system is either reversible or a topological Markov chain. As the main summands, these principles contain the integrals over invariant measures and the Kolmogorov–Sinai entropy. In the paper we derive the variational principle for an arbitrary dynamical system. It gives the explicit description of the Legendre dual object to the spectral potential. It is shown that in general this principle contains not the Kolmogorov–Sinai entropy but a new invariant of entropy type—the t-entropy.
The Rayleigh–Taylor (RT) instability develops and leads to turbulence when a heavy fluid falls under the action of gravity through a light one. We consider a model in which the RT instability is accompanied by a reactive transformation between the fluids. We study the model using direct numerical simulations (DNSs), focusing on the effect of the reaction (flame) on the turbulent mixing. We discuss ‘slow’ reactions in which the characteristic reaction time exceeds the temporal scale of the RT instability, τ ≫ tinst. In the early turbulent stage, tinst ≲ t ≲ τ, effects of the flame are distributed over a maturing mixing zone, whose development is weakly influenced by the reaction. At t ≳ τ, the fully mixed zone transforms into a conglomerate of pure-fluid patches of sizes proportional to the mixing zone width. In this ‘stirred flame’ regime, temperature fluctuations are consumed by reactions in the regions separating the pure-fluid patches. This DNS-based qualitative description is followed by a phenomenology suggesting that thin turbulent flame is of a single-fractal character, and thus distribution of the temperature field is strongly intermittent.
Electrical conductivity of open-cell metallic foam calculated by Dharmasena and Wadley using the tetrakaidecahedral cell-based model with ligaments of constant or varying triangular cross sections along the cell edges was mistakenly concluded to strongly overestimate measured electrical conductivities of the open-cell aluminum foams (ERG Duocel). Accurate analysis shows that the model with triangular ligaments on the contrary underestimates the experimental results. A tetrakaidecahedral cell-based model that takes into account the particular ligament geometry, which is determined by constant mean curvature of ligament surfaces, is shown to explain the experimental data of electrical conductivity of the Duocel foams ranging in relative density from 4% to 12%.
Investigations on standing wave (SW) interferometry come in focus of interest in the course of ongoing miniaturization of high precision length measurement systems. A key problem within these efforts is the development of a transparent ultra-thin photodetector for sampling the intensity profile of the generated SW. Group III-materials are promising candidates to ensure a good photodetector performance combined with the required optical transparency. In this work, we report on the interrelation of strain and dislocation density along with the influence of the structural properties on the sensitivity of double-heterostructure III-nitride photodetectors grown by molecular beam and metal organic vapour phase epitaxy.
This article deals with the first Hopf bifurcation of a cylinder flow, and more particularly with the properties of the unsteady periodic Kármán vortex street regime that sets in for supercritical Reynolds numbers Re > 46. Barkley (Europhys. Lett. vol.75, 2006, p. 750) has recently studied the linear properties of the associated mean flow, i.e. the flow which is obtained by a time average of this unsteady periodic flow. He observed, thanks to a global mode analysis, that the mean flow is marginally stable and that the eigenfrequencies associated with the global modes of the mean flow fit the Strouhal to Reynolds experimental function well in the range 46 < Re < 180. The aim of this article is to give a theoretical proof of this result near the bifurcation. For this, we do a global weakly nonlinear analysis valid in the vicinity of the critical Reynolds number Rec based on the small parameter ε = Rec−1 − Re−1 ≪ 1. We compute numerically the complex constants λ and μ′ which appear in the Stuart-Landau amplitude equation: dA/dt = ε λA − εμ′ A|A|2. Here A is the scalar complex amplitude of the critical global mode. By analysing carefully the nonlinear interactions yielding the term μ′, we show for the cylinder flow that the mean flow is approximately marginally stable and that the linear dynamics of the mean flow yields the frequency of the saturated Stuart-Landau limit cycle. We will finally show that these results are not general, by studying the case of the bifurcation of an open cavity flow. In particular, we show that the mean flow in this case remains strongly unstable and that the frequencies associated with the eigenmodes do not exactly match those of the nonlinear unsteady periodic cavity flow. It will be demonstrated that two precise conditions must hold for a linear stability analysis of a mean flow to be relevant and useful.