Miracle-dz/UniFAR-FTUs huggingface dataset:
On diffusive instabilities of a rapidly rotating electrically conducting layer of compressible fluid of varying depth

Compressibility is added to Busse's (1976) study of convection in a rotating, electrically conducting layer of fluid, of varying depth, which is permeated by an azimuthal magnetic field that is orthogonal to both the rotation vector Ω and the gravitational acceleration g. On the basis of a linear theory, we investigate the kinds of infinitesimal wave motion which the fluid can support when the dominant balance of forces is between pressure gradient and Coriolis force, so that the Proudman–Taylor theorem holds. Unlike Busse, however, we assume that the fluid is statically stable in the sense that the temperature gradient is sub-adiabatic. In the absence of diffusion, these waves are dynamically neutral and take the form of Rossby waves modified by compressibility and magnetic field. The waves are examined in two limits, the adiabatic and the isothermal, and we define two distinct frequencies at which a pure Rossby wave can oscillate. When diffusion is restored, the disparity between these frequencies makes the fluid susceptible to overstability. We prove that all such amplifying waves must propagate eastward, i.e. in the direction of g ∧ Ω, irrespective of the sign of the depth gradient or the magnitudes of the diffusivities. The theorem does not apply if the fluid is incompressible or convectively unstable at the outset, and therefore does not contradict Busse's result that the direction of azimuthal propagation can be altered by the diffusivities. Nevertheless, we suggest reasons why Busse's method of regarding the imaginary part of the marginal stability equation as a dispersion relation is not in general a reliable one. We examine the instabilities by subjecting the neutral waves to a weakly diffusive perturbation. We discover, in particular, a new kind of magnetic instability which is crucially dependent upon both compressibility and the depth gradient. In agreement with the general result described above, the instability takes the form of a slow, eastward propagating, amplifying wave. The principal source for all instabilities is elastic energy, which cannot be tapped in a Boussinesq fluid, since the work done by compression is neglected under that approximation.
Shunting path formation in thin film structures

We present a model for shunt formation in thin films containing small volume fractions of conductive components, below the critical volume fraction of percolation theory. We show that in this regime shunting is due to almost rectilinear conductive paths, which is beyond the percolation theory framework. The criteria of rectilinear paths shunting versus the percolation cluster scenario are established. The time and temperature dependence of shunting statistics is predicted with possible applications in phase change memory and thin oxides.
Ab initio calculation of elastic properties of solid He under pressure

The high-pressure equation of state and elastic properties of solid He (He-4) have been calculated using density functional theory formulated in the framework of the exact muffin-tin orbitals method. The theoretical results, obtained within the generalized gradient approximation for the exchange-correlation functional, are in good agreement with the experimental data available for pressures between 13 GPa and 32 GPa. We predict that at 0 K the hexagonal phase of He remains mechanically and thermodynamically stable up to the highest pressure considered in the present study (similar to 150 GPa). The calculated anisotropy ratios of He are similar to those observed in the case of hexagonal metals with c/a similar to 1.63. On the other hand, we find that hydrostatic pressure has negligible effect on the anisotropy of He. This indicates that He can be used as a quasihydrostatic medium in high-pressure experiments up to at least 150 GPa.
Phase-referenced Stellar Interferometry at the Palomar Testbed Interferometer

We discuss implementation and testing of phase referencing at the Palomar Testbed Interferometer. A new instrument configuration provides a coherent integration of 10 or 20 ms on a bright star while stabilizing the fringe phase of a nearby (20″) and faint visual companion, allowing coherent integration times of at least 250 ms. Observations have been made of several visual binaries, including 16 Cyg AB (mK = 4.5 and mK = 4.6) and HD 173648/49 (mK = 4.3 and mK ∼ 5) to test the performance of the technique. These measurements also demonstrate that phase-referenced visibility measurements can be calibrated at the level of 3%–7%.
Testing the homogeneity of proportions for clustered binary data without knowing the correlation structure

A robust generalized score test for comparing groups of cluster binary data is proposed. This novel test is asymptotically valid for practically any underlying correlation configurations including the situation when correlation coefficients vary within or between clusters. This structure generally undermines the validity of the typical large sample properties of the method of maximum likelihood. Simulations and real data analysis are used to demonstrate the merit of this parametric robust method. Results show that our test is superior to two recently proposed test statistics advocated by other researchers.
Star and cluster formation in NGC 1275

Imaging of the nuclear region of the central galaxy in the Perseus Cluster, NGC 1275, has confirmed the existence of luminous, blue, and unresolved objects surrounding the nucleus. A search of archival material indicates that none of these objects has varied significantly over the past 8 yr and that no new bright objects have either appeared or disappeared. This eliminates either ultraluminous stars or supernovae as explanations of these sources; young and luminous star clusters are the most probable interpretation. Analysis of the photometry of these clusters suggests that they have formed continuously at least over the past several hundred million years, making it unlikely that a galaxy-galaxy interaction triggered the star formation
On the use of non-planar geometry to improve squeeze film rheometry

This paper presents an investigation into the effects of plate misalignment in oscillatory squeeze film analysis. In order to achieve an exact solution to the lubrication approximation including misalignment, the problem is considered in two dimensions where it is noted that plate force increases with plate skew angle. This behaviour is seen in experimental tests with a spherical plate but not for the case of flat plates. For flat plates, the force drops at low film heights and large tilt angles, which is understood to occur due to the position of the centre of pressure in the film. For the flat plates, the centre of pressure moves towards the edge of the plates, and in the three-dimensional situation this will create a shorter leak path thereby reducing the pressure in the film. From the understanding of the phenomena displayed, a conical geometry is proposed and tested which shows superior performance to the spherical surface when estimating the viscosity of a Newtonian fluid.
Systèmes d’Euler $p$-adiques et théorie d’Iwasawa

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Relaxation of soil variables in a regional climate model

A regional climate model, HIRHAM4, has been used with perfect boundary conditions fromthe ECMWF reanalysis project with an integration area covering the Mediterranean area. Themodel has been applied iteratively to one particular year, 1982, in the sense that boundaryconditions corresponding to that year were applied for several ‘‘years’’ (iterations); soil temperatureand moisture have been allowed to evolve through the simulation, whereas all atmosphericvariables have been reinitialized for each iteration. This way the soil variables, in particulartemperature, approach a stationary state with a vanishing net energy and water exchange withthe atmosphere. This approach over most of the integration area follows an exponential functionafter a transient period of not more than 1 year. This has the consequence that the asymptoticsoil temperature can be calculated after just 4 iterations of the model, resulting in an estimatemuch closer to equilibrium than the value after 4 years. Soil water is much more noisy andreaches approximate equilibrium after a few iterations; the extrapolation procedure for thisquantity is only recommended for arid regions. In general, the present experiment constitutesa method to distinguish between the transient behavior of slowly varying fields and the interannualvariability. The asymptotic state is independent of the initial soil state before the firstiteration and is therefore a property only of the model and of the forcing through the integrationperiod. This way of obtaining a ‘‘canonical’’ soil state may, for instance, have uses for modelinter-comparisons, as the arbitrary influence from initial transients has been removed. DOI: 10.1034/j.1600-0870.1999. 00010.x
Viscous fingering in a shear-thinning fluid

We study the Saffman–Taylor instability in a rectangular Hele-Shaw cell. The driven fluid is a dilute (or semidilute) polymer solution, with a viscosity that exhibits shear thinning. Other non-Newtonian properties such as elastic effects are negligible under the present experimental conditions; the system thus allows for separate investigation of the influence of shear thinning on the instability. The experiments show that, for weak shear-thinning, the results for the width of the fingers as a function of the capillary number collapse onto the universal curve for Newtonian fluids, provided the shear-thinning viscosity is used to calculate the capillary number. For stronger shear thinning, narrower fingers are found. The experiment allows also for a study of the applicability of Darcy’s law to shear thinning fluids. For Newtonian fluids, this law gives the finger velocity as a function of the pressure gradient. For weakly shear-thinning fluids, we find that an effective Darcy’s law, in which the constant v.
An update on heart disease risk associated with testosterone boosting medications

ABSTRACT Introduction: The cardiovascular (CV) safety of testosterone replacement therapy (TRT) remains a crucial issue in the management of subjects with late-onset hypogonadism. The authors systematically reviewed and discussed the available evidence focusing our analysis on heart-related issues. Areas covered: All the available data from prospective observational studies evaluating the role endogenous T levels on the risk of acute myocardial infarction (AMI) were collected and analyzed. In addition, the impact of TRT on heart-related diseases, as derived from pharmaco-epidemiological studies as well as from randomized placebo-controlled trials (RCTs), was also investigated. Expert opinion: Available evidence indicates that endogenous low T represents a risk factor of AMI incidence and its related mortality. TRT in hypogonadal patients is able to improve angina symptoms in subjects with ischemic heart diseases and exercise ability in patients with heart failure (HF). In addition, when prescribed according to the recommended dosage, TRT does not increase the risk of heart-related events.
Intercomparison Study of Cloud-to-Ground Lightning Flashes Observed by KARITLDS and KLDN at South Korea

Abstract Concern regarding lightning activity as a precursor of severe weather is increasing. Atmospheric electricity, including lightning phenomena, is one of most serious threats to successful space launch operations. The objective of this study was to evaluate the performance of two different lightning detection networks using a time–range correlation method. Understanding lightning detection network performance enables the weather forecaster to support decisions made regarding space launch operations. The relative detection efficiency (ReDE), observation ratio, ellipse area for 50% probability of location, number of sensors reporting (NSR), time difference, and distance, as parameters that predict system performance, were calculated with the time-range correlation method using cloud-to-ground (CG) flash data from the Korea Aerospace Research Institute Total Lightning Detection System (KARITLDS) and from the Korean Meteorology Administration Lightning Detection Network (KLDN). In this study, 15 thunder.
The First-Order Approach to the Continuous-Time Principal-Agent Problem with Exponential Utility

Abstract We develop the first-order approach to principal–agent problems under moral hazard in a continuous-time/dynamic setting with exponential utility. Using martingale methods from stochastic control, we derive necessary conditions for optimality in the agent′s problem which lead to a semi-martingale representation of the agent′s salary. We characterize controls which the principal can enforce by assigning suitable sharing rules. As a corollary, a simple convexity of a Hamiltonian is sufficient for the validity of the first-order approach in this dynamic setting. We also give both necessary and sufficient conditions for optimality for the agent′s and for the principal′s relaxed problem. Journal of Economic Literature Classification Numbers: C61, C73, D82.
DESIGN OF ONE-DIMENSIONAL CHAOTIC MAPS WITH PRESCRIBED STATISTICAL PROPERTIES

The statistical properties of a wide class of 1D piecewise linear Markov maps are compiled. The method used enables one to address analytically the inverse problem of designing a map with a prescribed correlation function. This class of piecewise linear maps is then used as a system of reference to analyze non-Markov piecewise linear maps and to design maps with given invariant measure and correlation function.
The structure of time-optimal trajectories for single-input systems in the plane: the general real analytic case

For arbitrary single-input real analytic systems in the plane, in which the control enters linearly, we prove that the time-optimal trajectories are finite concatenations of “bang-bang” and singular arcs, with local bounds on the number of switchings. No “nondegeneracy” assumption is made other than real-analyticity. The analysis proceeds by applying our previous results on nondegenerate $C^\infty $ systems to study the behavior of the trajectories on a neighborhood of every point, except for the points in a discrete set of “branch points. ” The branch points are then handled by a combination of control-theoretic arguments and the use of subanalytic set theory.
Convectional magnetic shielding

A general review of conventional magnetic shielding is presented. Several possible definitions of the term 'shielding factor' are given, taking into account the ferromagnetic nature of shielding materials. An alternative treatment of single-layer axial shielding for a cylindrical shield is presented and compared to a previous approximation. A new algorithm for calculating multi-layer axial shielding factors is derived and shown to be an improvement on existing methods when applied to some assemblies, including one reported for the first time.
Convection-driven Emergence of Small-Scale Magnetic Fields and their Role in Coronal Heating and Solar Wind Acceleration

Recent observations by the Solar Optical Telescope (SOT) on board Hinode have revealed that the surface of the Sun is, on average, covered with small-scale horizontal magnetic fields. Frequent emergence of horizontal magnetic flux on a granular scale is found in the quiet Sun and in plage regions. In this Letter we present the results of magnetohydrodynamic simulations that cover the upper convection zone and the corona. It is found that, even when the initial magnetic field is uniform and vertical, a disordered magnetic field is produced in the convection zone. The photospheric magnetic field is then characterized by strong vertical fields concentrated in the intergranular lanes and relatively weak, horizontal fields both in the granules and in the intergranular lanes. Occasionally, fragments with large magnetic fluxes are driven above the photosphere by the upward convective flows. These characteristics are consistent with the SOT observations. Moreover, the simulated flux emerging on a granular scale undergoes magnetic reconnection with the expanding vertical magnetic concentrations in the chromosphere. These reconnection events heat the local plasma and emit high-frequency waves that propagate into the corona. Such an interplay between the small-scale horizontal fields and the vertical flux may play an important role in coronal heating and wind acceleration in the Sun and stars.
Modeling of Turbulent Heat Transfer from an Array of Submerged Jets Impinging on a Solid Surface

ABSTRACT Heat transfer from a row of turbulent jets impinging on a stationary surface is investigated numerically with the finite-difference algorithm SIMPLE. The jet-to-jet interaction, the geometric parameters of the jet array, and the effect of Reynolds number are investigated. The jets under consideration are submerged, planar, and exit the nozzle with a developed velocity profile. This study focuses on the turbulent regime (Re j up to 72,300) using the Lam-Bremhorst version of the low-Re k–ϵ model. The notorious overpredictions in the stagnation region are alleviated using the Yap correction. Transition from laminar to turbulent flow is predicted via the Schmidt-Patankar production-term-modification model. Good comparison with experimental data is shown. Model results show that heat transfer is a maximum at the stagnation point and decreases with distance along the plate. At some point, heat transfer may exhibit an increase due to transition to turbulence. There is also a local maximum in Nusselt number at the midpoint between two jets, where the jets collide. The uniformity of heat transfer on the plate is a function of where and whether transition to turbulence occurs and the magnitudes of heat transfer at the stagnation and collision points. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.
Multiphoton detachment from negative ions: new theory versus experiment

In this paper we compare the results of our adiabatic theory (Gribakin and Kuchiev 1997 Phys. Rev. A 55 3760) with other theoretical and experimental results, mostly for the halogen negative ions. The theory is based on the Keldysh approach, and shows that the multiphoton detachment rates depend only on the asymptotic parameters A and of the bound-state radial wavefunction . Simple analytical expressions for the differential and total n-photon detachment cross sections are obtained. They allow us to estimate the cross sections for almost any negative ion. Our approach suggests a new physical interpretation of the nontrivial oscillatory behaviour of the differential cross sections that has been observed in experiments. It also predicts oscillations in the energy dependence of the total n-photon cross sections, due to interplay among different partial waves.
Multivariate bandwidth selection for local linear regression

The existence and properties of optimal bandwidths for multivariate local linear regression are established, using either a scalar bandwidth for all regressors or a diagonal bandwidth vector that has a different bandwidth for each regressor. Both involve functionals of the derivatives of the unknown multivariate regression function. Estimating these functionals is difficult primarily because they contain multivariate derivatives. In this paper, an estimator of the multivariate second derivative is obtained via local cubic regression with most cross‐terms left out. This estimator has the optimal rate of convergence but is simpler and uses much less computing time than the full local estimator. Using this as a pilot estimator, we obtain plug‐in formulae for the optimal bandwidth, both scalar and diagonal, for multivariate local linear regression. As a simpler alternative, we also provide rule‐of‐thumb bandwidth selectors. All these bandwidths have satisfactory performance in our simulation study.
Student understanding of wave behavior at a boundary: The limiting case of reflection at fixed and free ends

We investigated student understanding of wave behavior at a boundary in the context of pulses and periodic waves in water and in elastic media. The participants were science and engineering majors in introductory calculus-based physics. We document several conceptual and reasoning difficulties and describe the refinement and assessment of our instructional materials. The results show significant improvements in student learning of some aspects of superposition and reflection, but some reasoning patterns that yield incorrect predictions persist. Among these difficulties is the tendency to employ simple rule-based approaches in cases in which a systematic application of the superposition principle is necessary to predict the motion of points in the medium over an interval of time.
Scanning an Interplanetary Magnetic Cloud Using High-Energy Protons

We applied a numerical modeling of the solar energetic particle (SEP) event inside a magnetic cloud in the solar wind to analyze the 17-22 MeV proton flux anisotropy observed on 1998 May 2 with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO), when SOHO was inside a magnetic cloud associated with a previous coronal mass ejection (CME). The analysis revealed a strong intermittency of the SEP transport parameters when different magnetic tubes were convected past the spacecraft. The estimated mean free path value varies over 1 order of magnitude, from ~2 to ~20 AU. The SEP event has been modeled with a prolonged injection of particles from a new CME into the previous ejecta comprising a set of magnetic loops. Both the prompt, direct proton flux and the delayed, counterstreaming flux were observed in the beginning and maximum phase of the event, but then the counterstreaming flux waned. The lack of counterstreaming protons can be explained either by the very fast escape of high-energy protons from the magnetic cloud, before they could complete one bounce in a narrow loop, or by the proton injection predominantly into one leg of a wide loop. An imprint of the magnetic compression at the leading part of the CME can be also found in the proton flux anisotropy data of SOHO/ERNE. These findings illustrate how high-precision anisotropy measurements and a numerical modeling can provide a kind of probe for the CME structure.