Flow invariants in a channel obstructed by a line of inclined rods

An experiment is conducted in a rectangular channel obstructed by a transverse line of four inclined cylindrical rods. The pressure on the surface of a central rod and the pressure drop through the channel are measured varying the inclination angle of the rods. Three assemblies of rods with different diameters are tested. The measurements were analyzed applying momentum conservation principles and semi-empirical considerations. Several invariant dimensionless groups of parameters relating the pressure at key locations of the system with characteristic dimensions of the rods are produced. It was found that the independence principle holds for most of the Euler numbers characterizing the pressure at different locations, that is, the group is independent of the inclination angle provided that the inlet velocity projection normal to the rods is used to non-dimensionalize the pressure. The resulting semi-empirical correlations can be useful for designing similar hydraulic units.Fil: Herrero, Víctor Angel. Universidad Austral. Facultad de Ingenieria. Laboratorio de Investigacion Desarrollo y Transferencia. - Comision de Investigaciones Cientificas de la Provincia de Buenos Aires. Laboratorio de Investigacion Desarrollo y Transferencia.; ArgentinaFil: Ferrari, Hernan Javier. Universidad Austral. Facultad de Ingenieria. Laboratorio de Investigacion Desarrollo y Transferencia. - Comision de Investigaciones Cientificas de la Provincia de Buenos Aires. Laboratorio de Investigacion Desarrollo y Transferencia.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Marino, R.. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Clausse, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Ezeiza; Argentin

Kinematic stability and simulations of the variational two-fluid model for slug flow

The two-fluid short-wave theory (TF-SWT) mode of the one-dimensional two-fluid model (TFM) [A. Clausse and M. Lopez de Bertodano, "Natural modes of the two-fluid model of two-phase flow,"Phys. Fluids 33, 033324 (2021)] showed that the incompressible kinematic and Kelvin-Helmholtz instabilities are the source of the long-standing ill-posed question. Here, the stability of the short wave mode is analyzed to obtain an unstable incompressible well-posed TFM for vertical slug flow, where inertial coupling and drag play the key role. Then, a computational method is implemented to perform non-linear simulations of slug waves. Linear stability analyses, i.e., characteristics and dispersion, of a variational TF-SWT for vertical slug flows are presented. The current TFM is constituted with a lumped-parameter model of inertial coupling between the Taylor bubble and the liquid. A characteristic analysis shows that this conservative model is parabolic, and it provides a base upon which other models can be constructed, including short-wave damping mechanisms, like vortex dynamics. The dispersion analysis shows that depending on the interfacial drag, the problem can be kinematic unstable. A new kinematic condition in terms of the inertial coupling and the interfacial drag is derived that is consistent with previous theoretical and experimental results. The material waves, which are predicted by linear stability theory, then develop into nonlinear slug waveforms that are captured by the numerical simulations. These and the horizontal stratified flow waves of previous research illustrate the TFM capability to model interfacial structures that behave like waves. Otherwise, when the physics of the TF-SWT waves is ignored, the model is ill-posed.Fil: Clausse, Alejandro. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina. Pontificia Universidad Católica Argentina "Santa María de los Buenos Aires"; ArgentinaFil: Chetty, K.. Purdue University. School Of Nuclear Engineering; Estados UnidosFil: Buchanan, J.. Naval Nuclear Laboratory; Estados UnidosFil: Ram, R.. Purdue University. School Of Nuclear Engineering; Estados UnidosFil: Lopez de Bertodano, M.. Purdue University. School Of Nuclear Engineering; Estados Unido

Electrical conductivity of dispersions: from dry foams to dilute suspensions

We present new data for the electrical conductivity of foams in which the liquid fraction ranges from two to eighty percent. We compare with a comprehensive collection of prior data, and we model all results with simple empirical formul\ae. We achieve a unified description that applies equally to dry foams and emulsions, where the droplets are highly compressed, as well as to dilute suspensions of spherical particles, where the particle separation is large. In the former limit, Lemlich's result is recovered; in the latter limit, Maxwell's result is recovered

Autómatas celulares sobre grafos de nodos irregulares: aplicación a la simulación de escurrimientos superficiales en zonas de llanura

El modelo AQUA-GRAPH [4] de escurrimiento superficial específico para zonas de llanuras se basa en la tecnología de autómatas celulares (AC), de amplia utilización en la simulación de procesos naturales. Este modelo se implementa sobre una estructura de datos grafo. Este diseño da una mayor flexibilidad al modelo de escurrimiento y al simulador permitiendo realizar diferentes tipos de discretizaciones con el objeto de mejorar la performance y dar diferentes niveles de detalle a la simulación. En el presente trabajo se perfeccionó el autómata AQUA-GRAPH para poder trabajar con células de diferente tamaño teniendo en cuenta el área y el volumen de agua acumulado en cada celda. Sobre este esquema se desarrolló un algoritmo para discretizar el terreno agrupando conjuntos de celdas con similares características en celdas de mayor área. Se utilizaron diferentes criterios para fusionar celdas como, diferencia de altura, gradiente acumulado y curvatura acumulada. Se evaluaron estas modificaciones sobre escenarios reales y analizando la sensibilidad a los diferentes parámetros. Los resultados permitieron mejorar notablemente la performance de las simulaciones al reducir la cantidad de elementos a tratar sin perder precisión o nivel de detalle en las zonas más importantes.Red de Universidades con Carreras en Informática (RedUNCI

Spectral and spatial imaging of the Be+sdO binary phi Persei

The rapidly rotating Be star phi Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution images of phi Persei made possible by new capabilities in longbaseline interferometry at near-IR and visible wavelengths. We observed phi Persei with the MIRC and VEGA instruments of the CHARA Array. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9.6+-0.3Msol and 1.2+-0.2Msol for the Be primary and subdwarf secondary, respectively. The inferred distance (186 +- 3 pc), kinematical properties, and evolutionary state are consistent with membership of phi Persei in the alpha Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the phi Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk.Comment: 16 pages, 6 figures, 1 Anne

Tests with a Carlina-type diluted telescope; Primary coherencing

Studies are under way to propose a new generation of post-VLTI interferometers. The Carlina concept studied at the Haute- Provence Observatory is one of the proposed solutions. It consists in an optical interferometer configured like a diluted version of the Arecibo radio telescope: above the diluted primary mirror made of fixed cospherical segments, a helium balloon (or cables suspended between two mountains), carries a gondola containing the focal optics. Since 2003, we have been building a technical demonstrator of this diluted telescope. First fringes were obtained in May 2004 with two closely-spaced primary segments and a CCD on the focal gondola. We have been testing the whole optical train with three primary mirrors. The main aim of this article is to describe the metrology that we have conceived, and tested under the helium balloon to align the primary mirrors separate by 5-10 m on the ground with an accuracy of a few microns. The servo loop stabilizes the mirror of metrology under the helium balloon with an accuracy better than 5 mm while it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have obtained the white fringes of metrology; i.e., the three mirrors are aligned (cospherized) with an accuracy of {\approx} 1 micron. We show data proving the stability of fringes over 15 minutes, therefore providing evidence that the mechanical parts are stabilized within a few microns. This is an important step that demonstrates the feasibility of building a diluted telescope using cables strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could be one of the first members of a new class of telescopes named diluted telescopes.Comment: 18 pages, 17 figures, A&A, accepte

Validación experimental de un modelo que describe la etapa axial de un plasma focus sin electrodo externo

Se presenta una modificación al modelo de la barredora de nieve tradicional donde se supone que la lámina se forma sobre el aislante y tiene un espesor , pudiendo luego evolucionar tanto en la dirección radial como en la axial. La evolución se realiza planteando la ecuación de conservación de la cantidad de movimiento en la cual, para tener en cuenta la curvatura de la lámina, se agrega un segundo parámetro (eficiencia de transferencia de impulso, ) en el término que contabiliza la cantidad de movimiento cedida al gas barrido. El modelo incluye la ecuación del circuito eléctrico y el sistema de ecuaciones resultante se resuelve numéricamente. Los parámetros libres se ajustan a partir de mediciones de la cinemática de la lámina obtenidas con una sonda magnética inductiva en un equipo Plasma Focus con un electrodo central de 9,5 mm de diámetro y 90 mm de longitud. El segundo electrodo está formado por un disco plano sobre el que asienta el aislante y el electrodo central. Los gases empleados fueron hidrógeno y nitrógeno, en el rango 0,5 – 10 mbar. Se encuentra que tanto como decrecen al aumentar la presión del gas de llenado.An extension to the standard Snow Plow model is presented which assumes that the current sheath is formed along the insulator with a width , and it is then able to evolve both in the axial and radial directions. This evolution is modelled taking into account the conservation of momentum, but a second parameter is included (momentum transfer efficiency ) in order to consider the curvature of the current sheath. The model also includes an equation for the electric circuit. The resulting set of equations is solved numerically. Both parameters are fitted using measurements of sheath cinematics taken with magnetic probes. A Plasma Focus device with a central electrode of 9.5 mm in diameter, 90 mm long and with no outer electrode was used. The filling gas was either hydrogen or nitrogen and the filling pressure was varied in the range 0.5 – 10 mbar. We found that both and decrease as the filling gas pressure is increased.ISSN 0327-358

The Carlina-type diluted telescope: Stellar fringes on Deneb

Context. The performance of interferometers has largely been increased over the last ten years. But the number of observable objects is still limited due to the low sensitivity and imaging capability of the current facilities. Studies have been done to propose a new generation of interferometers. Aims. The Carlina concept studied at the Haute-Provence Observatory consists in an optical interferometer configured as a diluted version of the Arecibo radio telescope: above the diluted primary mirror made of fixed co-spherical segments, a helium balloon or cables suspended between two mountains and/or pylons, carries a gondola containing the focal optics. This concept does not require delay lines. Methods. Since 2003, we have been building a technical demonstrator of this diluted telescope. The main goals of this project were to find the opto-mechanical solutions to stabilize the optics attached under cables at several tens of meters above the ground, and to characterize this diluted telescope under real conditions. In 2012, we have obtained metrology fringes, and co-spherized the primary mirrors within one micron accuracy. In 2013, we have tested the whole optical train: servo loop, metrology, and the focal gondola. Results. We obtained stellar fringes on Deneb in September 2013. In this paper, we present the characteristics of these observations: quality of the guiding, S /N reached, and possible improvements for a future system. Conclusions. It is an important step that demonstrates the feasibility of building a diluted telescope using cables strained between cliffs or pylons. Carlina, like the MMT or LBT, could be one of the first members of a new class of telescopes named Large Diluted Telescopes. Its optical architecture has many advantages for future projects: Planet Formation Imager, Post-ELTs, Interferometer in space.Comment: 8 pages, 7 figures, Astronomy & Astrophysic

Chromosphere of K giant stars Geometrical extent and spatial structure detection

We aim to constrain the geometrical extent of the chromosphere of non-binary K giant stars and detect any spatial structures in the chromosphere. We performed observations with the CHARA interferometer and the VEGA beam combiner at optical wavelengths. We observed seven non-binary K giant stars. We measured the ratio of the radii of the photosphere to the chromosphere using the interferometric measurements in the Halpha and the Ca II infrared triplet line cores. For beta Ceti, spectro-interferometric observations are compared to an non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including a chromosphere. The NLTE computations provide line intensities and contribution functions that indicate the relative locations where the line cores are formed and can constrain the size of the limb-darkened disk of the stars with chromospheres. We measured the angular diameter of seven K giant stars and deduced their fundamental parameters: effective temperatures, radii, luminosities, and masses. We determined the geometrical extent of the chromosphere for four giant stars. The chromosphere extents obtained range between 16% to 47% of the stellar radius. The NLTE computations confirm that the Ca II/849 nm line core is deeper in the chromosphere of ? Cet than either of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version of a semi-empirical model atmosphere derived by fitting the Ca II triplet line cores of this star. In four of our targets, we also detect the signature of a differential signal showing the presence of asymmetries in the chromospheres. Conclusions. It is the first time that geometrical extents and structure in the chromospheres of non-binary K giant stars are determined by interferometry. These observations provide strong constrains on stellar atmosphere models.Comment: 10 pages, 12 figure