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

Modelización de fenómenos físicos en aplicaciones de computación gráfica en tiempo real

Cuando se desarrollan aplicaciones de realidad virtual, uno de los principales desafíos es lograr realismo e inmersión a un costo computacional aceptable. Con las placas gráficas actuales es posible obtener imágenes muy cercanas a la realidad, aunque también se debe incluir comportamiento físico para lograr escenas que brinden una verdadera inmersión. Para cumplir este objetivo, se han desarrollado en los últimos años varios motores físicos implementados en forma de bibliotecas (ODE, NGD, TPE, BPL1) o en placas dedicadas a la simulación de modelos físicos (por ejemplo, el producto de nVIDIA® denominado PhysX); los cuales permiten agregar comportamiento físico a escenas tridimensionales. Es posible que los usuarios deseen realizar pruebas de rendimiento y/o precisión a cada motor físico para determinar cual de ellos se ajusta mejor a sus necesidades; o utilizar distintas funcionalidades de cada uno en una misma aplicación, pero ocurre que cada uno de estos motores necesita una configuración distinta de inicialización y muchas veces la funcionalidad implementada difiere entre uno y otro, haciendo que el código implementado sea obsoleto, requiriendo de un esfuerzo considerable para portar la aplicación a otro motor. Para solucionar dichos problemas, el presente trabajo se centra principalmente en crear una capa de abstracción o PAL (del inglés, Physics Abstraction Layer) que sea utilizada por un motor gráfico, la cual brinde una comunicación fluida con diversos motores físicos maximizando la reutilización de código y permita cambiar el uso entre ellos.Eje: Computación gráfica, Imágenes y VisualizaciónRed de Universidades con Carreras en Informática (RedUNCI

SearchCal: a Virtual Observatory tool for searching calibrators in optical long baseline interferometry. I: The bright object case

In long baseline interferometry, the raw fringe contrast must be calibrated to obtain the true visibility and then those observables that can be interpreted in terms of astrophysical parameters. The selection of suitable calibration stars is crucial for obtaining the ultimate precision of interferometric instruments like the VLTI. We have developed software SearchCal that builds an evolutive catalog of stars suitable as calibrators within any given user-defined angular distance and magnitude around the scientific target. We present the first version of SearchCal dedicated to the bright-object case V<=10; K<=5). Star catalogs available at the CDS are consulted via web requests. They provide all the useful information for selecting of calibrators. Missing photometries are computed with an accuracy of 0.1 mag and the missing angular diameters are calculated with a precision better than 10%. For each star the squared visibility is computed by taking the wavelength and the maximum baseline of the foreseen observation into account.} SearchCal is integrated into ASPRO, the interferometric observing preparation software developed by the JMMC, available at the address: http://mariotti.fr

The fundamental parameters of the roAp star γ\gamma Equulei

Physical processes working in the stellar interiors as well as the evolution of stars depend on some fundamental stellar properties, such as mass, radius, luminosity, and chemical abundances. A classical way to test stellar interior models is to compare the predicted and observed location of a star on theoretical evolutionary tracks in a H-R diagram. This requires the best possible determinations of stellar mass, radius, luminosity and abundances. To derive its fundamental parameters, we observed the well-known rapidly oscillating Ap star, $\gamma$ Equ, using the visible spectro-interferometer VEGA installed on the optical CHARA array. We computed the calibrated squared visibility and derived the limb-darkened diameter. We used the whole energy flux distribution, the parallax and this angular diameter to determine the luminosity and the effective temperature of the star. We obtained a limb-darkened angular diameter of 0.564~$\pm$~0.017~mas and deduced a radius of $R$~=~2.20~$\pm$~0.12~${\rm R_{\odot}}$. Without considering the multiple nature of the system, we derived a bolometric flux of $(3.12\pm 0.21)\times 10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of 7364~$\pm$~235~K, which is below the effective temperature that has been previously determined. Under the same conditions we found a luminosity of $L$~=~12.8~$\pm$~1.4~${\rm L_{\odot}}$. When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can be, respectively, up to $\sim$~100~K and up to $\sim$~0.8~L$_\odot$ smaller than the values mentioned above.These new values of the radius and effective temperature should bring further constraints on the asteroseismic modelling of the star.Comment: Accepted by A&

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

Modelización de fenómenos físicos en aplicaciones de computación gráfica en tiempo real

Cuando se desarrollan aplicaciones de realidad virtual, uno de los principales desafíos es lograr realismo e inmersión a un costo computacional aceptable. Con las placas gráficas actuales es posible obtener imágenes muy cercanas a la realidad, aunque también se debe incluir comportamiento físico para lograr escenas que brinden una verdadera inmersión. Para cumplir este objetivo, se han desarrollado en los últimos años varios motores físicos implementados en forma de bibliotecas (ODE, NGD, TPE, BPL1) o en placas dedicadas a la simulación de modelos físicos (por ejemplo, el producto de nVIDIA® denominado PhysX); los cuales permiten agregar comportamiento físico a escenas tridimensionales. Es posible que los usuarios deseen realizar pruebas de rendimiento y/o precisión a cada motor físico para determinar cual de ellos se ajusta mejor a sus necesidades; o utilizar distintas funcionalidades de cada uno en una misma aplicación, pero ocurre que cada uno de estos motores necesita una configuración distinta de inicialización y muchas veces la funcionalidad implementada difiere entre uno y otro, haciendo que el código implementado sea obsoleto, requiriendo de un esfuerzo considerable para portar la aplicación a otro motor. Para solucionar dichos problemas, el presente trabajo se centra principalmente en crear una capa de abstracción o PAL (del inglés, Physics Abstraction Layer) que sea utilizada por un motor gráfico, la cual brinde una comunicación fluida con diversos motores físicos maximizando la reutilización de código y permita cambiar el uso entre ellos.Eje: Computación gráfica, Imágenes y VisualizaciónRed de Universidades con Carreras en Informática (RedUNCI

Revealing the inclined circumstellar disk in the UX Orionis system KK Ophiuchi

This is the final version of the article. Available from EDP Sciences via the DOI in this record.Aims. We study the inner sub-AU region of the circumstellar environment of the UX Ori-type star KK Oph with near-infrared VLTI/AMBER interferometry. We are particularly interested in the inclination of the star-disk system, and we use this information to test the current standard picture for UX Ori stars. Methods. We recorded spectrally dispersed (R ~ 35) interferograms in the near-infrared H and K bands with the VLTI/AMBER instrument. The derived visibilities, closure phases, and the spectral energy distribution of KK Oph were compared with two-dimensional geometric and radiative transfer models (RADMC). Results. We obtained visibilities at four different position angles. Using two-dimensional geometric models, we derive an axis ratio ~3.0 corresponding to an inclination of ~70°. A fitted inclined ring model leads to a ring radius of 2.8 ± 0.2 mas, corresponding to 0.44 ± 0.03 AU at a distance of 160 pc, which is larger than the dust sublimation radius of ~0.1 AU predicted for a dust sublimation temperature of 1500 K. Our derived two-dimensional RADMC model consists of a circumstellar disk with an inclination angle of ~70° and an additional dust envelope. Conclusions. The finding of an ~70° inclined disk around KK Oph is consistent with the prediction that UX Ori objects are seen under large inclination angles, and orbiting clouds in the line of sight cause the observed variability. Furthermore, our results suggest that the orbit of the companion KK Oph B and the disk plane are coplanar.A. Kreplin was supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. V.G. and L.T. were supported in part by the grant of the Presidium of RAS P 21 and grant NSh. – 1625.2012.2. They also thank the Max-Planck-Society for the support during their stay in Bonn. This research has made use of NASA’s Astrophysics Data System Bibliographic Services

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