Steady state solutions for a lubrication two-fluid flow

International audienceIn this paper, we describe possible solutions for a stationary flow of two superposed fluids between two close surfaces in relative motion. Physically, this study is within the lubrication framework, in which it is of interest to predict the relative positions of the lubricant and the air in the device. Mathematically, we observe that this problem corresponds to finding the interface between the two fluids, and we prove that it is equivalent to solve some polynomial equation. We solve this equation using an original method of polynomial resolution. First, we check that our results are consistent with previous work. Next, we use this solution to answer some physically relevant questions related to the lubrication setting. For instance, we obtain theoretical and numerical results enabling to predict the apparition of a full film with respect to physical parameters (fluxes, shear velocity, . . . ). In particular, we present a figure giving the number of stationary solutions depending on the physical parameters. Moreover, in the last part, we give some indications for a better understanding of the multi-fluid case

The Maxwell-Stefan diffusion limit for a kinetic model of mixtures

International audienceWe consider the non-reactive fully elastic Boltzmann equation for mixtures. We deduce that, under the standard diffusive scaling, its limit for vanishing Mach and Knudsen numbers is the Maxwell-Stefan model for a multicomponent gaseous mixture

Venus wind map at cloud top level with the MTR/THEMIS visible spectrometer. I. Instrumental performance and first results

Solar light gets scattered at cloud top level in Venus' atmosphere, in the visible range, which corresponds to the altitude of 67 km. We present Doppler velocity measurements performed with the high resolution spectrometer MTR of the Solar telescope THEMIS (Teide Observatory, Canary Island) on the sodium D2 solar line (5890 \AA). Observations lasted only 49 min because of cloudy weather. However, we could assess the instrumental velocity sensitivity, 31 m/s per pixel of 1 arcsec, and give a value of the amplitude of zonal wind at equator at 151 +/- 16 m/s.Comment: 17 pages, 12 figure

Full Scale Proton Beam Impact Testing of new CERN Collimators and Validation of a Numerical Approach for Future Operation

New collimators are being produced at CERN in the framework of a large particle accelerator upgrade project to protect beam lines against stray particles. Their movable jaws hold low density absorbers with tight geometric requirements, while being able to withstand direct proton beam impacts. Such events induce considerable thermo-mechanical loads, leading to complex structural responses, which make the numerical analysis challenging. Hence, an experiment has been developed to validate the jaw design under representative conditions and to acquire online results to enhance the numerical models. Two jaws have been impacted by high-intensity proton beams in a dedicated facility at CERN and have recreated the worst possible scenario in future operation. The analysis of online results coupled to post-irradiation examinations have demonstrated that the jaw response remains in the elastic domain. However, they have also highlighted how sensitive the jaw geometry is to its mounting support inside the collimator. Proton beam impacts, as well as handling activities, may alter the jaw flatness tolerance value by $\pm$ 70 ${\mu}$m, whereas the flatness tolerance requirement is 200 ${\mu}$m. In spite of having validated the jaw design for this application, the study points out numerical limitations caused by the difficulties in describing complex geometries and boundary conditions with such unprecedented requirements.Comment: 22 pages, 17 figures, Prepared for submission to JINS

E´ chelle diagrams and period spacings of g modes in: Doradus stars from four years of Kepler observations

We use photometry from the Kepler Mission to study oscillations in Doradus stars. Some stars show remarkably clear sequences of g modes and we use period ´echelle diagrams to measure period spacings and identifyrotationally split multiplets with ` = 1 and ` = 2.We find small deviations from regular period spacings that arise from the gradient in the chemical composition just outside the convective core. We also find stars for which the period spacing shows a strong linear trend as a function of period, consistent with relatively rapid rotation. Overall, th

Study of a low Mach nuclear core model for two-phase flows with phase transition I: stiffened gas law

International audienceIn this paper, we are interested in modelling the flow of the coolant (water) in a nuclear reactor core. To this end, we use a monodimensional low Mach number model coupled to the stiffened gas law. We take into account potential phase transitions by a single equation of state which describes both pure and mixture phases. In some particular cases, we give analytical steady and/or unsteady solutions which provide qualitative information about the flow. In the second part of the paper, we introduce two variants of a numerical scheme based on the method of characteristics to simulate this model. We study and verify numerically the properties of these schemes. We finally present numerical simulations of a loss of flow accident (LOFA) induced by a coolant pump trip event

Downflows in sunspot umbral dots

We study the velocity field of umbral dots at a resolution of 0.14". Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0.25", velocities of up to 1000 m/s, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields.Comment: Final published version. For best quality figures, please download the PS versio

Numerical method for the 2D simulation of the respiration

International audienceIn this article we are interested in the simulation of the air flow in the bronchial tree. The model we use has already been described by Baffico, Grandmont and Maury and is based on a three part description of the respiratory tract. This model leads, after time discretization, to a Stokes system with non standard dissipative boundary conditions that cannot be easily and directly implemented in most FEM software, in particular in FreeFEM++. The objective is here to provide a new numerical method that could be implemented in any softwares. After describing the method, we illustrate it by two-dimensional simulations

On the structure of the Sun and alpha Centauri A and B in the light of seismic and non-seismic constraints

The small separation (delta nu 01, delta nu 02 and delta nu 13) between the oscillations with low degree l is dependent primarily on the sound speed profile within the stellar core, where nuclear evolution occurs. The detection of such oscillations for a star offers a very good opportunity to determine the stage of its nuclear evolution, and hence its age. In this context, we investigate the Sun and alpha Cen A and B. For alpha Cen A and B, each of the small separations delta nu 01, delta nu 02 and delta nu 13 gives a different age. Therefore, in our fitting process, we also employ the second difference, defined as nu n2 - 2 nu n1 + nu n0, which is 2 delta nu 01- delta nu 02. In addition to this, we also use frequency ratio (nu n0/ nu n2). For the Sun, these expressions areequivalent and give an age of about 4.9-5.0 Gyr. For alpha Cen A and B, however, the small separation and the second difference give very different ages. This conflict may be solved by the detection of oscillation frequencies that can be measured much more precisely than the current frequencies. When we fit the models to the observations, we find (i) Z 0=0.020, t=3.50 Gyr and M B=1.006 Msun from the small separations delta nu 01, delta nu 02 and delta nu 13 of alpha Cen B; and (ii) a variety of solutions from the non-seismic constraints and delta nu 02 of alpha Cen A and B, in which the masses of alpha Cen A and B are slightly modified and the age of the system is about 5.2-5.3 Gyr. For Z=0.025, the closest masses we find to the observed masses are M B=0.922 Msun and M A=1.115 Msun.The differences between these masses and the corresponding observed masses are about 0.01 Msun.Comment: 9 Pages and 9 Figure