5 results on '"Gastine, T."'
Search Results
2. Gravity darkening in late-type stars I. The Coriolis effect.
- Author
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Raynaud, R., Rieutord, M., Petitdemange, L., Gastine, T., and Putigny, B.
- Subjects
STELLAR rotation ,GRAVITY ,ASTRONOMICAL photometry ,CORIOLIS force ,HEAT transfer ,CIRCUMSTELLAR matter - Abstract
Context. Recent interferometric data have been used to constrain the brightness distribution at the surface of nearby stars, in particular the so-called gravity darkening that makes fast rotating stars brighter at their poles than at their equator. However, good models of gravity darkening are missing for stars that posses a convective envelope. Aim. In order to better understand how rotation affects the heat transfer in stellar convective envelopes, we focus on the heat flux distribution in latitude at the outer surface of numerical models. Methods. We carry out a systematic parameter study of three-dimensional, direct numerical simulations of anelastic convection in rotating spherical shells. As a first step, we neglect the centrifugal acceleration and retain only the Coriolis force. The fluid instability is driven by a fixed entropy drop between the inner and outer boundaries where stress-free boundary conditions are applied for the velocity field. Restricting our investigations to hydrodynamical models with a thermal Prandtl number fixed to unity, we consider both thick and thin (solar-like) shells, and vary the stratification over three orders of magnitude. We measure the heat transfer efficiency in terms of the Nusselt number, defined as the output luminosity normalised by the conductive state luminosity. Results. We report diverse Nusselt number profiles in latitude, ranging from brighter (usually at the onset of convection) to darker equator and uniform profiles. We find that the variations of the surface brightness are mainly controlled by the surface value of the local Rossby number: when the Coriolis force dominates the dynamics, the heat flux is weakened in the equatorial region by the zonal wind and enhanced at the poles by convective motions inside the tangent cylinder. In the presence of a strong background density stratification however, as expected in real stars, the increase of the local Rossby number in the outer layers leads to uniformisation of the surface heat flux distribution. [ABSTRACT FROM AUTHOR]
- Published
- 2018
- Full Text
- View/download PDF
3. Direct numerical simulations of the ?-mechanism. I. Radial modes in the purely radiative case
- Author
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Gastine, T., Dintrans, B., Laboratoire Astrophysique de Toulouse-Tarbes (LATT), Centre National de la Recherche Scientifique (CNRS)-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Toulouse III - Paul Sabatier (UT3), and Université Fédérale Toulouse Midi-Pyrénées
- Subjects
[PHYS.ASTR.CO]Physics [physics]/Astrophysics [astro-ph]/Cosmology and Extra-Galactic Astrophysics [astro-ph.CO] ,[SDU.ASTR]Sciences of the Universe [physics]/Astrophysics [astro-ph] ,instabilities ,hydrodynamics ,waves ,stars: oscillations ,methods: numerical - Abstract
15 pages, 15 figures, 1 table, accepted for publication in A&A; International audience; Context: We present a purely-radiative hydrodynamical model of the ?-mechanism that sustains radial oscillations in Cepheid variables. Aims: We determine the physical conditions favourable for the ?-mechanism to occur inside a layer, with a configurable conductivity-hollow. We complete nonlinear direct numerical simulations (DNS) that initiate from these most favourable conditions. Methods: We compare the results of a linear-stability analysis, applied to radial modes using a spectral solver, and a DNS, which is developed from a high-order finite difference code. Results: We find that by changing the location and shape of the hollow, we can generate well-defined instability strips. For a given position in the layer, the amplitude and width of the hollow appear to be key parameters to vary to attain unstable modes driven by the ?-mechanism. The DNS, starting from the favourable conditions, confirm both the growth rates and the structures of linearly-unstable modes. Nonlinear saturation is produced by intricate couplings between excited fundamental mode and higher damped overtones.
- Published
- 2008
4. Effect of shear and magnetic field on the heat-transfer efficiency of convection in rotating spherical shells.
- Author
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Yadav, R. K., Gastine, T., Christensen, U. R., Duarte, L. D. V., and Reiners, A.
- Subjects
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HEAT transfer , *SPHERICAL shells (Engineering) , *CONVECTIVE flow , *PROTOTYPES , *COMPUTER simulation , *HYDRODYNAMICS - Abstract
We study rotating thermal convection in spherical shells as prototype for flow in the cores of terrestrial planets, gas planets or in stars. We base our analysis on a set of about 450 direct numerical simulations of the (magneto)hydrodynamic equations under the Boussinesq approximation. The Ekman number ranges from 10-3 to 10-5. The supercriticality of the convection reaches about 1000 in some models. Four sets of simulations are considered: non-magnetic simulations and dynamo simulations with either free-slip or no-slip flow boundary conditions. The non-magnetic setup with free-slip boundaries generates the strongest zonal flows. Both non-magnetic simulations with no-slip flow boundary conditions and self-consistent dynamos with free-slip boundaries have drastically reduced zonal-flows. Suppression of shear leads to a substantial gain in heat-transfer efficiency, increasing by a factor of 3 in some cases. Such efficiency enhancement occurs as long as the convection is significantly influenced by rotation. At higher convective driving the heat-transfer efficiency tends towards that of the classical non-rotating Rayleigh-Bénard system. Analysis of the latitudinal distribution of heat flow at the outer boundary reveals that the shear is most effective at suppressing heat-transfer in the equatorial regions. Simulations with convection zones of different thickness show that the zonal flows become less energetic in thicker shells, and, therefore, their effect on heat-transfer efficiency decreases. Furthermore, we explore the influence of the magnetic field on the nonzonal flow components of the convection. For this we compare the heat-transfer efficiency of no-slip non-magnetic cases with that of the no-slip dynamo simulations. We find that at E = 10-5 magnetic field significantly affects the convection and a maximum gain of about 30 per cent (as compared to the non-magnetic case) in heat-transfer efficiency is obtained for an Elsasser number of about 3. Our analysis motivates us to speculate that convection in the polar regions in dynamos at E = 10-5 is probably in a 'magnetostrophic' regime. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
5. Anelastic convection-driven dynamo benchmarks
- Author
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Jones, C.A., Boronski, P., Brun, A.S., Glatzmaier, G.A., Gastine, T., Miesch, M.S., and Wicht, J.
- Subjects
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INTERNAL friction , *CONVECTION (Astrophysics) , *HYDRODYNAMICS , *ELECTRIC generators , *ROTATIONAL motion , *RAYLEIGH number - Abstract
Abstract: Benchmark solutions for fully nonlinear anelastic compressible convection and dynamo action in a rotating spherical shell are proposed. Three benchmarks are specified. The first is a purely hydrodynamic case, which is steady in a uniformly drifting frame. The second is a self-excited saturated dynamo solution, also steady in a drifting frame. The third is again a self-excited dynamo but is unsteady in time, and it has a higher Rayleigh number than the steady dynamo benchmark. Four independent codes have been tested against these benchmarks, and very satisfactory agreement has been found. This provides an accurate reference standard against which new anelastic codes can be tested. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
- View/download PDF
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