arXiv subjects

J. -P. Zahn

Publications and source records attributed to J. -P. Zahn.

22 records · Page 2Linked to original sources

arXiv2003

On hydrodynamic shear turbulence in Keplerian disks: via transient growth to bypass transition

This paper deals with the problem of hydrodynamic shear turbulence in non-magnetized Keplerian disks. We wish to draw attention to a route to hydrodynamic turbulence which seems to be little known by the astrophysical community, but which has been intensively discussed among fluid dynamicists during the past decade. In this so-called `bypass' concept for the onset of turbulence, perturbations undergo a transient growth, and they may reach an amplitude that is sufficiently large to allow positive feedback through nonlinear interactions. This transient growth is linear in nature, and thus it differs in principle from the well-known nonlinear instability. We describe the type of perturbations that according to this process are the most likely to lead to turbulence, namely non-axisymmetric vortex mode perturbations in the two dimensional limit. We show that the apparently inhibiting action of the Coriolis force on the dynamics of such vortical perturbations is substantially diminished due to the pressure perturbations, contrary to current opinion. We stress the similarity of the turbulent processes in Keplerian disks and in Cartesian flows and conclude that the prevalent skepticism of the astrophysical community on the occurrence of hydrodynamic shear turbulence in such disks is not founded.

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arXiv2002

Seismic tests for solar models with tachocline mixing

We have computed accurate 1-D solar models including both a macroscopic mixing process in the solar tachocline as well as up-to-date microscopic physical ingredients. Using sound speed and density profiles inferred through primary inversion of the solar oscillation frequencies coupled with the equation of thermal equilibrium, we have extracted the temperature and hydrogen abundance profiles. These inferred quantities place strong constraints on our theoretical models in terms of the extent and strength of our macroscopic mixing, on the photospheric heavy elements abundance, on the nuclear reaction rates such as $S_{11}$ and $S_{34}$ and on the efficiency of the microscopic diffusion. We find a good overall agreement between the seismic Sun and our models if we introduce a macroscopic mixing in the tachocline and allow for variation within their uncertainties of the main physical ingredients. From our study we deduce that the solar hydrogen abundance at the solar age is $X_{\rm inv}=0.732\pm 0.001$ and that based on the $^9$Be photospheric depletion, the maximum extent of mixing in the tachocline is 5% of the solar radius. The nuclear reaction rate for the fundamental $pp$ reaction is found to be $S_{11}(0)=4.06\pm 0.07$ $10^{-25}$ MeV barns, i.e., 1.5% higher than the present theoretical determination. The predicted solar neutrino fluxes are discussed in the light of the new SNO/SuperKamiokande results.

astro-ph↗

arXiv2000

Shear turbulence beneath the solar tachocline

Helioseismic inversions of the Sun's internal angular velocity profile show that the rotation changes from differential in latitude in the convection zone to almost uniform in the radiative region below. The transition occurs in a thin layer, the tachocline, which is the seat of strong shear in the vertical direction. In this Note we examine whether this rotation profile can lead to shear turbulence at the top of the radiation zone. By using the standard solar model, we show that such turbulence can be generated only in a narrow region 0.695R_sun < r < 0.713R_sun at the equator and even in narrower layers at higher latitudes. We conclude that the turbulence generated by this vertical shear is unlikely to play a significant role in the transport of matter and angular momentum, and that other mechanisms must be invoked to achieve this.

astro-ph↗

arXiv1996

Anisotropic diffusion and shear instabilities

We examine the role of anisotropic turbulence on the shear instabilities in a stratified flow. Such turbulence is expected to occur in the radiative interiors of stars, due to their differential rotation and their strong stratification, and the turbulent transport associated with it will be much stronger in the horizontal than in the vertical direction. It will thus weaken the restoring force which is caused by the gradient of mean molecular weight ($μ$). We find that the critical shear which is able to overcome the $μ$-gradient is substantially reduced by this anisotropic turbulence, and we derive an estimate for the resulting turbulent diffusivity in the vertical direction.

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