Physics – Geophysics
Scientific paper
2010-10-19
Geophysical \& Astrophysical Fluid Dynamics 104, 2-3 (2010) 221-248
Physics
Geophysics
Scientific paper
10.1080/03091920903550955
We numerically investigate the efficiency of a spherical Couette flow at generating a self-sustained magnetic field. No dynamo action occurs for axisymmetric flow while we always found a dynamo when non-axisymmetric hydrodynamical instabilities are excited. Without rotation of the outer sphere, typical critical magnetic Reynolds numbers $Rm_c$ are of the order of a few thousands. They increase as the mechanical forcing imposed by the inner core on the flow increases (Reynolds number $Re$). Namely, no dynamo is found if the magnetic Prandtl number $Pm=Rm/Re$ is less than a critical value $Pm_c\sim 1$. Oscillating quadrupolar dynamos are present in the vicinity of the dynamo onset. Saturated magnetic fields obtained in supercritical regimes (either $Re>2 Re_c$ or $Pm>2Pm_c$) correspond to the equipartition between magnetic and kinetic energies. A global rotation of the system (Ekman numbers $E=10^{-3}, 10^{-4}$) yields to a slight decrease (factor 2) of the critical magnetic Prandtl number, but we find a peculiar regime where dynamo action may be obtained for relatively low magnetic Reynolds numbers ($Rm_c\sim 300$). In this dynamical regime (Rossby number $Ro\sim -1$, spheres in opposite direction) at a moderate Ekman number ($E=10^{-3}$), a enhanced shear layer around the inner core might explain the decrease of the dynamo threshold. For lower $E$ ($E=10^{-4}$) this internal shear layer becomes unstable, leading to small scales fluctuations, and the favorable dynamo regime is lost. We also model the effect of ferromagnetic boundary conditions. Their presence have only a small impact on the dynamo onset but clearly enhance the saturated magnetic field in the ferromagnetic parts. Implications for experimental studies are discussed.
Cardin Philippe
Guervilly Céline
No associations
LandOfFree
Numerical Simulations of Dynamos Generated in Spherical Couette Flows does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Numerical Simulations of Dynamos Generated in Spherical Couette Flows, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Numerical Simulations of Dynamos Generated in Spherical Couette Flows will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-268699