25 research outputs found
Effect of metallic walls on dynamos generated by laminar boundary-driven flow in a spherical domain
We present a numerical study of dynamo action in a conducting fluid encased
in a metallic spherical shell. Motions in the fluid are driven by differential
rotation of the outer metallic shell, which we refer to as "the wall". The two
hemispheres of the wall are held in counter-rotation, producing a steady,
axisymmetric interior flow consisting of differential rotation and a two-cell
meridional circulation with radial inflow in the equatorial plane. From
previous studies, this type of flow is known to maintain a stationary
equatorial dipole by dynamo action if the magnetic Reynolds number is larger
than about 300 and if the outer boundary is electrically insulating. We vary
independently the thickness, electrical conductivity, and magnetic permeability
of the wall to determine their effect on the dynamo action. The main results
are: (a) Increasing the conductivity of the wall hinders the dynamo by allowing
eddy currents within the wall, which are induced by the relative motion of the
equatorial dipole field and the wall. This processes can be viewed as a skin
effect or, equivalently, as the tearing apart of the dipole by the differential
rotation of the wall, to which the field lines are anchored by high
conductivity. (b) Increasing the magnetic permeability of the wall favors
dynamo action by constraining the magnetic field lines in the fluid to be
normal to the wall, thereby decoupling the fluid from any induction in the
wall. (c) Decreasing the wall thickness limits the amplitude of the eddy
currents, and is therefore favorable for dynamo action, provided that the wall
is thinner than the skin depth. We explicitly demonstrate these effects of the
wall properties on the dynamo field by deriving an effective boundary condition
in the limit of vanishing wall thickness.Comment: accepted for publication in Physical Review
A self-consistent model of the solar tachocline
We present a local but fully nonlinear model of the solar tachocline, using
three-dimensional direct numerical simulations. The tachocline forms naturally
as a statistically steady balance between Coriolis, pressure, buoyancy and
Lorentz forces beneath a turbulent convection zone. Uniform rotation is
maintained in the radiation zone by a primordial magnetic field, which is
confined by meridional flows in the tachocline and convection zone. Such
balanced dynamics has previously been found in idealised laminar models, but
never in fully self-consistent numerical simulations.Comment: Accepted for publication in the Astrophysical Journa
Double-diffusive instabilities of a shear-generated magnetic layer
Previous theoretical work has speculated about the existence of
double-diffusive magnetic buoyancy instabilities of a dynamically evolving
horizontal magnetic layer generated by the interaction of forced vertically
sheared velocity and a background vertical magnetic field. Here we confirm
numerically that if the ratio of the magnetic to thermal diffusivities is
sufficiently low then such instabilities can indeed exist, even for high
Richardson number shear flows. Magnetic buoyancy may therefore occur via this
mechanism for parameters that are likely to be relevant to the solar
tachocline, where regular magnetic buoyancy instabilities are unlikely.Comment: Submitted to ApJ
The Evolution of a Double Diffusive Magnetic Buoyancy Instability
Recently, Silvers, Vasil, Brummell, & Proctor (2009), using numerical
simulations, confirmed the existence of a double diffusive magnetic buoyancy
instability of a layer of horizontal magnetic field produced by the interaction
of a shear velocity field with a weak vertical field. Here, we demonstrate the
longer term nonlinear evolution of such an instability in the simulations. We
find that a quasi two-dimensional interchange instability rides (or "surfs") on
the growing shear-induced background downstream field gradients. The region of
activity expands since three-dimensional perturbations remain unstable in the
wake of this upward-moving activity front, and so the three-dimensional nature
becomes more noticeable with time.Comment: 9 pages; 3 figures; accepted to appear in IAU symposium 27
On the dynamical interaction between overshooting convection and an underlying dipole magnetic field -- I. The non-dynamo regime
Motivated by the dynamics in the deep interiors of many stars, we study the
interaction between overshooting convection and the large-scale poloidal fields
residing in radiative zones. We have run a suite of 3D Boussinesq numerical
calculations in a spherical shell that consists of a convection zone with an
underlying stable region that initially compactly contains a dipole field. By
varying the strength of the convective driving, we find that, in the less
turbulent regime, convection acts as turbulent diffusion that removes the field
faster than solely molecular diffusion would do. However, in the more turbulent
regime, turbulent pumping becomes more efficient and partially counteracts
turbulent diffusion, leading to a local accumulation of the field below the
overshoot region. These simulations suggest that dipole fields might be
confined in underlying stable regions by highly turbulent convective motions at
stellar parameters. The confinement is of large-scale field in an average sense
and we show that it is reasonably modeled by mean-field ideas. Our findings are
particularly interesting for certain models of the Sun, which require a
large-scale, poloidal magnetic field to be confined in the solar radiative zone
in order to explain simultaneously the uniform rotation of the latter and the
thinness of the solar tachocline.Comment: Accepted to MNRAS, 14 figure
Turbulent Compressible Convection with Rotation
The effects of Coriolis forces on compressible convection are studied using three-dimensional numerical simulations carried out within a local modified f-plane model. The physics is simplified by considering a perfect gas occupying a rectilinear domain placed tangentially to a rotating sphere at various latitudes, through which a destabilizing heat flux is driven. The resulting convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers, evaluating conditions where the influence of rotation is both weak and strong. Given the computational demands of these high-resolution simulations, the parameter space is explored sparsely to ascertain the differences between laminar and turbulent rotating convection. The first paper in this series examines the effects of rotation on the flow structure within the convection, its evolution, and some consequences for mixing. Subsequent papers consider the large-scale mean shear flows that are generated by the convection, and the effects of rotation on the convective energetics and transport properties. It is found here that the structure of rotating turbulent convection is similar to earlier nonrotating studies, with a laminar, cellular surface network disguising a fully turbulent interior punctuated by vertically coherent structures. However, the temporal signature of the surface flows is modified by inertial motions to yield new cellular evolution patterns and an overall increase in the mobility of the network. The turbulent convection contains vortex tubes of many scales, including large-scale coherent structures spanning the full vertical extent of the domain involving multiple density scale heights. Remarkably, such structures align with the rotation vector via the influence of Coriolis forces on turbulent motions, in contrast with the zonal tilting of streamlines found in laminar flows. Such novel turbulent mechanisms alter the correlations which drive mean shearing flows and affect the convective transport properties. In contrast to this large-scale anisotropy, small-scale vortex tubes at greater depths are randomly orientated by the rotational mixing of momentum, leading to an increased degree of isotropy on the medium to small scales of motion there. Rotation also influences the thermodynamic mixing properties of the convection. In particular, interaction of the larger coherent vortices causes a loss of correlation between the vertical velocity and the temperature leaving a mean stratification which is not isentropic
Self-consistent simulations of a von K\'arm\'an type dynamo in a spherical domain with metallic walls
We have performed numerical simulations of boundary-driven dynamos using a
three-dimensional non-linear magnetohydrodynamical model in a spherical shell
geometry. A conducting fluid of magnetic Prandtl number Pm=0.01 is driven into
motion by the counter-rotation of the two hemispheric walls. The resulting flow
is of von K\'arm\'an type, consisting of a layer of zonal velocity close to the
outer wall and a secondary meridional circulation. Above a certain forcing
threshold, the mean flow is unstable to non-axisymmetric motions within an
equatorial belt. For fixed forcing above this threshold, we have studied the
dynamo properties of this flow. The presence of a conducting outer wall is
essential to the existence of a dynamo at these parameters. We have therefore
studied the effect of changing the material parameters of the wall (magnetic
permeability, electrical conductivity, and thickness) on the dynamo. In common
with previous studies, we find that dynamos are obtained only when either the
conductivity or the permeability is sufficiently large. However, we find that
the effect of these two parameters on the dynamo process are different and can
even compete to the detriment of the dynamo. Our self-consistent approach allow
us to analyze in detail the dynamo feedback loop. The dynamos we obtain are
typically dominated by an axisymmetric toroidal magnetic field and an axial
dipole component. We show that the ability of the outer shear layer to produce
a strong toroidal field depends critically on the presence of a conducting
outer wall, which shields the fluid from the vacuum outside. The generation of
the axisymmetric poloidal field, on the other hand, occurs in the equatorial
belt and does not depend on the wall properties.Comment: accepted for publication in Physical Review
Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae
The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot
High mobility In0.75Ga0.25As quantum wells in an InAs phonon lattice
InGaAs based devices are great complements to silicon for CMOS, as they provide an increased carrier saturation velocity, lower operating voltage and reduced power dissipation (International technology roadmap for semiconductors (www.itrs2.net)). In this work we show that In0.75Ga0.25As quantum wells with a high mobility, 15 000 to 20 000 cm2V-1s-1at ambient temperature, show an InAs-like phonon with an energy of 28.8 meV, frequency of 232 cm-1that dominates the polar-optical mode scattering from  ∼70 K to 300 K. The measured optical phonon frequency is insensitive to the carrier density modulated with a surface gate or LED illumination. We model the electron scattering mechanisms as a function of temperature and identify mechanisms that limit the electron mobility in In0.75Ga0.25As quantum wells. Background impurity scattering starts to dominate for temperatures  <100 K. In the high mobility In0.75Ga0.25As quantum well, GaAs-like phonons do not couple to the electron gas unlike the case of In0.53Ga0.47As quantum wells