337 research outputs found
Wave-driven dynamo action in spherical magnetohydrodynamic systems
Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically
forced two-vortex flow inside a sphere are reported. The simulations are
performed in the intermediate regime between the laminar flow and developed
turbulence where a hydrodynamic instability is found to generate internal waves
with a characteristic m=2 zonal wave number. It is shown that this
time-periodic flow acts as a dynamo although snapshots of the flow as well as
the mean flow are not dynamos. The magnetic fields' growth rate exhibits
resonance effects depending on the wave frequency. Furthermore, a cyclic
self-killing and self-recovering dynamo based on the relative alignment of the
velocity and magnetic fields is presented. The phenomena are explained in terms
of a mixing of non-orthogonal eigenstates of the time dependent linear operator
of the magnetic induction equation. The potential relevance of this mechanism
to dynamo experiments is discussed.Comment: 11 pages, 13 figure
Some Unusual Properties of Turbulent Convection and Dynamos in Rotating Spherical Shells
The dynamics of convecting fluids in rotating spherical shells is governed at
Prandtl numbers of the order unity by the interaction between differential
rotation and roll-like convection eddies. While the differential rotation is
driven by the Reynolds stresses of the eddies, its shearing action inhibits
convection and causes phenomena such as localized convection and turbulent
relaxation oscillations. The response of the system is enriched in the case of
dynamo action. Lorentz forces may brake either entirely or partially the
geostrophic differential rotation and give rise to two rather different dynamo
states. Bistability of turbulent dynamos exists for magnetic Prandtl numbers of
the order unity. While the ratios between mean magnetic and kinetic energies
differ by a factor of 5 or more for the two dynamo states, the mean convective
heat transports are nearly the same. They are much larger than in the absence
of a magnetic field.Comment: To appear in Procs. IUTAM Symposium on Turbulence in the Atmosphere
and Oceans, 08-7 = GA.06-0
Towards an experimental von Karman dynamo: numerical studies for an optimized design
Numerical studies of a kinematic dynamo based on von Karman type flows
between two counterrotating disks in a finite cylinder are reported. The flow
has been optimized using a water model experiment, varying the driving
impellers configuration. A solution leading to dynamo action for the mean flow
has been found. This solution may be achieved in VKS2, the new sodium
experiment to be performed in Cadarache, France. The optimization process is
described and discussed, then the effects of adding a stationary conducting
layer around the flow on the threshold, on the shape of the neutral mode and on
the magnetic energy balance are studied. Finally, the possible processes
involved into kinematic dynamo action in a von Karman flow are reviewed and
discussed. Among the possible processes we highlight the joint effect of the
boundary-layer radial velocity shear and of the Ohmic dissipation localized at
the flow/outer-shell boundary
Dynamo action at low magnetic Prandtl numbers: mean flow vs. fully turbulent motion
We compute numerically the threshold for dynamo action in Taylor-Green
swirling flows. Kinematic calculations, for which the flow field is fixed to
its time averaged profile, are compared to dynamical runs for which both the
Navier-Stokes and the induction equations are jointly solved. The kinematic
instability is found to have two branches, for all explored Reynolds numbers.
The dynamical dynamo threshold follows these branches: at low Reynolds number
it lies within the low branch while at high kinetic Reynolds number it is close
to the high branch.Comment: 4 pages, 4 figure
Reduction of velocity fluctuations in a turbulent flow of gallium by an external magnetic field
The magnetic field of planets or stars is generated by the motion of a
conducting fluid through a dynamo instability. The saturation of the magnetic
field occurs through the reaction of the Lorentz force on the flow. In relation
to this phenomenon, we study the effect of a magnetic field on a turbulent flow
of liquid Gallium. The measurement of electric potential differences provides a
signal related to the local velocity fluctuations. We observe a reduction of
velocity fluctuations at all frequencies in the spectrum when the magnetic
field is increased.Comment: accepted for Physical Review
Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds
The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools
Stable water isotopologue ratios in fog and cloud droplets of liquid clouds are not size-dependent
In this work, we present the first observations of stable water isotopologue ratios in cloud droplets of different sizes collected simultaneously. We address the question whether the isotope ratio of droplets in a liquid cloud varies as a function of droplet size. Samples were collected from a ground intercepted cloud (= fog) during the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) using a three-stage Caltech Active Strand Cloud water Collector (CASCC). An instrument test revealed that no artificial isotopic fractionation occurs during sample collection with the CASCC. Furthermore, we could experimentally confirm the hypothesis that the δ values of cloud droplets of the relevant droplet sizes (μm-range) were not significantly different and thus can be assumed to be in isotopic equilibrium immediately with the surrounding water vapor. However, during the dissolution period of the cloud, when the supersaturation inside the cloud decreased and the cloud began to clear, differences in isotope ratios of the different droplet sizes tended to be larger. This is likely to result from the cloud's heterogeneity, implying that larger and smaller cloud droplets have been collected at different moments in time, delivering isotope ratios from different collection times
Patterns of convection in rotating spherical shells
Patterns of convection in internally heated, self-gravitating rotating
spherical fluid shells are investigated through numerical simulations. While
turbulent states are of primary interest in planetary and stellar applications
the present paper emphasizes more regular dynamical features at Rayleigh
numbers not far above threshold which are similar to those which might be
observed in laboratory or space experiments. Amplitude vacillations and spatial
modulations of convection columns are common features at moderate and large
Prandtl numbers. In the low Prandtl number regime equatorially attached
convection evolves differently with increasing Rayleigh number and exhibits an
early transition into a chaotic state. Relationships of the dynamical features
to coherent structures in fully turbulent convection states are emphasized
Asymptotic and numerical solutions of the initial value problem in rotating planetary fluid cores
Copyright © 2010 The Royal Astronomical SocietyAn initial state of fluid motion in planetary cores or atmospheres, excited, for example, by the giant impact of an asteroid or an earthquake and then damped by viscous dissipation, decays towards the state of rigid-body rotation. The process of how the initial state approaches the final state, the initial value problem, is investigated both analytically and numerically for rotating fluid spheres. We derive an explicit asymptotic expression for the time-dependent solution of the initial value problem valid for an asymptotically small Ekman number E. We also perform a fully numerical analysis to simulate time-dependent solutions of the initial value problem for a small value of E. Comparison between the asymptotic solution and the corresponding numerical simulation shows a satisfactory quantitative agreement. For the purpose of illustrating why spherical geometry represents an intricate and exceptional case, we also briefly discuss the initial value problem in a rotating fluid channel. Geophysical and planetary physical implications of the result are also discussed
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