266 research outputs found
Test-field method for mean-field coefficients with MHD background
Aims: The test-field method for computing turbulent transport coefficients
from simulations of hydromagnetic flows is extended to the regime with a
magnetohydrodynamic (MHD) background. Methods: A generalized set of test
equations is derived using both the induction equation and a modified momentum
equation. By employing an additional set of auxiliary equations, we derive
linear equations describing the response of the system to a set of prescribed
test fields. Purely magnetic and MHD backgrounds are emulated by applying an
electromotive force in the induction equation analogously to the ponderomotive
force in the momentum equation. Both forces are chosen to have Roberts
flow-like geometry. Results: Examples with an MHD background are studied where
the previously used quasi-kinematic test-field method breaks down. In cases
with homogeneous mean fields it is shown that the generalized test-field method
produces the same results as the imposed-field method, where the field-aligned
component of the actual electromotive force from the simulation is used.
Furthermore, results for the turbulent diffusivity tensor are given, which are
inaccessible to the imposed-field method. For MHD backgrounds, new mean-field
effects are found that depend on the occurrence of cross-correlations between
magnetic and velocity fluctuations. For strong imposed fields, is
found to be quenched proportional to the fourth power of the field strength,
regardless of the type of background studied.Comment: 17 pages, 10 figures, submitted to Astronomy & Astrophysic
Mean--field electrodynamics: Critical analysis of various analytical approaches to the mean electromotive force
There are various analytical approaches to the mean electromotive force crucial in mean--field electrodynamics, with
and
being velocity and magnetic field fluctuations. In most cases the
traditional approach, restricted to the second--order correlation
approximation, has been used. Its validity is only guaranteed for a range of
conditions, which is narrow in view of many applications, e.g., in
astrophysics. With the intention to have a wider range of applicability other
approaches have been proposed which make use of the so--called
--approximation, reducing correlations of third order in and
to such of second order. After explaining some basic features of the
traditional approach a critical analysis of the approaches of that kind is
given. It is shown that they lead in some cases to results which are in clear
conflict with those of the traditional approach. It is argued that this
indicates shortcomings of the --approaches and poses serious restrictions
to their applicability. These shortcomings do not result from the basic
assumption of the --approximation. Instead, they seem to originate in
some simplifications made in order to derive without really solving
the equations governing and . A starting point for a new
approach is described which avoids the conflict.Comment: 32 pages, no figures; accepted by Geophys. Astrophys. Fluid Dynam. A
quenching formula for \alpha and a section on comparisons with numerical
simulations added; references amended; changes in presentation and languag
Fanning out of the -mode in presence of nonuniform magnetic fields
We show that in the presence of a harmonically varying magnetic field the
fundamental or -mode in a stratified layer is altered in such a way that it
fans out in the diagnostic diagram, but with mode power also within
the fan. In our simulations, the surface is defined by a temperature and
density jump in a piecewise isothermal layer. Unlike our previous work (Singh
et al. 2014) where a uniform magnetic field was considered, we employ here a
nonuniform magnetic field together with hydromagnetic turbulence at length
scales much smaller than those of the magnetic fields. The expansion of the
-mode is stronger for fields confined to the layer below the surface. In
some of those cases, the diagram also reveals a new class of low
frequency vertical stripes at multiples of twice the horizontal wavenumber of
the background magnetic field. We argue that the study of the -mode
expansion might be a new and sensitive tool to determining subsurface magnetic
fields with longitudinal periodicity.Comment: 6 pages, 4 figures, submitted to Astrophysical Journal Letter
Small mammal community structure (Rodentia: Muridae) in Punda Maria, Kruger National Park, South Africa : Causes and consequences
The population dynamics of small mammal populations in Punda Maria, Kruger National Park, South Africa was investigated with respect to community size and structure, seed predation and dispersal of several trees. Species populations, community structure and habitat attributes were measured at six sites comprising of 3 habitat types (2 sites per habitat); namely Acaciagrassland, North-facing and South-facing sloped sites. Seasonal variation was investigated at the Acacia sites during late summer (April) and winter months (July). Five habitat attributes were measured; percentages of total cover and herbage, plant litter depth (cm), height of dominant vegetation type measured (m) and edaphic condition were recorded at 15 randomly selected traps per site. Almonds (Prunus dulcis), baobabs (Adansonia digitata), mopane (Colophospermum mopane), and Acacia tortilis seeds were used in the predation and dispersal experiment. North- and South-facing sites were dominated by Aethomys sp. while Acacia sites were dominated by Mastomys natalensis. Seed predation was high at the North-facing slopes, with a significant difference in predation on A. tortilis seeds. Seed dispersal activity was low; no , scatter hoarding was identified; only larder hoarding was observed by Aethomys sp. and M. natalensis at North-facing sites and Acacia sites respectively. Trampling by megaherbivore activity impacted vegetation structure and has negatively affected rodent population and ground activity, particularly at the Acacia sites
Feeding preferences of Tripneustes gratilla Linnaeus Echinodermata Echinodea and the prospect for sea urchin aquaculture in South Africa
Sea urchin aquaculture for human food is a major industry, and there are prospects for the industry in South Africa. Feeding preferences of the sea urchin Tripneustes gratilla were tested by two different methods; a touch-preference test and a 48-hour consumption test. A touchpreference is established when the sea urchins' lantern teeth touched an algal species. The 48-hour consumption test was performed with paired combinations using four species of fresh algae. It was hypothesized that the preferred species would be similar in the touch-preference test and the consumption test. Five algal species were used in the touch-preference test; namely, Ecklonia maxima (kelp), Gigartina polycarpa, Grateloupia capensi, Porphyra capensis, and U/va rigida. Four of these species were used in the paired consumption test; namely, E. maxima, G. polycarpa, P. capensis and U. rigida. We found that E. maxima was most preferred in the touch-test followed by P. capensis and U. rigida. This result did not correlate with our consumption test. Here, U. rigida was significantly most preferred followed by E. maxima. Overall, the result is significant in relation to the prospect of echinoculture in South Africa as Ulva may provide a valuable alternate feed or supplement to this new industry. Due to the increase in time provided for the paired consumption test, we found that the paired consumption test was more conclusive in providing a preference for Tripneustes gratilla. Our results agree with literature which suggests that Tripneustes gratilla is a genera list feeder; however, Tripneustes gratilla shows definite preferences. Ulva spp. has been grown on a number of abalone farms in South Africa. Integrated aquaculture using T. gratilla and U. rigida as a biofilter and alternate feed is a very promising idea for echinoculture in South Africa
Properties of - and -modes in hydromagnetic turbulence
With the ultimate aim of using the fundamental or -mode to study
helioseismic aspects of turbulence-generated magnetic flux concentrations, we
use randomly forced hydromagnetic simulations of a piecewise isothermal layer
in two dimensions with reflecting boundaries at top and bottom. We compute
numerically diagnostic wavenumber-frequency diagrams of the vertical velocity
at the interface between the denser gas below and the less dense gas above. For
an Alfv\'en-to-sound speed ratio of about 0.1, a 5% frequency increase of the
-mode can be measured when -, where is the
horizontal wavenumber and is the pressure scale height at the
surface. Since the solar radius is about 2000 times larger than ,
the corresponding spherical harmonic degree would be 6000-8000. For weaker
fields, a -dependent frequency decrease by the turbulent motions becomes
dominant. For vertical magnetic fields, the frequency is enhanced for
, but decreased relative to its nonmagnetic value for
.Comment: 17 pages, 22 figures, Version accepted in MNRA
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