132 research outputs found
Testing turbulent closure models with convection simulations
We compare simple analytical closure models of homogeneous turbulent
Boussinesq convection for stellar applications with three-dimensional
simulations. We use simple analytical closure models to compute the fluxes of
angular momentum and heat as a function of rotation rate measured by the Taylor
number. We also investigate cases with varying angles between the angular
velocity and gravity vectors, corresponding to locating the computational
domain at different latitudes ranging from the pole to the equator of the star.
We perform three-dimensional numerical simulations in the same parameter
regimes for comparison. The free parameters appearing in the closure models are
calibrated by two fitting methods using simulation data. Unique determination
of the closure parameters is possible only in the non-rotating case or when the
system is placed at the pole. In the other cases the fit procedures yield
somewhat differing results. The quality of the closure is tested by
substituting the resulting coefficients back into the closure model and
comparing with the simulation results. To eliminate the possibilities that the
results obtained depend on the aspect ratio of the simulation domain or suffer
from too small Rayleigh numbers we performed runs varying these parameters. The
simulation data for the Reynolds stress and heat fluxes broadly agree with
previous compressible simulations. The closure works fairly well with slow and
fast rotation but its quality degrades for intermediate rotation rates. We find
that the closure parameters depend not only on rotation rate but also on
latitude. The weak dependence on Rayleigh number and the aspect ratio of the
domain indicates that our results are generally validComment: 21 pages, 9 figures, submitted to Astron. Nach
Angular momentum transport in convectively unstable shear flows
Angular momentum transport owing to hydrodynamic turbulent convection is
studied using local three dimensional numerical simulations employing the
shearing box approximation. We determine the turbulent viscosity from
non-rotating runs over a range of values of the shear parameter and use a
simple analytical model in order to extract the non-diffusive contribution
(Lambda-effect) to the stress in runs where rotation is included. Our results
suggest that the turbulent viscosity is of the order of the mixing length
estimate and weakly affected by rotation. The Lambda-effect is non-zero and a
factor of 2-4 smaller than the turbulent viscosity in the slow rotation regime.
We demonstrate that for Keplerian shear, the angular momentum transport can
change sign and be outward when the rotation period is greater than the
turnover time, i.e. when the Coriolis number is below unity. This result seems
to be relatively independent of the value of the Rayleigh number.Comment: 10 pages, 12 figures, published version. Version with higher
resolution figures can be found at http://www.helsinki.fi/~kapyla/publ.htm
The alpha-effect in rotating convection: a comparison of numerical simulations
Numerical simulations are an important tool in furthering our understanding
of turbulent dynamo action, a process that occurs in a vast range of
astrophysical bodies. It is important in all computational work that
comparisons are made between different codes and, if non-trivial differences
arise, that these are explained. Kapyla et al (2010: MNRAS 402, 1458) describe
an attempt to reproduce the results of Hughes & Proctor (2009: PRL 102, 044501)
and, by employing a different methodology, they arrive at very different
conclusions concerning the mean electromotive force and the generation of
large-scale fields. Here we describe why the simulations of Kapyla et al (2010)
are simply not suitable for a meaningful comparison, since they solve different
equations, at different parameter values and with different boundary
conditions. Furthermore we describe why the interpretation of Kapyla et al
(2010) of the calculation of the alpha-effect is inappropriate and argue that
the generation of large-scale magnetic fields by turbulent convection remains a
problematic issue.Comment: Submitted to MNRAS. 5 pages, 3 figure
Reynolds stresses from hydrodynamic turbulence with shear and rotation
To study the Reynolds stresses which describe turbulent momentum transport
from turbulence affected by large-scale shear and rotation. Three-dimensional
numerical simulations are used to study turbulent transport under the
influences of large-scale shear and rotation in homogeneous, isotropically
forced turbulence. We study three cases: one with only shear, and two others
where in addition to shear, rotation is present. These cases differ by the
angle (0 or 90\degr) the rotation vector makes with respect to the z-direction.
Two subsets of runs are performed with both values of \theta where either
rotation or shear is kept constant. When only shear is present, the
off-diagonal stress can be described by turbulent viscosity whereas if the
system also rotates, nondiffusive contributions (\Lambda-effect) to the stress
can arise. Comparison of the direct simulations are made with analytical
results from a simple closure model. We find that the turbulent viscosity is of
the order of the first order smoothing result in the parameter regime studied
and that for sufficiently large Reynolds numbers the Strouhal number,
describing the ratio of correlation to turnover times, is roughly 1.5. This is
consistent with the closure model based on the minimal tau-approximation which
produces a reasonable fit to the simulation data for similar Strouhal numbers.
In the cases where rotation is present, separating the diffusive and
nondiffusive components of the stress turns out to be challenging but taking
the results at face value, we can obtain nondiffusive contributions of the
order of 0.1 times the turbulent viscosity. We also find that the simple
closure model is able to reproduce most of the qualitative features of the
numerical results provided that the Strouhal number is of the order of unity.Comment: 19 pages, 12 figures, published versio
Turbulent transport in hydromagnetic flows
The predictive power of mean-field theory is emphasized by comparing theory
with simulations under controlled conditions. The recently developed test-field
method is used to extract turbulent transport coefficients both in kinematic as
well as nonlinear and quasi-kinematic cases. A striking example of the
quasi-kinematic method is provided by magnetic buoyancy-driven flows that
produce an alpha effect and turbulent diffusion.Comment: 17 pages, 6 figures, topical issue of Physica Scripta on turbulent
mixing and beyon
Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
The turbulent magnetic diffusivity tensor is determined in the presence of
rotation or shear. The question is addressed whether dynamo action from the
shear-current effect can explain large-scale magnetic field generation found in
simulations with shear. For this purpose a set of evolution equations for the
response to imposed test fields is solved with turbulent and mean motions
calculated from the momentum and continuity equations. The corresponding
results for the electromotive force are used to calculate turbulent transport
coefficients. The diagonal components of the turbulent magnetic diffusivity
tensor are found to be very close together, but their values increase slightly
with increasing shear and decrease with increasing rotation rate. In the
presence of shear, the sign of the two off-diagonal components of the turbulent
magnetic diffusion tensor is the same and opposite to the sign of the shear.
This implies that dynamo action from the shear--current effect is impossible,
except perhaps for high magnetic Reynolds numbers. However, even though there
is no alpha effect on the average, the components of the alpha tensor display
Gaussian fluctuations around zero. These fluctuations are strong enough to
drive an incoherent alpha--shear dynamo. The incoherent shear--current effect,
on the other hand, is found to be subdominant.Comment: 12 pages, 13 figures, improved version, accepted by Ap
Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model
Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large
scale field growth to total magnetic helicity evolution best predict the
saturation of LSDs seen in simulations. For the simplest so called "{\alpha}2"
LSDs in periodic boxes, the electromotive force driving LSD growth depends on
the difference between the time-integrated kinetic and current helicity
associated with fluctuations. When the system is helically kinetically forced
(KF), the growth of the large scale helical field is accompanied by growth of
small scale magnetic (and current) helicity which ultimately quench the LSD.
Here, using both simulations and theory, we study the complementary
magnetically forced(MF) case in which the system is forced with an electric
field that supplies magnetic helicity. For this MF case, the kinetic helicity
becomes the back-reactor that saturates the LSD. Simulations of both MF and KF
cases can be approximately modeled with the same equations of magnetic helicity
evolution, but with complementary initial conditions. A key difference between
KF and MF cases is that the helical large scale field in the MF case grows with
the same sign of injected magnetic helicity, whereas the large and small scale
magnetic helicities grow with opposite sign for the KF case. The MF case can
arise even when the thermal pressure is approximately smaller than the magnetic
pressure, and requires only that helical small scale magnetic fluctuations
dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in
accretion discs and Babcock models of the solar dynamo are actually MF LSDs.Comment: 12 pages, 34 figure
The preference and costs of sleeping under light at night in forest and urban great tits
Artificial light at night (ALAN) is an increasing phenomenon associated with worldwide urbanization. In birds, broad-spectrum white ALAN can have disruptive effects on activity patterns, metabolism, stress response and immune function. There has been growing research on whether the use of alternative light spectra can reduce these negative effects, but surprisingly, there has been no study to determine which light spectrum birds prefer. To test such a preference, we gave urban and forest great tits (Parus major) the choice where to roost using pairwise combinations of darkness, white light or green dim light at night (1.5 lux). Birds preferred to sleep under artificial light instead of darkness, and green was preferred over white light. In a subsequent experiment, we investigated the consequence of sleeping under a particular light condition, and measured birds' daily activity levels, daily energy expenditure (DEE), oxalic acid as a biomarker for sleep debt and cognitive abilities. White light affected activity patterns more than green light. Moreover, there was an origin-dependent response to spectral composition: in urban birds, the total daily activity and night activity did not differ between white and green light, while forest birds were more active under white than green light. We also found that individuals who slept under white and green light had higher DEE. However, there were no differences in oxalic acid levels or cognitive abilities between light treatments. Thus, we argue that in naive birds that had never encountered light at night, white light might disrupt circadian rhythms more than green light. However, it is possible that the negative effects of ALAN on sleep and cognition might be observed only under intensities higher than 1.5 lux. These results suggest that reducing the intensity of light pollution as well as tuning the spectrum towards long wavelengths may considerably reduce its impact
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