22,827 research outputs found
Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications
We investigate single-fluid magnetohydrodynamics (MHD) with anisotropic viscosity,
often referred to as Braginskii MHD, with a particular eye to solar coronal applications.
First, we examine the full Braginskii viscous tensor in the single-fluid limit. We pay
particular attention to how the Braginskii tensor behaves as the magnetic field strength
vanishes. The solar corona contains a magnetic field with a complex and evolving
topology, so the viscosity must revert to its isotropic form when the field strength is zero,
e.g. at null points. We highlight that the standard form in which the Braginskii tensor
is written is not suitable for inclusion in simulations as singularities in the individual
terms can develop. Instead, an altered form, where the parallel and perpendicular tensors
are combined, provides the required asymptotic behaviour in the weak-field limit. We
implement this combined form of the tensor into the Lare3D code, which is widely used
for coronal simulations. Since our main focus is the viscous heating of the solar corona,
we drop the drift terms of the Braginskii tensor. In a stressed null point simulation,
we discover that small-scale structures, which develop very close to the null, lead to
anisotropic viscous heating at the null itself (that is, heating due to the anisotropic
terms in the viscosity tensor). The null point simulation we present has a much higher
resolution than many other simulations containing null points so this excess heating is
a practical concern in coronal simulations. To remedy this unwanted heating at the null
point, we develop a model for the viscosity tensor that captures the most important
physics of viscosity in the corona: parallel viscosity for strong field and isotropic viscosity
at null points. We derive a continuum model of viscosity where momentum transport,
described by this viscosity model, has the magnetic field as its preferred orientation.
When the field strength is zero, there is no preferred direction for momentum transport
and viscosity reverts to the standard isotropic form. The most general viscous stress
tensor of a (single-fluid) plasma satisfying these conditions is found. It is shown that
the Braginskii model, without the drift terms, is a specialization of the general model.
Performing the stressed null point simulation with this simplified model of viscosity
reveals very similar heating profiles compared to the full Braginskii model. The new
model, however, does not produce anisotropic heating at the null point, as required.
Since the vast majority of coronal simulations use only isotropic viscosity, we perform the
stressed null point simulation with isotropic viscosity and compare the heating profiles
to those of the anisotropic models. It is shown than the fully isotropic viscosity can
over-estimate the viscous heating by an order of magnitude
Exit polling and racial bloc voting: Combining individual-level and RC ecological data
Despite its shortcomings, cross-level or ecological inference remains a
necessary part of some areas of quantitative inference, including in United
States voting rights litigation. Ecological inference suffers from a lack of
identification that, most agree, is best addressed by incorporating
individual-level data into the model. In this paper we test the limits of such
an incorporation by attempting it in the context of drawing inferences about
racial voting patterns using a combination of an exit poll and precinct-level
ecological data; accurate information about racial voting patterns is needed to
assess triggers in voting rights laws that can determine the composition of
United States legislative bodies. Specifically, we extend and study a hybrid
model that addresses two-way tables of arbitrary dimension. We apply the hybrid
model to an exit poll we administered in the City of Boston in 2008. Using the
resulting data as well as simulation, we compare the performance of a pure
ecological estimator, pure survey estimators using various sampling schemes and
our hybrid. We conclude that the hybrid estimator offers substantial benefits
by enabling substantive inferences about voting patterns not practicably
available without its use.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS353 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Maneuvering and vibration control of flexible spacecraft
Equations of motion, control strategy, perturbation, rigid-body maneuvers, quasi-modal equations, and vibration control are discussed for flexible spacecraft
In-flight boundary-layer measurements on a hollow cylinder at a Mach number of 3.0
Skin temperatures, shear forces, surface static pressures, boundary layer pitot pressures, and boundary layer total temperatures were measured on the external surface of a hollow cylinder that was 3.04 meters long and 0.437 meter in diameter and was mounted beneath the fuselage of the YF-12A airplane. The data were obtained at a nominal free stream Mach number of 3.0 (a local Mach number of 2.9) and at wall to recovery temperature ratios of 0.66 to 0.91. The local Reynolds number had a nominal value of 4,300,000 per meter. Heat transfer coefficients and skin friction coefficients were derived from skin temperature time histories and shear force measurements, respectively. In addition, boundary layer velocity profiles were derived from pitot pressure measurements, and a Reynolds analogy factor was obtained from the heat transfer and skin friction measurements. The measured data are compared with several boundary layer prediction methods
In-flight Compressible Turbulent Boundary Layer Measurements on a Hollow Cylinder at a Mach Number of 3.0
Skin temperatures, shearing forces, surface static pressures, and boundary layer pitot pressures and total temperatures were measured on a hollow cylinder 3.04 meters long and 0.437 meter in diameter mounted beneath the fuselage of the YF-12A airplane. The data were obtained at a nominal free stream Mach number of 3.0 and at wall-to-recovery temperature ratios of 0.66 to 0.91. The free stream Reynolds number had a minimal value of 4.2 million per meter. Heat transfer coefficients and skin friction coefficients were derived from skin temperature time histories and shear force measurements, respectively. Boundary layer velocity profiles were derived from pitot pressure measurements, and a Reynolds analogy factor of 1.11 was obtained from the measured heat transfer and skin friction data. The skin friction coefficients predicted by the theory of van Driest were in excellent agreement with the measurements. Theoretical heat transfer coefficients, in the form of Stanton numbers calculated by using a modified Reynolds analogy between skin friction and heat transfer, were compared with measured values. The measured velocity profiles were compared to Coles' incompressible law-of-the-wall profile
When is capital enough to get female microenterprises growing? Evidence from a randomized experiment in Ghana
Standard models of investment predict that credit-constrained firms should grow rapidly when given additional capital, and that how this capital is provided should not effect decisions to invest in the business or consume the capital. We randomly gave cash and in-kind grants to male- and female-owned microenterprises in urban Ghana. Our findings cast doubt on the ability of capital alone to stimulate the growth of female microenterprises. First, while the average treatment effects of the in-kind grants are large and positive for both males and females, the gain in profits is almost zero for women with initial profits below the median, suggesting that capital alone is not enough to grow subsistence enterprises owned by women. Second, for women we strongly reject equality of the cash and in-kind grants;
only in-kind grants lead to growth in business profits. The results for men also suggest a lower impact of cash, but differences between cash and in-kind grants are less robust. The difference in the effects of cash and in-kind grants is associated more with a lack of self-control than with external pressure. As a result, the manner in which funding is provided affects microenterprise growth
The Jury System in Contemporary Ireland: In the Shadow of a Troubled Past
Jackson et al discuss the distinctive features of criminal trial by jury in Ireland, both north and south, to explain how the jury continues to survive within modern Ireland and how it also has managed to decline in significance
Active Object Localization in Visual Situations
We describe a method for performing active localization of objects in
instances of visual situations. A visual situation is an abstract
concept---e.g., "a boxing match", "a birthday party", "walking the dog",
"waiting for a bus"---whose image instantiations are linked more by their
common spatial and semantic structure than by low-level visual similarity. Our
system combines given and learned knowledge of the structure of a particular
situation, and adapts that knowledge to a new situation instance as it actively
searches for objects. More specifically, the system learns a set of probability
distributions describing spatial and other relationships among relevant
objects. The system uses those distributions to iteratively sample object
proposals on a test image, but also continually uses information from those
object proposals to adaptively modify the distributions based on what the
system has detected. We test our approach's ability to efficiently localize
objects, using a situation-specific image dataset created by our group. We
compare the results with several baselines and variations on our method, and
demonstrate the strong benefit of using situation knowledge and active
context-driven localization. Finally, we contrast our method with several other
approaches that use context as well as active search for object localization in
images.Comment: 14 page
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