7,451 research outputs found
Can conduction induce convection? The non-linear saturation of buoyancy instabilities in dilute plasmas
We study the effects of anisotropic thermal conduction on low-collisionality,
astrophysical plasmas using two and three-dimensional magnetohydrodynamic
simulations. For weak magnetic fields, dilute plasmas are buoyantly unstable
for either sign of the temperature gradient: the heat-flux-driven buoyancy
instability (HBI) operates when the temperature increases with radius while the
magnetothermal instability (MTI) operates in the opposite limit. In contrast to
previous results, we show that, in the presence of a sustained temperature
gradient, the MTI drives strong turbulence and operates as an efficient
magnetic dynamo (akin to standard, adiabatic convection). Together, the
turbulent and magnetic energies contribute up to ~10% of the pressure support
in the plasma. In addition, the MTI drives a large convective heat flux, ~1.5%
of rho c_s^3. These findings are robust even in the presence of an external
source of strong turbulence. Our results on the nonlinear saturation of the HBI
are consistent with previous studies but we explain physically why the HBI
saturates quiescently by re-orienting the magnetic field (suppressing the
conductive heat flux through the plasma), while the MTI saturates by generating
sustained turbulence. We also systematically study how an external source of
turbulence affects the saturation of the HBI: such turbulence can disrupt the
HBI only on scales where the shearing rate of the turbulence is faster than the
growth rate of the HBI. In particular, our results provide a simple mapping
between the level of turbulence in a plasma and the effective isotropic thermal
conductivity. We discuss the astrophysical implications of these findings, with
a particular focus on the intracluster medium of galaxy clusters.Comment: 18 pages, 14 figures. Submitted to MNRA
Issues in Evaluating Health Department Web-Based Data Query Systems: Working Papers
Compiles papers on conceptual and methodological topics to consider in evaluating state health department systems that provide aggregate data online, such as taxonomy, logic models, indicators, and design. Includes surveys and examples of evaluations
The Dynamics of Rayleigh-Taylor Stable and Unstable Contact Discontinuities with Anisotropic Thermal Conduction
We study the effects of anisotropic thermal conduction along magnetic field
lines on an accelerated contact discontinuity in a weakly collisional plasma.
We first perform a linear stability analysis similar to that used to derive the
Rayleigh-Taylor instability (RTI) dispersion relation. We find that anisotropic
conduction is only important for compressible modes, as incompressible modes
are isothermal. Modes grow faster in the presence of anisotropic conduction,
but growth rates do not change by more than a factor of order unity. We next
run fully non-linear numerical simulations of a contact discontinuity with
anisotropic conduction. The non-linear evolution can be thought of as a
superposition of three physical effects: temperature diffusion due to vertical
conduction, the RTI, and the heat flux driven buoyancy instability (HBI). In
simulations with RTI-stable contact discontinuities, the temperature
discontinuity spreads due to vertical heat conduction. This occurs even for
initially horizontal magnetic fields due to the initial vertical velocity
perturbation and numerical mixing across the interface. The HBI slows this
temperature diffusion by reorienting initially vertical magnetic field lines to
a more horizontal geometry. In simulations with RTI-unstable contact
discontinuities, the dynamics are initially governed by temperature diffusion,
but the RTI becomes increasingly important at late times. We discuss the
possible application of these results to supernova remnants, solar prominences,
and cold fronts in galaxy clusters.Comment: 18 pages, 15 figures, submitted to MNRA
Anisotropic Thermal Conduction and the Cooling Flow Problem in Galaxy Clusters
We examine the long-standing cooling flow problem in galaxy clusters with 3D
MHD simulations of isolated clusters including radiative cooling and
anisotropic thermal conduction along magnetic field lines. The central regions
of the intracluster medium (ICM) can have cooling timescales of ~200 Myr or
shorter--in order to prevent a cooling catastrophe the ICM must be heated by
some mechanism such as AGN feedback or thermal conduction from the thermal
reservoir at large radii. The cores of galaxy clusters are linearly unstable to
the heat-flux-driven buoyancy instability (HBI), which significantly changes
the thermodynamics of the cluster core. The HBI is a convective,
buoyancy-driven instability that rearranges the magnetic field to be
preferentially perpendicular to the temperature gradient. For a wide range of
parameters, our simulations demonstrate that in the presence of the HBI, the
effective radial thermal conductivity is reduced to less than 10% of the full
Spitzer conductivity. With this suppression of conductive heating, the cooling
catastrophe occurs on a timescale comparable to the central cooling time of the
cluster. Thermal conduction alone is thus unlikely to stabilize clusters with
low central entropies and short central cooling timescales. High central
entropy clusters have sufficiently long cooling times that conduction can help
stave off the cooling catastrophe for cosmologically interesting timescales.Comment: Submitted to ApJ, 14 pages, 14 figure
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Cognitive barriers during monitoring-based commissioning of buildings
Monitoring-based commissioning (MBCx) is a continuous building energy management process used to optimize energy performance in buildings. Although monitoring-based commissioning (MBCx) can reduce energy waste by up to 20%, many buildings still underperform due to issues such as unnoticed system faults and inefficient operational procedures. While there are technical barriers that impede the MBCx process, such as data quality, the focuses of this paper are the non-technical, behavioral and organizational, barriers that contribute to issues initiating and implementing MBCx. In particular, this paper discusses cognitive biases, which can lead to suboptimal outcomes in energy efficiency decisions, resulting in missed opportunities for energy savings. This paper provides evidence of cognitive biases in decisions during the MBCx process using qualitative data from over 40 public and private sector organizations. The results describe barriers resulting from cognitive biases, listed in descending order of occurrence, including: risk aversion, social norms, choice overload, status quo bias, information overload, professional bias, and temporal discounting. Building practitioners can use these results to better understand potential cognitive biases, in turn allowing them to establish best practices and make more informed decisions. Researchers can use these results to empirically test specific decision interventions and facilitate more energy efficient decisions
Comparison of a linear and a nonlinear washout for motion simulators utilizing objective and subjective data from CTOL transport landing approaches
Objective and subjective data gathered in the processes of comparing a linear and a nonlinear washout for motion simulators reveal that there is no difference in the pilot performance measurements used during instrument landing system (ILS) approaches with a Boeing 737 conventional takeoff and landing (CTOL) airplane between fixed base, linear washout, and nonlinear washout operations. However, the subjective opinions of the pilots reveal an important advance in motion cue presentation. The advance is not in the increased cue available over a linear filter for the same amount of motion base travel but rather in the elimination of false rotational rate cues presented by linear filters
Evaluation of a linear washout for simulator motion cue presentation during landing approach
The comparison of a fixed-base versus a five-degree-of-freedom motion base simulation of a 737 conventional take-off and landing (CTOL) aircraft performing instrument landing system (ILS) landing approaches was used to evaluate a linear motion washout technique. The fact that the pilots felt that the addition of motion increased the pilot workload and this increase was not reflected in the objective data results, indicates that motion cues, as presented, are not a contributing factor to root-mean-square (rms) performance during the landing approach task. Subjective results from standard maneuvering about straight-and-level flight for specific motion cue evaluation revealed that the longitudinal channels (pitch and surge) possibly the yaw channel produce acceptable motions. The roll cue representation, involving both roll and sway channels, was found to be inadequate for large roll inputs, as used for example, in turn entries
Buoyancy Instabilities in Galaxy Clusters: Convection Due to Adiabatic Cosmic Rays and Anisotropic Thermal Conduction
Using a linear stability analysis and two and three-dimensional nonlinear
simulations, we study the physics of buoyancy instabilities in a combined
thermal and relativistic (cosmic ray) plasma, motivated by the application to
clusters of galaxies. We argue that cosmic ray diffusion is likely to be slow
compared to the buoyancy time on large length scales, so that cosmic rays are
effectively adiabatic. If the cosmic ray pressure is of
the thermal pressure, and the cosmic ray entropy (;
is the thermal plasma density) decreases outwards, cosmic rays drive an
adiabatic convective instability analogous to Schwarzschild convection in
stars. Global simulations of galaxy cluster cores show that this instability
saturates by reducing the cosmic ray entropy gradient and driving efficient
convection and turbulent mixing. At larger radii in cluster cores, the thermal
plasma is unstable to the heat flux-driven buoyancy instability (HBI), a
convective instability generated by anisotropic thermal conduction and a
background conductive heat flux. Cosmic-ray driven convection and the HBI may
contribute to redistributing metals produced by Type 1a supernovae in clusters.
Our calculations demonstrate that adiabatic simulations of galaxy clusters can
artificially suppress the mixing of thermal and relativistic plasma;
anisotropic thermal conduction allows more efficient mixing, which may
contribute to cosmic rays being distributed throughout the cluster volume.Comment: submitted to ApJ; 15 pages and 12 figures; abstract shortened to < 24
lines; for high resolution movies see
http://astro.berkeley.edu/~psharma/clustermovie.htm
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