1,912 research outputs found
Plasma Relaxation and Topological Aspects in Hall Magnetohydrodynamics
Parker's formulation of isotopological plasma relaxation process in
magnetohydrodynamics (MHD) is extended to Hall MHD. The torsion coefficient
alpha in the Hall MHD Beltrami condition turns out now to be proportional to
the "potential vorticity." The Hall MHD Beltrami condition becomes equivalent
to the "potential vorticity" conservation equation in two-dimensional (2D)
hydrodynamics if the Hall MHD Lagrange multiplier beta is taken to be
proportional to the "potential vorticity" as well. The winding pattern of the
magnetic field lines in Hall MHD then appears to evolve in the same way as
"potential vorticity" lines in 2D hydrodynamics
Estimation of turbulence dissipation rate and its variability from sonic anemometer and wind Doppler lidar during the XPIA field campaign
Despite turbulence being a fundamental transport process in the boundary
layer, the capability of current numerical models to represent it is
undermined by the limits of the adopted assumptions, notably that of local
equilibrium. Here we leverage the potential of extensive observations in
determining the variability in turbulence dissipation rate (ϵ).
These observations can provide insights towards the understanding of the
scales at which the major assumption of local equilibrium between generation
and dissipation of turbulence is invalid. Typically, observations of
ϵ require time- and labor-intensive measurements from sonic and/or
hot-wire anemometers. We explore the capability of wind Doppler lidars to
provide measurements of ϵ. We refine and extend an existing method
to accommodate different atmospheric stability conditions. To validate our
approach, we estimate ϵ from four wind Doppler lidars during the
3-month XPIA campaign at the Boulder Atmospheric Observatory (Colorado), and
we assess the uncertainty of the proposed method by data intercomparison
with sonic anemometer measurements of ϵ. Our analysis of this
extensive dataset provides understanding of the climatology of turbulence
dissipation over the course of the campaign. Further, the variability in
ϵ with atmospheric stability, height, and wind speed is also
assessed. Finally, we present how ϵ increases as nocturnal
turbulence is generated during low-level jet events.</p
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Characterization of flow recirculation zones at the Perdigão site using multi-lidar measurements
Because flow recirculation can generate significant amounts of turbulence, it
can impact the success of wind energy projects. This study uses unique
Doppler lidar observations to quantify occurrences of flow recirculation on
lee sides of ridges. An extensive dataset of observations of flow over
complex terrain is available from the Perdigão 2017 field campaign over a
period of 3 months. The campaign site was selected because of the unique
terrain feature of two nearly parallel ridges with a valley-to-ridge-top
height difference of about 200 m and a ridge-to-ridge distance of 1.4 km.
Six scanning Doppler lidars probed the flow field in several vertical planes
orthogonal to the ridges using range–height indicator scans. With this lidar
setup, we achieved vertical scans of the recirculation zone at three
positions along two parallel ridges. We construct a method to identify flow
recirculation zones in the scans, as well as define characteristics of these
zones. According to our data analysis, flow recirculation, with reverse flow
wind speeds greater than 0.5 m s−1, occurs over 50 % of the time
when the wind direction is perpendicular to the direction of the ridges.
Atmospheric conditions, such as atmospheric stability and wind speed, affect
the occurrence of flow recirculation. Flow recirculation occurs more
frequently during periods with wind speeds above 8 m s−1.
Recirculation within the valley affects the mean wind and turbulence fields
at turbine heights on the downwind ridge in magnitudes significant for wind
resource assessment.</p
Seasonal variability of wake impacts on US mid-Atlantic offshore wind plant power production
The mid-Atlantic will experience rapid wind plant development due to its promising wind resource located near large population centers. Wind turbines and wind plants create wakes, or regions of reduced wind speed, that may negatively affect downwind turbines and plants. We evaluate wake variability and annual energy production with the first yearlong modeling assessment using the Weather Research and Forecasting model, deploying 12 MW turbines across the domain at a density of 3.14 MW km−2, matching the planned density of 3 MW km−2. Using a series of simulations with no wind plants, one wind plant, and complete build-out of lease areas, we calculate wake effects and distinguish the effect of wakes generated internally within one plant from those generated externally between plants. We also provide a first step towards uncertainty quantification by testing the amount of added turbulence kinetic energy (TKE) by 0 % and 100 %. We provide a sensitivity analysis by additionally comparing 25 % and 50 % for a short case study period. The strongest wakes, propagating 55 km, occur in summertime stable stratification, just when New England's grid demand peaks in summer. The seasonal variability of wakes in this offshore region is much stronger than the diurnal variability of wakes. Overall, yearlong simulated wake impacts reduce power output by a range between 38.2 % and 34.1 % (for 0 %–100 % added TKE). Internal wakes cause greater yearlong power losses, from 29.2 % to 25.7 %, compared to external wakes, from 14.7 % to 13.4 %. The overall impact is different from the linear sum of internal wakes and external wakes due to non-linear processes. Additional simulations quantify wake uncertainty by modifying the added amount of turbulent kinetic energy from wind turbines, introducing power output variability of 3.8 %. Finally, we compare annual energy production to New England grid demand and find that the lease areas can supply 58.8 % to 61.2 % of annual load. We note that the results of this assessment are not intended to make nor are they suitable to make commercial judgments about specific wind projects.</p
Return to driving after traumatic brain injury : a British perspective
Primary Objective: to identify current legal situation, and professional practice in assisting persons with traumatic brain injury (TBI) to return to safe driving after injury.
Methods and Procedures
A brief review of relevant literature, a description of the current statutory and quasi-statutory authorities regulating return to driving after TBI in the UK, and a description of the nature and resolution of clinical and practical dilemmas facing professionals helping return to safe driving after TBI. Each of the 15 UK mobility centres was contacted and literature requested; in addition a representative of each centre responded to a structured telephone survey.
Main Outcome and Results: The current situation in Great Britain is described, with a brief analysis of the strengths and weaknesses both of the current statutory situation, and also the practical situation (driving centres), with suggestions for improvements in practice.
Conclusion
Although brain injury may cause serious limitations in driving ability, previous drivers are not routinely assessed or advised regarding return to driving after TBI
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Spatial and temporal variability of turbulence dissipation rate in complex terrain
To improve parameterizations of the turbulence dissipation rate (ϵ)
in numerical weather prediction models, the temporal and spatial variability
of ϵ must be assessed. In this study, we explore influences on the
variability of ϵ at various scales in the Columbia River Gorge
during the WFIP2 field experiment between 2015 and 2017. We calculate
ϵ from five sonic anemometers all deployed in a ∼4 km2
area as well as
from two scanning Doppler lidars and four profiling
Doppler lidars, whose locations span a ∼300 km wide region.
We retrieve ϵ from the sonic anemometers using the second-order
structure function method, from the scanning lidars with the azimuth
structure function approach, and from the profiling lidars with a novel
technique using the variance of the line-of-sight velocity. The turbulence
dissipation rate shows large spatial variability, even at the microscale,
especially during nighttime stable conditions. Orographic features have a
strong impact on the variability of ϵ, with the correlation between
ϵ at different stations being highly influenced by terrain.
ϵ shows larger values in sites located downwind of complex
orographic structures or in wind farm wakes. A clear diurnal cycle in
ϵ is found, with daytime convective conditions determining values
over an order of magnitude higher than nighttime stable conditions.
ϵ also shows a distinct seasonal cycle, with differences greater
than an order of magnitude between average ϵ values in summer and
winter.</p
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
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Turbine Inflow Characterization at the National Wind Technology Center: Preprint
Utility-scale wind turbines operate in dynamic flows that can vary significantly over timescales from less than a second to several years. To better understand the inflow to utility-scale turbines, two inflow towers were installed and commissioned at the National Renewable Energy Laboratory's (NREL) National Wind Technology Center near Boulder, Colorado, in 2011. These towers are 135 m tall and instrumented with a combination of sonic anemometers, cup anemometers, wind vanes, and temperature measurements to characterize the inflow wind speed and direction, turbulence, stability and thermal stratification to two utility-scale turbines. Herein, we present variations in mean and turbulent wind parameters with height, atmospheric stability, and as a function of wind direction that could be important for turbine operation as well as persistence of turbine wakes. Wind speed, turbulence intensity, and dissipation are all factors that affect turbine performance. Our results shown that these all vary with height across the rotor disk, demonstrating the importance of measuring atmospheric conditions that influence wind turbine performance at multiple heights in the rotor disk, rather than relying on extrapolation from lower levels
Coronal mass ejections as expanding force-free structures
We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic
structures and find the self-similar dynamics of configurations with spatially
constant \alpha, where {\bf J} =\alpha {\bf B}, in spherical and cylindrical
geometries, expanding spheromaks and expanding Lundquist fields
correspondingly. The field structures remain force-free, under the conventional
non-relativistic assumption that the dynamical effects of the inductive
electric fields can be neglected. While keeping the internal magnetic field
structure of the stationary solutions, expansion leads to complicated internal
velocities and rotation, induced by inductive electric field. The structures
depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured
at a given moment, and thus are applicable to arbitrary expansion laws. In case
of cylindrical Lundquist fields, the flux conservation requires that both axial
and radial expansion proceed with equal rates. In accordance with observations,
the model predicts that the maximum magnetic field is reached before the
spacecraft reaches the geometric center of a CME.Comment: 19 pages, 9 Figures, accepted by Solar Physic
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