2,514 research outputs found
Trkalian fields and Radon transformation
We write the spherical curl transformation for Trkalian fields using
differential forms. Then we consider Radon transform of these fields. The Radon
transform of a Trkalian field satisfies a corresponding eigenvalue equation on
a sphere in transform space. The field can be reconstructed using knowledge of
the Radon transform on a canonical hemisphere. We consider relation of the
Radon transformation with Biot-Savart integral operator and discuss its
transform introducing Radon-Biot- Savart operator. The Radon transform of a
Trkalian field is an eigenvector of this operator. We also present an Ampere
law type relation for these fields. We apply these to Lundquist solution. We
present a Chandrasekhar-Kendall type solution of the corresponding equation in
the transform space. Lastly, we focus on the Euclidean topologically massive
Abelian gauge theory. The Radon transform of an anti-self-dual field is related
by antipodal map on this sphere to the transform of the self-dual field
obtained by inverting space coordinates. The Lundquist solution provides an
example of quantization of topological mass in this context.Comment: 23 page
Guiding of Rydberg atoms in a high-gradient magnetic guide
We study the guiding of Rb 59D Rydberg atoms in a linear,
high-gradient, two-wire magnetic guide. Time delayed microwave ionization and
ion detection are used to probe the Rydberg atom motion. We observe guiding of
Rydberg atoms over a period of 5 ms following excitation. The decay time of the
guided atom signal is about five times that of the initial state. We attribute
the lifetime increase to an initial phase of -changing collisions and
thermally induced Rydberg-Rydberg transitions. Detailed simulations of Rydberg
atom guiding reproduce most experimental observations and offer insight into
the internal-state evolution
Possible geopotential improvement from satellite altimetry
Possible geopotential improvement from satellite altimetr
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
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Evaluation of the wind farm parameterization in the Weather Research and Forecasting model (version 3.8.1) with meteorological and turbine power data
Abstract. Forecasts of wind power production are necessary to facilitate the integration of wind energy into power grids, and these forecasts should incorporate the impact of wind turbine wakes. This paper focuses on a case study of four diurnal cycles with significant power production, and assesses the skill of the wind farm parameterization (WFP) distributed with the Weather Research and Forecasting (WRF) model version 3.8.1, as well as its sensitivity to model configuration. After validating the simulated ambient flow with observations, we quantify the value of the WFP as it accounts for wake impacts on power production of downwind turbines. We also illustrate that a vertical grid with nominally 12-m vertical resolution is necessary for reproducing the observed power production, with statistical significance. Further, the WFP overestimates wake effects and hence underestimates downwind power production during high wind speed and low turbulence conditions. We also find the WFP performance is independent of atmospheric stability, the number of wind turbines per model grid cell, and the upwind-downwind position of turbines. Rather, the ability of the WFP to predict power production is most dependent on the skill of the WRF model in simulating the ambient wind speed.
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US East Coast lidar measurements show offshore wind turbines will encounter very low atmospheric turbulence
© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bodini, N., Lundquist, J. K., & Kirincich, A. US East Coast lidar measurements show offshore wind turbines will encounter very low atmospheric turbulence. Geophysical Research Letters, 46(10), (2019):5582-5591, doi:10.1029/2019GL082636.The rapid growth of offshore wind energy requires accurate modeling of the wind resource, which can be depleted by wind farm wakes. Turbulence dissipation rate (ϵ) governs the accuracy of model predictions of hub‐height wind speed and the development and erosion of wakes. Here we assess the variability of turbulence kinetic energy and ϵ using 13 months of observations from a profiling lidar deployed on a platform off the Massachusetts coast. Offshore, ϵ is 2 orders of magnitude smaller than onshore, with a subtle diurnal cycle. Wind direction influences the annual cycle of turbulence, with larger values in winter when the wind flows from the land, and smaller values in summer, when the wind flows from open ocean. Because of the weak turbulence, wind plant wakes will be stronger and persist farther downwind in summer.Collection of the wind data was funded by the Massachusetts Clean Energy Center through agreements with WHOI and AWS Truepower. The authors appreciate the efforts of the MVCO/ASIT technicians and AWS staff who collected the data. This analysis was supported by the National Science Foundation CAREER Award (AGS‐1554055) to J. K. L. and N. B., and by internal funds from WHOI for A. K. This work was authored (in part) by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract DE‐AC36‐08GO28308. Funding was provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid‐up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. The lidar observations used here are described at https://www.masscec.com/masscec-metocean-data-initiative, and available at https://doi.org/10.26025/1912/24050. The postprocessed data and the scripts used for the Figures of the present paper can be found at https://github.com/nicolabodini/GRL_OffshoreTurbulence.2019-11-0
On the jets, kinks, and spheromaks formed by a planar magnetized coaxial gun
Measurements of the various plasma configurations produced by a planar
magnetized coaxial gun provide insight into the magnetic topology evolution
resulting from magnetic helicity injection. Important features of the
experiments are a very simple coaxial gun design so that all observed
geometrical complexity is due to the intrinsic physical dynamics rather than
the source shape and use of a fast multiple-frame digital camera which provides
direct imaging of topologically complex shapes and dynamics. Three key
experimental findings were obtained: (1) formation of an axial collimated jet
[Hsu and Bellan, Mon. Not. R. Astron. Soc. 334, 257 (2002)] that is consistent
with a magnetohydrodynamic description of astrophysical jets, (2)
identification of the kink instability when this jet satisfies the
Kruskal-Shafranov limit, and (3) the nonlinear properties of the kink
instability providing a conversion of toroidal to poloidal flux as required for
spheromak formation by a coaxial magnetized source [Hsu and Bellan, Phys. Rev.
Lett. 90, 215002 (2003)]. A new interpretation is proposed for how the n=1
central column instability provides flux amplification during spheromak
formation and sustainment, and it is shown that jet collimation can occur
within one rotation of the background poloidal field.Comment: Physics of Plasmas (accepted
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An LES-based airborne Doppler lidar simulator and its application to wind profiling in inhomogeneous flow conditions
Wind profiling by Doppler lidar is common practice and highly useful in a wide range of applications. Airborne Doppler lidar can provide additional insights relative to ground-based systems by allowing for spatially distributed and targeted measurements. Providing a link between theory and measurement, a first large eddy simulation (LES)-based airborne Doppler lidar simulator (ADLS) has been developed. Simulated measurements are conducted based on LES wind fields, considering the coordinate and geometric transformations applicable to real-world measurements. The ADLS provides added value as the input truth used to create the measurements is known exactly, which is nearly impossible in real-world situations. Thus, valuable insight can be gained into measurement system characteristics as well as retrieval strategies.
As an example application, airborne Doppler lidar wind profiling is investigated using the ADLS. For commonly used airborne velocity azimuth display (AVAD) techniques, flow homogeneity is assumed throughout the retrieval volume, a condition which is violated in turbulent boundary layer flow. Assuming an ideal measurement system, the ADLS allows to isolate and evaluate the error in wind profiling which occurs due to the violation of the flow homogeneity assumption. Overall, the ADLS demonstrates that wind profiling is possible in turbulent wind field conditions with reasonable errors (root mean squared error of 0.36 m s−1 for wind speed when using a commonly used system setup and retrieval strategy for the conditions investigated). Nevertheless, flow inhomogeneity, e.g., due to boundary layer turbulence, can cause an important contribution to wind profiling error and is non-negligible. Results suggest that airborne Doppler lidar wind profiling at low wind speeds (<5ms −1) can be biased, if conducted in regions of inhomogeneous flow conditions
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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