2,563 research outputs found
The determination of the global average OH concentration using a deuteroethane tracer
It is proposed to measure the decreasing global concentration of an OH reactive isotopic tracer, G sub 2 D sub 6, after its introduction into the troposphere in a manner to facilitate uniform global mixing. Analyses at the level of 2 x 10 to the -19th power fraction, corresponding to one kg uniformly distributed globally, should be possible by a combination of cryogenic absorption techniques to separate ethane from air and high sensitivity isotopic analysis of ethane by mass spectrometry. Aliquots of C sub 2 D sub 6 totaling one kg would be introduced to numerous southern and northern latitudes over a 10 day period in order to achieve a uniform global concentration within 3 to 6 months by the normal atmospheric circulation. Then samples of air of 1000 l (STP) would be collected periodically at a tropical and temperate zone location in each hemisphere and spiked with a known amount of another isotopic species of ethane, C-13 sub 2 H sub 6, at the level of 10 to the -11th power mole fraction. After separation of the ethanes from air, the absolute concentration of C sub 2 D sub 6 would be analyzed using the Argonne 100-inch radius mass spectrometer
A concurrent precursor inflow method for Large Eddy Simulations and applications to finite length wind farms
In order to enable simulations of developing wind turbine array boundary
layers with highly realistic inflow conditions a concurrent precursor method
for Large Eddy Simulations is proposed. In this method we consider two domains
simultaneously, i.e. in one domain a turbulent Atmospheric Boundary Layer (ABL)
without wind turbines is simulated in order to generate the turbulent inflow
conditions for a second domain in which the wind turbines are placed. The
benefit of this approach is that a) it avoids the need for large databases in
which the turbulent inflow conditions are stored and the correspondingly slow
I/O operations and b) we are sure that the simulations are not negatively
affected by statically swept fixed inflow fields or synthetic fields lacking
the proper ABL coherent structures. Sample applications are presented, in
which, in agreement with field data a strong decrease of the power output of
downstream wind-turbines with respect to the first row of wind-turbines is
observed for perfectly aligned inflow.Comment: 13 pages, 5 figure
Generalized coupled wake boundary layer model: applications and comparisons with field and LES data for two wind-farms
We describe a generalization of the Coupled Wake Boundary Layer (CWBL) model
for wind-farms that can be used to evaluate the performance of wind-farms under
arbitrary wind inflow directions whereas the original CWBL model (Stevens et
al., J. Renewable and Sustainable Energy 7, 023115 (2015)) focused on aligned
or staggered wind-farms. The generalized CWBL approach combines an analytical
Jensen wake model with a "top-down" boundary layer model coupled through an
iterative determination of the wake expansion coefficient and an effective wake
coverage area for which the velocity at hub-height obtained using both models
converges in the "deep-array" portion (fully developed region) of the
wind-farm. The approach accounts for the effect of the wind direction by
enforcing the coupling for each wind direction. Here we present detailed
comparisons of model predictions with LES results and field measurements for
the Horns Rev and Nysted wind-farms operating over a wide range of wind inflow
directions. Our results demonstrate that two-way coupling between the Jensen
wake model and a "top-down" model enables the generalized CWBL model to predict
the "deep-array" performance of a wind-farm better than the Jensen wake model
alone. The results also show that the new generalization allows us to study a
much larger class of wind-farms than the original CWBL model, which increases
the utility of the approach for wind-farm designers.Comment: 17 pages, 11 figure
Modeling space-time correlations of velocity fluctuations in wind farms
An analytical model for the streamwise velocity space-time correlations in
turbulent flows is derived and applied to the special case of velocity
fluctuations in large wind farms. The model is based on the Kraichnan-Tennekes
random sweeping hypothesis, capturing the decorrelation in time while including
a mean wind velocity in the streamwise direction. In the resulting model, the
streamwise velocity space-time correlation is expressed as a convolution of the
pure space correlation with an analytical temporal decorrelation kernel. Hence,
the spatio-temporal structure of velocity fluctuations in wind farms can be
derived from the spatial correlations only. We then explore the applicability
of the model to predict spatio-temporal correlations in turbulent flows in wind
farms. Comparisons of the model with data from a large eddy simulation of flow
in a large, spatially periodic wind farm are performed, where needed model
parameters such as spatial and temporal integral scales and spatial
correlations are determined from the large eddy simulation. Good agreement is
obtained between the model and large eddy simulation data showing that spatial
data may be used to model the full temporal structure of fluctuations in wind
farms.Comment: Submitted to Wind Energ
Spatio-temporal spectra in the logarithmic layer of wall turbulence: large-eddy simulations and simple models
Motivated by the need to characterize the spatio-temporal structure of
turbulence in wall-bounded flows, we study wavenumber-frequency spectra of the
streamwise velocity component based on large-eddy simulation (LES) data. The
LES data are used to measure spectra as a function of the two wall-parallel
wavenumbers and the frequency in the equilibrium (logarithmic) layer. We then
reformulate one of the simplest models that is able to reproduce the
observations: the random sweeping model with a Gaussian large-scale fluctuating
velocity and with additional mean flow. Comparison with LES data shows that the
model captures the observed temporal decorrelation, which is related to the
Doppler broadening of frequencies. We furthermore introduce a parameterization
for the entire wavenumber-frequency spectrum , where
, are the streamwise and spanwise wavenumbers, is the
frequency and is the distance to the wall. The results are found to be in
good agreement with LES data
Coupled wake boundary layer model of wind-farms
We present and test the coupled wake boundary layer (CWBL) model that
describes the distribution of the power output in a wind-farm. The model
couples the traditional, industry-standard wake model approach with a
"top-down" model for the overall wind-farm boundary layer structure. This wake
model captures the effect of turbine positioning, while the "top-down" portion
of the model adds the interactions between the wind-turbine wakes and the
atmospheric boundary layer. Each portion of the model requires specification of
a parameter that is not known a-priori. For the wake model, the wake expansion
coefficient is required, while the "top-down" model requires an effective
spanwise turbine spacing within which the model's momentum balance is relevant.
The wake expansion coefficient is obtained by matching the predicted mean
velocity at the turbine from both approaches, while the effective spanwise
turbine spacing depends on turbine positioning and thus can be determined from
the wake model. Coupling of the constitutive components of the CWBL model is
achieved by iterating these parameters until convergence is reached. We
illustrate the performance of the model by applying it to both developing
wind-farms including entrance effects and to fully developed (deep-array)
conditions. Comparisons of the CWBL model predictions with results from a suite
of large eddy simulations (LES) shows that the model closely represents the
results obtained in these high-fidelity numerical simulations. A comparison
with measured power degradation at the Horns Rev and Nysted wind-farms shows
that the model can also be successfully applied to real wind-farms.Comment: 25 pages, 21 figures, submitted to Journal of Renewable and
Sustainable Energy on July 18, 201
The Drosophila genome nexus: a population genomic resource of 623 Drosophila melanogaster genomes, including 197 from a single ancestral range population.
Hundreds of wild-derived Drosophila melanogaster genomes have been published, but rigorous comparisons across data sets are precluded by differences in alignment methodology. The most common approach to reference-based genome assembly is a single round of alignment followed by quality filtering and variant detection. We evaluated variations and extensions of this approach and settled on an assembly strategy that utilizes two alignment programs and incorporates both substitutions and short indels to construct an updated reference for a second round of mapping prior to final variant detection. Utilizing this approach, we reassembled published D. melanogaster population genomic data sets and added unpublished genomes from several sub-Saharan populations. Most notably, we present aligned data from phase 3 of the Drosophila Population Genomics Project (DPGP3), which provides 197 genomes from a single ancestral range population of D. melanogaster (from Zambia). The large sample size, high genetic diversity, and potentially simpler demographic history of the DPGP3 sample will make this a highly valuable resource for fundamental population genetic research. The complete set of assemblies described here, termed the Drosophila Genome Nexus, presently comprises 623 consistently aligned genomes and is publicly available in multiple formats with supporting documentation and bioinformatic tools. This resource will greatly facilitate population genomic analysis in this model species by reducing the methodological differences between data sets
Improving problem solving with simple interventions
Although problem solving is a major goal for most science educators, many still rely on the demonstration method as an approach to teach it. This remains the case even though most are not happy with the results. Using a web-based problem delivery system to track students’ performance, we have investigated the effects of collaborative learning, and concept mapping on student problem solving ability. We find that student ability in general can be improved by about 10% after a group problem solving intervention. Furthermore we find differences in improvement depending upon the students’ level of logical thinking and gender
Temporal structure of aggregate power fluctuations in large-eddy simulations of extended wind-farms
Fluctuations represent a major challenge for the incorporation of electric
power from large wind-farms into power grids. Wind farm power output fluctuates
strongly in time, over various time scales. Understanding these fluctuations,
especially their spatio-temporal characteristics, is particularly important for
the design of backup power systems that must be readily available in
conjunction with wind-farms. In this work we analyze the power fluctuations
associated with the wind-input variability at scales between minutes to several
hours, using large eddy simulations (LES) of extended wind-parks, interacting
with the atmospheric boundary layer. LES studies enable careful control of
parameters and availability of wind-velocities simultaneously across the entire
wind-farm. The present study focuses on neutral atmospheric conditions and flat
terrain, using actuator-disk representations of the individual wind-turbines.
We consider power from various aggregates of wind-turbines such as the total
average power signal, or signals from sub-averages within the wind-farm.
Non-trivial correlations are observed due to the complex interactions between
turbines placed downstream of each other, and they lead to noticeable spectral
peaks at frequencies associated with the inter-turbine spacings when the
wind-direction is completely fixed. In that case we observe that the frequency
spectra of the total wind-farm output show a decay that follows approximately a
power-law scaling regime, qualitatively consistent with some
observations made in field-scale operational wind-parks (Apt, 2007). We find
that these features are still observed when the wind-speed varies in magnitude.
However, significant changes in the wind-direction over time tend to smooth out
the observed spectral peak and reduce the extent of the observed
power-law.Comment: 20 pages, 15 figure
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