809 research outputs found
Workshop on Drought Forecasting for Northeast Brazil
Precipitation forecasting parameters for northeast Brazil were developed. Hydrological, sociological, and economic aspects were examined. A drought forecasting model is presented
The Reader, the text, the Context: An Exploration of a Choreographed Response to Literature
Much current theory about response to literature stresses the reader's active role in constructing meaning, with reader, text, and context affecting the responses of individual readers (Beach, 1993). Response to literature, like most classroom interaction, tends to take a linguistic form. In a supportive classroom environment, however, a range of response media can potentially mediate students' transactions with literature. The present exploratory study used stimulated recall to elicit a retrospective account from two alternative school students who choreographed a dance to depict their understanding of the relationship between the two central characters in a short story. In their account they indicate that in composing their text they (a) initiated their interpretation by empathizing with the characters, (b) represented the characters' relationship through spatial images and configurations, and (c) used the psychological tool of dance to both represent and develop their thinking about the story. Their thought and activity were further mediated by the social context of learning, including the communication genres of the classroom, their own interaction, their teacher's intervention, and the stimulated recall interview itself. Their account illustrates the way in which reader, text, and context participate in a complex transaction when readers construct meaning for literature. Their experience also illustrates the ways in which the values of an instructional setting influence the extent to which learners may take advantage of the psychological tools available to them for growth.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Atmospheric circulation of tidally locked exoplanets: a suite of benchmark tests for dynamical solvers
The complexity of atmospheric modelling and its inherent non-linearity,
together with the limited amount of data of exoplanets available, motivate
model intercomparisons and benchmark tests. In the geophysical community, the
Held-Suarez test is a standard benchmark for comparing dynamical core
simulations of the Earth's atmosphere with different solvers, based on
statistically-averaged flow quantities. In the present study, we perform
analogues of the Held-Suarez test for tidally-locked exoplanets with the
GFDL-Princeton Flexible Modeling System (FMS) by subjecting both the spectral
and finite difference dynamical cores to a suite of tests, including the
standard benchmark for Earth, a hypothetical tidally-locked Earth, a "shallow"
hot Jupiter model and a "deep" model of HD 209458b. We find qualitative and
quantitative agreement between the solvers for the Earth, tidally-locked Earth
and shallow hot Jupiter benchmarks, but the agreement is less than satisfactory
for the deep model of HD 209458b. Further investigation reveals that closer
agreement may be attained by arbitrarily adjusting the values of the horizontal
dissipation parameters in the two solvers, but it remains the case that the
magnitude of the horizontal dissipation is not easily specified from first
principles. Irrespective of radiative transfer or chemical composition
considerations, our study points to limitations in our ability to accurately
model hot Jupiter atmospheres with meteorological solvers at the level of ten
percent for the temperature field and several tens of percent for the velocity
field. Direct wind measurements should thus be particularly constraining for
the models. Our suite of benchmark tests also provides a reference point for
researchers wishing to adapt their codes to study the atmospheric circulation
regimes of tidally-locked Earths/Neptunes/Jupiters.Comment: Accepted by MNRAS, 23 pages, 17 figures, 2 tables. No changes from
previous version, except MNRAS wants no hyphen in the title. Sample movies of
simulations are available at http://www.phys.ethz.ch/~kheng/fms
Large eddy simulation of two-dimensional isotropic turbulence
Large eddy simulation (LES) of forced, homogeneous, isotropic,
two-dimensional (2D) turbulence in the energy transfer subrange is the subject
of this paper. A difficulty specific to this LES and its subgrid scale (SGS)
representation is in that the energy source resides in high wave number modes
excluded in simulations. Therefore, the SGS scheme in this case should assume
the function of the energy source. In addition, the controversial requirements
to ensure direct enstrophy transfer and inverse energy transfer make the
conventional scheme of positive and dissipative eddy viscosity inapplicable to
2D turbulence. It is shown that these requirements can be reconciled by
utilizing a two-parametric viscosity introduced by Kraichnan (1976) that
accounts for the energy and enstrophy exchange between the resolved and subgrid
scale modes in a way consistent with the dynamics of 2D turbulence; it is
negative on large scales, positive on small scales and complies with the basic
conservation laws for energy and enstrophy. Different implementations of the
two-parametric viscosity for LES of 2D turbulence were considered. It was found
that if kept constant, this viscosity results in unstable numerical scheme.
Therefore, another scheme was advanced in which the two-parametric viscosity
depends on the flow field. In addition, to extend simulations beyond the limits
imposed by the finiteness of computational domain, a large scale drag was
introduced. The resulting LES exhibited remarkable and fast convergence to the
solution obtained in the preceding direct numerical simulations (DNS) by
Chekhlov et al. (1994) while the flow parameters were in good agreement with
their DNS counterparts. Also, good agreement with the Kolmogorov theory was
found. This LES could be continued virtually indefinitely. Then, a simplifiedComment: 34 pages plain tex + 18 postscript figures separately, uses auxilary
djnlx.tex fil
Numerical studies towards practical large-eddy simulation
Large-eddy simulation developments and validations are presented for an
improved simulation of turbulent internal flows. Numerical methods are proposed
according to two competing criteria: numerical qualities (precision and
spectral characteristics), and adaptability to complex configurations. First,
methods are tested on academic test-cases, in order to abridge with fundamental
studies. Consistent results are obtained using adaptable finite volume method,
with higher order advection fluxes, implicit grid filtering and "low-cost"
shear-improved Smagorinsky model. This analysis particularly focuses on mean
flow, fluctuations, two-point correlations and spectra. Moreover, it is shown
that exponential averaging is a promising tool for LES implementation in
complex geometry with deterministic unsteadiness. Finally, adaptability of the
method is demonstrated by application to a configuration representative of
blade-tip clearance flow in a turbomachine
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Weather, climate and the nature of predictability
The prediction and simulation of future weather and climate is a key ingredient in good weather risk management. This chapter briefly reviews the nature and underlying sources of predictability on timescales from hours-ahead to centuries-ahead. The traditional distinction between ‘weather’ and ‘climate’ predictions is described, and the role of recent scientific developments in driving a convergence of these two classic problems is highlighted. The chapter concludes by outlining and comparing the two main strategies used for creating weather and climate predictions, and discussing the challenges of using predictions in quantitative applications
Large-eddy simulation of buoyant plane plumes
In the present study a turbulent plane plume is examined by means of numerical simulations. Results are compared with experiments and results from integral models based on an entrainment assumption. The objective of this research is to determine wheter a large-eddy simulation can be applied in this case and what subgrid scale model performs best
Leray and LANS- modeling of turbulent mixing
Mathematical regularisation of the nonlinear terms in the Navier-Stokes
equations provides a systematic approach to deriving subgrid closures for
numerical simulations of turbulent flow. By construction, these subgrid
closures imply existence and uniqueness of strong solutions to the
corresponding modelled system of equations. We will consider the large eddy
interpretation of two such mathematical regularisation principles, i.e., Leray
and LANS regularisation. The Leray principle introduces a {\bfi
smoothed transport velocity} as part of the regularised convective
nonlinearity. The LANS principle extends the Leray formulation in a
natural way in which a {\bfi filtered Kelvin circulation theorem},
incorporating the smoothed transport velocity, is explicitly satisfied. These
regularisation principles give rise to implied subgrid closures which will be
applied in large eddy simulation of turbulent mixing. Comparison with filtered
direct numerical simulation data, and with predictions obtained from popular
dynamic eddy-viscosity modelling, shows that these mathematical regularisation
models are considerably more accurate, at a lower computational cost.Comment: 42 pages, 12 figure
Anomalous Scaling of Structure Functions and Dynamic Constraints on Turbulence Simulations
The connection between anomalous scaling of structure functions
(intermittency) and numerical methods for turbulence simulations is discussed.
It is argued that the computational work for direct numerical simulations (DNS)
of fully developed turbulence increases as , and not as
expected from Kolmogorov's theory, where is a large-scale Reynolds number.
Various relations for the moments of acceleration and velocity derivatives are
derived. An infinite set of exact constraints on dynamically consistent subgrid
models for Large Eddy Simulations (LES) is derived from the Navier-Stokes
equations, and some problems of principle associated with existing LES models
are highlighted.Comment: 18 page
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