8,578 research outputs found
Thorpe method applied to planetary boundary layer data
Turbulence affects the dynamics of atmospheric processes by enhancing the transport of mass, heat, humidity and pollutants. The global objective of our work is to analyze some direct turbulent descriptors which reflect the mixing
processes in the atmospheric boundary layer (ABL). In this paper we present results related to the Thorpe displacements dT , the maximum Thorpe displacement (dT )max and the Thorpe scale LT , the Ozmidov scale and their time evolution in the ABL during a day cycle. A tethered balloon was used to obtain vertical profiles of the atmospheric physical magnitudes up to 1000m. We discuss the vertical
and horizontal variability and how different descriptors are related to atmospheric mixing
Graph Theory Data for Topological Quantum Chemistry
Topological phases of noninteracting particles are distinguished by global
properties of their band structure and eigenfunctions in momentum space. On the
other hand, group theory as conventionally applied to solid-state physics
focuses only on properties which are local (at high symmetry points, lines, and
planes) in the Brillouin zone. To bridge this gap, we have previously [B.
Bradlyn et al., Nature 547, 298--305 (2017)] mapped the problem of constructing
global band structures out of local data to a graph construction problem. In
this paper, we provide the explicit data and formulate the necessary algorithms
to produce all topologically distinct graphs. Furthermore, we show how to apply
these algorithms to certain "elementary" band structures highlighted in the
aforementioned reference, and so identified and tabulated all orbital types and
lattices that can give rise to topologically disconnected band structures.
Finally, we show how to use the newly developed BANDREP program on the Bilbao
Crystallographic Server to access the results of our computation.Comment: v1: 29 Pages, 13 Figures. Explains how to access the data presented
in arXiv:1703.02050 v2: Accepted version. References updated, figures
improve
Filtered deterministic waves and analysis of the fractal dimension of the components of the wind velocity
The difficulty in developing models for waves in turbulent flows is a key problem in the analysis of the complexity of turbulence. We present a method to find and filter perturbations that are generated by the flow of deterministic waves from the power spectrum in the atmospheric boundary layer (ABL). The perturbation model proposed assumes that the amplitude and frequency of such waves decay with time exponentially. For illustrative purposes, we apply the technique to three time series of wind velocities obtained with a sonic anemometer. This analytical procedure allows us to filter waves of the proposed structure with a 99% significance level in the power spectrum. We have applied the same method to 540 such wind series, all painting similar results. We then compare the fractal dimension of the original series to those from which the waves have been removed. We find that the fractal dimension of the filtered waves is slightly less than that of the original series. Finally, we consider the fractal dimension of the studied series as a function of the
length-scales and dissipation rate of kinetic energy per unit mass. Our results suggest an increase of fractal dimension with both length-scale and dissipation rate of kinetic energy
Building Blocks of Topological Quantum Chemistry: Elementary Band Representations
The link between chemical orbitals described by local degrees of freedom and
band theory, which is defined in momentum space, was proposed by Zak several
decades ago for spinless systems with and without time-reversal in his theory
of "elementary" band representations. In Nature 547, 298-305 (2017), we
introduced the generalization of this theory to the experimentally relevant
situation of spin-orbit coupled systems with time-reversal symmetry and proved
that all bands that do not transform as band representations are topological.
Here, we give the full details of this construction. We prove that elementary
band representations are either connected as bands in the Brillouin zone and
are described by localized Wannier orbitals respecting the symmetries of the
lattice (including time-reversal when applicable), or, if disconnected,
describe topological insulators. We then show how to generate a band
representation from a particular Wyckoff position and determine which Wyckoff
positions generate elementary band representations for all space groups. This
theory applies to spinful and spinless systems, in all dimensions, with and
without time reversal. We introduce a homotopic notion of equivalence and show
that it results in a finer classification of topological phases than approaches
based only on the symmetry of wavefunctions at special points in the Brillouin
zone. Utilizing a mapping of the band connectivity into a graph theory problem,
which we introduced in Nature 547, 298-305 (2017), we show in companion papers
which Wyckoff positions can generate disconnected elementary band
representations, furnishing a natural avenue for a systematic materials search.Comment: 15+9 pages, 4 figures; v2: minor corrections; v3: updated references
(published version
Band Connectivity for Topological Quantum Chemistry: Band Structures As A Graph Theory Problem
The conventional theory of solids is well suited to describing band
structures locally near isolated points in momentum space, but struggles to
capture the full, global picture necessary for understanding topological
phenomena. In part of a recent paper [B. Bradlyn et al., Nature 547, 298
(2017)], we have introduced the way to overcome this difficulty by formulating
the problem of sewing together many disconnected local "k-dot-p" band
structures across the Brillouin zone in terms of graph theory. In the current
manuscript we give the details of our full theoretical construction. We show
that crystal symmetries strongly constrain the allowed connectivities of energy
bands, and we employ graph-theoretic techniques such as graph connectivity to
enumerate all the solutions to these constraints. The tools of graph theory
allow us to identify disconnected groups of bands in these solutions, and so
identify topologically distinct insulating phases.Comment: 19 pages. Companion paper to arXiv:1703.02050 and arXiv:1706.08529
v2: Accepted version, minor typos corrected and references added. Now
19+epsilon page
Topological quantum chemistry
The past decade's apparent success in predicting and experimentally
discovering distinct classes of topological insulators (TIs) and semimetals
masks a fundamental shortcoming: out of 200,000 stoichiometric compounds extant
in material databases, only several hundred of them are topologically
nontrivial. Are TIs that esoteric, or does this reflect a fundamental problem
with the current piecemeal approach to finding them? To address this, we
propose a new and complete electronic band theory that highlights the link
between topology and local chemical bonding, and combines this with the
conventional band theory of electrons. Topological Quantum Chemistry is a
description of the universal global properties of all possible band structures
and materials, comprised of a graph theoretical description of momentum space
and a dual group theoretical description in real space. We classify the
possible band structures for all 230 crystal symmetry groups that arise from
local atomic orbitals, and show which are topologically nontrivial. We show how
our topological band theory sheds new light on known TIs, and demonstrate the
power of our method to predict a plethora of new TIs.Comment: v1: 8 pages + 40 pages supplemenetary material. Previously submitted
v2: ~ Published version. 11 pages + 79 pages supplementary material.
Descriptions of the data used in this paper can be found in arXiv:1706.08529
and arXiv:1706.09272. All data can be accessed via the Bilbao
Crystallographic Server (http://cryst.ehu.es). Two additional papers
elaborating on the general theory currently in pre
Package âSixSigmaâ
This package contains functions and utilities to performStatistical Analyses in the Six Sigma way. Through the DMAICcycle (Define, Measure, Analyze, Improve, Control), you can
manage several Quality Management studies: Gage R&R, CapabilityAnalysis, Control Charts, Loss Function Analysis, etc. Dataframes used in the book ââSix Sigma with Rââ (Springer, 2012) are also included in the package
An experimental model of mixing processes generated by an array of top-heavy turbulent plumes
The mixing process of two fluids of unequal density generated by the evolution of an array of forced turbulent plumes is studied in the laboratory. The corresponding qualitative conclusions and the quantitative results based on measures of the density field and of the height of the fluid layers are described. The partial mixing process is characterized and analyzed, and the conclusions of this analysis are related to the mixing efficiency and the volume of the final mixed layer as functions of the Atwood number, which ranges from 0.010 to 0.134. An exponential fit is used to evaluate the mixing efficiency versus the Atwood showing the role of initial conditions on mixing efficiency variability
Anaerobic Thermophilic Colonization of Porous Support
Biofilm development in an open-pore sintered glass material (SIRAN) was studied using a laboratory-scale anaerobic fixed-film bioreactor under thermophilic conditions.
The startup and performance of that bioreactor, operating on distillery waste water feed (vinasses), were also studied.
Results obtained indicated that stepped organic loading during initial bioreactor start-up reduced the periods of adaptation in the colonization process and micro-organism attachment and biofilm formation was accelerated by the surface characteristics of
the carrier. The results obtained by operating with stepped organic loading (& = 2.0 kg mâ3 dâ1 COD) over a period of 70 days suggest that a stable operation of the process (90% COD removal) and high density of biomass immobilized on the support (20 kgVSatt per m3 SIRAN) is achieved. Epifluorescence microscopy demonstrated that, initially, attached growth was developed in crevices, where biomass was protected from shear forces and, finally, SIRAN was completely covered and biofilm developed on the entire SIRAN particles
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