928 research outputs found
PBC design for voltage regulation in buck converters with parametric uncertainties
This paper addresses the problem of voltage output regulation in DC Buck converters from the passivity-based control (PBC) point of view. The PBC takes advantage of the natural port-Hamiltonian representation of dynamic equations of the buck converter, to design a feedback controller with proportionalintegral gains, that allows to guarantee stability conditions in the sense of Lyapunov for closed-loop operation. The design of the controller is based on the incremental dynamic model of the buck converter. The PBC approach considers unknown resistive loads in the controller design without degrading the dynamic performance of the controller. In addition, the proposed approach allows to design a controller regardless the buck parameters (capacitance and inductance) which makes it robust to parametric uncertainties. Sliding planes and classical PI control methods are used for comparing the proposed PBC method. All simulations have been performed in MATLAB software by using SymPowerSystems library
Model for Glass Transition in a Binary fluid from a Mode Coupling approach
We consider the Mode Coupling Theory (MCT) of Glass transition for a Binary
fluid. The Equations of Nonlinear Fluctuating Hydrodynamics are obtained with a
proper choice of the slow variables corresponding to the conservation laws. The
resulting model equations are solved in the long time limit to locate the
dynamic transition. The transition point from our model is considerably higher
than predicted in existing MCT models for binary systems. This is in agreement
with what is seen in Computer Simulation of binary fluids. fluids.Comment: 9 Pages, 3 Figure
Synthesis and Characterization of Aero-Eutectic Graphite Obtained by Solidification and Its Application in Energy Storage: Cathodes for Lithium Oxygen Batteries
Aero-eutectic graphite can be defined as a new light material with hierarchically structured porosity. It is obtained from the solidification of gray cast irons, followed by the dissolution of the ferrous matrix by an acidic sequence. The result is a continuous and interconnected network of graphite sheets with varied dimensions randomly oriented. X-ray diffraction characterization has revealed graphite crystallographic planes (002), (100), (101), (102) and (004), while the surface area measured by BET and Langmuir methods has been determined in the order of 90 m2 g−1 and 336 m2 g−1, respectively. The process of obtaining eutectic aero-graphite also allows the deposit of Cu nanofilms and TiC particles. Aero-eutectic graphite has been tested as cathode in Li–O2 batteries as it has been prepared, without the addition of binders or conductive carbons, showing an appropriate contact with the electrolyte, so that the oxygen reduction and evolution reactions may develop satisfactorily. In the discharge-charge galvanostatic tests, the battery accomplishes 20 complete cycles with area capacity limited to 1.2 mAh cm−2
The impact of unloading stresses on post-caldera magma intrusions
Calderas represent morphological depressions several kilometers in diameter, and the unloaded crustal stresses they produce can form rapidly (e.g. Pinatubo, 1990) or slowly (e.g. Hawaii, 2018). Active calderas are known as sites of persistent magma intrusions, and yet the dynamics of their shallow plumbing system is not well constrained. We use scaled laboratory experiments to study how experimental intrusions are created beneath a caldera by injecting dyed water (magma analogue) into the base of an elastic gelatin solid (crust analogue) with a cylindrical cavity in its surface to mimic a caldera-like topography. The evolving dike geometry and stress field were qualitatively determined using polarized light, and digital image correlation allowed the incremental and total strain to be quantified by tracking passive-tracer particles in the gelatin that fluoresced in a thin 2D vertical laser sheet. Our results show that the unloaded stress field from a caldera can cause a divergence of vertical dikes, and leads to circumferential dikes and cone sheets. When the caldera was large the initially vertical dike became arrested, then grew laterally via circumferentially-propagating en echelon segments; these eventually joined to complete a cone sheet that was parallel to, but extended outside and beneath, the large caldera. When the caldera was small, a circumferential dike erupted, producing a short fissure which was outside, but parallel to, the caldera. We suggest that the distinct curved geometry, velocity, strain and stress characteristics of circumferential dikes and cone sheets can be used to interpret the origin and growth of post-caldera magmatism and the likelihood of eruption in caldera systems
The physical activity and health status of two generations of Black South African professional women
Increased health risks associated with physical inactivity in the Black population have been reported in recent years. Black women, suffering the highest levels of inactivity, overweight and obesity, are at greatest risk of developing chronic diseases of lifestyle. This explorativedescriptive study investigated the physical activity patterns and health status of two generations of Black professional women, reflecting pre-democracy and post-democracy age groups. Quantitative measures were used, including the ActiGraph GT1M accelerometer, the Global Physical Activity Questionnaire and the Health-Promoting Lifestyle Profile. Sample groups comprised teachers, nurses, social workers and public sector managers. Participants aged between 35 and 45 years were allocated to the older generation group (n = 111), whilst those aged between 18 and 21 years (students in the mentioned professional fields) were allocated to the younger generation group (n = 69). The results indicated that these women displayed lower levels of health-promoting behavioural practices than expected, significantly lower levels of physical activity and significantly higher levels of overweight and obesity than the South African norms. The observation that the younger group appeared to be replicating the patterns of the older women is a cause of concern. Greater compliance to health-promoting behaviours was expected in this group owing to participants’ professional involvement in health, education and social development fields. Wide-ranging initiatives are necessary to promote physical activity and health amongst the Black female population in South Africa.
Opsomming
Gedurende die afgelope jare het navorsing onder die Swart bevolking ʼn toename in gesondheidsrisiko’s wat met fisieke onaktiwiteit geassosieer is, getoon. Swart vroue, wat die hoogste vlakke van onaktiwiteit, oorgewig en obesiteit toon, blyk ook die grootste risiko te loop om leefstylverwante chroniese siektes te ontwikkel. Hierdie ondersoekendbeskrywende studie het die fisieke aktiwiteitspatrone en gesondheidstatus van twee generasies van Swart professionele vroue ondersoek. Die tweegenerasiesteekproef verteenwoordig voor- en na-demokrasie-ouderdomsgroepe. Kwantitatiewe meetinstrumente, naamlik die ActiGraph GT1M versnellingsmeter, Globale Fisieke Aktiwiteitsvraelys en die Gesondheidsbevorderende Leefstylprofiel is gebruik. Die steekproef het onderwysers, verpleegkundiges, maatskaplike werkers en bestuurders in die openbare sektor ingesluit. Deelnemers tussen 35 en 45 jaar is aan die ouergenerasie-toetsgroep toegewys (n = 111), terwyl dié tussen 18 en 21 jaar (studente in bogenoemde studierigtings) aan die jongergenerasietoetsgroep toegewys is (n = 69). Die resultate het gewys dat die Swart professionele vroue laer as verwagte gesondheidsbevorderende gedrag, betekenisvol laer vlakke van fisieke aktiwiteit en betekenisvol hoër vlakke van oorgewig en obesiteit toon as die Suid-Afrikaanse norme. Die waarneming dat die jonger groep dieselfde gedragspatrone as die ouer vroue getoon het, is ʼn bron van kommer. ʼn Groter nakoming van gesondheidsbevorderende gedrag is onder die jonger groep vroue verwag, aangesien hulle hul opleiding in die professionele sektor van gesondheid, onderwys en maatskaplike werk ontvang. ʼn Holistiese benadering is nodig om fisieke aktiwiteit en gesondheid onder Swart Suid-Afrikaanse vrouens te bevorder
Equivariant geometric K-homology for compact Lie group actions
Let G be a compact Lie-group, X a compact G-CW-complex. We define equivariant
geometric K-homology groups K^G_*(X), using an obvious equivariant version of
the (M,E,f)-picture of Baum-Douglas for K-homology. We define explicit natural
transformations to and from equivariant K-homology defined via KK-theory (the
"official" equivariant K-homology groups) and show that these are isomorphism.Comment: 25 pages. v2: some mistakes corrected, more detail added, Michael
Walter as author added. To appear in Abhandlungen aus dem Mathematischen
Seminar der Universit\"at Hambur
Light propagation in non-trivial QED vacua
Within the framework of effective action QED, we derive the light cone
condition for homogeneous non-trivial QED vacua in the geometric optics
approximation. Our result generalizes the ``unified formula'' suggested by
Latorre, Pascual and Tarrach and allows for the calculation of velocity shifts
and refractive indices for soft photons travelling through these vacua.
Furthermore, we clarify the connection between the light velocity shift and the
scale anomaly. This study motivates the introduction of a so-called effective
action charge that characterizes the velocity modifying properties of the
vacuum. Several applications are given concerning vacuum modifications caused
by, e.g., strong fields, Casimir systems and high temperature.Comment: 13 pages, REVTeX, 3 figures, to appear in Phys. Rev.
Phylogenetic lineages in the Botryosphaeriaceae
Botryosphaeria is a species-rich genus with a cosmopolitan
distribution, commonly associated with dieback and cankers of woody plants. As
many as 18 anamorph genera have been associated with Botryosphaeria,
most of which have been reduced to synonymy under Diplodia (conidia
mostly ovoid, pigmented, thick-walled), or Fusicoccum (conidia mostly
fusoid, hyaline, thin-walled). However, there are numerous conidial anamorphs
having morphological characteristics intermediate between Diplodia
and Fusicoccum, and there are several records of species outside the
Botryosphaeriaceae that have anamorphs apparently typical of
Botryosphaeria s.str. Recent studies have also linked
Botryosphaeria to species with pigmented, septate ascospores, and
Dothiorella anamorphs, or Fusicoccum anamorphs with
Dichomera synanamorphs. The aim of this study was to employ DNA
sequence data of the 28S rDNA to resolve apparent lineages within the
Botryosphaeriaceae. From these data, 12 clades are recognised. Two of
these lineages clustered outside the Botryosphaeriaceae, namely
Diplodia-like anamorphs occurring on maize, which are best
accommodated in Stenocarpella (Diaporthales), as well as an
unresolved clade including species of
Camarosporium/Microdiplodia. We recognise 10 lineages within
the Botryosphaeriaceae, including an unresolved clade
(Diplodia/Lasiodiplodia/Tiarosporella),
Botryosphaeria s.str. (Fusicoccum anamorphs),
Macrophomina, Neoscytalidium gen. nov.,
Dothidotthia (Dothiorella anamorphs), Neofusicoccum
gen. nov. (Botryosphaeria-like teleomorphs, Dichomera-like
synanamorphs), Pseudofusicoccum gen. nov., Saccharata
(Fusicoccum- and Diplodia-like synanamorphs),
“Botryosphaeria” quercuum
(Diplodia-like anamorph), and Guignardia
(Phyllosticta anamorphs). Separate teleomorph and anamorph names are
not provided for newly introduced genera, even where both morphs are known.
The taxonomy of some clades and isolates (e.g. B. mamane) remains
unresolved due to the absence of ex-type cultures
Approach to equilibrium for a class of random quantum models of infinite range
We consider random generalizations of a quantum model of infinite range
introduced by Emch and Radin. The generalization allows a neat extension from
the class of absolutely summable lattice potentials to the optimal class
of square summable potentials first considered by Khanin and Sinai and
generalised by van Enter and van Hemmen. The approach to equilibrium in the
case of a Gaussian distribution is proved to be faster than for a Bernoulli
distribution for both short-range and long-range lattice potentials. While
exponential decay to equilibrium is excluded in the nonrandom case, it is
proved to occur for both short and long range potentials for Gaussian
distributions, and for potentials of class in the Bernoulli case. Open
problems are discussed.Comment: 10 pages, no figures. This last version, to appear in J. Stat. Phys.,
corrects some minor errors and includes additional references and comments on
the relation to experiment
Entire solutions of hydrodynamical equations with exponential dissipation
We consider a modification of the three-dimensional Navier--Stokes equations
and other hydrodynamical evolution equations with space-periodic initial
conditions in which the usual Laplacian of the dissipation operator is replaced
by an operator whose Fourier symbol grows exponentially as \ue ^{|k|/\kd} at
high wavenumbers . Using estimates in suitable classes of analytic
functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients
decay faster than \ue ^{-C(k/\kd) \ln (|k|/\kd)} for any . The
same result holds for the one-dimensional Burgers equation with exponential
dissipation but can be improved: heuristic arguments and very precise
simulations, analyzed by the method of asymptotic extrapolation of van der
Hoeven, indicate that the leading-order asymptotics is precisely of the above
form with . The same behavior with a universal constant
is conjectured for the Navier--Stokes equations with exponential
dissipation in any space dimension. This universality prevents the strong
growth of intermittency in the far dissipation range which is obtained for
ordinary Navier--Stokes turbulence. Possible applications to improved spectral
simulations are briefly discussed.Comment: 29 pages, 3 figures, Comm. Math. Phys., in pres
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