2,631 research outputs found
Non-Oberbeck-Boussinesq effects in two-dimensional Rayleigh-Benard convection in glycerol
We numerically analyze Non-Oberbeck-Boussinesq (NOB) effects in
two-dimensional Rayleigh-Benard flow in glycerol, which shows a dramatic change
in the viscosity with temperature. The results are presented both as functions
of the Rayleigh number (Ra) up to (for fixed temperature difference
between the top and bottom plates) and as functions of
"non-Oberbeck-Boussinesqness'' or "NOBness'' () up to 50 K (for fixed
Ra). For this large NOBness the center temperature is more than 5 K
larger than the arithmetic mean temperature between top and bottom plate
and only weakly depends on Ra. To physically account for the NOB deviations of
the Nusselt numbers from its Oberbeck-Boussinesq values, we apply the
decomposition of into the product of two effects, namely
first the change in the sum of the top and bottom thermal BL thicknesses, and
second the shift of the center temperature as compared to . While
for water the origin of the deviation is totally dominated by the second
effect (cf. Ahlers et al., J. Fluid Mech. 569, pp. 409 (2006)) for glycerol the
first effect is dominating, in spite of the large increase of as compared
to .Comment: 6 pages, 7 figure
Extended phase diagram of the Lorenz model
The parameter dependence of the various attractive solutions of the three
variable nonlinear Lorenz model equations for thermal convection in
Rayleigh-B\'enard flow is studied. Its bifurcation structure has commonly been
investigated as a function of r, the normalized Rayleigh number, at fixed
Prandtl number \sigma. The present work extends the analysis to the entire
(r,\sigma) parameter plane. An onion like periodic pattern is found which is
due to the alternating stability of symmetric and non-symmetric periodic
orbits. This periodic pattern is explained by considering non-trivial limits of
large r and \sigma. In addition to the limit which was previously analyzed by
Sparrow, we identify two more distinct asymptotic regimes in which either
\sigma/r or \sigma^2/r is constant. In both limits the dynamics is
approximately described by Airy functions whence the periodicity in parameter
space can be calculated analytically. Furthermore, some observations about
sequences of bifurcations and coexistence of attractors, periodic as well as
chaotic, are reported.Comment: 36 pages, 20 figure
Classification of phase transitions of finite Bose-Einstein condensates in power law traps by Fisher zeros
We present a detailed description of a classification scheme for phase
transitions in finite systems based on the distribution of Fisher zeros of the
canonical partition function in the complex temperature plane. We apply this
scheme to finite Bose-systems in power law traps within a semi-analytic
approach with a continuous one-particle density of states for different values of and to a three dimensional harmonically
confined ideal Bose-gas with discrete energy levels. Our results indicate that
the order of the Bose-Einstein condensation phase transition sensitively
depends on the confining potential.Comment: 7 pages, 9 eps-figures, For recent information on physics of small
systems see "http://www.smallsystems.de
Visualization of Coherent Destruction of Tunneling in an Optical Double Well System
We report on a direct visualization of coherent destruction of tunneling
(CDT) of light waves in a double well system which provides an optical analog
of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and Hanggi
[Phys. Rev. Lett. {\bf 67}, 516 (1991)]. The driven double well, realized by
two periodically-curved waveguides in an Er:Yb-doped glass, is designed so that
spatial light propagation exactly mimics the coherent space-time dynamics of
matter waves in a driven double-well potential governed by the Schr\"{o}dinger
equation. The fluorescence of Er ions is exploited to image the spatial
evolution of light in the two wells, clearly demonstrating suppression of light
tunneling for special ratios between frequency and amplitude of the driving
field.Comment: final versio
Fluxtube model atmospheres and Stokes V zero-crossing wavelengths
First results of the inversion of Stokes I and V profiles from plage regions
near disk center are presented. Both low and high spatial resolution spectra of
FeI 6301.5 and FeI 6302.5 A obtained with the Advanced Stokes Polarimeter (ASP)
have been considered for analysis. The thin flux tube approximation,
implemented in an LTE inversion code based on response functions, is used to
describe unresolved magnetic elements. The code allows the simultaneous and
consistent inference of all atmospheric quantities determining the radiative
transfer with the sole assumption of hydrostatic equilibrium. By considering
velocity gradients within the tubes we are able to match the full ASP Stokes
profiles. The magnetic atmospheres derived from the inversion are characterized
by the absence of significant motions in high layers and strong velocity
gradients in deeper layers. These are essential to reproduce the asymmetries of
the observed profiles. Our scenario predicts a shift of the Stokes V
zero-crossing wavelengths which is indeed present in observations made with the
Fourier Transform Spectrometer.Comment: To appear in ApJ Letters (1997) (in press
How the Concept of Dignity Is Relevant to the Study of Energy Poverty and Energy Justice
Since the concept of energy poverty first emerged, studies have combined normative orientations, analytical approaches and policy review to engage with energy deprivation as a problematic feature of contemporary societies. Over the past decade, this scholarship has aimed to conceptualize the normative grounds for critique, empirical work and policy design when engaging with the interplay of social life and energy systems. Scholars now include dynamic and complex concepts such as energy vulnerability and energy deprivation and are shifting toward the incorporation of social-philosophical justice concepts. However, in most of these writings on energy equality or energy justice, material aspects like access to (clean) energy, affordable energy costs, and material deprivation are in the foreground. This resonates with the energy poverty literature's emphasis on energy poverty as a material deprivation (Longhurst and Hargreaves, 2019). The way that energy poverty can result in financial stress, cold homes, poor health and the need to cut other basic expenditures is well-explored, but the less tangible, non-material deprivations stemming from energy poverty are less well-captured. We instead find it beneficial to also focus on the less tangible, non-material deprivations which have not yet been captured conceptually, and argue that the concept of dignity can be a pathway to investigate them. We aim to demonstrate how “dignity” can add to the normative orientations of energy poverty and energy justice research, and complement existing frames. With an empirical position in Europe we will draw from own empirical data and existing literature to illustrate how households living in energy poverty, or being cut off from energy provision, experience dignity violations. Copyright © 2021 Grossmann and Trubina.The paper stems - among others - from collaborative work within COST ActionEuropean Energy Poverty: Agenda Co-Creation and Knowledge Innovation (ENGAGER, 2017-2021, CA16232) funded by European Cooperation in Science and Technology-www.cost.eu. The authors also wish to acknowledge the help of Leona Sandmann, Robert Franke, and Helene Oettel with empirical work. We thank Ute Dubois and Malgorzata Dereniowska for allowing us to use individual quotes from interviews they conducted
Variable Curvature Slab Molecular Dynamics as a Method to Determine Surface Stress
A thin plate or slab, prepared so that opposite faces have different surface
stresses, will bend as a result of the stress difference. We have developed a
classical molecular dynamics (MD) formulation where (similar in spirit to
constant-pressure MD) the curvature of the slab enters as an additional
dynamical degree of freedom. The equations of motion of the atoms have been
modified according to a variable metric, and an additional equation of motion
for the curvature is introduced. We demonstrate the method to Au surfaces, both
clean and covered with Pb adsorbates, using many-body glue potentials.
Applications to stepped surfaces, deconstruction and other surface phenomena
are under study.Comment: 16 pages, 8 figures, REVTeX, submitted to Physical Review
Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles
The dynamics of electrons in ac driven double quantum dots have been
extensively analyzed by means of Floquet theory. In these systems, coherent
destruction of tunneling has been shown to occur for certain ac field
parameters. In the present work we analyze, by means of Floquet theory, the
electron dynamics of a triple quantum dot in series attached to electric
contacts, where the central dot position oscillates. In particular, we analyze
the quasienergy spectrum of this ac driven nanoelectromechanical system, as a
function of the intensity and frequency of the ac field and of external dc
voltages. For strong driving fields, we derive, by means of perturbation
theory, analytical expressions for the quasienergies of the driven oscillator
system. From this analysis we discuss the conditions for coherent destruction
of tunneling (CDT) to occur as a function of detuning and field parameters. For
zero detuning, and from the invariance of the Floquet Hamiltonian under a
generalized parity transformation, we find analytical expressions describing
the symmetry properties of the Fourier components of the Floquet states under
such transformation. By using these expressions, we show that in the vicinity
of the CDT condition, the quasienergy spectrum exhibits exact crossings which
can be characterized by the parity properties of the corresponding
eigenvectors
Confinement in the Deconfined Phase: A numerical study with a cluster algorithm
We have previously found analytically a very unusual and unexpected form of
confinement in SU(3) Yang-Mills theory. This confinement occurs in the
deconfined phase of the theory. The free energy of a single static test quark
diverges, even though it is contained in deconfined bulk phase and there is no
QCD string present. This phenomenon occurs in cylindrical volumes with a
certain choice of spatial boundary conditions. We examine numerically an
effective model for the Yang-Mills theory and, using a cluster algorithm, we
observe this unusual confinement. We also find a new way to determine the
interface tension of domain walls separating distinct bulk phases.Comment: LaTex, 14 pages, 4 figure
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