2,128 research outputs found
Design and optimization of electrochemical microreactors for continuous electrosynthesis
The study focuses on the design and construction, as well as the theoretical and experimental optimization of electrochemical filter press microreactors for the electrosynthesis of molecules with a high added value. The main characteristics of these devices are firstly a high-specific electrochemical area to increase conversion and selectivity, and secondly the shape and size of themicrochannels designed for a uniform residence time distribution of the fluid. A heat exchanger is integrated into the microstructured electrode to rapidly remove (or supply) the heat required in exo- or endothermic reactions. The microreactors designed are used to perform-specific electrosynthesis reactions such as thermodynamically unfavorable reactions (continuous NADH regeneration), or reactions with high enthalpy changes
Games on graphs with a public signal monitoring
We study pure Nash equilibria in games on graphs with an imperfect monitoring
based on a public signal. In such games, deviations and players responsible for
those deviations can be hard to detect and track. We propose a generic
epistemic game abstraction, which conveniently allows to represent the
knowledge of the players about these deviations, and give a characterization of
Nash equilibria in terms of winning strategies in the abstraction. We then use
the abstraction to develop algorithms for some payoff functions.Comment: 28 page
Implications of Cosmological Gamma-Ray Absorption II. Modification of gamma-ray spectra
Bearing on the model for the time-dependent metagalactic radiation field
developed in the first paper of this series, we compute the gamma-ray
attenuation due to pair production in photon-photon scattering. Emphasis is on
the effects of varying the star formation rate and the fraction of UV radiation
assumed to escape from the star forming regions, the latter being important
mainly for high-redshift sources. Conversely, we investigate how the
metagalactic radiation field can be measured from the gamma-ray pair creation
cutoff as a function of redshift, the Fazio-Stecker relation. For three
observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of
gamma-ray attenuation on their spectra in detail.Comment: 10 pages, 6 figures, accepted by A&
Constrained Dynamics of Universally Coupled Massive Spin 2-spin 0 Gravities
The 2-parameter family of massive variants of Einstein's gravity (on a
Minkowski background) found by Ogievetsky and Polubarinov by excluding lower
spins can also be derived using universal coupling. A Dirac-Bergmann
constrained dynamics analysis seems not to have been presented for these
theories, the Freund-Maheshwari-Schonberg special case, or any other massive
gravity beyond the linear level treated by Marzban, Whiting and van Dam. Here
the Dirac-Bergmann apparatus is applied to these theories. A few remarks are
made on the question of positive energy. Being bimetric, massive gravities have
a causality puzzle, but it appears soluble by the introduction and judicious
use of gauge freedom.Comment: 6 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Wind-Current Feedback Is an Energy Sink For Oceanic Internal Waves
Internal waves contain a large amount of energy in the ocean and are an important source of turbulent mixing. Ocean mixing is relevant for climate because it drives vertical transport of water, heat, carbon and other tracers. Understanding the life cycle of internal waves, from generation to dissipation, is therefore important for improving the representation of ocean mixing in climate models. Here, we provide evidence from a regional realistic numerical simulation in the northeastern Pacific that the wind can play an important role in damping internal waves through current feedback. This results in a reduction of 67% of wind power input at near-inertial frequencies in the region of study. Wind-current feedback also provides a net energy sink for internal tides, removing energy at a rate of 0.2 mW/m2 on average, corresponding to 8% of the local internal tide generation at the Mendocino ridge. The temporal variability and modal distribution of this energy sink are also investigated
Type I D-branes in an H-flux and twisted KO-theory
Witten has argued that charges of Type I D-branes in the presence of an
H-flux, take values in twisted KO-theory. We begin with the study of real
bundle gerbes and their holonomy. We then introduce the notion of real bundle
gerbe KO-theory which we establish is a geometric realization of twisted
KO-theory. We examine the relation with twisted K-theory, the Chern character
and provide some examples. We conclude with some open problems.Comment: 23 pages, Latex2e, 2 new references adde
Primordial Black Hole Formation during First-Order Phase Transitions
Primordial black holes (PBHs) may form in the early universe when
pre-existing adiabatic density fluctuations enter into the cosmological horizon
and recollapse. It has been suggested that PBH formation may be facilitated
when fluctuations enter into the horizon during a strongly first-order phase
transition which proceeds in approximate equilibrium. We employ
general-relativistic hydrodynamics numerical simulations in order to follow the
collapse of density fluctuations during first-order phase transitions. We find
that during late stages of the collapse fluctuations separate into two regimes,
an inner part existing exclusively in the high-energy density phase with energy
density , surrounded by an outer part which exists
exclusively in the low-energy density phase with energy density , where is the latent heat of the transition. We confirm that the
fluctuation density threshold required for the
formation of PBHs during first-order transitions decreases with increasing
and falls below that for PBH formation during ordinary radiation dominated
epochs. Our results imply that, in case PBHs form at all in the early universe,
their mass spectrum is likely dominated by the approximate horizon masses
during epochs when the universe undergoes phase transitions.Comment: 8 pages, 4 figures, revtex style, submitted to PR
First Astronomical Use of Multiplexed Transition Edge Bolometers
We present performance results based on the first astronomical use of multiplexed superconducting bolometers. The Fabry-Perot Interferometer Bolometer Research Experiment
(FIBRE) is a broadband submillimeter spectrometer that achieved first light in June 2001 at the Caltech Submillimeter Observatory (CSO). FIBRE'S detectors are superconducting transition edge sensor (TES) bolometers read out by a SQUID multiplexer. The Fabry-Perot uses a low
resolution grating to order sort the incoming light. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing 5 orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering Δλ/λ= 1/7 at a resolution of δλ/λ ≈ 1/1200 can be acquired. This spectral resolution is sufficient to resolve
Doppler-broadened line emission from external galaxies. FIBRE operates in the 350 µm and 450 µm bands. These bands cover line emission from the important star formation tracers neutral
carbon [Cl] and carbon monoxide (CO). We have verified that the multiplexed bolometers are
photon noise limited even with the low power present in moderate resolution spectrometry
Remote Internal Wave Forcing of Regional Ocean Simulations Near the U.S. West Coast
Low mode internal waves are able to propagate across ocean basins and modulate ocean dynamics thousands of kilometers away from their generation sites. In this study, the impact of remotely generated internal waves on the internal wave energetics near the U.S. West Coast is investigated with realistically forced regional ocean simulations. At the open boundaries, we impose high-frequency oceanic state variables obtained from a global ocean simulation with realistic atmospheric and astronomical tidal forcing. We use the Discrete Fourier Transform (DFT) technique in separating ingoing and outgoing internal tide energy fluxes at the open boundaries in order to quantify internal tide reflections. Although internal tide reflections are reduced with increasing sponge viscosity and/or sponge layer width, reflection coefficients (λ) can be as high as 73%. In the presence of remote internal waves, the model variance and spatial correlations become more in agreement with both mooring and altimetry datasets. The results confirm that an improved internal wave continuum can be achieved in regional models with remote internal wave forcing at the open boundaries. However, care should be taken to avoid excessive reflections of internal waves from the interior at these boundaries
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