349 research outputs found
Flame Enhancement and Quenching in Fluid Flows
We perform direct numerical simulations (DNS) of an advected scalar field
which diffuses and reacts according to a nonlinear reaction law. The objective
is to study how the bulk burning rate of the reaction is affected by an imposed
flow. In particular, we are interested in comparing the numerical results with
recently predicted analytical upper and lower bounds. We focus on reaction
enhancement and quenching phenomena for two classes of imposed model flows with
different geometries: periodic shear flow and cellular flow. We are primarily
interested in the fast advection regime. We find that the bulk burning rate v
in a shear flow satisfies v ~ a*U+b where U is the typical flow velocity and a
is a constant depending on the relationship between the oscillation length
scale of the flow and laminar front thickness. For cellular flow, we obtain v ~
U^{1/4}. We also study flame extinction (quenching) for an ignition-type
reaction law and compactly supported initial data for the scalar field. We find
that in a shear flow the flame of the size W can be typically quenched by a
flow with amplitude U ~ alpha*W. The constant alpha depends on the geometry of
the flow and tends to infinity if the flow profile has a plateau larger than a
critical size. In a cellular flow, we find that the advection strength required
for quenching is U ~ W^4 if the cell size is smaller than a critical value.Comment: 14 pages, 20 figures, revtex4, submitted to Combustion Theory and
Modellin
On the existence of traveling waves in the 3D Boussinesq system
We extend earlier work on traveling waves in premixed flames in a
gravitationally stratified medium, subject to the Boussinesq approximation. For
three-dimensional channels not aligned with the gravity direction and under the
Dirichlet boundary conditions in the fluid velocity, it is shown that a
non-planar traveling wave, corresponding to a non-zero reaction, exists, under
an explicit condition relating the geometry of the crossection of the channel
to the magnitude of the Prandtl and Rayleigh numbers, or when the advection
term in the flow equations is neglected.Comment: 15 pages, to appear in Communications in Mathematical Physic
Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities
Photoinduced Kerr rotation by more than radians is demonstrated in
planar quantum well microcavity in the strong coupling regime. This result is
close to the predicted theoretical maximum of . It is achieved by
engineering microcavity parameters such that the optical impedance matching
condition is reached at the smallest negative detuning between exciton
resonance and the cavity mode. This ensures the optimum combination of the
exciton induced optical non-linearity and the enhancement of the Kerr angle by
the cavity. Comprehensive analysis of the polarization state of the light in
this regime shows that both renormalization of the exciton energy and the
saturation of the excitonic resonance contribute to the observed optical
nonlinearities.Comment: Shortened version prepared to submit in Phys. Rev. Letter
Nuclear spin-lattice relaxation in p-type GaAs
Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied
using a three-stage experimental protocol including optical pumping and
measuring the difference of the nuclear spin polarization before and after a
dark interval of variable length. This method allows us to measure the
spin-lattice relaxation time of optically pumped nuclei "in the dark",
that is, in the absence of illumination. The measured values fall into
the sub-second time range, being three orders of magnitude shorter than in
earlier studied n-type GaAs. The drastic difference is further emphasized by
magnetic-field and temperature dependences of in p-GaAs, showing no
similarity to those in n-GaAs. This unexpected behavior is explained within a
developed theoretical model involving quadrupole relaxation of nuclear spins,
which is induced by electric fields within closely spaced donor-acceptor pairs.Comment: 9 pages, 8 figure
Model Flames in the Boussinesq Limit: The Effects of Feedback
We have studied the fully nonlinear behavior of pre-mixed flames in a
gravitationally stratified medium, subject to the Boussinesq approximation. Key
results include the establishment of criterion for when such flames propagate
as simple planar flames; elucidation of scaling laws for the effective flame
speed; and a study of the stability properties of these flames. The simplicity
of some of our scalings results suggests that analytical work may further
advance our understandings of buoyant flames.Comment: 11 pages, 14 figures, RevTex, gzipped tar fil
Value uncaptured perspective for sustainable business model innovation
Sustainability has become one of the key factors for long-term business success. Recent research and practice show that business model innovation is a promising approach for improving sustainability in manufacturing firms. To date business models have been examined mostly from the perspectives of value proposition, value capture, value creation and delivery. There is a need for a more comprehensive understanding of value in order to promote sustainability. This paper proposes value uncaptured as a new perspective for sustainable business model innovation, and develops four forms of value uncaptured: value surplus, value absence, value missed and value destroyed. This paper also proposes a framework of using value uncaptured for sustainable business model innovation. This framework has been validated in case studies in six product-service systems firms with advanced manufacturing technologies. The empirical studies identify 26 main sources of value uncaptured throughout the product life cycle. The findings show that (a) the new perspective of value uncaptured can help manufacturing firms understand the negative aspects of their business models; (b) the four forms and 26 sources can assist firms in identifying value uncaptured in a structured way; and (c) the identified value uncaptured can trigger the discovery of new value opportunities which lead to new business models with higher sustainable value. This paper extends the existing studies on business models from the perspectives of value proposition, capture, creation and delivery, to value uncaptured. The proposed framework has helped firms identify value opportunities that trigger the innovation of sustainable business models. Therefore, this paper contributes to both theory and practice in the field of sustainable business models innovation.This paper builds on the work undertaken on the UK EPSRC Centre for Innovative Manufacturing in Industrial Sustainability (EP/I033351/1), Europe-China High Value Engineering Network (EC-HVEN) (PIRSES-GA-2011-295130), the EPSRC Business Models for Sustainable Industrial Systems (BM4SIS) (EP/L019914/1), and “SustainValue” project funded by the European Community's Seventh Framework Programme (FP7/2007-2013)
Middle Eastern Everyday Life in S.S. Kondurushkin’s Magazine Essays at Beginning of Twentieth Century
The problem of the presentation of the Middle Eastern everyday life in the early prose of S.S. Kondurushkin (1874—1919), an active participant in the literary and social life of Russia at the turn of the XIX—XX centuries is examined in the article. On the basis of the essay cycle “From Wanderings in Syria” and other works of the early 1900s, an analysis of the methods of recreating the Middle Eastern everyday life is presented. The narrative strategies that underlie the Middle Eastern narrative of the Russian traveler are studied. This makes it possible to clarify the characteristics of the genre of the travel sketch in the general context of Russian literature at the turn of the century, which determines the scientific novelty and relevance of the article. The authors examine in detail the essays “Greeks in Palestine and Syria”, “Terra incognita”, “La Bayadere”, “Akulina in Tripoli”. The consistent change of the exposing discourse of political journal-ism to various forms of “ethnographic” narrative are substantiated in the article. The tasks set required the use of traditional methods of academic literary criticism, as well as techniques of cultural linguistics, imagology, imperial and colonial studies. A number of archival materials are introduced into scientific circulation, in particular, letters from N.K. Mikhailovsky addressed to S.S.Kondurushkin
Complexity of dipolar exciton Mott transition in GaN/(AlGa)N nanostructures
The Mott transition from a dipolar excitonic liquid to an electron-hole
plasma is demonstrated in a wide GaN/(Al,Ga)N quantum well at K by means
of spatially-resolved magneto-photoluminescence spectroscopy. Increasing
optical excitation density we drive the system from the excitonic state,
characterized by a diamagnetic behavior and thus a quadratic energy dependence
on the magnetic field, to the unbound electron-hole state, characterized by a
linear shift of the emission energy with the magnetic field. The complexity of
the system requires to take into account both the density-dependence of the
exciton binding energy and the exciton-exciton interaction and correlation
energy that are of the same order of magnitude. We estimate the carrier density
at Mott transition as cm and
address the role played by excitonic correlations in this process. Our results
strongly rely on the spatial resolution of the photoluminescence and the
assessment of the carrier transport. We show, that in contrast to
GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly
quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N
the band parameters are such that the transport is preserved up to T.Comment: 15 pages 13 figure
Orbital dynamics of "smart dust" devices with solar radiation pressure and drag
This paper investigates how perturbations due to asymmetric solar radiation pressure, in the presence of Earth shadow, and atmospheric drag can be balanced to obtain long-lived Earth centred orbits for swarms of micro-scale 'smart dust' devices, without the use of active control. The secular variation of Keplerian elements is expressed analytically through an averaging technique. Families of solutions are then identified where Sun-synchronous apse-line precession is achieved passively to maintain asymmetric solar radiation pressure. The long-term orbit evolution is characterized by librational motion, progressively decaying due to the non-conservative effect of atmospheric drag. Long-lived orbits can then be designed through the interaction of energy gain from asymmetric solar radiation pressure and energy dissipation due to drag. In this way, the usual short drag lifetime of such high area-to-mass spacecraft can be greatly extended (and indeed selected). In addition, the effect of atmospheric drag can be exploited to ensure the rapid end-of-life decay of such devices, thus preventing long-lived orbit debris
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