141,217 research outputs found
Combustion instabilities: mating dance of chemical, combustion, and combustor dynamics
Combustion instabilities exist as consequences of
interactions among three classes of phenomena: chemistry and chemical dynamics; combustion dynamics; and combustor dynamics. These dynamical processes take place simultaneously in widely different spatial scales characterized by lengths roughly in the ratios (10^(-3)
- 10^(-6)):1:(10^3-10^6). However, due to the wide differences in the associated characteristic velocities, the corresponding time scales are all close. The instabilities in question are observed as oscillations having a time scale in the range of natural acoustic oscillations. The apparent dominance of that single macroscopic time scale must not be permitted to obscure the fact that the relevant physical processes occur on three disparate length scales. Hence, understanding combustion instabilities at the practical level of design and successful operation is ultimately based on understanding three distinct sorts of dynamics
Imaging With Nature: Compressive Imaging Using a Multiply Scattering Medium
The recent theory of compressive sensing leverages upon the structure of
signals to acquire them with much fewer measurements than was previously
thought necessary, and certainly well below the traditional Nyquist-Shannon
sampling rate. However, most implementations developed to take advantage of
this framework revolve around controlling the measurements with carefully
engineered material or acquisition sequences. Instead, we use the natural
randomness of wave propagation through multiply scattering media as an optimal
and instantaneous compressive imaging mechanism. Waves reflected from an object
are detected after propagation through a well-characterized complex medium.
Each local measurement thus contains global information about the object,
yielding a purely analog compressive sensing method. We experimentally
demonstrate the effectiveness of the proposed approach for optical imaging by
using a 300-micrometer thick layer of white paint as the compressive imaging
device. Scattering media are thus promising candidates for designing efficient
and compact compressive imagers.Comment: 17 pages, 8 figure
Convolutional neural networks: a magic bullet for gravitational-wave detection?
In the last few years, machine learning techniques, in particular
convolutional neural networks, have been investigated as a method to replace or
complement traditional matched filtering techniques that are used to detect the
gravitational-wave signature of merging black holes. However, to date, these
methods have not yet been successfully applied to the analysis of long
stretches of data recorded by the Advanced LIGO and Virgo gravitational-wave
observatories. In this work, we critically examine the use of convolutional
neural networks as a tool to search for merging black holes. We identify the
strengths and limitations of this approach, highlight some common pitfalls in
translating between machine learning and gravitational-wave astronomy, and
discuss the interdisciplinary challenges. In particular, we explain in detail
why convolutional neural networks alone cannot be used to claim a statistically
significant gravitational-wave detection. However, we demonstrate how they can
still be used to rapidly flag the times of potential signals in the data for a
more detailed follow-up. Our convolutional neural network architecture as well
as the proposed performance metrics are better suited for this task than a
standard binary classifications scheme. A detailed evaluation of our approach
on Advanced LIGO data demonstrates the potential of such systems as trigger
generators. Finally, we sound a note of caution by constructing adversarial
examples, which showcase interesting "failure modes" of our model, where inputs
with no visible resemblance to real gravitational-wave signals are identified
as such by the network with high confidence.Comment: First two authors contributed equally; appeared at Phys. Rev.
Approximate analysis and stability of pressure oscillations in ramjets
This paper summarizes work accomplished during the past five years on analysis of stability related to
recent experimental results on combustion instabilities in dump combustors. The primary purpose is to provide
the information in a form useful to those concerned with design and development of operational systems. Thus
most substantial details are omitted; the material is presented in a qualitative fashion
Modeling for Active Control of Combustion and Thermally Driven Oscillations
Organized oscillations excited and sustained by high densities of energy release in combustion chambers have long caused serious problems in development of propulsion systems. The amplitudes often become sufficiently large to cause unacceptable structural vibrations. Because the oscillations are self-excited, they reach limiting amplitudes (limit cycles) only because of the action of nonlinear processes. Traditionally, satisfactory behavior
has been achieved through a combination of trial-and-error
design and testing, with control always involving passive means: geometrical modifications, changes of propellant composition, or devices to enhance dissipation of acoustic energy. Active control has been applied only to small-scale laboratory devices, but the limited success suggests the possibility of serious applications to full-scale propulsion systems. Realization of that potential rests on further experimental work, combined with deeper understanding of the mechanisms causing the oscillations and of the physical behavior of the systems. Effective design of active control systems will require faithful modeling of the relevant processes over broad frequency ranges covering the spectra of natural modes. This paper will cover the general character of the linear and nonlinear behavior of combustion systems, with special attention to acoustics and the mechanisms of excitation.
The discussion is intended to supplement the paper by Doyle et al. concerned primarily with controls issues and the observed behavior of simple laboratory devices
Neighbourhoodies: courageous community, colours, blazing bling and defiant delight
As we see a global culture appear across the planet identity politics simultaneously gravitate towards issues of the local. In society’s top strata people strive to live in posh areas with the right postal code. Subversive counterculture activists try to keep their own multi-ethnic spaces free from yuppies who in turn try to gentrify the same areas into authentic bohemian-chic quarters. In the urban fringes gangs protect their territory and even tattoo their hood names as a sign of authentic pride. Caught in the line of fire of identity politics is the hoodie, an average street-style garment, the canvas on which social conflicts and criminal stigmata are drawn, but also where local pride and reconciliation can be brought about, inspired by its connection to the resonance of musical milieus. In a time of liquid consumerism and fear, the habitus of the hoodie seems to frame a problematic identity which has been exposed in the ban on such garments in some British malls. The Neighburhoodies expands on a practice-based endeavour where fashion students from London College of Fashion reflected on their glocal London identities through the design of a special hoodie - a Neighbourhoodie
Beyond Green: The Arts as a Catalyst for Sustainability
The creative sector has played a significant role in efforts to raise awareness of the impacts of climate change and encourage sustainable social, economic, and environmental practices worldwide. Many artists and cultural organizations have embarked on remarkable projects that make us reflect on our behaviors, our carbon footprints, and the claims of infinite growth based on finite resources. Sometimes treading a fine line between arts and advocacy, they have sparked extraordinary collaborations that reveal new ways of living together on a shared planet. The 'art of the possible' will become even more relevant as 2016 dawns - bringing the challenge of how to implement the Sustainable Development Goals and the Climate Change Agreement adopted at the end of 2015. Yet with negotiations overshadowed by scientific controversy, political polemic and geographic polarization, individuals can easily lose faith in their own ability to shape change beyond the hyperlocal level. Against this challenging backdrop, could the arts and creative practice become a particle accelerator - to shift mindsets, embrace new ways of sharing space and resources, and catalyze more creative leadership in the public and private spheres? The goal of this Salzburg Global Seminar session was to build on path-breaking cultural initiatives to advance international and cross-sectoral links between existing arts and sustainability activities around the world, encourage bolder awareness-raising efforts, and recommend strategic approaches for making innovative grassroots to scale for greater, longer-term impact
A review of the ONR/NAVAIR research option combustion instabilities in compact ramjets, 1983-1988
This paper consists of two parts summarizing two portions of the ONR/NAVAIR Research Option. The option began in 1983 and continued for five years, involving 11 organizations. Simultaneously, similar or related programs supported by other agencies or institutions were being carried out in several other places. Results of those programs have been briefly summarized in five papers collected in a document to be published by C.P.L.A. This paper contains two of the five papers in that document. Here we cover the subjects of approximate analyses and stability; and large-scale structures and passive control. The first is concerned chiefly with an analytical framework constructed on the basis of observations; it is intended to provide a means of correlating and interpreting data, and predicting the stability of motions in a combustion chamber. The second is a summary of recent experimental work directed to understanding the flows in dump combustors of the sort used in modern ramjet engines. Much relevant material is not included here, but may be found in the remaining papers of the document cited above. For completeness, we note briefly the substance of those reports.
In their summary "Spray Combustion Processes in Ramjet Combustion Instability," Bowman (Stanford), Law (University of California, Davis) and Sirignano (University of California, Irvine) review several aspects of spray combustion relevant to combustion instabilities. The objectives of the works were: (1) to determine the
effect of spray characteristics on the energy release pattern in a dump combustor and the subsequent effects on
combustion instability; (2) to gain a fundamental understanding of the coupling of the spray vaporization process with an unsteady flow field; and (3) to investigate methods for controlling and enhancing spray vaporization
rates in liquid-fueled ramjets. During the past five years considerable progress has been made in applying methods of computational fluid dynamics to the flow in a dump combustor including consequences of energy release due to combustion processes. Jou has summarized work done at Flow Research, Inc. and at the Naval Research Laboratory in his
paper "A Summary Report on Large-Eddy Simulations of Pressure Oscillations in a Ramjet Combustor." The serious effects of combustion instabilities on the inlets of ramjet engines were discovered in the late 1970's in experimental work at the Aeropropulsion Laboratory, Wright Field, the Naval Weapons Center and the Marquardt Company. The most thorough laboratory work on the unsteady behavior of inlets has been accomplished at the McDonnell-Douglas Research Laboratory by Sajben who has reviewed the subject in his
paper "The Role of Inlet in Ramjet Pressure Oscillations.
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