1,364 research outputs found
Further analysis of the effects of baffles on combustion instability
A computerized analytical model, developed to predict the effects of baffles on combustion instability, was modified in an effort to improve the ability to properly predict stability effects. The model was modified: (1) to replace a single spatially-averaged response factor by separate values for each baffle compartment; (2) to calculate the axial component of the acoustic energy flux, and (3) to permit analysis of traveling waves in a thin annular chamber. Allowance for separate average response factors in each baffle compartment was found to significantly affect the predicted results. With this modification, an optimum baffle length was predicted which gave maximum stability
Analysis of the effects of baffles on combustion instability
An analytical model has been developed for predicting the effects of baffles on combustion instability. This model has been developed by coupling an acoustic analysis of the wave motion within baffled chambers with a model for the oscillatory combustion response of a propellant droplet developed by Heidmann. A computer program was developed for numerical solution of the resultant coupled equations. Diagnostic calculations were made to determine the reasons for the improper prediction. These calculations showed that the chosen method of representing the combustion response was a very poor approximation. At the end of the program, attempts were made to minimize this effect but the model still improperly predicts the stability trends. Therefore, it is recommended that additional analysis be done with an improved approximation
Analysis of combustion instability in liquid propellant engines with or without acoustic cavities
Analytical studies have been made of the relative combustion stability of various propellant combinations when used with hardware configurations representative of current design practices and with or without acoustic cavities. Two combustion instability models, a Priem-type model and a modification of the Northern Research and Engineering (NREC) instability model, were used to predict the variation in engine stability with changes in operating conditions, hardware characteristics or propellant combination, exclusive of acoustic cavity effects. The NREC model was developed for turbojet engines but is applicable to liquid propellant engines. A steady-state combustion model was used to predict the needed input for the instability models. In addition, preliminary development was completed on a new model to predict the influence of an acoustic cavity with specific allowance for the effects the nozzle, steady flow and combustion
Volatiles in protoplanetary disks
Volatiles are compounds with low sublimation temperatures, and they make up
most of the condensible mass in typical planet-forming environments. They
consist of relatively small, often hydrogenated, molecules based on the
abundant elements carbon, nitrogen and oxygen. Volatiles are central to the
process of planet formation, forming the backbone of a rich chemistry that sets
the initial conditions for the formation of planetary atmospheres, and act as a
solid mass reservoir catalyzing the formation of planets and planetesimals.
This growth has been driven by rapid advances in observations and models of
protoplanetary disks, and by a deepening understanding of the cosmochemistry of
the solar system. Indeed, it is only in the past few years that representative
samples of molecules have been discovered in great abundance throughout
protoplanetary disks - enough to begin building a complete budget for the most
abundant elements after hydrogen and helium. The spatial distributions of key
volatiles are being mapped, snow lines are directly seen and quantified, and
distinct chemical regions within protoplanetary disks are being identified,
characterized and modeled. Theoretical processes invoked to explain the solar
system record are now being observationally constrained in protoplanetary
disks, including transport of icy bodies and concentration of bulk
condensibles. The balance between chemical reset - processing of inner disk
material strong enough to destroy its memory of past chemistry, and inheritance
- the chemically gentle accretion of pristine material from the interstellar
medium in the outer disk, ultimately determines the final composition of
pre-planetary matter. This chapter focuses on making the first steps toward
understanding whether the planet formation processes that led to our solar
system are universal.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Structural Heat Intercept, Insulation and Vibration Evaluation Rig (SHIIVER)
The Structural Heat Intercept, Insulation, and Vibration Evaluation Rig (SHIIVER) is a large scale cryogenic fluid management (CFM) test bed designed to scale CFM technologies for inclusion on large, in-space stages. A part of the evolvable Cryogenics (eCryo) project, SHIIVER is a technology development task that is supportive of future exploration propulsion needs. Technologies developed under the eCryo Project will play a critical role in enabling increasingly longer duration in-space missions beyond Low Earth Orbit (LEO)
RNA-seq: technical variability and sampling
<p>Abstract</p> <p>Background</p> <p>RNA-seq is revolutionizing the way we study transcriptomes. mRNA can be surveyed without prior knowledge of gene transcripts. Alternative splicing of transcript isoforms and the identification of previously unknown exons are being reported. Initial reports of differences in exon usage, and splicing between samples as well as quantitative differences among samples are beginning to surface. Biological variation has been reported to be larger than technical variation. In addition, technical variation has been reported to be in line with expectations due to random sampling. However, strategies for dealing with technical variation will differ depending on the magnitude. The size of technical variance, and the role of sampling are examined in this manuscript.</p> <p>Results</p> <p>In this study three independent Solexa/Illumina experiments containing technical replicates are analyzed. When coverage is low, large disagreements between technical replicates are apparent. Exon detection between technical replicates is highly variable when the coverage is less than 5 reads per nucleotide and estimates of gene expression are more likely to disagree when coverage is low. Although large disagreements in the estimates of expression are observed at all levels of coverage.</p> <p>Conclusions</p> <p>Technical variability is too high to ignore. Technical variability results in inconsistent detection of exons at low levels of coverage. Further, the estimate of the relative abundance of a transcript can substantially disagree, even when coverage levels are high. This may be due to the low sampling fraction and if so, it will persist as an issue needing to be addressed in experimental design even as the next wave of technology produces larger numbers of reads. We provide practical recommendations for dealing with the technical variability, without dramatic cost increases.</p
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