1,265 research outputs found
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Regulation of Wages and Hours Prior to 1938
Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure
Enabling adaptive scientific workflows via trigger detection
Next generation architectures necessitate a shift away from traditional
workflows in which the simulation state is saved at prescribed frequencies for
post-processing analysis. While the need to shift to in~situ workflows has been
acknowledged for some time, much of the current research is focused on static
workflows, where the analysis that would have been done as a post-process is
performed concurrently with the simulation at user-prescribed frequencies.
Recently, research efforts are striving to enable adaptive workflows, in which
the frequency, composition, and execution of computational and data
manipulation steps dynamically depend on the state of the simulation. Adapting
the workflow to the state of simulation in such a data-driven fashion puts
extremely strict efficiency requirements on the analysis capabilities that are
used to identify the transitions in the workflow. In this paper we build upon
earlier work on trigger detection using sublinear techniques to drive adaptive
workflows. Here we propose a methodology to detect the time when sudden heat
release occurs in simulations of turbulent combustion. Our proposed method
provides an alternative metric that can be used along with our former metric to
increase the robustness of trigger detection. We show the effectiveness of our
metric empirically for predicting heat release for two use cases.Comment: arXiv admin note: substantial text overlap with arXiv:1506.0825
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Meet the Scientists
Scientific visualization has become an active area of research. Most researchers and students in the field of visualization, however, do not have access to data sets generated by the state-of-the-art sim- ulations. In the case they have access to some of these data sets, they often do not get to directly interact with the scientists who generated the data sets. This interaction is crucial for obtaining the understanding of what scientists really need to get out of their data sets and what visualization functionalities are missing in ex- isting visualization software tools. This panel will provide such interaction. Through the DOE SciDAC Institute for Ultrascale Vi- sualization (Ultravis Institute) [6], scientists in representative areas fromastrophysics, combustion, to plasma physicswill be sponsored to attend the Visualization 2007 Conference and participate in the panel. Each scientist will describe his/her application, data sets, and the corresponding visualization and data analysis needs and challenges, and then answer questions. By making their data sets openly available through the Ultravis Institute after the Conference, more visualization researchers will be given the chance to work on the problems truly faced by the scientists. The panel will thus pro- vide these researchers the correct understanding of the problems, and subsequently help accelerate the development of the field of scientific visualization. The following sections introduce the four participating scientists and their application areas
Exercise Training and Heart Failure: A Review of the Literature
Exercise and cardiac rehabilitation have been underused therapy options for patients with congestive heart failure despite being recommended in international guidelines and being covered by Medicare in the US. This article reviews the evidence behind this treatment strategy and details current trials that will contribute to the evidence base
Diva: A Declarative and Reactive Language for In-Situ Visualization
The use of adaptive workflow management for in situ visualization and
analysis has been a growing trend in large-scale scientific simulations.
However, coordinating adaptive workflows with traditional procedural
programming languages can be difficult because system flow is determined by
unpredictable scientific phenomena, which often appear in an unknown order and
can evade event handling. This makes the implementation of adaptive workflows
tedious and error-prone. Recently, reactive and declarative programming
paradigms have been recognized as well-suited solutions to similar problems in
other domains. However, there is a dearth of research on adapting these
approaches to in situ visualization and analysis. With this paper, we present a
language design and runtime system for developing adaptive systems through a
declarative and reactive programming paradigm. We illustrate how an adaptive
workflow programming system is implemented using our approach and demonstrate
it with a use case from a combustion simulation.Comment: 11 pages, 5 figures, 6 listings, 1 table, to be published in LDAV
2020. The article has gone through 2 major revisions: Emphasized
contributions, features and examples. Addressed connections between DIVA and
FRP. In sec. 3, we fixed a design flaw and addressed it in sec. 3.3-3.4.
Re-designed sec. 5 with a more concrete example and benchmark results.
Simplified the syntax of DIV
Evaluation of finite difference based asynchronous partial differential equations solver for reacting flows
Next-generation exascale machines with extreme levels of parallelism will
provide massive computing resources for large scale numerical simulations of
complex physical systems at unprecedented parameter ranges. However, novel
numerical methods, scalable algorithms and re-design of current state-of-the
art numerical solvers are required for scaling to these machines with minimal
overheads. One such approach for partial differential equations based solvers
involves computation of spatial derivatives with possibly delayed or
asynchronous data using high-order asynchrony-tolerant (AT) schemes to
facilitate mitigation of communication and synchronization bottlenecks without
affecting the numerical accuracy. In the present study, an effective
methodology of implementing temporal discretization using a multi-stage
Runge-Kutta method with AT schemes is presented. Together these schemes are
used to perform asynchronous simulations of canonical reacting flow problems,
demonstrated in one-dimension including auto-ignition of a premixture, premixed
flame propagation and non-premixed autoignition. Simulation results show that
the AT schemes incur very small numerical errors in all key quantities of
interest including stiff intermediate species despite delayed data at
processing element (PE) boundaries. For simulations of supersonic flows, the
degraded numerical accuracy of well-known shock-resolving WENO (weighted
essentially non-oscillatory) schemes when used with relaxed synchronization is
also discussed. To overcome this loss of accuracy, high-order AT-WENO schemes
are derived and tested on linear and non-linear equations. Finally the novel
AT-WENO schemes are demonstrated in the propagation of a detonation wave with
delays at PE boundaries
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Terascale High-Fidelity Simulations of Turbulent Combustion with Detailed Chemistry
The TSTC project is a multi-university collaborative effort to develop a high-fidelity turbulent reacting flow simulation capability utilizing terascale, massively parallel computer technology. The main paradigm of our approach is direct numerical simulation (DNS) featuring highest temporal and spatial accuracy, allowing quantitative observations of the fine-scale physics found in turbulent reacting flows as well as providing a useful tool for development of sub-models needed in device-level simulations. The code named S3D, developed and shared with Chen and coworkers at Sandia National Laboratories, has been enhanced with new numerical algorithms and physical models to provide predictive capabilities for spray dynamics, combustion, and pollutant formation processes in turbulent combustion. Major accomplishments include improved characteristic boundary conditions, fundamental studies of auto-ignition in turbulent stratified reactant mixtures, flame-wall interaction, and turbulent flame extinction by water spray. The overarching scientific issue in our recent investigations is to characterize criticality phenomena (ignition/extinction) in turbulent combustion, thereby developing unified criteria to identify ignition and extinction conditions. The computational development under TSTC has enabled the recent large-scale 3D turbulent combustion simulations conducted at Sandia National Laboratories
Fetal Metabolic Adaptations to Cardiovascular Stress in Twin-Twin Transfusion Syndrome
Monochorionic-diamniotic twin pregnancies are susceptible to unique complications arising from a single placenta shared by two fetuses. Twin-twin transfusion syndrome (TTTS) is a constellation of disturbances caused by unequal blood flow within the shared placenta giving rise to a major hemodynamic imbalance between the twins. Here, we applied TTTS as a model to uncover fetal metabolic adaptations to cardiovascular stress. We compared untargeted metabolomic analyses of amniotic fluid samples from severe TTTS cases vs. singleton controls. Amniotic fluid metabolites demonstrated alterations in fatty acid, glucose, and steroid hormone metabolism in TTTS. Among TTTS cases, unsupervised principal component analysis revealed two distinct clusters of disease defined by levels of glucose metabolites, amino acids, urea, and redox status. Our results suggest that the human fetal heart can adapt to hemodynamic stress by modulating its glucose metabolism and identify potential differences in the ability of individual fetuses to respond to cardiovascular stress
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