Publications

Abstract not provided.

In this position paper, we argue for improved fault-tolerance of an MPI code by introducing lightweight virtualization into the MPI interface. In particular, we outline key-value store semantics for MPI send/recv calls, thereby creating a far more expressive programming model. The general message passing semantics and imperative style of MPI application codes would remain essentially unchanged. However, the additional expressiblity of the programming model 1) enables the underlying transport layer to handle faulttolerance more transparently to the application developer, and 2) provides an evolutionary code path towards more declarative asynchronous programming models. The core contribution of this paper is an initial implementation of the DHARMA transport layer that provides the new, required functionality to support the MPI key-value store model.

Application resilience is a key challenge that must be ad-dressed in order to realize the exascale vision. Previous work has shown that online recovery, even when done in a global manner (i.e., involving all processes), can dramatically re-duce the overhead of failures when compared to the more traditional approach of terminating the job and restarting it from the last stored checkpoint. In this paper we suggest going one step further, and explore how local recovery can be used for certain classes of applications to reduce the over-heads due to failures. Specifically we study the feasibility of local recovery for stencil-based parallel applications and we show how multiple independent failures can be masked to effectively reduce the impact on the total time to solution.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

This paper reports the results of a joint experimental and numerical study of the flow characteristics and flame structure of a hydrogen rich jet injected normal to a turbulent, vitiated crossflow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H2/CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air crossflow, the present conditions lead to a burner-attached flame that initiates uniformly around the burner edge. Significant asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/crossflow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the inert and reacting JICF. The paper concludes with an analysis of the ignition, flame characteristics, and global structure of the burner-attached flame. Chemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is examined by computing the Takeno flame index, which shows that ~70% of the heat release occurs in non-premixed regions.

Three-dimensional direct numerical simulation results of a transverse syngas fuel jet in turbulent cross-flow of air are analyzed to study the influence of varying volume fractions of CO relative to H2 in the fuel composition on the near field flame stabilization. The mean flame stabilizes at a similar location for CO-lean and CO-rich cases despite the trend suggested by their laminar flame speed, which is higher for the CO-lean condition. To identify local mixtures having favorable mixture conditions for flame stabilization, explosive zones are defined using a chemical explosive mode timescale. The explosive zones related to flame stabilization are located in relatively low velocity regions. The explosive zones are characterized by excess hydrogen transported solely by differential diffusion, in the absence of intense turbulent mixing or scalar dissipation rate. The conditional averages show that differential diffusion is negatively correlated with turbulent mixing. Moreover, the local turbulent Reynolds number is insufficient to estimate the magnitude of the differential diffusion effect. Alternatively, the Karlovitz number provides a better indicator of the importance of differential diffusion. A comparison of the variations of differential diffusion, turbulent mixing, heat release rate and probability of encountering explosive zones demonstrates that differential diffusion predominantly plays an important role for mixture preparation and initiation of chemical reactions, closely followed by intense chemical reactions sustained by sufficient downstream turbulent mixing. The mechanism by which differential diffusion contributes to mixture preparation is investigated using the Takeno Flame Index. The mean Flame Index, based on the combined fuel species, shows that the overall extent of premixing is not intense in the upstream regions. However, the Flame Index computed based on individual contribution of H2 or CO species reveals that hydrogen contributes significantly to premixing, particularly in explosive zones in the upstream leeward region, i.e. at the preferred flame stabilization location. Therefore, a small amount of H2 diffuses much faster than CO, creating relatively homogeneous mixture pockets depending on the competition with turbulent mixing. These pockets, together with high H2 reactivity, contribute to stabilizing the flame at a consistent location regardless of the CO concentration in the fuel for the present range of DNS conditions.

This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity. Results indicate that At the Reynolds numbers of the simulations (Returb = 600, Re = 8000) choice of molecular diffusion models affects significantly the temperature and concentration fields;Assuming Le = 1 for all species predicts temperatures up to 250 K higher than the physically realistic multi-component model;Faster molecular transport of lighter species changes the local concentration field and affects reaction pathways and chemical kinetics. A possible explanation for these observations is provided in terms of species diffusion velocity that is a strong function of gradients: thus, at sufficiently large Reynolds numbers, gradients and their effects tend to be large. The preliminary conclusion from these simulations seems to indicate molecular diffusion as the third important mechanism active in flames besides convective transport and kinetics. If confirmed by further DNS and measurements, molecular transport in high intensity turbulent flames will have to be realistically modeled to accurately predict emissions (gaseous and particulates) and other combustor performance metrics.

Abstract not provided.

The topology of turbulent premixed flames is analysed using data from Direct Numerical Simulation (DNS), with emphasis on the statistical geometry of flame-flame interaction. A general method for obtaining the critical points of line, surface and volume fields is outlined, and the method is applied to isosurfaces of reaction progress variable in a DNS configuration involving a pair of freely-propagating hydrogen-air flames in a field of intense shear-generated turbulence. A complete set of possible flame-interaction topologies is derived using the eigenvalues of the scalar Hessian, and the topologies are parametrised using a pair of shape factors. The frequency of occurrence of each type of topology is evaluated from the DNS dataset for two different Damköhler numbers. Different types of flame-interaction topology are found to be favoured in various regions of the turbulent flame, and the physical significance of each interaction is discussed.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Extreme-scale computing will bring significant changes to high performance computing system architectures. In particular, the increased number of system components is creating a need for software to demonstrate 'pervasive parallelism' and resiliency. Asynchronous, many-task programming models show promise in addressing both the scalability and resiliency challenges, however, they introduce an enormously challenging distributed, resilient consistency problem. In this work, we explore the viability of resilient collective communication in task scheduling and work stealing and, through simulation with SST/macro, the performance of these collectives on speculative extreme-scale architectures.

Abstract not provided.

Abstract not provided.

Abstract not provided.