Near neighbor search in nonmetric space

23 pages (this version is cleaner that the previous report and includes empirical results for the Smith-Waterman distance).We consider the computational problem of the Near Neighbor Search (NNS) in nonmetric spaces. Nonmetric spaces are the generalization of the metric spaces because they do not require the triangular inequality assumption. Nonmetric spaces are important because many similarity measures (between images, proteins, etc) do not verify the triangular inequality. We show the nonmetric situation calls for different evaluation criterions of NNS algorithms. As a first attempt, to our knowledge, to perform general nonmetric NNS, we introduce such evaluation criterions. The insights provided by those criterions lead us to introduce a new category of search structures, called "densitrees", that extend the classical metric tree algorithm for the nonmetric NNS. Against well established datasets, our preliminary empirical results lead us to a counter-intuitive conclusion: "the triangular inequality has only a secondary contribution on the efficiency metric". Additionally, the densitrees, although very naively implemented, performs reasonably well both in metric and nonmetric situations

Sketch based Distributed Garbage Collection, Theory and Empirical Elements

8 pagesObject-Oriented Programming (OOP) is a pillar of actual, and most probably future, software engineering. Within the advances for OOP, Garbage Collection (GC) is one the major improvements that brought both large productivity and reliability benefits. Yet, to our knowledge, no widely used distributed systems benefit from a complete Distributed Garbage Collector (DGC) at this time. We believe that this situation is due, a least partly, to actual DGC performance issues. In this paper, we introduce the idea of sketch based DGC where object-graph fragments are sketched rather than explicitly represented. We prove, under reasonable assumptions that sketched messages are smaller up to one order of magnitude than their explicit counterparts. Those results apply to most of state-of-art DGC methods. In the case of the Veiga and Ferreira DGC algorithm, the improvement is more than a factor~4 under very limited assumptions

LES of knocking in engines using dual heat transfer and two-step reduced schemes

Large Eddy Simulation of knocking in piston engines requires high-fidelity physical models and numerical techniques. The need to capture temperature fields with high precision to predict autoignition is an additional critical constraint compared to existing LES in engines. The present work presents advances for LES of knocking in two fields: (1) a Conjugate Heat Transfer (CHT) technique is implemented to compute the flow within the engine over successive cycles with LES together with the temperature field within the cylinder head walls and the valves and (2) a reduced two-step scheme is used to predict both propagating premixed flames as well as autoignition times over a wide range of equivalence ratios, pressures and temperatures. The paper focuses on CHT which is critical for knocking because the gas temperature field is controlled by the wall temperature field and knocking is sensitive to small temperature changes. The CHT LES is compared to classical LES where the temperatures of the head and the valves are supposed to be homogeneous and imposed empirically. Results show that the skin temperature field (which is a result of the CHT LES while it is a user input for classical LES) is complex and controls knocking events. While the results of the CHT LES are obviously better because they suppress a large part of the empirical specification of the wall temperatures, this study also reveals a difficult and crucial element of the CHT approach: the description of exhaust valves cooling which are in contact with the engine head for part of the cycle and not in the rest of the cycle, leading to difficulties for heat transfer descriptions between valves and head. The CHT method is successfully applied to an engine studied at IFP Energies Nouvelles where knocking characteristics have been studied over a wide range of conditions

Impacts on thin elastic sheets

International audienceWe study transverse impacts of rigid objects on a free elastic membrane, using thin circular sheets of natural rubber as experimental models. After impact, two distinct axisymmetric waves propagate in and on the sheet. First a tensile wave travels at sound speed leaving behind the wave front a stretched domain. Then, a transverse wave propagates on the stretched area at a lower speed. In the stretched area, geometrical confinement induces compressive circumferential stresses leading to a buckling instability, giving rise to radial wrinkles. We report on a set of experiments and theoretical remarks on the conditions of occurrence of these wrinkles, their dynamics and wavelength

LES of explosions in venting chamber: A test case for premixed turbulent combustion models

This paper presents a new experimental and Large Eddy Simulation (LES) database to study upscaling effects in vented gas explosions. The propagation of premixed flames in three setups of increasing size is investigated experimentally and numerically. The baseline model is the well-known laboratory-scale combustion chamber from Sydney (Kent et al., 2005; Masri et al., 2012); two exact replicas at scales 6 and 24.4 were set up by GexCon (Bergen, Norway). The volume ratio of the three setups varies from 1 to more than 10,000, a variation unseen in previous experiments, allowing the exploration of a large range of Reynolds and Damköhler numbers. LES of gaseous fully premixed flames have been performed on the three configurations, under different operating conditions, varying the number of obstacles in the chamber, their position and the type of fuel (hydrogen, propane and methane). Particular attention is paid to the influence of the turbulent combustion model on the results (overpressure, flame front speed) comparing two different algebraic sub-grid scale models, the closures of Colin et al. (2000) and Charlette et al. (2002), used in conjunction with a thickened flame approach. Mesh dependency is checked by performing a highly resolved LES on the small-scale case. For a given scale and with a fixed model constant, LES results agree with experimental results, for all geometric arrangement of the obstacles and all fuels. However, when switching from small-scale cases to medium-scale or large-scale cases this conclusion does not hold, illustrating one of the main deficiencies of these algebraic models, namely the need for an a priori fitting of the model parameters. Although this database was initially designed for safety studies, it is also a difficult test for turbulent combustion models

Large-Eddy Simulation and experimental study of cycle-to-cycle variations of stable and unstable operating points in a spark ignition engine

This article presents a comparison between experiments and Large-Eddy Simulation (LES) of a spark ignition engine on two operating points: a stable one characterized by low cycle-to-cycle variations (CCV) and an unstable one with high CCV. In order to match the experimental cycle sample, 75 full cycles (with combustion) are computed by LES. LES results are compared with experiments by means of pressure signals in the intake and exhaust ducts, in-cylinder pressure, chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF). Results show that LES is able to: (1) reproduce the flame behavior in both cases (low and high CCV) in terms of position, shape and timing; (2) distinguish a stable point from an unstable one; (3) predict quantitatively the CCV levels of the two fired operating points. For the unstable case, part of the observed CCV is due to incomplete combustion. The results are then used to analyze the incomplete combustion phenomenon which occurs for some cycles of the unstable point and propose modification of the spark location to control CCV

Asynchronous, complete and distributed garbage collection in the NGrid project

15 pagesThe distributed garbage collection (DGC) problem can be divided in two parts: 1) acyclic garbage collection solved by the Reference Listing method 2) cyclic garbage collection solved by the Graph Summarizer method. This reports improvements brought by the NGrid DGC project

Large Eddy Simulation of Vented Deflagration

In order to understand gas explosion phenomena in industrial buildings, a reduced-scale vented combustion 6 chamber is investigated numerically. In this configuration, a flame is ignited in an initially quiescent flammable mixture and 7 propagates past solid obstacles, generating a strong pressure increase. The aim of this numerical study is twofold: The first 8 objective is to show how large eddy simulation manages to reproduce the parameters of critical relevance for this multiscale 9 problem, in particular the overpressure generated during the flame propagation. The second objective is to highlight that, even if 10 large- to small-scale turbulence effects play a crucial role in the flame development and the resulting overpressure, it is also 11 needed to correctly account for thermo-diffusive scale phenomena

Numerical Methods and Turbulence Modeling for LES of Piston Engines: Impact on Flow Motion and Combustion

In this article, Large Eddy Simulations (LES) of Spark Ignition (SI) engines are performed to evaluate the impact of the numerical set-up on the predicted flow motion and combustion process. Due to the high complexity and computational cost of such simulations, the classical set-up commonly includes "low" order numerical schemes (typically first or second-order accurate in time and space) as well as simple turbulence models (such as the well known constant coefficient Smagorinsky model (Smagorinsky J. (1963) Mon. Weather Rev. 91, 99-164). The scope of this paper is to evaluate the feasibility and the potential benefits of using high precision methods for engine simulations, relying on higher order numerical methods and state-of-the-art Sub-Grid-Scale (SGS) models. For this purpose, two high order convection schemes from the Two-step Taylor Galerkin (TTG) family (Colin and Rudgyard (2000) J. Comput. Phys. 162, 338-371) and several SGS turbulence models, namely Dynamic Smagorinsky (Germano et al. (1991) Phys. Fluids 3, 1760-1765) and sigma (Baya Toda et al. (2010) Proc. Summer Program 2010, Stanford, Center for Turbulence Research, NASA Ames/Stanford Univ., pp. 193-202) are considered to improve the accuracy of the classically used Lax-Wendroff (LW) (Lax and Wendroff (1964) Commun. Pure Appl. Math. 17, 381-398) - Smagorinsky set-up. This evaluation is performed considering two different engine configurations from IFP Energies nouvelles. The first one is the naturally aspirated four-valve spark-ignited F7P engine which benefits from an exhaustive experimental and numerical characterization. The second one, called Ecosural, is a highly supercharged spark-ignited engine. Unique realizations of engine cycles have been simulated for each set-up starting from the same initial conditions and the comparison is made with experimental and previous numerical results for the F7P configuration. For the Ecosural engine, experimental results are not available yet and only qualitative comparisons are performed to enforce the analysis and conclusions made on the F7P configuration. Regarding SGS models, only slight differences are found at the aerodynamic level even if sigma allows a better resolution of small structures of the velocity field. However, all results are in cycle-to-cycle variability envelopes from Granet (Granet et al. (2012) Combust. Flame 159, 1562-1575) and these single cycle computations don’t permit to distinguish clear improvements on macroscopic parameters such as resolved kinetic energy, heat release or mean in-cylinder pressure. Concerning numerical schemes, TTG schemes also allow a slighlty better resolution of small scale vortices but global quantities such as resolved kinetic energy and SGS viscosity are comparable. Nevertheless, clear differences appear between the different schemes in the combustion stroke. This is attributed to a better resolution of the flame-turbulence interaction process during the free flame propagation period, leading to an increase of the resolved part of heat release. It is also shown in this paper that an adjustment of the efficiency constant in the Thickened Flame (TF) model is compulsory to account for the over dissipation of the smallest resolved structures ifLWis used. In the light of these conclusions an hybrid setup, called ES O2 (Engine Stroke Optimal Order), which consists in using TTGC during combustion and LW elsewhere is proposed and applied to the two engines configurations. Results are in good agreement with the ones obtained in the case of a full TTGC simulation, while the CPU (Central Processing Unit) cost increase is only about 10% compared to LW. The accuracy of LW seems therefore to be sufficient for pure aerodynamic phases, while the use of TTGC only during combustion permits an improvement in the LES quality. The hybrid ES O2 method thus appears as an attractive approach to improve further calculations accuracy without being greatly penalized by additional CPU costs in multi-cycle simulations

Molecular Simulations of Supercritical Fluid Permeation through Disordered Microporous Carbons

International audienceFluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature