Cosmological Simulations on a Grid of Computers

The work presented in this paper aims at restricting the input parameter values of the semi-analytical model used in GALICS and MOMAF, so as to derive which parameters influence the most the results, e.g., star formation, feedback and halo recycling efficiencies, etc. Our approach is to proceed empirically: we run lots of simulations and derive the correct ranges of values. The computation time needed is so large, that we need to run on a grid of computers. Hence, we model GALICS and MOMAF execution time and output files size, and run the simulation using a grid middleware: DIET. All the complexity of accessing resources, scheduling simulations and managing data is harnessed by DIET and hidden behind a web portal accessible to the users.Comment: Accepted and Published in AIP Conference Proceedings 1241, 2010, pages 816-82

Modelization for the Deployment of a Hierarchical Middleware on a Homogeneous Platform

Accessing the power of distributed resources can nowadays easily be done using a middleware based on a client/server approach. Several architectures exist for those middlewares. The most scalable ones rely on a hierarchical design. Determining the best shape for the hierarchy, the one giving the best throughput of services, is not an easy task. We first propose a computation and communication model for such hierarchical middleware. Our model takes into account the deployment of several services in the hierarchy. Then, based on this model, we propose an algorithm for automatically constructing a hierarchy. This algorithm aims at offering the users the best obtained to requested throughput ratio, while providing fairness on this ratio for the different kind of services, and using as few resources as possible. Finally, we compare our model with experimental results on a real middleware called DIET

Optimal transient growth and very large-scale structures in turbulent boundary layers

International audienceThe optimal energy growth of perturbations sustained by a zero pressure gradient turbulent boundary is computed using the eddy viscosity associated with the turbulent mean flow. it is found that even if all the considered turbulent mean profiles are linearly stable, they support transient energy growths. The most amplified perturbations are streamwise uniform and correspond to streamwise streaks originated by streamwise vortices. For sufficiently large Reynolds numbers two distinct peaks of the optimal growth exist, respectively scaling in inner and outer units. The optimal structures associated with the peak scaling in inner units correspond well with the most probable streaks and vortices observed in the buffer layer, and their moderate energy growth is independent of the Reynolds number. The energy growth associated with the peak scaling in outer units is larger than that of the inner peak and scales linearly with an effective turbulent Reynolds number Formed with the maximum eddy viscosity and a modified Rotta Clauser length based on the momentum thickness. The corresponding optimal perturbations consist of very large scale structures with a spanwise wavelength of the order of 8 delta. The associated optimal streaks scale in outer variables in the outer region and in wall units in the inner region of the boundary layer, in which they are proportional to the mean flow velocity, These outer streaks protrude far into the near wall region, having still 50% of their maximum amplitude at y(1) = 20. The amplification of very large scale structures appears to be a robust feature of the turbulent boundary layer: optimal perturbations with spanwise wavelengths ranging from 4 delta to 15 delta can all reach 80% of the overall optimal peak growth

Optimal Transient Growth and Very Large-Scale Structures in Zero-Pressure Gradient Turbulent Boundary Layers

International audienceWe are interested in the optimal energy growth of perturbations sustained by a zero pressure gradient turbulent boundary layer. We use the mean flow proposed by Monkewitz et al. (2007), the turbulence dynamics being modeled by an eddy viscosity added in the disturbance equations following the approach of del Alamo and Jimenez (2006), or Pujals et al. (2009) in the turbulent channel flow case. Although all the considered turbulent mean profiles are linearly stable, they support transient energy growths due to the non-normality of the operator. We find that the most amplified perturbations are streamwise uniform and correspond to streamwise vortices evolving into streamwise streaks. Consistently with the study of del Alamo and Jimenez (2006), we find that two distinct peaks of the optimal growth exist for sufficiently large Reynolds numbers: a primary one scaling in outer units and a secondary one scaling in wall units. The optimal structures associated with the peak scaling in wall units correspond well to the most probable streaks observed in the buffer layer and their moderate energy growth is independent of the Reynolds number. The energy growth associated with the peak scaling in outer units is larger than that of the inner peak. The optimal perturbations associated with this primary peak consist in very large-scale structures with a spanwise wavelength of the order of 8. Since such very large-scale structures have not been observed yet in turbulent shear flows, preliminary experiments aiming at forcing such structures and studying their growth have been conducted. We find that large-scale turbulent streaks can be forced using well-shaped roughness elements embedded in the boundary layer. Their amplitude can reach about 13.5% of the free-stream velocity before decaying

Cosmological Simulations using Grid Middleware

One way to access the aggregated power of a collection of heterogeneous machines is to use a grid middleware, such as DIET, GridSolve or NINF. It addresses the problem of monitoring the resources, of handling the submissions of jobs and as an example the inherent transfer of input and output data, in place of the user. In this paper we present how to run cosmological simulations using the RAMSES application along with the DIET middleware. We will describe how to write the corresponding DIET client and server. The remainder of the paper is organized as follows: Section 2 presents the DIET middleware. Section 3 describes the RAMSES cosmological software and simulations, and how to interface it with DIET. We show how to write a client and a server in Section 4. Finally, Section 5 presents the experiments realized on Grid'5000, the French Research Grid, and we conclude in Section 6.Comment: submitted Nov 200

A Self-Stabilizing K-Clustering Algorithm Using an Arbitrary Metric (Revised Version of RR2008-31)

32 pagesMobile ad hoc networks as well as grid platforms are distributed, changing, and error prone environments. Communication costs within such infrastructure can be improved, or at least bounded, by using k-clustering. A k-clustering of a graph, is a partition of the nodes into disjoint sets, called clusters, in which every node is distance at most k from a designated node in its cluster, called the clusterhead. A self-stabilizing asynchronous distributed algorithm is given for constructing a k-clustering of a connected network of processes with unique IDs and weighted edges. The algorithm is comparison-based, takes O(nk) time, and uses O(log n + log k) space per process, where n is the size of the network. This is the first distributed solution to the k-clustering problem on weighted graphs

A note on optimal transient growth in turbulent channel flows

International audienceWe compute the optimal transient growth of perturbations sustained by a turbulent channel flow following the same approach recently used by del Álamo and Jiménez [J. Fluid Mech.559, 205 (2006)]. Contrary to this previous analysis, we use generalized Orr-Sommerfeld and Squire operators consistent with previous investigations of mean flows with variable viscosity. The optimal perturbations are streamwise vortices evolving into streamwise streaks. In accordance with del Álamo and Jiménez, it is found that for very elongated structures and for sufficiently large Reynolds numbers, the optimal energy growth presents a primary peak in the spanwise wavelength, scaling in outer units, and a secondary peak scaling in inner units and corresponding to λ+z≈100. Contrary to the previous results, however, it is found that the maximum energy growth associated with the primary peak increases with the Reynolds number. This growth, in a first approximation, scales linearly with an effective Reynolds number based on the centerline velocity, the channel half width and the maximum eddyviscosity associated. The optimal streaks associated with the primary peak have an optimal spacing of λz=4h and scale in outer units in the outer region and in wall units in the near wall region, where they still have up to 50% of their maximum amplitude near y+=10. © 2009 American Institute of Physics

Modelization for the Deployment of a Hierarchical Middleware on a Heterogeneous Platform

Accessing the power of distributed resources can nowadays easily be done using a middleware based on a client/server approach. Several architectures exist for those middlewares. The most scalable ones rely on a hierarchical design. Determining the best shape for the hierarchy, the one giving the best throughput of services, is not an easy task. We first propose a computation and communication model for such hierarchical middleware. Our model takes into account the deployment of several services in the hierarchy. Then, based on this model, we propose algorithms for automatically constructing a hierarchy on two kind of heterogeneous platforms: communication homogeneous/computation heterogeneous platforms, and fully heterogeneous platforms. The proposed algorithm aim at offering the users the best obtained to requested throughput ratio, while providing fairness on this ratio for the different kind of services, and using as few resources as possible. For each kind of platforms, we compare our model with experimental results on a real middleware called Diet.De nos jours, l'accès à des ressources distribuées peut être réalisé aisément en utilisant un intergiciel se basant sur une approche client/serveur. Différentes architectures existent pour de tels intergiciels. Ceux passant le mieux à l'échelle utilisent une hiérarchie d'agents. Déterminer quelle est la meilleure hiérarchie, c'est à dire celle qui fournira le meilleur débit au niveau des services, n'est pas une tâche aisée. Nous proposons tout d'abord un modèle de calcul et de communication pour de tels intergiciels hiérarchiques. Notre modèle prend en compte le déploiement de plusieurs services au sein de la hiérarchie. Puis, en nous basant sur le modèle, nous proposons des algorithmes pour construire automatiquement la hiérarchie sur différents types de plates-formes: des plates-formes avec des communications homogènes et des puissances de calcul hétérogènes, ou des plates-formes complètement hétérogènes. Les algorithmes visent à offrir aux utilisateurs le meilleur ratio entre le débit demandé, et le débit fourni, tout en utilisant le moins de ressources possible. Pour chaque type de plate-forme, nous comparons notre modèle à des résultats expérimentaux obtenus avec l'intergiciel de grille DIET

A note on optimal transient growth in turbulent channel flows

A measuring principle for qualitative and quantitative analyses of three-dimensional unsteady flows is presented. The principle is based on colour coding of the flow volume under consideration. Coloured light sheets are generated and used to illuminate the flow volume. Consecutive light sheets of different colours are scanned over the volume within a small interval of time. Thus, the volume is sliced and colour-coded quasi-instantaneously. With this technique, the 3D position of a particle in the volume can be identified by a 2D image and an associated colour. Since most optical flow measuring systems are based on tracers, colour coding allows the application of 2D image recorders to register 3D flow information. The paper discusses the state-of-the-art of this principle for three-dimensional flow analyses and gives information about applicability and limitations