FMM-based vortex method for simulation of isotropic turbulence on GPUs, compared with a spectral method

The Lagrangian vortex method offers an alternative numerical approach for direct numerical simulation of turbulence. The fact that it uses the fast multipole method (FMM)--a hierarchical algorithm for N-body problems with highly scalable parallel implementations--as numerical engine makes it a potentially good candidate for exascale systems. However, there have been few validation studies of Lagrangian vortex simulations and the insufficient comparisons against standard DNS codes has left ample room for skepticism. This paper presents a comparison between a Lagrangian vortex method and a pseudo-spectral method for the simulation of decaying homogeneous isotropic turbulence. This flow field is chosen despite the fact that it is not the most favorable flow problem for particle methods (which shine in wake flows or where vorticity is compact), due to the fact that it is ideal for the quantitative validation of DNS codes. We use a 256^3 grid with Re_lambda=50 and 100 and look at the turbulence statistics, including high-order moments. The focus is on the effect of the various parameters in the vortex method, e.g., order of FMM series expansion, frequency of reinitialization, overlap ratio and time step. The vortex method uses an FMM code (exaFMM) that runs on GPU hardware using CUDA, while the spectral code (hit3d) runs on CPU only. Results indicate that, for this application (and with the current code implementations), the spectral method is an order of magnitude faster than the vortex method when using a single GPU for the FMM and six CPU cores for the FFT

Factors influencing success of donor funded pastoral projects in the Sahel.

Most livestock and range management projects in the Sahelian countries of Africa have failed to achieve their objectives. Many reasons have been given to explain the failure, but these reasons usually fall short of covering the whole environmental picture and concentrate on the factors internal to the projects. They do not provide complete explanations of why pastoral programs fail because they tend to over-emphasize the isolation of pastoral communities from their wider societies and do not take into consideration the whole socio-economic and political environment. The purpose of this study was to more constructively explain why pastoral development projects succeed or fail. Pastoral projects funded by World Bank and USAID in six Sahelian countries of Africa were studied. Metaevaluation was used to study all of the factors affecting the outcome of pastoral projects. A survey of project evaluators and summaries of project reports and evaluation activities were used as the main sources of data. The results indicate that outcomes of pastoral projects do depend upon the technological, sociocultural, and environmental dimensions of the development situation. But the outcomes are more affected by the policies of governments and their institutions than by any other factor. Factors such as uncertain funding, conflicting goals and objectives, and complicated bureaucratic procedures, generally mean that projects are not implemented as planned, changes are made as the projects are implemented, or implementation never occurs

Couplage de méthodes numériques en simulation directe d'écoulements incompressibles

This work is devoted to the developement of methods based on lagrangian numerical schemes as alternative or complement to the classical Eulerian methods in the context of direct numerical simulation of incompressible viscous flows around obstacls. We are interested in hybrid techniques blending finite-differences and vortex methods. When the domain is decomposed in distinct subdomains ,which are traited either by Eulerian or Lagrangian Methods, high order interpolation formulas allow to realise the consistent interface conditions between the subdomains. We are able to design domain decomposition techniques of Euler/Lagrange or Lagrange/Lagrange type which combine (velocity-pressure) and (velocity-vorticity) formulation. The different methods are validated on a number of fluid flows (driven cavity, rebound of vortex dipole, backward-facing step, flow past a cylinder) and are compared with high order finite-difference methods.Ce travail est consacré au développement des méthodes lagrangiennes comme alternatives ou compléments aux méthodes euleriennes conventionnelles pour la simulation d'écoulements incompressibles en présence d'obstacles. On s'intéresse en particulier à des techniques ou des solveurs eulériens et lagrangiens cohabitent dans le même domaine de calcul mais traitent différents termes des équations de Navier-Stokes, ainsi qu'à des techniques de décomposition de domaines ou différents solveurs sont utilisés dans chaques sous-domaines. Lorsque les méthodes euleriennes et lagrangiennes cohabitent dans le même domaine de calcul (méthode V.I.C.), les formules de passage particules-grilles permettent de représenter la vorticité avec la même précision sur une grille fixe et sur la grille lagrangienne. Les méthodes V.I.C. ainsi obtenues combinent stabilité et précision et fournissent une alternative avantageuse aux méthodes différences-finies pour des écoulements confinés. Lorsque le domaine de calcul est décomposé en sous-domaines distincts traités par méthodes lagrangiennes et par méthodes euleriennes, l'interpolation d'ordre élevé permet de réaliser des conditions d'interface consistantes entre les différents sous-domaines. On dispose alors de méthodes de calcul avec décomposition en sous-domaines, de type Euler/Lagrange ou Lagrange/Lagrange, et résolution en formulation (vitesse-tourbillon)/(vitesse-tourbillon) ou (vitesse-pression)/(vitesse-tourbillon). Les différentes méthodes développées ici sont testées sur plusieurs types d'écoulements (cavité entrainée, rebond de dipôles de vorticité, écoulements dans une conduite et sur une marche, écoulements autour d'obstacles) et comparées à des méthodes de différences-finies d'ordre élevé

Multi-purpose regridding in vortex methods

This paper is concerned by regridding strategies for accurate vortex calculations. We first demonstrate on several test cases of two and three dimensional, inviscid as well as viscous flows, that, when based on high order interpolation formulas, these technics do allow to maintain accuracy for long time simulations. We then show that, by alternating regridding on several non-conforming grids, one can easily design particle-based domain decomposition algorithms. This is illustrated by the example of a flow past two cylinders

Simulation of vehicle aerodynamics using a vortex element method

Recent developments of the 3-D Lagrangian vortex element method for bluff body flows are presented. In this approach attached boundary layer regions are modelled using infinitely thin vortex sheets while Lagrangian vortex elements are used for the separation regions and the wake. Preliminary results for the flow past a simplified generic truck geometry are presented. Further developments, aimed at the development of a hybrid Eulerian-Lagrangian solver, are briefly introduced

Applications of an hybrid particle-grid penalization method for the DNS and passive control of bluff-body flows

International audienceIn this work, a hybrid particle-grid method coupled with a penalization technique is introduced in order to compute Direct Numerical Simulations in three dimensions. The method is validated with the literature for the flow past a sphere and a hemisphere. The approach is extented to solid-porous-fluid media and applied to passive flow control for the hemisphere using porous coatings