Visualization and analysis of SPH data

Advances in graphics hardware in recent years have led not only to a huge growth in the speed at which 3D data can be rendered, but also to a marked change in the way in which different data types can be displayed. In particular, point based rendering techniques have benefited from the advent of vertex and fragment shaders on the GPU which allow simple point primitives to be displayed not just as dots, but rather as complex entities in their own right. We present a simple way of displaying arbitrary 2D slices through 3D SPH data by evaluating the SPH kernel on the GPU and accumulating the contributions from individual particles intersecting a slice plane into a texture. The resulting textured plane can then be displayed alongside the particle based data. Combining 2D slices and 3D views in an interactive way improves perception of the underlying physics and speeds up the development cycle of simulation code. In addition to rendering particles themselves, we can improve visualization by generating particle trails to show motion history, glyphs to show vector fields, transparency to enhance or diminish areas of high/low interest and multiple views of the same or different data for comparative visualization. We combine these techniques with interactive control or arbitrary scalar parameters and animation through time to produce a feature rich environment for exploration of SPH data

High performance computing 3D SPH model: Sphere impacting the free-surface of water

In this work, an analysis based on a three-dimensional parallelized SPH model developed by ECN and applied to free surface impact simulations is presented. The aim of this work is to show that SPH simulations can be performed on huge computer as EPFL IBM Blue Gene/L with 8'192 cores. This paper presents improvements concerning namely the memory consumption, which remains quite subtle because of the variable-H scheme constraints. These improvements have made possible the simulation of test cases involving tens of millions of particles computed by using more than thousand cores. Furthermore, pv-meshless developed by CSCS, is used to show the pressure field and the effect of impact

SPH High-Performance Computing simulations of rigid solids impacting the free-surface of water

Numerical simulations of water entries based on a three-dimensional parallelized Smoothed Particle Hydrodynamics (SPH) model developed by Ecole Centrale Nantes are presented. The aim of the paper is to show how such SPH simulations of complex 3D problems involving a free surface can be performed on a super computer like the IBM Blue Gene/L with 8,192 cores of Ecole polytechnique fédérale de Lausanne. The present paper thus presents the different techniques which had to be included into the SPH model to make possible such simulations. Memory handling, in particular, is a quite subtle issue because of constraints due to the use of a variable-h scheme. These improvements made possible the simulation of test cases involving hundreds of million particles computed by using thousands of cores. Speedup and efficiency of these parallel calculations are studied. The model capabilities are illustrated in the paper for two water entry problems, firstly, on a simple test case involving a sphere impacting the free surface at high velocity; and secondly, on a complex 3D geometry involving a ship hull impacting the free surface in forced motion. Sensitivity to spatial resolution is investigated as well in the case of the sphere water entry, and the flow analysis is performed by comparing both experimental and theoretical reference results

Numerical Simulation of Nonlinear Self Oscillations of a Full Load Vortex Rope

Self excited instabilities or oscillations of a cavitating full load vortex rope occur due to an interaction between the gas volume and the acoustic waves. From the onset of the oscillations, the amplitudes grow until they reach a maximum, called the “limit cycle”. The aim of this paper is to predict and to simulate this full load instability with its corresponding “limit cycle”. The test case is a reduced scale model installed on test rig in the Laboratory for Hydraulic Machines at the EPFL. An advanced hydro acoustic vortex rope model is developed to take into account the energy dissipation due to thermodynamic exchange between the gas and the surrounding liquid. Three key hydro acoustic parameters are set up using both steady CFD simulations and analytical models. First of all, parameters are assumed to be constant and time domain simulation is divergent without reaching the limit cycle. However frequency of instability is well predicted. Then inclusion of nonlinear parameters is found to lead to a limit cycle of finite amplitude. Prediction is compared with results from experiments and is in good agreement. It is shown that nonlinearity of the viscoelastic damping parameter, modelling the energy dissipation, is decisive to reach the limit cycle. Moreover, an energy approach is developed to understand the interaction process between the mass source and the system dissipation to reach the equilibrium at the limit cycle. It brings out that over one period the dissipation can provide energy to the system whereas the mass source dissipates to ensure the equilibrium

Surface Roughness Impact on Francis Turbine Performances and Prediction of Efficiency Step Up

In the process of turbine modernizations, the investigation of the influences of water passage roughness on radial flow machine performance is crucial and validates the efficiency step up between reduced scale model and prototype. This study presents the specific losses per component of a Francis turbine, which are estimated by CFD simulation. Simulations are performed for different water passage surface roughness heights, which represents the equivalent sand grain roughness height. As a result, the boundary layer logarithmic velocity profile still exists for rough walls, but moves closer to the wall. Consequently, the wall friction depends not only on roughness height but also on its shape and distribution. The specific losses are determined by CFD numerical simulations for every component of the prototype, taking into account its own specific sand grain roughness height. The model efficiency step up between reduced scale model and prototype value is finally computed by the assessment of specific losses on prototype and by evaluating specific losses for a reduced scale model with smooth walls. Furthermore, surveys of rough walls of each component were performed during the geometry recovery on the prototype and comparisons are made with experimental data from the EPFL Laboratory for Hydraulic Machines reduced scale model measurements. This study underlines that if rough walls are considered, the CFD approach estimates well the local friction loss coefficient. It is clear that by considering sand grain roughness heights in CFD simulations, it forms a significant part of the global performance estimation. The availability of the efficiency field measurements provides a unique opportunity to assess the CFD method in view of a systematic approach for turbine modernization step up evaluation. Moreover, this paper states that CFD is a very promising tool for future evaluation of turbine performance transposition from the model scale to the prototype

Phénomènes en proche paroi et stabilité de la colonne pour un arc électrique dans les disjoncteurs basse tension

Low-tension circuit breakers generally rely on the extinction of the electric arc created when the current-carrying contacts separate. The dynamics of the arc has been studied for a number of years using numerical simulations of the coupled fluid dynamics/electromagnetic governing equations. One of the objectives of this thesis is to improve numerical simulation near the wall, where there is a thin boundary layer whose accurate numerical description requires fine meshes, and hence long computing times, unless special measures are taken. Different simulations, based on an existing code, have aided in the development of a two region model, with one region away from the wall and another representing the boundary layer. In the external zone, the full equations are used, but the boundary conditions at the wall are modified by the existence of the thin wall-layer. This external flow forms the outer boundary conditions for a boundary layer calculation. As part of own numerical studies, we have also compared results of simulations with those of experiments. The second topic of the thesis is the stability of an arc column. A basic arc state id perturbed and the growth or decay of the perturbations with time used to determine the stability or otherwise of that basic state. The basic state used was a simple, steady, axisymmetric arc of infinite length, but we found that, in general, to maintain such an arc steady, some means of extracting the heat generated by the arc was necessary, a requirement which was met by introducing a sink of mass at the axis. A study of such arcs, cooled by the influx from outside, showed a variety of families of different solutions, for a given pressure and electric filed strength. Having selected a particular basic state, the linearized equations were solved for the normal modes and their eigenvalues used to examine stability. Different types of unstable and stable modes were found, which were interpreted in terms of magnetic diffusion, acoustics and unstable couples modes