Coupling of the meshless SPH-ALE method with a finite volume method

This paper presents a novel coupling algorithm for a Smoothed Particle Hydrodynamics-Arbitrary Lagrange Euler (SPH-ALE) and a mesh-based Finite Volume (FV) method where information is transferred in two ways. On the one hand, we use the FV calculation points as SPH neighbors in the regions where the mesh is overlapping the SPH-ALE particles. On the other hand, the boundary conditions for the FV domain are interpolated from the SPH-ALE particles, similar to what is done in the CHIMERA method of overlapping meshes. In contrast to the CHIMERA method, interpolation is not performed on a structured grid but on a set of unstructured points. Hence, an interpolation technique for scattered data is used. The approach is carefully validated by means of well-known academic testcases that show very encouraging results. Our final aim is the simulation of transient flows in hydraulic machines

Simulation des écoulements à surface libre dans les turbines Pelton par une méthode hybride SPH-ALE

International audienceAn Arbitrary Lagrange Euler (ALE) description of fluid flows is used together with the meshless numerical method Smoothed Particle Hydrodynamics (SPH) to simulate free surface flows. The ALE description leads to an hybrid method that can be closely connected to the finite volume approach. It is then possible to adapt some common techniques like upwind schemes and preconditioning to remedy some of the well known drawbacks of SPH like stability and accuracy. An efficient boundary treatment based on a proper upwinding of fluid information at the boundary surface is settled. The resulting SPH-ALE numerical method is applied to simulate free surface flows encountered in Pelton turbines.La méthode numérique sans maillage Smoothed Particle Hydrodynamics (SPH) est modifiée par l'adoption d'une description Arbitrary Lagrange Euler (ALE) des écoulements fluides, dans le but de simuler des écoulements à surface libre. Le formalisme ALE conduit à une méthode numérique hybride s'apparentant sur de nombreux points à une approche volumes finis. Il est alors possible d'adapter des techniques numériques courantes comme les schémas décentrés et le préconditionnement pour résoudre certains défauts majeurs de la méthode SPH, comme la stabilité numérique ou le manque de précision. Par ailleurs, le traitement des conditions limites est réalisé par un décentrement approprié des informations fluides sur les surfaces frontières. La méthode numérique SPH-ALE résultante est appliquée à la simulation d'écoulements à surface libre tels que ceux rencontrés dans les turbines Pelton

Simulation numérique de l'érosion par impacts répétés de gouttes d'eau à grand vitesse sur les augets de turbine Pelton

Le travail présenté ici concerne la simulation numérique d'un impact de goutte d'eau sur un massif axisymétrique en acier. Les calculs fluide/structure sont séparés : d'un côté le calcul fluide est réalisé par la méthode SPH-ALE et concerne la chute d'une goutte sur un mur rigide, et de l'autre côté, le calcul de structure est effectué par éléments finis avec une pression imitant l'impact de la goutte

Prediction and numerical simulation of droplet impact erosion on metallic structure

Hydraulic turbines can undergo severe damage during operation, because of low quality water or detrimental flow conditions. Damage induces maintenance costs and power production losses, and can also endanger safety of installations. Hydropower plants operators and turbine manufacturers are interested in extending overhaul periods by reducing damage intensity and protecting turbine components with surface treatments, but accurate and reliable prediction of damage is however missing. The present work is related to the erosion arising from repeated impacts of high speed water droplets on specific parts of Pelton turbines. Indeed for high head Pelton units, the jet of water is composed of a liquid core surrounded by droplets. Observations show that regions of impact of these droplets exhibit specific erosion patterns. The aim of the work consists in understanding the corresponding erosion mechanisms through detailed numerical simulation (micro-scale) of the impacts of high speed liquid droplets on turbine components. These results will then be transposed at the machine level (macro-scale) in order to predict the damage along the life cycle of the turbine. When a high velocity water droplet with small diameter impacts a rigid surface, the ``water-hammer'' pressure due to inertial effect appears in the water droplet at the central contact zone, though the maximum pressure occurs on the envelope of the contact area and may be far higher than water-hammer pressure. The impact causes the traveling of a shock wave across the droplet, and lateral jetting occurs by compression when the wave front overtakes the contact area. Concerning the structure, erosion has been found to be due to fatigue cracking. First, material grains are weakened during an ``incubation'' phase. After a large number of impacts, micro-cracks emerge and lead to ejection or fracture of grains, what is called ``amplification'' phase. Numerical simulations are performed subsequently to understand and get a more detailed analysis. The entire simulation is modeled in explicit dynamics with a strong 2-way coupler which is energy conservative at the interface. The solid domain is computed by the finite elements method with Europlexus code, and the fluid one is discretized by Smoothed Particle Hydrodynamics with an Arbitrary Lagrangien-Eulerian description by an in-house software. Thanks to these simulations, the pressure peak on the contact surface can be found. Results are in line with literature and the impulse of the impact allows to locate the most loaded zones of the area. The fatigue-based mechanism is validated by observing the change of sign of hydrostatic stress. Finally, a post-processing erosion program developed with a simple damage criterion provides the location of the most eroded zones of the structure during a loading cycle. The stress range of the transient simulation is computed and gives a number of cycles to failure by a S-N curve for each Gauss point of the solid mesh. A number of cycles is chosen, which allows to make some elements eroded then removed from the mesh for further droplet impact simulations. This post-processing makes some strong assumptions for the number of cycles: every droplet falls with the same velocity, angle, diameter, density at the same place and water is cleaned between each impact. Moreover we assume that geometry does not change during the chosen number of cycles. These first results will be a strong basis for a sensitivity analysis on main impact parameters (droplets diameter and velocity). It is also planned to investigate the influence of a thin water layer set on the solid surface to mimic the wet environment, and a multi-layer material to take into account the coated surface of Pelton buckets

A partitioned approach for the coupling of SPH-ALE and FE methods for transient FSI problems with incompatible time-steps

Une technique de couplage des sous-domaines garantissant la conservation de l'energie à l'interface préalablement développée permet de réaliser des simulations fluide-structure précises et stables. Si l'on utilise des intégrateurs temporels explicites pour réaliser ce couplage, il devient nécessaire de pouvoir intégrer chaque sous-domaine avec un pas de temps distinct afin d'éviter que les contraintes de pas de temps d'un sous-domaine soient imposées aux autres sous-domaines du calcul. Le but de cette étude est de mettre en place une méthode permettant d'intégrer chaque sous-domaine avec des pas de temps différents tout en respectant les propriétés de conservation de l'energie à l'interface

Simulation des écoulements à surface libre dans les turbines Pelton par une méthode hybride SPH-ALE

International audienceAn Arbitrary Lagrange Euler (ALE) description of fluid flows is used together with the meshless numerical method Smoothed Particle Hydrodynamics (SPH) to simulate free surface flows. The ALE description leads to an hybrid method that can be closely connected to the finite volume approach. It is then possible to adapt some common techniques like upwind schemes and preconditioning to remedy some of the well known drawbacks of SPH like stability and accuracy. An efficient boundary treatment based on a proper upwinding of fluid information at the boundary surface is settled. The resulting SPH-ALE numerical method is applied to simulate free surface flows encountered in Pelton turbines.La méthode numérique sans maillage Smoothed Particle Hydrodynamics (SPH) est modifiée par l'adoption d'une description Arbitrary Lagrange Euler (ALE) des écoulements fluides, dans le but de simuler des écoulements à surface libre. Le formalisme ALE conduit à une méthode numérique hybride s'apparentant sur de nombreux points à une approche volumes finis. Il est alors possible d'adapter des techniques numériques courantes comme les schémas décentrés et le préconditionnement pour résoudre certains défauts majeurs de la méthode SPH, comme la stabilité numérique ou le manque de précision. Par ailleurs, le traitement des conditions limites est réalisé par un décentrement approprié des informations fluides sur les surfaces frontières. La méthode numérique SPH-ALE résultante est appliquée à la simulation d'écoulements à surface libre tels que ceux rencontrés dans les turbines Pelton

Coupling of the meshless SPH-ALE method with a finite volume method

This paper presents a novel coupling algorithm for a Smoothed Particle Hydrodynamics-Arbitrary Lagrange Euler (SPH-ALE) and a mesh-based Finite Volume (FV) method where information is transferred in two ways. On the one hand, we use the FV calculation points as SPH neighbors in the regions where the mesh is overlapping the SPH-ALE particles. On the other hand, the boundary conditions for the FV domain are interpolated from the SPH-ALE particles, similar to what is done in the CHIMERA method of overlapping meshes. In contrast to the CHIMERA method, interpolation is not performed on a structured grid but on a set of unstructured points. Hence, an interpolation technique for scattered data is used. The approach is carefully validated by means of well-known academic testcases that show very encouraging results. Our final aim is the simulation of transient flows in hydraulic machines

Hybridized guard particles for Adaptive Particle Refinement

International audienceThe Adaptive Particle Refinement (APR) is a promising local refinement technique, which results in a coupling between two overlapping SPH domains. Each domain is a uniform particle distribution composed by either larger particles (mothers) or smaller particles (daughters). The coupling is mediated by guard particles that act as boundary conditions at the interface between the two domains. An oscillating 2D channel test case is simulated. It is found that the APR method has a lack of robustness in Lagrangian simulations of flows going back and forth. The coupling appears to be particularly sensitive to the guard particles layout. The present paper aims at improving the APR method robustness, performing a guard particles selfregulated resettlement to maintain a suitable tesselation. Therefore, the guard particles are hybridized with both interpolated and SPH computed fields. This method is called Hybridized Adaptive Particle Refinement (HAPR). At first, a 2D shock tube test case is performed for APR and HAPR methods validation, with satisfactory results in comparison with both analytic and fully-refined SPH solutions. Then, the oscillating channel test case is used for APR and HAPR methods comparison. The proposed remodeling improves the robustness

A partitioned approach for the coupling of SPH and FE methods for transient nonlinear FSI problems with incompatible time-steps

International audienceWe propose a method to couple smoothed particle hydrodynamics and finite elements methods for nonlinear transient fluid–structure interaction simulations by adopting different time‐steps depending on the fluid or solid sub‐domains. These developments were motivated by the need to simulate highly non‐linear and sudden phenomena requiring the use of explicit time integrators on both sub‐domains (explicit Newmark for the solid and Runge–Kutta 2 for the fluid). However, due to critical time‐step required for the stability of the explicit time integrators in, it becomes important to be able to integrate each sub‐domain with a different time‐step while respecting the features that a previously developed mono time‐step coupling algorithm offered. For this matter, a dual‐Schur decomposition method originally proposed for structural dynamics was considered, allowing to couple time integrators of the Newmark family with different time‐steps with the use of Lagrange multipliers. Copyright © 2016 John Wiley & Sons, Ltd