RANS and LES simulations at partial load in Francis turbines: Three-dimensional topology and dynamic behaviour of inter-blade vortices

Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions

RANS and LES Simulations at Partial Load in Francis Turbines: Three-Dimensional Topology and Dynamic Behaviour of Inter- Blade Vortices

International audienceHydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions

Numerical simulation and analysis at partial load in Francis turbines: Three-dimensional topology and frequency signature of inter-blade vortices

International audienceHydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. A previous experimental study [S. Bouajila et al., IOP Conf. Ser.: Earth Environ. Sci., 2016] has shown that the appearance of the inter-blade vortices can be correlated with a large-band frequency signature in the pressure fluctuations measured on the blades. The numerical simulations presented in this paper focused on the prediction of this frequency signature as well as on the analysis of its origin

Les sciences sociales pour penser les acteurs dans les didactiques des sciences sociales

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Introduction

Dans les didactiques de l’histoire, de la géographie et de l’éducation à la citoyenneté, il est encore peu fréquent d’étudier l’enseignement et l’apprentissage en s’appuyant sur des figures et catégories construites de l’acteur et de l’action. Le colloque international dont cet ouvrage est issu invitait ses participants à réfléchir à la place et aux statuts des acteurs dans les recherches conduites au sein de ces didactiques disciplinaires. Les rares synthèses existantes dans le champ des did..

Acteurs et action. Perspectives en didactiques de l'histoire et de la géographie

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Conclusion de la partie 1

Ces quatre premiers articles éclairent l’action didactique à partir de l’appropriation par les élèves de situations d’enseignement qui leur sont proposées sur le mode de la rupture avec les fonctionnements classiques. Ils nous informent sur la façon dont ces situations vont se traduire pour eux en termes d’incertitude sur le sens de l’action en cours, sur les ressources qu’ils mobilisent face à cette incertitude, enfin, pour les chercheurs, sur les efforts d’analyse qui paraissent nécessaires..

Conclusion de la partie 2

La série de ces sept études donne à penser la question de la place et du rôle des acteurs dans nos didactiques en partant de l’enseignant. Elle nous renseigne en premier lieu sur les contraintes qu’il tend à subir, comme le signale le titre choisi pour cette partie de l’ouvrage, tout en caractérisant le rôle central qu’il joue et qui fait de lui le moteur de l’action dans la classe. Pourtant, c’est l’image d’un acteur incertain qui transparaît aussi dans chacune de ces études, loin de celle d..

Les sciences sociales pour penser les acteurs dans les didactiques des sciences sociales

International audienc

Les sciences sociales pour penser les acteurs dans les didactiques des sciences sociales

International audienc