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Person# Audrey Paulette Solange Maertens

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Erosion

Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another locatio

Soil erosion

Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water,

Multiscale modeling

Multiscale modeling or multiscale mathematics is the field of solving problems that have important features at multiple scales of time and/or space. Important problems include multiscale modeling of

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Siamak Alimirzazadeh, François Avellan, Ebrahim Jahanbakhsh, Takashi Kumashiro, Sebastián Camilo Leguizamón Sarmiento, Audrey Paulette Solange Maertens, Christian Vessaz

The Pelton turbine is an impulse turbine typically installed for high head hydroelectric power plants. For a site with a rated head H and discharge Q, a higher specific speed turbine results in a more compact generating unit with reduced manufacturing costs but requires a larger number of jets. However, by increasing the number of jets and specific speed, the water jets tend to interfere, creating a significant energy loss. In the present research, the interaction between two adjacent jets in a six-jet Pelton runner is simulated using a GPU-accelerated particle-based in-house solver based on the 3-D Finite Volume Particle Method (FVPM). The numerical simulations are performed at eight operating points ranging from N/NBEP = 0.89 to N/NBEP = 1.31; where N is the runner rotational speed, and BEP is the Best Efficiency Point. The torque and efficiency trends, as well as the speed range in which the jets interfere, are well-captured, which provides confidence in the use of the numerical simulations for the design optimization of Pelton turbines. The simulations, in particular, evidence a significant torque and efficiency drop at high rotational speeds, due to jet interference. Furthermore, jet disturbance yields load fluctuations at rotational speeds both lower and higher than the NBEP, which is likely to amplify fatigue damage. Both phenomena are worth considering that in the design process of a Pelton machine.

2020Siamak Alimirzazadeh, François Avellan, Ebrahim Jahanbakhsh, Sebastián Camilo Leguizamón Sarmiento, Audrey Paulette Solange Maertens

The erosion of hydraulic machines by solid particle impacts is a widespread problem that leads to outage for expensive repairs, efficiency reduction, and cavitation enhancement. Numerical simulations can be used to study the phenomenon, provided they feature accurate thermomechanical and contact modeling. Numerical investigations often assume that the impacting particles are spherical and rigid, with only some recent studies modeling them as elastic polyhedrons or spheres. However, in the specific case of the erosion of hydraulic machines, particles are far from spherical or polyhedral and are less rigid than the base material. The present investigation focuses on the effect of the particle shape and elasticity on the erosion of oxygen-free copper and martensitic stainless steel 13Cr-4Ni impacted by quartz sediments. First, a novel algorithm to generate realistically-shaped sediment discretizations is presented, bypassing the need to use simplified shapes such as polyhedrons. The algorithm is shown to produce particle discretizations of predefined characteristic size that closely follow the objective sphericity value selected, which can cover the full range of sphericity values found in real sediments. Then, the effect of the particle elasticity on the impact damage is investigated, revealing that an error of up to 38 % is introduced by assuming that the particles are rigid. The effect of the particle shape is then assessed. For the case of copper, sharp sediments generate an increase in damage per unit mass of up to 225 % with respect to spherical particles; a comparable effect is expected on the erosion rate. For the martensitic stainless steel, the shape effect is similar in character but significantly weaker in magnitude. The results are analyzed and explained in terms of the known erosion mechanisms and their dependence on the particle shape, the material ductility and hardness.

Siamak Alimirzazadeh, François Avellan, Ebrahim Jahanbakhsh, Takashi Kumashiro, Sebastián Camilo Leguizamón Sarmiento, Audrey Paulette Solange Maertens

The numerical investigation of the unsteady flow patterns around a Pelton bucket can be helpful to improve the overall turbine efficiency by optimizing the bucket design based on identified loss mechanisms. Since the flow is highly turbulent, modeling the effect of turbulence can bring about improved predictions. In this paper, two RANS-based eddy viscosity models (namely the standard and realizable k-epsilon) have been implemented as a module in a particle-based in-house solver, GPU-SHPEROS. A scalable wall function based on the log-law has been utilized to model the flow in the near-wall region. The solver has been accelerated on GPUs and is based on the Finite Volume Particle Method (FVPM), which is a locally conservative and consistent particle-based method including many of the attractive features of both particle-based methods (e.g. SPH) and conventional mesh-based methods (e.g. FVM). As a mesh-five method based on the Arbitrary Lagrangian Eulerian (ALE) formulation, FVPM is robust in handling five surface flows and large boundary deformations, such as the ones found in rotating Pelton buckets. The validation of the turbulence models implementation within FVPM is presented for internal and free surface flows. Then, the effectiveness of the turbulence models in the case of rotating Pelton buckets is assessed by comparing the predicted torque time histories to experimental data acquired on a model-scale test rig.

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