9 research outputs found

    Experimental investigation of cavitation-induced erosion around a surface-mounted bluff body

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    The objective of this study is to investigate the collapsing behavior of cavitation, which leads to the erosion of material. An experimental examination was conducted in a channel with a semi-circular cylinder obstacle, which serves as a “vortex cavity” generator. Cavitation was achieved by employing a range of pressure differences over the test section and a high-speed camera was used to observe the cavitation behavior. The flow field behind the semi-circular cylinder was investigated as a characteristic example of bluff bodies that exhibit a distinct, separated vortex flowin their wake. The cases with the bluff body were also compared to the ones without the bluff body. Erosion tests were performed using paint (stencil ink). The intensity of cavitation is characterized by the cavitation number (σ); the lower the cavitation number, the higher the cavitation intensity. The erosion (removal of paint) after 40\ua0min of operation revealed distinct and repeatable results. For a high cavitation number, a large number of von Karman-vortex-like cavities are shed downstream of the obstacle. This results in a higher number of collapse events and, ultimately, more erosion. On the other hand, at lower cavitation numbers, the erosion took place at the cavity\u27s closure line. It was seen that with the increase in cavitation intensity, the erosion area increases. Moreover, the bluff body obstacle promotes and localizes cavitation-induced erosion on the sample plate compared to the cases without the bluff body. This ultimately means that in the cases with the bluff body, less power is required in the system to cause erosion. The erosion patterns caused by the bluff body cavitation are more repeatable compared to the cases without the bluff body due to the localized cavitation load. The erosion pattern from the paint test is also compared with a material loss test (30\ua0h of operation). A very good qualitative agreement is found between the two tests, with the paint test requiring approximately two orders of magnitude less running time of the facility. We demonstrate that paint tests, combined with this geometry, provide an efficient and economical way to investigate erosion patterns compared to expensive material loss tests

    Experimental study of cavitation erosion around a surface-mounted semi-circular cylinder

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    The objective of this study is to investigate the collapsing behavior of cavitation, which leads to erosion. For this purpose, an experimental investigation was performed in a channel with a semi-circular cylinder obstacle at the Hydraulic Laboratory of ANDRITZ HYDRO in Vevey. Cavitation was achieved by employing a range of pressure differences over the test section. The obstacle promotes and localizes cavitation-induced erosion. The flow field behind the semi-circular cylinder was investigated as a characteristic example of bluff bodies, which exhibit a distinct separated vortex flow in their wake. A high-speed camera observed the cavitation behavior. At the same time, erosion tests were performed using paint (stencil ink). The intensity of cavitation is described by the cavitation number (σ), the lower the cavitation number, the higher the cavitation intensity. Three erosive cases at different cavitation numbers are presented here. The erosion (removal of paint) after 40-60 mins of operation revealed distinct and repeatable results. These results will serve as validation data for numerical studies. For a high cavitation number, a large number of Karman-vortex-like cavities are shed downstream of the obstacle. This results in a higher number of collapse events and ultimately more erosion. On the other hand, at lower cavitation numbers the erosion took place at the closure line of the cavity. We demonstrate that paint tests in combination with this geometry provide an efficient and economical way to investigate erosion patterns compared to expensive material loss tests

    DĂ©veloppement d’une mĂ©thode couplĂ©e SPH-ALE / Volumes Finis pour la simulation des Ă©coulements transitoires dans les machines hydrauliques

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    The increased use of intermittent forms of renewable energy like wind and solar energy produces fluctuations in the electric grid that have to be compensated. For this reason, hydraulic machines like turbines and pumps are more often operated under non-conventional operating conditions and are submitted to frequent starts and stops. This type of operating conditions has important consequences on the life cycle of the machines. It is thus of paramount importance that transient flows at off-design conditions are properly taken into account in the design phase and numerical simulation is an appropriate way to do so. The present study aims at developing a flexible coupling method of the meshbased Finite Volume Method (FVM) and the meshless Smoothed Particle Hydrodynamics - Arbitrary Lagrange Euler (SPH-ALE) method, which can be used as a tool for the investigation of transient phenomena in hydraulic machines. SPH-ALE is very well adapted for the simulation of highly dynamic flows with moving geometries but has difficulties to correctly represent rapidly changing gradients of the field variables. Particle refinement is difficult to implement, especially if particles are refined in an anisotropic way. FV methods are well established in CFD because of their accuracy and stability. However, they can be tedious for simulations with moving geometries and often necessitate an interface between moving and static parts of the mesh which introduces additional errors. To overcome the shortcomings of both methods, a coupling method is developed that uses a decomposition of the computational domain into regions where the physical field variables are computed by the FV method, regions where they are computed by SPH-ALE and overlapping regions where the information is transferred from the FV domain to the SPH domain and vice versa. In the overlapping regions FV calculation points are used as neighbors for the SPH integration in space. At the boundaries of the FV mesh, velocity and pressure are interpolated from the SPH particles by means of scattered data interpolation techniques, similarly to Chimera methods for overlapping grids. For this study, an existing SPH-ALE software of the ANDRITZ Group is used. A weakly compressible FV solver is implemented into this software that discretizes the same form of the Navier-Stokes equations than the SPH-ALE solver. Similar to the present SPH-ALE method, Riemann solvers with reconstructed states, obtained by a MUSCL scheme, are employed. Moreover, adaptations and improvements of the SPH-ALE solver itself are made, which are important for the coupling and for the simulation of internal flows in general. Thus, subsonic inlet and outlet conditions are implemented. Furthermore, a correction method of the kernel gradient is presented that ensures zeroth order consistency of the SPH-ALE approximation of the divergence of the convective fluxes. The correction improves greatly the SPH pressure field on non-uniform particle distributions. The implemented coupled method is successfully validated by means of inviscid academic one-dimensional and two-dimensional testcases like a shock tube case, Taylor-Green vortices and the flow around a symmetric NACA airfoil with particles in Eulerian description. Furthermore, the coupling provides a possibility to implement outlet boundary conditions to Lagrangian moving SPH particles. It is then applied to the simulation of transient flows in rotor stator systems in 2D with moving particles.L'utilisation croissante des sources d'Ă©nergie renouvelable avec une grande volatilitĂ© de production, comme l'Ă©nergie Ă©olienne et solaire, conduit Ă  des fluctuations dans le rĂ©seau Ă©lectrique qui doivent ĂȘtre compensĂ©es. Pour cette raison les machines hydrauliques, turbines et pompes, sont plus souvent opĂ©rĂ©es dans les rĂ©gimes de fonctionnement hors fonctionnement nominal et la frĂ©quence des phases de dĂ©marrage et arrĂȘt augmente. Ce type de fonctionnement peut avoir des consĂ©quences importantes sur le cycle de vie des machines. Il est donc essentiel de prendre en compte l'Ă©coulement dans les phases transitoires lors de la conception de la machine et la simulation numĂ©rique des Ă©coulements est un outil adaptĂ© pour cela. La prĂ©sente Ă©tude a pour objectif de dĂ©velopper une mĂ©thode de couplage flexible qui combine la mĂ©thode Ă  maillage volumes finis (VF) et la mĂ©thode sans maillage Smoothed Particle Hydrodynamics - Arbitrary Lagrange Euler (SPH-ALE). Cette mĂ©thode couplĂ©e peut ĂȘtre utilisĂ©e comme outil pour l'investigation des phĂ©nomĂšnes transitoires dans les machines hydrauliques. SPH-ALE est particuliĂšrement bien adaptĂ© aux simulations des Ă©coulements fortement dynamiques avec des gĂ©omĂ©tries mobiles mais elle a des difficultĂ©s pour calculer des forts gradients de pression et vitesse. Un raffinement de particules est difficile Ă  implĂ©menter, surtout si les particules doivent ĂȘtre raffinĂ©es de maniĂšre anisotrope. Les mĂ©thodes volumes finis (VF) sont Ă©tablies pour les simulations numĂ©riques d'Ă©coulements grĂące Ă  leur stabilitĂ© et prĂ©cision. Par contre, elles peuvent ĂȘtre lourdes pour les simulations avec des gĂ©omĂ©tries mobiles et demandent souvent une interface entre des parties mobiles et statiques du maillage ce qui gĂ©nĂšre des erreurs supplĂ©mentaires. Pour combiner les deux approches complĂ©mentaires, une mĂ©thode de couplage a Ă©tĂ© dĂ©veloppĂ©e qui dĂ©compose le domaine de calcul en zones oĂč la vitesse et la pression sont calculĂ©es par la mĂ©thode VF, en zones oĂč elles sont obtenues par SPH-ALE et en zones de recouvrement oĂč les informations sont transfĂ©rĂ©es de la zone VF Ă  la zone SPH et inversement. Dans les zones de recouvrement les points de calcul VF sont utilisĂ©s comme voisins pour l'intĂ©gration en espace des particules SPH. Aux limites du maillage VF la vitesse et la pression sont interpolĂ©es des particules SPH, similairement aux mĂ©thodes ChimĂ©re des maillages recouvrants. Un logiciel SPH-ALE existant du groupe ANDRITZ est utilisĂ© pour cette Ă©tude. Un solveur VF faiblement compressible est implĂ©mentĂ© dans ce logiciel. Le solveur discrĂ©tise la mĂȘme forme des Ă©quations de Navier-Stokes que le solveur SPH-ALE. Des solveurs de Riemann avec des Ă©tats reconstruits par la mĂ©thode MUSCL sont employĂ©s. En outre, le solveur SPH-ALE est amĂ©liorĂ© et adaptĂ© aux Ă©coulements internes. Pour cette raison des conditions Ă  l'entrĂ©e et Ă  la sortie du type subsonique sont implĂ©mentĂ©es. Du plus, une mĂ©thode de correction du gradient de la fonction kernel est prĂ©sentĂ©e qui amĂ©liore la prĂ©cision du champ de pression, notamment si les particules ne sont pas distribuĂ©es rĂ©guliĂšrement. La mĂ©thode couplĂ©e est validĂ©e Ă  l'aide des cas test acadĂ©miques en unidimensionnel et en bidimensionnel, comme le cas de tube Ă  choc, les tourbillons de Taylor-Green et l'Ă©coulement autour d'une aube symĂ©trique du type NACA avec des particules en description eulĂ©rienne. En outre, le couplage offre la possibilitĂ© d'imposer des conditions Ă  la sortie aux particules lagrangiennes. La mĂ©thode est appliquĂ©e aux simulations d'Ă©coulement transitoire en 2D avec des particules qui se dĂ©placent en suivant les gĂ©omĂ©tries mobiles

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

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    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 ïŹnal aim is the simulation of transient ïŹ‚ows in hydraulic machines

    Worldwide trends in hypertension prevalence and progress in treatment and control from 1990 to 2019: a pooled analysis of 1201 population-representative studies with 104 million participants

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    International audienceBACKGROUND - Hypertension can be detected at the primary health-care level and low-cost treatments can effectively control hypertension. We aimed to measure the prevalence of hypertension and progress in its detection, treatment, and control from 1990 to 2019 for 200 countries and territories. METHODS - We used data from 1990 to 2019 on people aged 30–79 years from population-representative studies with measurement of blood pressure and data on blood pressure treatment. We defined hypertension as having systolic blood pressure 140 mm Hg or greater, diastolic blood pressure 90 mm Hg or greater, or taking medication for hypertension. We applied a Bayesian hierarchical model to estimate the prevalence of hypertension and the proportion of people with hypertension who had a previous diagnosis (detection), who were taking medication for hypertension (treatment), and whose hypertension was controlled to below 140/90 mm Hg (control). The model allowed for trends over time to be non-linear and to vary by age. FINDINGS - The number of people aged 30–79 years with hypertension doubled from 1990 to 2019, from 331 (95% credible interval 306–359) million women and 317 (292–344) million men in 1990 to 626 (584–668) million women and 652 (604–698) million men in 2019, despite stable global age-standardised prevalence. In 2019, age-standardised hypertension prevalence was lowest in Canada and Peru for both men and women; in Taiwan, South Korea, Japan, and some countries in western Europe including Switzerland, Spain, and the UK for women; and in several low-income and middle-income countries such as Eritrea, Bangladesh, Ethiopia, and Solomon Islands for men. Hypertension prevalence surpassed 50% for women in two countries and men in nine countries, in central and eastern Europe, central Asia, Oceania, and Latin America. Globally, 59% (55–62) of women and 49% (46–52) of men with hypertension reported a previous diagnosis of hypertension in 2019, and 47% (43–51) of women and 38% (35–41) of men were treated. Control rates among people with hypertension in 2019 were 23% (20–27) for women and 18% (16–21) for men. In 2019, treatment and control rates were highest in South Korea, Canada, and Iceland (treatment >70%; control >50%), followed by the USA, Costa Rica, Germany, Portugal, and Taiwan. Treatment rates were less than 25% for women and less than 20% for men in Nepal, Indonesia, and some countries in sub-Saharan Africa and Oceania. Control rates were below 10% for women and men in these countries and for men in some countries in north Africa, central and south Asia, and eastern Europe. Treatment and control rates have improved in most countries since 1990, but we found little change in most countries in sub-Saharan Africa and Oceania. Improvements were largest in high-income countries, central Europe, and some upper-middle-income and recently high-income countries including Costa Rica, Taiwan, Kazakhstan, South Africa, Brazil, Chile, Turkey, and Iran. INTERPRETATION - Improvements in the detection, treatment, and control of hypertension have varied substantially across countries, with some middle-income countries now outperforming most high-income nations. The dual approach of reducing hypertension prevalence through primary prevention and enhancing its treatment and control is achievable not only in high-income countries but also in low-income and middle-income settings

    Worldwide trends in hypertension prevalence and progress in treatment and control from 1990 to 2019: a pooled analysis of 1201 population-representative studies with 104 million participants

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