Modèle multi-bulles pour la cavitation : comparaison avec un modèle basé sur l'équation de Rayleigh-Plesset

Dans le modèle multi-bulles pour la cavitation, on considère deux bulles non identiques localisées dans un volume de contrôle. La présence de deux bulles introduit une instabilité. Le modèle intègre le constat expérimental de disparition très précoce de petites bulles d'une part et d'autre part la prise en compte de l'interaction entre les bulles par la possibilité d'échange de volume de vide entre des bulles voisines. Dans cette étude nous comparons dans un cas simple le modèle multi-bulles avec un modèle basé sur l'équation de Rayleigh-Plesset où on prend en compte plus que deux bulles

Experimental study of a cavitating centrifugal pump during fast starts-up

The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behavior is reached. The pump transient behavior induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behavior during rapid starting periods. Three different types of fast start-up behaviors have been identified. According to the final operating point, the start-up is characterized either by a single drop of the delivery static pressure, by several low-frequency drops, or by a water hammer phenomenon that can be observed both a the inlet and outlet of the pump. A physical analysis is proposed to explain these three different types of transient flow behavior

"Low Cost" Approaches for the Prediction of Rotating Instabilities in the Vaneless Diffuser of a Radial Flow Pump

The instabilities, occurring in pumps operating at partial flow rates, may directly cause several negative effects, such as vibration, noise, mechanical damage. The present study is focused on the investigation of the instabilities developing in the vaneless diffuser of a radial flow pump. The prediction of this kind of complex phenomena by numerical simulation usually involves complex (3D unsteady) and expensive calculations. The main goal of this paper is to propose and evaluate some “low cost” numerical or analytical approaches to predict the arising and the characteristics of rotating stall instabilities. For that purpose, two-dimensional numerical unsteady calculations were used. The boundary condition for the diffuser inlet was based on the flow field at the impeller outlet and static pressure was set at the diffuser outlet. The simulations have been managed for several flow rates conditions. The results of the numerical simulation are discussed and compared to i/ an already existing database (including PIV and pressure measurements), ii/ results of a 2D linear stability analysis

Investigation of the Vortex Ring Transition using Scanning Tomo-PIV

The transition of a vortex ring at ReΓ = 5030 is studied by time-resolved scanning tomographic PIV technique. The transition process is first analyzed through flow quantities such as circulation and vorticity components. Using the volumetric measurement technique, vortical organization of the vortex ring at early and late transition stages are visualized respectively. Focus is paid to the instability phenomenon associated with transition. The present 4D flow data allows analysis of the temporal evolution of the wavenumber spectra. The dominant wavenumbers in transition are identified and the growth of their amplitude is revealed. The vortex ring transition is finally studied through the particle trajectories. A phase difference between the axial velocity and radial velocity is found in the beginning of transition, however, it is subject to change following the progression of transition. Statistical analysis on the velocity components helps to identify the aft portion of the inner ring as the one that is first to lose the original phase relation in velocity, which is caused by the secondary vortical activity during transition

Static pressure recovery analysis in the vane island diffuser of a centrifugal pump

The overall performance of a vane-island type diffuser of a centrifugal pump model was obtained by means of directional probe traverses. These measurements were performed in an air model of a real hydraulic pump for five volume flow rates. Directional probe traverses are performed with a classical three-hole probe to cover most of the complete inlet section of the diffuser from hub to shroud and from pressure to suction side. Existing Particle image velocimetry (PIV) measurement results are also used to compare probe measurement results between the inlet and outlet throats of vane island diffuser at mid-span. Some assistance from already existing unsteady calculation, including leakage effects, is used to evaluate the numerical approach capability and to correctly define the mean initial conditions at impeller’s outlet section. Pressure recovery and the measured total pressure loss levels inside this particular vane diffuser geometry are then calculated. Detailed analysis of the flow structure at the inlet section of the vane island diffuser is presented to focus on pressure evolution inside the entire diffuser section for different flow rates. The combined effects of incidence angle and blockage distributions along hub to shroud direction are found to play an important role on loss distribution in such a diffuser

Socio-economic assessement of farmers' vulnerability as water users subject to global change stressors in the hard rock area of southern India. The SHIVA ANR project

International audienceDemand for vulnerability assessments is growing in policy-making circles, to support the choice of appropriate measures and policies to reduce the vulnerability of water users and resources. Through the SHIVA ANR project, we are seeking a method to assess and map the vulnerability of farmers in southern India to both climate and socioeconomic changes, and secondly, to assess the costs and benefits associated with trends farmers' vulnerability in the medium and long-term. The project is focusing on southern India 's hard rock area, as in the geological context, both surface and ground water resources are naturally limited. We are also focusing on farming populations as these are the main water users in the area and rely exclusively on groundwater. The area covers southern India's semi-arid zone, where the rainfall gradient ranges from 600 mm to 1100 mm. Vulnerability is expected to vary according to local climatic conditions but also the socioeconomic characteristics of farming households. The SHIVA research team has been divided into six thematic groups in order to address the different scientific issues : downscaling the regional climate scenario, farm area projections, vulnerability assessments and quantification, vulnerability mapping, hydrological modelling and upscaling, and vulnerability impact assessements. Our approach is multidisciplinary to cater for for numerous inherent themes, and integrated to cater for vulnerability as a dynamic and multidimensional concept. The project 's first results after 10 months of research are presented below

Douarnenez, port de pêche

Dazin A. Douarnenez, port de pêche. In: Annales de Géographie, t. 35, n°194, 1926. pp. 179-181

Data reduction problems using a 3 holes directional pressure probe to investigate mean flow characteristics in the vaneless gap between impeller and diffuser radial pump

Among several different measurement techniques that have been already performed and presented in a radial impeller pump model including PIV, a directional pressure probe has been used to obtain mean velocity field and stagnation pressure between impeller outlet and the inlet vaned diffuser sections. These measurements are supposed to get more information not only about global pump head coefficient including vaned diffuser ones but also about impeller performances itself. Pressure probe information is affected by rotor-stator interactions and impeller rotation, and this paper presents a way to explain and correct pressure probe indications in order to achieve a better evaluation of overall impeller mean performances. The use of unsteady RANS calculation results is found to be a useful way to perform better data reduction analysis for this purpos

Modelling of a radial pump fast startup with the CATHARE-3 Code and analyse of the loop response

International audienceThe french alternative energies and atomic energy commission (CEA) is currently doingresearch in the pump modelling field. A predictive transient, two-phase flow rotodynamicpump model has been developed in the CATHARE-3 code. Flow inside parts of the pump(suction, impeller, diffuser, volute) is computed according to a one-dimensional discretization.A mean flow path has to be defined for such a modelling. Validation has startedduring years 2017-18 at the component scale by comparison to an existing experimentaldatabase. The ability of the model to predict head and torque as a function of time duringa 1-second pump fast start-up has been proved when imposing rotational speed and flowrate evolutions. The present study aims at extending the validation at the system scale awhole loop is modelled