No abstract available

[en] The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

[en] In this paper for ideally plastic materials the influence of high temperature cavitation damage upon creep at steady increasing loads is investigated. The damage function A(t) enters a constitutive equation for plastic flow through an effective stress sigmasub(e). For given loading conditions the latter is derived from the solution of Hart's tensile test equation. The case of time linear increase in load (F = constant) and in true stress (s = constant) is investigated. The creep equations for cavitating as well as for non-cavitating materials are derived and the volume change during creep at F = constant are calculated. (orig./RK)

[en] The aim of the present paper is to investigate the influence of high temperature cavitation upon the creep behavior of ideally plastic materials subjected to tensile load cycling. The oscillating tensile stress is replaced by a static equivalent stress which is a function of the loading conditions. Creep equations are derived for cavitating and non-cavitating specimens respectively. The volume change for constant load amplitude cycling is calculated. The main results derived for different loading procedures are also compared. (orig./RK)

[en] The phenomenon of cavitation, often perceived as merely a nuisance, may have far more serious implications. These are discussed here. They include noise, loss of performance and damage to the constituent parts. (author)

[en] The cavitation bubble cluster and the velocity field in the focal region of a lithotripter are investigated. Images of the bubble cluster were captured which show the bubbles increasing in size over approximately 500 μs before collapsing rapidly. Velocity measurements were obtained using particle image velocimetry. A few milliseconds after the inception of cavitation the velocity field consisted of localized flows with maximum velocities and shear rates of the order of 100 mm s^-1 and 50 s^-1 respectively. At times 40 ms after cavitation an average uni-directional flow of the order of 10 mm s^-1 was observed. The mechanism by which the flow is generated is also considered

[en] Cavitation erosion is a major cause of the petroleum equipment hydraulic erosion, which leads to the metal weight loss of the equipment and its breakdown, which can be followed by the full stop of the plant or company work. The probability of the metal weight loss and equipment failure can be reduced by the use of special protective coatings or rivets, made of the sacrificial metals, the use of which significantly increases the service life and the production equipment reliability. The article investigates the cavitation erosion effect, occurred under the condition of the advanced hydrodynamic cavitation on the hydrodynamic cavitation reactor. This article presents the results of the experiments and recommendations for increasing the operational resource. (paper)

[en] The primary aim of the project was to increase cavitation performance of a big mixed–flow pump. The CFD software was used as a main research tool. The two kinds of numerical models were used to solve this problem – both were presented. The results were verified on the basis of comparison with experimental results obtained for real and model pumps. The interesting way of estimating cavitation performance was presented. The pump characteristics before and after the modernisation were shown. (paper)

No abstract available

[en] The possibility of initiating cold fusion reactions by ultrasonic cavitation in heavy water is studied. A time-varying emission of neutrons is observed during cavitation in D₂O on a titanium vibrator that shows traces of cavitation-induced erosion (having been open-quotes conditionedclose quotes in D₂O for 20 h). The effect is furthest above the natural background level (12σ) during cavitation in a suspension of disperse LaNi₅D_x particles in D₂O