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Experimentální systém pro výzkum kavitačních jevů v hydraulických tlumičích rázů
Müller, M. ; Prášil, L. ; Zima, Patrik
The article describes the experimental system for investigation of growth, activity and collapse of cavitation bubbles or structures in a hydraulic shock absorber. The experimental chamber allows to insert a real shock absorber piston and install acoustic/optical measurement devices including bubble spectrometer, a PVDF hydrophone for the measurement of acoustic emission from the collapsing bubbles and a high-speed camera combined with laser sheet illumination. A specially designed high-resolution mirror lens can be installed in the chamber to investigate the bubbles in the micrometer range.
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Klasická nukleační teorie a modelování kavitačních jevů
Němec, Tomáš
The description of the nucleation of bubbles and their growth in cavitation processes suitable for computational fluid dynamics (CFD) simulations is investigated. The number of newly created bubbles and their sizes are treated empirically in most of the numerical codes. We show that the Classical Nucleation Theory (CNT) can be used to calculate the nucleation probabilities in water. Also a simple model of the growth of the nucleated bubbles is described. The CNT predictions of the nucleation rate show promising results and together with the bubble growth model can be used to enhance the precision of CFD modeling of cavitating flows.
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Nucleation theory for the description of cavitation in water
Němec, Tomáš ; Maršík, František
The Classical Nucleation Theory (CNT) is a powerful tool for the description of nucleation processes whose development started more than a century ago. It proved its strength above all in dealing with the condensation phenomena, and it is quite accurate when describing not only the nucleation in pure water vapor but also evaluating the influence of admixtures, e.g. sulfuric acid, alcohols, mineral salts. Such a multicomponent approach was not yet applied in the case of cavitation processes; the CNT predictions are restricted to pure water, and the role of other components of the cavitating medium are implemented ad-hoc. This paper describes the influence of a gas dissolved in water (e.g. oxygen, nitrogen, carbon dioxide) on the nucleation process in cavitation.
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Kavitační bublina v blízkosti pevné stěny
Müller, M. ; Zima, Patrik
The article presents the results of the calculation of the cavitation bubble collapse near a solid boundary. The solution is obtained using the FLUENT CFD software. The VOF (Volume Of Fluid) model is applied to study the behavior of the interphase between the liquid and the bubble content. The calculations were performed for three different distances of the bubble from the solid wall. The evolution of the bubble radius is obtained from the phase interface contours. The bubble jet velocity and its thickness are determined for all bubble positions.
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Experimentální výzkum kavitační bubliny ve vodě
Müller, M. ; Zima, Patrik
The article presents the results of experimental investigation of laser-induced cavitation bubble in water. The cavitation bubble is generated using the Q-switched Nd-YAG laser and recorded by a multi-exposure method, which allows to record the bubble at different phases into single frame.
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CFD analýza kavitačních jevů v hydraulických strojích
Sedlář, M. ; Zima, Patrik
The paper deals with the CFD analysis of cavitation phenomena in the hydraulic machinery. Both the ANSYS CFX 11.0 package and an in-house software developed in the SIGMA Research and Development Institute and the Institute of Thermomechanics, AS CR v.v.i. are used to study three types of cavitation problems: calculation of the NPSH3 curves, modelling of the cavitation instabilities and prediction of the cavitation erosion.
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Kavitace a nukleace bublin ve vodě s příměsemi
Zima, Patrik ; Maršík, František ; Sedlář, M.
The latest advances in modeling of cavitation in water flows and experimental investigation of bubble nucleation in a cavitation nozzle will be presented. The emphasis will be put on presenting the results of the recent measurements with cavitation nozzle and the critical assessment of the applicability of the cavitation nozzle principle for determining the size of critical bubble radii. Some results of numerical modeling of cavitation in pumps will be summarized.
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Simulace kavitační nestability v induktoru se 3 lopatkami
Sedlář, M. ; Maršík, František ; Zima, Patrik
Axial-flow inducers are used to improve cavitation performance of radial-flow or mixed-flow pumps. There is a wide range of cavitation instabilities, which can be observed during inducer operation, even at the design flow rate. The type of cavitation instability is determined by the frequency analysis. Two cavitation models were used to determine the frequency range of four different types of cavitation instabilities – CEV and VOF built in the ANSYS CFX software – for a 3-blade inducer and suction impeller of a multistage pump. The calculated values of NPSH3 were compared with experimental data for SST, SST-SAS and RSM SSG turbulence models for fully unsteady solutions. It was found that the SST-SAS model provides the most realistic picture of the backflow vortices and their oscillations.
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Rozložení energie v kavitační bublině během jejího růstu a kolapsu
Müller, M. ; Maršík, František ; Garen, W.
Shock wave generation during the bubble growth and collapse and the corresponding energy separation were investigated experimentally. The calculation of the shock wave energy was based on the experimentally determined shock wave velocity profile. The possibility of using the similarity solution to determine the shock wave intensity is discussed. The velocity field around the collapsing bubble was visualized and compared with the numerical simulation using the commercial FLUENT code.
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