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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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Prediction of turbulent flow over the inclined step by means of various two-equation turbulence models
Janeček, Vladislav ; Příhoda, Jaromír
The work deals with the numerical simulation of turbulent flow over an inclined step in a channel of the rectangular cross-section according to measurement of Makiola (1992). The simulation was carried out by means of the commercial software ANSYS FLUENT with users defined functions. Various two-equation turbulence models based on the k-eps formulation were tested with the aim to estimate their ability to predict the extent of the separation region and energy losses due to inclined step with the angle 15 deg. Numerical results were compared with experimental data from LDA measurements. Besides, determined energy losses were compared with losses in a channel with a backward-facing step.
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Prospects of digital holographic interferometry in heat transfer measurement
Doleček, R. ; Lédl, Vít ; Kopecký, V. ; Psota, P. ; Václavík, Jan ; Vít, Tomáš
In the measurement of heat transfer problem, a holographic interferometry (HI) is often used mainly because of its unique properties and very high adaptability to different sort of problems under investigation. Although in the area of vibration or deformation measurement of solid body the standard HI is replaced massively by digital holography in the case of refractive index distribution the situation is quite opposite. In this paper we present two different types of digital holographic interferometers. The interferometers with different sensitivities have been build and successfully tested.
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Erozní potenciál bublinkové kavitace ve vodním čerpadle – numerická simulace a experiment
Zima, Patrik ; Sedlář, M. ; Müller, M.
A model assessing erosive potential of cavitation bubbles for 3D steady-state pump flow in the traveling cavitation mode is proposed. The 3D Navier-Stokes code for turbulent pump flow is coupled with the Rayleigh-Plesset equation by means of two-way coupling. The energy dissipated between two successive bubble rebounds is estimated to assess the erosive potential. The model is validated for a 2D hydrofoil in the cavitation tunnel and then employed for a 3D mixed-flow water pump impeller. The numerical results indicate the dominant effect of the first collapse and are in good agreement with the experiment.
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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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Experimental Investigation of Throttling Process Affected by Gas Impurities
Vinš, Václav ; Vacek, V.
The throttling process in a small-diameter tube – capillary tube – was experimentally investigated in this study. A special testing capillary tube equipped with precise temperature and pressure sensors was used to describe the effect of gas impurities, i.e. non-condensing gases (nitrogen in our case), on the flow of throttled refrigerant R218. Total pressure at the onset of vaporization was increased by partial pressure of contaminating gas. Therefore, the two-phase flow of gas-contaminated refrigerant started to be generated notably earlier than in the case of pure refrigerant flow. Mass flow rate of refrigerant delivered through capillary tube decreased even by 20 % in some cases. The experimental data were compared with a numerical simulation of pure refrigerant flow in capillary tube.
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Studie fluidických snímačů rychlosti útku
Tesař, Václav
Measuring the speed of an unsteady axial motion of weft thread in a shuttle-less loom is a a task considered impossible to perform. It was solved by the author using principles based on colliding air jets. Further development and search for other principles described in this contribution was aimed at increasing sensitivity of the contact-less fluidic sensing.
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Struktury vyvolané nestabilitou v impaktním proudění
Tesař, Václav ; Něnička, Václav
Authors investigated by flow visualisation an air jet impinging on a perpendicular flat plate. The jet was supplied with tiny water droplets, produced by condensation, which caused scattering of laser light illuminating the section of the jet in meridian plane. A high-speed camera, operating in an adjustable phase relationship with acoustic signals triggering the formation of the instabilities was used to record the visualisation images. These were then processed using advanced procedures aimed at identifying the structures.
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