Abstract. The presence of air in the liquid causes the dynamic system behaviour. When solve to issue of the dynamics we often meet problems of cavitation. Cavitation is an undesirable phenomenon, since it causes a disruption of the surrounding material and material destruction. Cavitation is accompanied by loud sound effects and reduces the efficiency of such pumps, etc. Therefore, it is desirable to model systems in which the cavitation might occur. A typical example is a solution of water hammer.

The article deals with experimental investigation of water cavitation in the convergent-divergent nozzle of rectangular cross-section. In practice, a quick and simple determination of cavitation is essential, especially if it is basic cavitation or cavitation generated additionally by the air being sucked. Air influences the formation, development and size of the cavity area in hydraulic elements. Removal or reduction of the cavity area is possible by structural changes of the element. In case of the cavitation with the suction air, it is necessary to find the source of the air and seal it. The pressure gradient, the flow, the oxygen content in the tank, and hence the air dissolved in the water, the air flow rate, the noise intensity and the vibration velocity on the nozzle wall were measured on laboratory equipment. From the selected measurements the frequency spectrum of the variation of the water flow of the cavity with cavitation without air saturation and with air saturation was compared and evaluated.

Cavitation is undesirable phenomenon occurring mainly in the flow of water in pumps and turbines, and therefore it is necessary to pay attention to it. The purpose is to explore the possibility of the mathematical modelling of the cavitation using Singhal cavitation model, which solves the multiphase flow of water and vapour. The issue can be solved taking into account the non-condensable gas (air). Singhal cavitation model was selected on the basis of good experience in the modelling of dynamic cavitation changes in the Laval nozzle [2], [10]. This article presents two alternatives. The first variant is testing a mathematical model for flow in a simple geometry of the cone. The second variant resolves the formation of cavitation rope behind vortex generator and the results are compared with experiment.

Mathematical modeling is applied as an effective tool for prediction of cavitation in hydraulic components and systems. A multiphase mathematical model based on the change in phase between water and vapor is typically used to investigate the cavitation flow. However, dissolved air can significantly affect the cavitation. This study proposes a new approach based on a multiphase turbulent mathematical model by adding the air into the mixture to solve the dynamics of cavitation. To clearly assess the significance of air in the multiphase model, four variants of the mixture are investigated (water; water and vapor; water and air; and water, vapor, and air together). The software of the computational fluid dynamics ANSYS Fluent was applied to numerically solve the proposed mathematical models. The influence of gaseous components is analyzed through evaluation of hydraulic parameters and spectral characteristics of the cavitation bubble. To verify the proposed mathematical models, a hydraulic water circuit was built to generate cavitation in a transparent Venturi nozzle. Cavitation in the experiment was identified by measuring the flow rate, static pressure, and noise and visualized with a camera. The numerical results of the extended multiphase flow confirmed very good agreement with experimentally obtained basic hydraulic parameters and frequency-related characteristics. Knowledge obtained from the multiphase mathematical model of cavitation can be applied to cavitation in the oil flow (pump suction and flow through the valve) in future research, where the effect of the air on cavitation is more important than the effect of vapor.

The paper deals with the determination of characteristic as dependence of the valve pressure drop on the flow rate, flow characteristic and cavitation conditions in case of water flow in the flow control valve. Emphasis is put on the utilization of simple, available relationships and measuring for identification of the basic valve coefficients, e.g. loss coefficient, flow rate coefficient and cavitation factor. These coefficients are used for designing of pipe circuits. In this paper there is defined methodology for determining those coefficients and is applied to the modified cone of flow control valve for verification the linear flow characteristic. It is necessary to consider the fact that in various countries the modifications of coefficients are preferred and it is therefore necessary to specify them.