Study On Cloud Cavitation Evolution And Its Flow Characteristics

Cloud cavitation is one of the most common cavitation phenomena in the field ofengineering, which exhibits strong unsteady characteristics in its formation and developmentprocess. It can be the source of vibration, noise and erosion that are harmful to hydraulicmachines, propellers and hydraulic facilities. As a result, the control of cloud cavitation isalways a hot issue in cavitation research. However, since the development of cloud cavitation isassociated with turbulence and vortex flow which are still tough problems in fluid mechanics,there has been few knowledge on it for the moment. Therefore, the study of cloud cavitationformation and development can be helpful not only in the deeper understanding of itsmechanism but also in control of its damage in engineering application. In present thesis,experimental and numerical methods are employed to investigate the development andcharacteristics of cloud cavitation shedded by sheet cavitation around hydrofoils.Firstly, cavitation in two dimensional and three dimensional twisted hydrofoils with steadyand homogeneous incoming flow were observed by high speed video technique respectively,which provided the proper condition to study the large scale cloud cavitation. Then thegeneration and development mechanism of cloud cavitation at certain attack angle andcavitation number was analysed. The results confirmed that cloud cavitation was generated bythe shedding of sheet cavitation and re-entrant jet was the main cause of shedding. In thedevelopment of cloud cavitation there existed a U-shaped vortex structure which had closerelationship with the development of cloud cavitation. In addition, PIV was used to distinguishthe difference between cavitating and noncavitating flows.And then numerical simulation of cloud cavitation around three dimensional twistedhydrofoil under certain experiment conditions was performed. The computational results ofcloud cavitation evolution are considerably consistent with experiments, which showed thepresent numerical method can obtain the main features in cloud cavitation development.Subsequently the numerical results were used to analyse the mechanism in cloud cavitationformation and development. It was revealed that the formation of cloud cavitation was relatedto the re-entrant flow under the bottom of sheet cavitation and the development of cloudcavitation was governed by a U-shaped vortex. The U-shaped structure of cloud cavitation wasexplained from the point of vortex dynamics and vortex kinematics. Finally, a generalmechanism for the generation and development of cloud cavitation was proposed.