Experimental and Numerical Investigations of Cavitation Control Using Cavitating-bubble Generators

Abstract

Cavitation is a physical phenomenon which consists of evaporation, bubble formation and collapse and plays an important role in many technical fields. Cavitation appears in ship technology due to its erosive effect on the ship’s rudder and propeller as a result of the collapse of cavitation bubbles near the wall surface. Cavitation can cause structural vibrations, noise emissions, and hydrodynamic efficiency reduction in the maritime sector and in hydraulic machinery systems. Measures to reduce or avoid undesirable cavitation-related consequences may importantly contribute to a better economic operation of ships and hydraulic machinery. This thesis deals with developing efficient passive flow control methods to control and suppress the deleterious effects of cavitation in different regimes. For this aim, a wedge-type miniature vortex generator so-called Cavitating-bubble Generator (CG) was developed and cylindrical-type miniature vortex generators so-called Cylindrical Cavitating-bubble Generators (CCGs) was proposed to control the cavitation and to stabilize the cavitation-induced instabilities. In this work, experimental investigations of these two cavitation passive control measures under different cavitation conditions such as cavitation inception, quasi-steady partial cavitation, unsteady cloud cavitation and cavitation surge regimes were performed. First, a high-speed visualization of cavitation around two test cases without cavitation control was performed to analyze the cavitation dynamics. Second, a particle image velocity technique was applied to measure the mean flow velocity profiles around the surface and in the wake region. Third, a hydroacoustic measurement was carried out to record local pressure pulsations in the wake region of the test cases. Then, the effects of the passive flow control methods on the qualitative parameters such as cavity structures in different cavitating regimes were studied by means of high-speed imaging. Finally, the effects of the cavitation control on the quantitative parameters such as pressure pulsations, velocity profiles and shedding frequency were analyzed and compared with the test cases without cavitation control. In addition, numerical investigations of unsteady cavitating flows were performed to study the effects of the cavitation control in details. First, unsteady cavitating flows around the test cases without cavitation control were computed. For modelling of the cavitating flows, an Euler-Euler method with volume of fluid method was used. Further on, a Partially-averaged Navier Stokes (PANS) for turbulence modelling was coupled with the cavitation model and implemented to an open source code. Second, the influence of cavitation model parameters such as different nuclei density, nuclei radius and effect of turbulence model coefficient on the dynamics of unsteady cloud cavitation were analyzed. The numerical results were validated with the experimental data obtained in this work and the experimental data from a benchmark study. Finally, the effects of different passive cavitation control methods on the qualitative and quantitative parameters of cavitation were analyzed and compared with the test cases without cavitation control. Fair qualitative and quantitative agreements were obtained with the experimental data. The results revealed that the implemented cavitation control methods are effective methods to mitigate the cavitation regions and to suppress the unsteady behavior of the cavitation. Overall, this work presents results towards the investigation of cavitation control and the finding may be used to further develop in scientific and industrial applications in future works.