Influence of Acoustic Interaction between Cavities That Generate Cavitation Noise

Doktorsavhandling, 2002

The thesis concerns high frequency cavitation noise generated by marine propellers, for example. The collapse of a cavity is forced by the surrounding pressure which comprises the pressure in the undisturbed flow, the pressure disturbance associated with the local non-cavitating flow and, finally, the pressure disturbance from the presence and motion of neighbouring cavities. This last contribution, dealt with here mainly as an acoustic interaction from the motion of neighbouring cavities, is the main subject of the present work. The primary aim is to make a preliminary study of the role of interaction between a few cavities on the generation of high frequency cavitation noise. Different cavitation processes, statistical distributions of parameters and initial conditions for interacting cavities were observed by high-speed films from experiments. The numerical part of the study concentrates on the central example of acoustic interaction between two spherical cavities. The influence of various parameters on an interaction model and on the interaction itself were studied. Monte Carlo simulations with random input data were made with the model to study the influence of interaction in collapse processes identified from experiments. Scaling laws were analysed and applied to interacting cavities. A main engineering conclusion drawn from the numerical simulations is that, provided the levels at the very highest frequencies are disregarded, the acoustic interaction between medium-sized dominating structures is not very important. This indicates that, if a numerical method can generate an acceptable size and time distribution of cavities and a realistic pressure forcing the collapse, it can also be expected to generate realistic noise levels up to medium-high frequencies. For prediction by model tests, the present study implies that, except for the very highest frequencies, the exact distribution in space and time of nearby cavities is not very critical. The numerical simulations indicate that, if a good estimate of the noise at the very highest frequencies is desired, an accurate simulation of both the acoustic interaction and the statistical properties of cloud cavitation is required.