Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
The dynamics and acoustics of conical bubble collapse occurring in a U-Tube device are studied. Two conical shapes are used, one with a smooth interior and another with a stepped interior in order to understand their associated phenomena such as: luminescence, bubble cloud formation, and emission of strong rebound pressures. High-speed video frames, waveforms from piezoelectric transducers and photomultipliers were acquired during the experimental runs. All data are synchronized on the same timeline. To explore the details of the acoustic energy dissipation, the piezoelectric waveforms were analyzed by means of Fourier transform and wavelets. Results show (I) with to respect to the frustum cone that: a) two mean flow structures are formed, one of them is a gas pocket and the other is a bubble cloud. The bubble cloud is produced when the concave meniscus is folded inwards due to fluid flow surrounding that divides the initial gas pocket in two, one remains trapped inside of meniscus area and the other is pushed towards to the cone end. These structures are the main location where the light emission occurs; (II) for the stepped cone that: a) asymmetrical counter-current flows are produced by the stepped boundaries, which generate several stagnation points.
From the detailed analysis of the signals acquired, two issues were found: the frequencies and scales associated with the shock waves emission onset and its propagation, as well as frequency bands in which there is an absence of energy. The detailed flow structures as jets and splashing effects also are considered.