Microbubbles and Cavitation: Microscales to Macroscales

Microbubbles are systemic to cavitation physics. They are present in all practical liquid flows, they provide nuclei for inception, their dynamics play a role in many of the phenomena and processes involved in cavitation and ultimately they are generated by cavitation collapse and condensation. Various techniques for the generation, measurement and dispersion of microbubbles for experimental modelling of cavitation nucleation and dynamics in hydrodynamics test facilities are presented. Controlled generation of microbubbles are also required for development and calibration of optical measurement techniques and for flow diagnostics. Several techniques for generation of mono-, and poly-disperse microbubble populations in the size range 1 to 100 microns are discussed. Microbubble concentrations generated by devices or from cavitation itself vary from 1000 bubbles per cubic cm down to 0.01, in addition to the size range, necessitating the use of several methods for measurement of both size range and concentration. Experimental results demonstrating the influence of nuclei populations on stable and unstable cavitation in canonical flows about spheres and hydrofoils are presented. Nuclei populations affect shedding mechanisms and modes, spectral content, amplitudes of forces and microbubble content in downstream wakes.