Experimental and theoretical study on cavitation inception and bubbly flow dynamics. Part 1: Design, development and operation of a cavitation susceptibility meter. Part 2: Linearized dynamics of bubbly and cavitating flows with bubble dynamics effects

Abstract

The first and main part of this work presents the design, development and operation of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the content of active cavitation nuclei in water samples. The pressure at the venturi throat is determined from the upstream pressure and the local flow velocity without corrections for viscous effects because the flow possesses a laminar potential core in all operational conditions. The detection of cavitation and the measurement of the flow velocity are carried out optically. The apparatus comprises a Laser Doppler Velocimeter for the measurement of the flow velocity and the detection of cavitation, a custom-made electronic Signal Processor for real time generation and temporary storage of the data and a computerized system for the final acquisition and reduction of the collected data. The various steps and considerations leading to the present design concept are discussed in detail and the implementation of the whole system is described in order to provide the all the information necessary for its calibration and operation. Finally, the results of application of the Cavitation Susceptibility Meter to the measurement of the water quality of tap water samples are presented and critically discussed with reference to other similar or alternative methods of cavitation nuclei detection and to the current state of knowledge on cavitation inception. The second part of the present work presents the results of an investigation on the linearized dynamics of two-phase bubbly flows with the inclusion of bubble dynamics effects. Two flow configurations have been studied: the time dependent one-dimensional flow of a spherical bubble cloud subject to harmonic excitation of the far field external pressure and the steady state two-dimensional flow of a bubbly mixture on a slender profile of arbitrary shape. The inclusion of bubble dynamic damping and of the relative motion between the two phases and the extension of the results to the case of arbitrary excitation are discussed when examining the second flow configuration. The simple linearized dynamical analysis developed so far clearly demonstrates the importance of the complex phenomena connected to the interaction of the dynamics of the bubbles with the flow and provides an introduction to the study of the same flows with non-linear bubble dynamics.