PIV-Based Acoustic Pressure Measurements of a Single Bubble near the Elastic Boundary

The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used to measure the temporal bubble shapes, transient flow structure, as well as the mid-span deflection of an elastic boundary. Results are presented for three different initial positions near an elastic boundary, which were compared with results obtained using a rigid boundary. A formula relating velocity and pressure was proposed to calculate the acoustic pressure contours surrounding a bubble based on the velocity field of the transient flow structure obtained using PIV. The results show the bubbles near the elastic boundary presented a “mushroom” bubble and an inverted cone bubble. Based on the PIV-measured acoustic pressure contours, a significant pressure difference is found between the elastic boundary and the underside of the bubble, which contributed to the formation of the “mushroom” bubble and inverted cone bubble. Furthermore, the bubbles had opposite migration direction near rigid and elastic boundaries, respectively. In detail, the bubble was repelled away from the elastic boundary and the bubble was attracted by the rigid boundary. The resultant force made up of a Bjerknes force and buoyancy force dominated the migration direction of the bubble.

Modeling of Non-Newtonian Flow in an Inverted Cone Foam Breaker

Foam formation is a widespread phenomenon and often a serious problem in fermentation processes. Inverted cones used as mechanical foam breakers are rotating devices that pump the fluid up and pulverize it at the edge. The shearing and centrifugal actions of such geometries can help to control foaming. In this study, a model was developed using Computational Fluid Dynamics (CFD), based on the non-Newtonian properties of foam, to describe and explain the action of inverted cones as foam breakers.