(297a) Characterization of Bubbly Two-Phase Flows Using PIV, Shadowgraphy and PTV Techniques

An experimental procedure based on a combination of Particle Image Velocimetry (PIV), Shadowgraphy and Particle Tracking Velocimetry (PTV) optical techniques is proposed for investigating bubbly flows. The procedure is applied to the turbulent two-phase flow arising from the normal impingement of a round free-surface water jet on a horizontal air-water interface. A fully developed turbulent jet, exiting a long pipe, ensured properly characterized inflow conditions. Particle image velocimetry (PIV) was used to evaluate the flow field beneath the interface, with and without air entrainment. Turbulent velocity fields of the continuous phase and dispersed phase were simultaneously measured and presented using Reynolds decomposition into mean and fluctuating components. The subsurface dynamics of bubble formation, evolution and flow behavior was also investigated. Time-resolved particle tracking velocimetry (TR-PTV) and shadowography techniques were used to measure the velocity and size distributions of the dispersed phase. An elaborate post processing procedure was used for background equalization, noise removal, threshold detection, gradient analysis and validation. The image processing and evaluation scheme included the detection of each bubble contour, centroid, shape, size and particle tracking-based velocity determination. A threshold level parameter was used to define the contours of the bubbles in the images. Edge height/slope validation criteria were used to accept well focused bubbles.