A shadowgraphy approach for the 3D Lagrangian description of bubbly flows

A 3D volumetric technique for measuring the evolution over time of the kinematic and geometric characteristics of the bubble population in multiphase flows at moderate void fraction is here proposed. The method is based on a shadowgraphy approach and requires a set of calibrated and synchronized cameras, each placed in front of a bright screen. Synchronized, 2D images of the bubbly flow are analyzed to extract the outline of the bubbles as seen from every camera. Then, each bubble is separately identified as a 3D volume described by the intersection of the cones having vertices on the optical center of each camera and passing through the contour of the bubble. Details about the implementation of the procedure, including the further refinements of the first rough bubble identification and the optimization of the number and geometric arrangement of the points of view, are reported against the results obtained on a reference set of spheres of known dimensions. Application on isolated bubbles demonstrates the ability of the procedure to extract quantitative and self-consistent information over time. These results are consolidated by a hint at a plunging jet test case with a significant void fraction, showing potential for application to situations of practical interest.