Three-dimensional flow characteristics of a turbulent bubbly jet near the source*

Time-resolved Lagrangian particle tracking (LPT) based measurement of a bubbly jet from a round source is presented in this study (Re ~ 3,000, D = 20 mm, α ~ 1%). To achieve accurate measurement of both phases, illuminating devices for (1) fluorescent tracer particles for water and (2) spherical bubbles from a bubble generator are operated with a short delay. The measurement is conducted for a near-field of the bubbly jet. LPT data is reconstructed into a regular grid with 2 mm spacing with Vortex-in-Cell sharp (VIC#) method. Reconstructed data are validated with a bubbly jet DNS result with a similar slip velocity and Reynolds number. Near the jet source, there is a region with an alternation of acceleration and deceleration. The centerline streamwise velocity of the bubbly jet eventually reached around 1.7 times the velocity at the source, and the potential core of the bubbly jet collapsed. Alternating accelerations attribute to the formation of a series of ring-like vortices. The vortices are unstable and diminish as the jet flows downstream. Formation of the vortical structure occurs more frequently than large-scale meandering of the jet but less frequently than bubble-attributed instability.