Complex vortex formation and aeration in air-water mixing flows in stirred tanks*

Three-dimensional (3D) direct numerical simulations are carried out of air-water mixing flows using pitched blade turbine in an open vessel. We useahybrid front-tracking/level-set solver for 3D parallel simulations of multiphase flows. The turbine is constructed through a module that defines immersed objects using a distance function that accounts for the objects interaction with the flow and with appropriate boundary conditions in the Navier-Stokes solution the velocity is corrected in order to satisfy a rigid body motion constraint. In addition to ordinary primary vortices occurring in any kind of rotating flow, our configuration also features several secondary vortices, vortex breakdown, blade-tip, and end-wall corner vortices. For certain turbine rotational speeds, aeration takes places; this corresponds to the breakup of at the air-water interface leading to the entrainment of bubbles into the water phase. The mechanisms underlying the entrainment, and bubble formation processes, are elucidated.