Wake structure of a single bubble rising in a two-dimensional column

Abstract

The formation-shedding mechanism of the turbulent wake induced by a single gas bubble rising in a two-dimensional column has been experimentally studied. Both qualitative flow visualization and quantitative full-field measurements using particle tracking technique have been conducted. The topological characteristics of phase-averaged and time-mean flow fields are studied in terms of the critical point theory. Contours for phase-averaged vorticity and time-mean Reynolds stresses are presented. The continuous supplies of small-scale shear layer vortices and turbulent stresses from the free shear layer to the wake are evident. Variation of circulation with the phase is given. Except at the time when the entrained free stream cuts through one side of the wake boundary and results in shedding of the Strouhal vortex, the supply-dispersion of positive and negative shear layer vortices from both sides of the free shear layer is found to be nearly equivalent. The descending velocity of Strouhal vortex in the near wake is examined to be less than $ {1 \\over 3}$ of the bubble rising velocity.