Accurate prediction of the highly-mixed flow in the near field of a surface ship is a challenging and active research topic in Computational Ship Hydrodynamics. The disparity in the time and length scales and the scales of entrainment dictates the use of bubble source and mixed-phase flow models in which the current state of the art models are ad hoc. This paper presents the air entrainment characteristics and multiphase turbulence modeling of the near-field flow of a canonical stern with the inclusion of simple geometry effects. Using state of the art Cartesian-grid numerical methods with the full field equations, high-resolution two-phase flow data sets of a canonical stern with three different half-beam to draft ratios are simulated down to the scales of bubble entrainment. These data sets are used as the foundation for: (i) characterization of wake structure and near-wake air entrainment of the stern; (ii) analysis of turbulent mass flux in the wake of the stern; and (iii) a priori testing of multiphase turbulence models for turbulent mass flux. We obtain results to show that these techniques enable analysis and physics-based parameterization of near-field air entrainment about surface ships for use in Computational Ship Hydrodynamics.