  Accurate models of turbulent buoyant flows are essential for the design of
nuclear reactors thermal hydraulics and passive safety systems. However,
available models fail to fully capture the physics of turbulent mixing when
buoyancy becomes predominant with respect to momentum. Therefore, high-fidelity
experiments of well-controlled fundamental flows are needed to develop and
validate more accurate models. We analyze experiments of positive and negative
turbulent buoyant jets, both in uniform and stratified environments, with the
aim of understanding the thermal hydraulics of turbulent mixing with variable
density and providing high-fidelity data for the development and validation of
turbulence models. Non-intrusive, simultaneous Particle Image Velocimetry and
Laser Induced Fluorescence measurements were carried out to acquire
instantaneous velocity and concentration fields on a vertical section parallel
to the axis of a jet in the self-similar region. The Refractive Index Matching
method was applied to measure high-resolution buoyant jets with up to 8.6%
density difference. These data are free of the typical errors that characterize
optical measurements of buoyancy driven flows (e.g., natural and mixed
convection) where the refractive index of the fluid is inhomogeneous throughout
the measurement domain. Turbulent statistics and entrainment of buoyant jets in
uniform and stratified environments are presented. These data are compared with
non-buoyant jets in uniform environment, as a reference to investigate the
effects of buoyancy and stratification on turbulent mixing. The results will be
used for the assessment of current turbulence models and as basis for the
development of a new one that captures turbulent mixing.
