Velocity- and scalar-field measurements of turbulent buoyant round jets in a two-layer stratified environment

Results reported in the literature have shown that the turbulence models currently implemented in Computational Fluid Dynamics (CFD) commercial codes (e.g. ANSYS-CFX, STAR-CCM+, and FLUENT) have a tendency to overestimate thermal stratification and underestimate turbulent mixing when buoyancy effects become dominant with respect to momentum effects. Also, standard Large Eddy Simulation (LES) models cannot fully capture the behavior of jets interacting with stratified environments because the assumption of turbulence isotropy of the smaller scales breaks down. Due to light diffraction and image distortion, it is challenging to apply non-intrusive optical flow measurements, like particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) to get experimental data for CFD validations when there are density variances involved in the flow. However, a Refractive Index Matching (RIM) technique has been recently developed in our lab, which allows us to perform high-resolution measurements of velocity- and scalar-fields for turbulent buoyant jet flow in presence of density differences as high as 8.6%. To form a fully turbulent round free jet, an experimental facility was designed with a jet nozzle diameter of 2 mm, located at the bottom of a cubic tank with 30 cm side length. The jet flow is established by a servo-engine-driven piston to eliminate possible fluctuations introduced by the motor. A high-fidelity synchronized PIV/PLIF system was utilized in conjunction with RIM to measure the velocity and concentrations fields in the self-similar regions of a jet flow with a density difference of 3.16% for aqueous solutions. With Reynolds numbers of 4,000 and 10,000, the jet impinging with a two-layer stably stratified environment are compared to the positively buoyant jet with lighter fluid injected into denser surroundings. Detailed quantifications of the measurement uncertainties are also carried out. The experimental results are presented in terms of turbulent statistics, and the analysis of jet penetration depths.