Experimental investigation of stable density-stratified confined mixing layers

The study of stratified flows provides important insight towards turbulent mixing in chemical and nuclear reactors, geophysical flows, and combustion processes. If the flow is stably stratified, buoyancy forces can impede the growth of Kelvin-Helmholtz (KH) instabilities, leading to a decay in turbulence production. In this work, stereo particle image velocimetry (PIV) measurements are performed in IDEFIX (HIgh gas Density differEnce Facility for mIXing) to characterize turbulent mixing mechanisms in shear-buoyancy driven flow. The facility consists of an open gas mixing loop that supplies two parallel gas flows with independent velocities (u1, u2) and independent densities (rho1, rho2) to a horizontal mixing chamber. The velocity and density ratios, R = (u1-u2)/(u1+u2) and A=(rho1-rho2)/(rho1+rho2), considered include R = 0.2, 0.4, 0.5, 0.6 and A = 0, 0.2, 0.4, respectively, where the half channel Reynolds number Re = 4400 is kept constant in the upper leg. The PIV measurements are used to characterize empirical growth laws across a wide parameter range. The density fields are estimated from continuity to provide an assessment of the local gradient Richardson number and identify different flow regimes based on the Miles-Howard criterion. Finally, proper orthogonal decomposition (POD) is applied to extract and compare the organization of coherent structures across different density ratios to further describe the different modes of turbulent mixing.