  Buoyant shear layers are encountered in many engineering and environmental
applications and have been studied by researchers in the context of experiments
and modeling for decades. Often, these flows have high Reynolds and Richardson
numbers, and this leads to significant/intractable space-time resolution
requirements for DNS or LES modeling. On the other hand, many of the important
physical mechanisms in these systems, such as stress anisotropy, wake
stabilization, and regime transition, inherently render eddy viscosity-based
RANS modeling inappropriate. Accordingly, we pursue second-moment closure
(SMC), i.e., full Reynolds stress/flux/variance modeling, for moderate Reynolds
number non-stratified and stratified shear layers for which DNS is possible. A
range of sub-model complexity is pursued for the diffusion of stresses, density
fluxes and variance, pressure strain and scrambling, and dissipation. These
sub-models are evaluated in terms of how well they are represented by DNS in
comparison to the exact Reynolds averaged terms, and how well they impact the
accuracy of the full RANS closure. For the non-stratified case, the SMC model
predicts the shear layer growth rate and Reynolds shear stress profiles
accurately. Stress anisotropy and budgets are captured only qualitatively.
Comparing DNS of exact and modeled terms, inconsistencies in model performance
and assumptions are observed, including inaccurate prediction of individual
statistics, non-negligible pressure diffusion, and dissipation anisotropy. For
the stratified case, shear layer and gradient Richardson number growth rates,
and stress, flux, and variance decay rates, are captured with less accuracy
than corresponding flow parameters in the non-stratified case. These studies
lead to several recommendations for model improvement.
