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Buoyant shear layers are encountered in many engineering and environmentalapplications and have been studied by researchers in the context of experimentsand modeling for decades. Often, these flows have high Reynolds and Richardsonnumbers, and this leads to significant/intractable space-time resolutionrequirements for DNS or LES modeling. On the other hand, many of the importantphysical mechanisms in these systems, such as stress anisotropy, wakestabilization, and regime transition, inherently render eddy viscosity-basedRANS modeling inappropriate. Accordingly, we pursue second-moment closure(SMC), i.e., full Reynolds stress/flux/variance modeling, for moderate Reynoldsnumber non-stratified and stratified shear layers for which DNS is possible. Arange of sub-model complexity is pursued for the diffusion of stresses, densityfluxes and variance, pressure strain and scrambling, and dissipation. Thesesub-models are evaluated in terms of how well they are represented by DNS incomparison to the exact Reynolds averaged terms, and how well they impact theaccuracy of the full RANS closure. For the non-stratified case, the SMC modelpredicts the shear layer growth rate and Reynolds shear stress profilesaccurately. Stress anisotropy and budgets are captured only qualitatively.Comparing DNS of exact and modeled terms, inconsistencies in model performanceand assumptions are observed, including inaccurate prediction of individualstatistics, non-negligible pressure diffusion, and dissipation anisotropy. Forthe stratified case, shear layer and gradient Richardson number growth rates,and stress, flux, and variance decay rates, are captured with less accuracythan corresponding flow parameters in the non-stratified case. These studieslead to several recommendations for model improvement.

Second Moment Closure Modeling and DNS of Stratified Shear Layers