Session S08: Turbulence: Shear Layers (5:45pm - 6:30pm CST)

Reynolds Stress/Flux/Variance Modeling and DNS of Stratified Shear Layers

Buoyant shear layers and wakes are often characterized by high Reynolds and Froude numbers, as well as very large computational space/time domain sizes, and these limit the applicability of DNS and LES modeling. On the other hand, many of the important physical mechanisms in these systems inherently render eddy viscosity-based RANS modeling inappropriate (e.g., stress/variance anisotropy/budgets, stabilization, regime transition), particularly at high Richardson numbers. Accordingly, we pursue Full Reynolds Stress/Flux/Variance modelling in this context. Here, we have applied such modeling to several lower Reynolds number non-stratified and stratified shear layers for which DNS data is available. 7-equation and 11-equation modeling is used, respectively. A range of sub-model complexity is applied for diffusion of stresses, density fluxes/variance, pressure strain/scrambling, and dissipation. We take the approach of evaluating these sub-models: 1) in terms of how the model itself it represented by DNS in comparison to the exact Reynolds averaged terms, and 2) in terms of how RANS carries over the predictive performance of the sub-models to the full model. It is found that there are numerous striking shortcomings with well established sub-models. Approaches to improve these are proposed and tested.   

Relation of Zero crossing with dissipation and Production scales in Free shear flows.

Statistical properties of zero crossing of plane jet and plane mixing layer and its relation with dissipation and production scales is studied. The variation of relation between these scales in cross stream direction is investigated using direct numerical simulation results. The ratio, $\Lambda/\lambda$ and $\Lambda/\lambda_{P}$ are found out to be of order unity. The probability density function of the time interval between successive zero crossing of stream-wise velocity has been found to be varying exponentially. Similar analysis has been performed for cross stream fluctuation components and Reynolds stress u'v' and results. Strong departure of these turbulent flows from Gaussianity still yielded these results like Gaussian signals as has been found for other flows like boundary layer, wakes and heated jets as well.   

Internal Regulation in Compressible Turbulent Shear Layers

High resolution simulations of temporally evolving mixing layers, for convective Mach numbers ranging from $M_c=0.2$ to $M_c=2.0$ with density ratios $s=1$ and $s=7$, are analyzed to characterize compressibility effects on the structure and evolution of turbulence in this compressible flow. Published experimental results are used to validate simulation results. Examination of the turbulence scales in the present data suggests an internal regulation mechanism. Correlated eddying motions were found to be limited by acoustic signal propagation. Eddy scales in all spatial directions are found to be a progressively smaller fraction of the overall mixing layer thickness with increasing $M_c$, forming independent layers of eddying motions at high $M_c$. The behavior of these length scales are interpreted in relation to the 'multi-layered' mixing proposed by Planch\'{e} (1992) and Day (1998), and the 'sonic eddy hypothesis' by Breidenthal (1992). These reduced spatial scales serve to reduce the effective velocity scale for turbulent motions, suppressed Reynolds stresses, TKE production and dissipation, and the mixing layer thickness growth rate. This talk will focus on this internal scaling based on the effective velocity difference seen by the eddies.