Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H28: Turbulence: Mixing II |
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Chair: Julia Ling, Stanford University Room: 2011 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H28.00001: DNS of Turbulent Mixing Layers Between Two Fluids of Large Density Difference Jon Baltzer, Daniel Livescu In numerous practical applications, shear layers exist between fluids of strongly differing densities. At high Atwood numbers, the large variations in density introduce important effects that have recently been observed in other flows (e.g., Livescu \& Ristorcelli, J. Fluid Mech., \textbf{605}, 145-180, 2008). To investigate the inertial variable density effects on the instability growth and structure of mixing layers, we first omit the buoyancy effects and perform very large Direct Numerical Simulations of planar mixing layers between two miscible fluids, each with different density. We consider initial disturbances for the DNS in light of linear stability analysis using a new, generalized form of the Orr-Sommerfeld equations, which includes the variable density effects. Based on the most unstable modes obtained from this analysis, DNS domain sizes are varied to accommodate different extents of mode pairing. The results display the overall statistical effects on the turbulence and mixing, as well as the structural differences, that occur as Atwood number is varied. In particular, the asymmetries introduced by the differences in the densities of the mixing layer streams are highlighted. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H28.00002: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 10:56AM - 11:09AM |
H28.00003: Favre-Averaged Turbulence Statistics in Variable Density Mixing of Buoyant Jets John Charonko, Kathy Prestridge Variable density mixing of a heavy fluid jet with lower density ambient fluid in a subsonic wind tunnel was experimentally studied using Particle Image Velocimetry and Planar Laser Induced Fluorescence to simultaneously measure velocity and density. Flows involving the mixing of fluids with large density ratios are important in a range of physical problems including atmospheric and oceanic flows, industrial processes, and inertial confinement fusion. Here we focus on buoyant jets with coflow. Results from two different Atwood numbers, 0.1 (Boussinesq limit) and 0.6 (non-Boussinesq case), reveal that buoyancy is important for most of the turbulent quantities measured. Statistical characteristics of the mixing important for modeling these flows such as the PDFs of density and density gradients, turbulent kinetic energy, Favre averaged Reynolds stress, turbulent mass flux velocity, density-specific volume correlation, and density power spectra were also examined and compared with previous direct numerical simulations. Additionally, a method for directly estimating Reynolds-averaged velocity statistics on a per-pixel basis is extended to Favre-averages, yielding improved accuracy and spatial resolution as compared to traditional post-processing of velocity and density fields. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H28.00004: Alignment of principal strain rates, vorticity, and scalar gradients in a turbulent nonpremixed jet flame Antonio Attili, Fabrizio Bisetti The alignment of vorticity and gradients of conserved and reactive scalars with the eigenvectors of the strain rate tensor (i.e., the principal strains) is investigated in a direct numerical simulation of a turbulent nonpremixed flame achieving a Taylor's scale Reynolds number in the range $100\le{\rm Re}_{\lambda}\le150$ $[$Attili {\it et al.} Comb. Flame, 161, 2014$]$. The vorticity vector displays a pronounced tendency to align with the direction of the intermediate strain. These alignment statistics are in almost perfect agreement with those in homogeneous isotropic turbulence $[$Ashurst {\it et al.} Physics of Fluids 30, 1987$]$ and differ significantly from the results obtained in other nonpremixed flames in which vorticity alignment with the most extensive strain was observed $[$Boratav {\it et al.} Physics of Fluids 8, 1996$]$. The gradients of conserved and reactive scalars align with the most compressive strain. It is worth noting that conditioning on the local values of the mixture fraction does not affect the statistics. Our results suggest that turbulence overshadows the effects of heat release and chemical reactions. This may be due to the larger Reynolds number achieved in the present study compared to that in previous works. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H28.00005: Modeling Reaction Rates in Variable-Density Turbulent Mixing Nicholas Denissen, Raymond Ristorcelli Modeling reactions in variable-density turbulent mixing is important for multi-physics applications such as Inertial Confinement Fusion (ICF). Reynolds--Averaged Navier--Stokes (RANS) models are an important tool in this research, and work is ongoing to improve their fidelity in complex flows. Connecting these models to the underlying multi-material mixing and thermonuclear reaction rates is essential. This talk describes the BHR family of turbulence models developed at Los Alamos National Laboratory (LANL), and the information they provide for characterizing turbulent mixing. Some exact relationships for reaction rates in variable-density turbulence will be presented and connected to the lower-order moments provided by variable-density RANS models. These provide model equations for initially pre-mixed and initially separated reactants. The strengths and limitations will be discussed, and examples will be shown from ICF-like hydrodynamic simulations in the LANL ASC code FLAG to assess the impact of these models. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H28.00006: Prediction of scalar gradient distributions under stretching and random aggregation processes: application to mixing in turbulent and porous media flows Tanguy Le Borgne, Peter Huck, Marco Dentz, Emmanuel Villermaux Scalar gradients play a key role in controlling mixing and reaction processes in natural and industrial flow systems. The stretching action of flow fields naturally organizes scalar fields into lamellar structures, whose elongation and aggregation determine the evolution of concentration distributions. In this context, the prediction of scalar gradient distributions requires quantifying the spatial correlation of concentration fields. For general stretching and aggregation processes, we derive theoretical predictions of the temporal evolution of the concentration increment PDFs over any spatial increments. This framework is shown to provide accurate predictions of concentration gradient distributions for a range of flow systems, including turbulent and porous media flows. In particular, the theory links intermittent scalar field properties to their random additive nature and consequent spatial organization. We argue that the analysis of the distribution of concentration increments over different spatial increments may be considered as a deconstruction of the basic lamella assemblage, revealing the elementary structures building concentration distributions in heterogeneous flows. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H28.00007: Hierarchical parcel-swapping (HiPS) representation of turbulent flow and mixing Alan Kerstein An economical representation of effects of turbulence on the time-evolving structure of diffusive scalar fields is obtained by introducing a hierarchical (tree) network connecting fluid parcels, with effects of turbulent advection represented by swapping pairs of sub-trees at rates determined by turbulence time scales associated with the sub-trees [1]. The fluid parcels reside at the base of the tree. The tree structure partitions the fluid parcels into adjacent pairs (or more generally, p-tuples). Adjacent parcels intermix at rates governed by diffusion time scales based on molecular diffusivities and parcel sizes. This simple procedure efficiently accomplishes long-standing objectives of turbulent mixing model development, such as generating physically based time histories of fluid-parcel nearest-neighbor encounters and the associated spatial structure of turbulent scalar fields. With the introduction of velocity components as well as scalars, this hierarchical parcel-swapping (HiPS) formulation becomes a self-contained flow simulation, as illustrated by its application to fully developed channel flow [2]. [1] A. R. Kerstein, J. Stat. Phys. \textbf{153}, 142-161 (2013). [2] A. R. Kerstein, J. Fluid Mech. \textbf{750}, 421-463 (2014). [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H28.00008: Backward tracking and Lagrangian passive scalar mixing in turbulence simulations D. Buaria, P.K Yeung, B.L. Sawford In many environmental problems the dispersion of contaminants with finite molecular diffusivity is closely related to the trajectories of molecules which undergo Brownian motion relative in the field. This invokes a Lagrangian view of mixing, which asks, for instance, how a pair of molecules far apart at earlier times may come together and cause a high local concentration of the diffusing material or property. We have implemented an efficient and statistically robust approach to extract backward statistics via the post-processing of trajectory data stored in direct numerical simulations with many millions of fluid particles and diffusing molecules. Results are obtained at Taylor scale Reynolds number 140 to 400 and Schmidt numbers from 0.001 to 1000. As expected the contrast between forward and backward dispersion is greater at higher Reynolds number where nonlinear turbulent transport is stronger. Subject to sampling, the Lagrangian data obtained agree well with Eulerian results on the production and dissipation of the variance of a passive scalar driven by a uniform mean gradient in isotropic turbulence. Extensions to multi-particle and multi-molecule clusters are also briefly addressed. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H28.00009: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 12:27PM - 12:40PM |
H28.00010: Effect of velocity ratio on coherent-structure dynamics in turbulent free shear layers Saikishan Suryanarayanan, Roddam Narasimha The relevance of the vortex-gas model to the large scale dynamics of temporally evolving turbulent free shear layers has been established by extensive simulations (Phys.Rev.E 89, 013009, 2014). The effects of velocity ratio ($r = U_2/U_1$) on shear layer dynamics are revealed by spatially evolving vortex-gas shear-layer simulations using a computational model based on Basu et al (Appl.Math.Modelling 19, 1995), but with a crucial improvement that ensures conservation of global circulation. The simulations show that the initial conditions and downstream boundaries can significantly affect the flow over substantial part of the domain, but the equilibrium spread rate is a universal function of $r$, and is within the experimental scatter. The spread in the $r = 0$ limit is higher than Galilean-transformed temporal value. The present 2D simulations at $r = 0$ show continuous growth of structures, while merger-dominated evolution is observed for $r = 0.23$ (and higher). These two mechanisms were observed across the same two values of $r$ in the experiments of D'Ovidio \& Coats (J.Fluid Mech 737, 2013), but the continuous growth was instead attributed to mixing-transition and 3D. The 2D mechanisms responsible for the observed continuous growth of structures are analyzed in detail. [Preview Abstract] |
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