Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FS: Turbulence: Mixing I |
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Chair: Lester Su, Johns Hopkins University Room: Salt Palace Convention Center Ballroom EG |
Monday, November 19, 2007 8:00AM - 8:13AM |
FS.00001: Modeling and Measurements of Multiscalar Subgrid-Scale Mixing for LES/FMDF Abhilash Chandy, Steven Frankel, Matt Dinger, Jian Cai, Chenning Tong, Campbell Carter One of the main challenges in applying the filtered mass density function (FMDF) approach in the context of large eddy simulation (LES) of turbulent reacting flows is related to the ability of the mixing model to accurately predict subgrid-scale multiscalar mixing of passive or active scalars. LES/FMDF predictions and preliminary experimental measurements for three-stream mixing in a turbulent jet featuring a co-annular jet of ethylene and acetone-doped air issuing into an air co-flow, geometrically and hydrodynamically similar to recently studied piloted jet flames, will be presented. Laser-based diagnostics are used to obtain instantaneous and spatially filtered mass fraction measurements of relevance to the FMDF and its statistics. LES/FMDF studies, based on high-fidelity numerical methods, will compare predictions obtained from different mixing models and explore sensitivities to mixing model parameters with a focus on the mechanical-to-scalar time scale ratio. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FS.00002: Vortex ring dynamics and mixing in crossflow Rajes Sau, Krishnan Mahesh DNS is used to study the effect of crossflow on the dynamics, entrainment and mixing of vortex rings issuing from a nozzle. Three distinct regimes are found to exist, depending on the velocity ratio ($r$) and stroke ratio ($L/D$). A hairpin vortex forms instead of a complete vortex ring at velocity ratios below approximately 2. More interestingly, for large $L/D$, a series of hairpin vortices are shed downstream. The shedding characteristics are periodic for very low $Re$ (e.g. 300). For higher velocity ratios, two regimes are obtained depending upon $L/D$. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of $r$, the transition stroke ratio approaches the `formation number' defined by Gharib et al. (1998). In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behavior is explained. The trailing column is found to contribute significantly to the overall mixing and entrainment. There is an optimal length of the trailing column for maximum entrainment. A classification map which categorizes the different regimes of ring dynamics, structure and entrainment is presented. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FS.00003: Experiments and simulations of molecular mixing in a reattaching shear layer Aristides Bonanos, Georgios Matheou, Paul Dimotakis Experiments and simulations of a turbulent, compressible, re-attaching shear layer were conducted in order to investigate molecular mixing and validate the computational model. Experiments were conducted in a two-stream blow-down type wind-tunnel, with the top stream having a Mach number of 0.5 and two lower-to-upper stream mass flux ratios: 0.22 and 0.11. The two streams were seeded with hydrogen and fluorine respectively and the heat release from the reaction was used as a tracer of the mixed fluid. Molecular mixing was measured using the ``flip experiment'' technique. The experiments were modeled with a compressible large-eddy simulation (LES) solver using the stretched-vortex subgrid-scale model for momentum and scalar transport. The results from the experiments and simulations will be presented and discussed. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FS.00004: Statistical Modeling of Turbulent Dispersion using a Near-Neighbor Implementation of a Local Mixing Model Sharadha Viswanathan, Stephen B. Pope Probability density function (PDF) calculations are reported for the dispersion from line sources in isotropic turbulence. These flows pose a significant challenge to statistical models, because the scalar length scale (of the initial plume) is much smaller than the turbulence integral scale. The PDF calculations are based on a new near-neighbor implementation of the interaction by exchange with the conditional mean (IECM) mixing model. The calculations are compared to the experimental data of Warhaft (1984) on single and pairs of line sources, and with the previous calculations of Sawford (2004). This establishes the accuracy of the new implementation of IECM. An array of line sources is also considered with comparison to the experimental data of Warhaft \& Lumley (1978), which show the dependence of the scalar variance decay rate on the array spacing relative to the turbulence integral scale. The near-neighbor implementation is applicable to other local mixing models, as arise, for example, in multiple mapping conditioning (Klimenko \& Pope 2003). In the particle method used to solve the modeled PDF equation, the near-neighbor implementation results in a particle's mixing with just one or two near neighbors (in the relevant space), and hence maximizes the localness of mixing. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FS.00005: ABSTRACT WITHDRAWN |
Monday, November 19, 2007 9:05AM - 9:18AM |
FS.00006: Quantitative multi-scalar planar imaging in turbulent jets Cody Brownell, Lester Su We apply planar imaging techniques to investigate scalar mixing in a three-species turbulent axisymmetric jet. The jet consists of helium, acetone vapor, and air, issuing into a slow air co-flow. The diagnostic methods applied here are planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering. The PLIF signal provides an absolute measure of the acetone vapor concentration. The jet composition is specified so that the Rayleigh scattering signal quantifies the concentration difference between the helium and acetone. Simultaneous application of these methods allows direct determination of all relevant mole fractions in the three-species system. The imaging results focus on the near-field of the flow. The jet Reynolds number varies from approximately 1000 to 3000. These multi-scalar measurements are particularly interesting to simulations of turbulent combustion that go beyond the conserved scalar approach to the mixing problem. Quantitative results presented include scalar differences and correlations, and joint statistics of the different scalars. We also discuss the roles played by the large-scale organization, and the intermittency, of the flow, and the effect of varying the Reynolds number. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FS.00007: Reynolds Number Effects on Near-Source Turbulent Mixing in Pipe Flow Timothy J. Dwyer, James Guilkey, Holly Oldroyd, Timothy A. Barber, Eric R. Pardyjak Transport and dispersion physics of turbulent pipe flow is of importance to many industrial processes as well as water quality and pollution dispersion studies. This experimental work examines Reynolds number (Re) effects on turbulent pipe flow mixing characteristics in the near field of a coaxial scalar injection. The experimental approach is designed to obtain simultaneous velocity and concentration field measurements using 2D PIV and planar light induced fluorescence (PLIF). The experimental apparatus consists of a recirculating pipe flow apparatus with a customized test section designed to limit laser light reflection. A number of turbulent pipe flow velocity and concentration studies are available in the literature, however few present combined data over a range of Reynolds numbers. This study provides velocity, concentration and covariance data for Re = 5300-60000 that are compared to previous work. Simultaneous mean and fluctuating velocity and concentration data are presented with a unique focus on the spatial variability of the turbulent fluxes of momentum and scalar concentration. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FS.00008: The directional dependence of scalar mixing downstream of a concentrated source in turbulent channel flow Jason Lepore, Laurent Mydlarski To determine the effect of an inhomogeneous turbulent velocity field on scalar mixing, the present work focuses on the directional dependence of scalar fields in inhomogeneous turbulence. The scalar field under consideration is that generated downstream of concentrated line sources in fully-developed, high-aspect-ratio turbulent channel flow. The latter is the simplest realization of an inhomogeneous flow because the inhomogeneity is confined to only one direction. The work is performed by i) measuring the scalar field emitted by a line source oriented {\it normal} to the channel walls, and ii) comparing these results with previous research\footnote{R.A. Lavertu and L. Mydlarski, 2005. {\it J. Fluid Mech.}, {\bf 528}, p. 135} studying the scalar field downstream of a line source oriented {\it parallel} to the channel walls. Given that the flow is inhomogeneous, the results of i) -- pertaining to the ``lateral'' dispersion -- must differ from those of ii), which pertain to the ``transverse'' dispersion. The scalar (temperature) field is generated by heating fine wires that traverse the channel and is measured by means of cold-wire thermometry. The downstream evolution of statistical moments and probability density functions will be compared to reveal the directional dependence of the scalar dispersion. [Preview Abstract] |
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