Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session AA: Turbulent Mixing I |
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Chair: Snezhana I. Abarzhi, University of Chicago Room: Long Beach Convention Center 101A |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AA.00001: LES of scalar transport in wave and wind-driven flows with large-scale structures Cigdem Akan, Andres Tejada-Martinez, Chester Grosch Near-surface scalar (mass) transport results from large-eddy simulation (LES) of wind-driven flow with and without full-depth Langmuir circulation (LC) are reported. LC is generated by wave-current interaction and consists of counter rotating vortices aligned in the direction of the wind. LES driven by wind and wave forcing conditions measured during field observations of full-depth LC by Gargett and Wells (Journal of Fluid Mechanics, 576, 27-61, 2007) shows that this large-scale, downwind-elongated structure increases surface mass transfer velocity (a measure of mass transfer efficiency) by approximately 60 percent with respect to a similar flow without surface wave effects (i.e. without LC). The LES will be used to test the accuracy of surface renewal-based parameterizations (models) in predicting surface transfer velocity increase in flows with LC. Statistical analysis of LES variables will be presented demonstrating that full-depth LC dominates near-surface mass transport as well as transport everywhere else in the water column. In the absence of LC, near-surface small eddies contribute significantly towards the mass transport at the air-water interface. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AA.00002: Large-eddy simulation of oxygen transport and depletion in waterbodies Carlo Scalo, Ugo Piomelli, Leon Boegman Dissolved oxygen (DO) in water plays an important role in lake and marine ecosystems. Agricultural runoff may spur excessive plant growth on the water surface; when the plants die they sink to the bottom of the water bodies and decompose, consuming oxygen. Significant environmental (and economic) damage may result from the loss of aquatic life caused by the oxygen depletion. The study of DO transport and depletion dynamics in water bodies has, therefore, become increasingly important. We study this phenomenon by large-eddy simulations performed at laboratory scale. ~The equations governing the transport of momentum and of a scalar (the DO) in the fluid are coupled to a biochemical model for DO depletion in the permeable sediment bed [Higashino et al., Water Res. (38) 1, 2004)], and to an equation for the fluid transpiration in the porous medium. The simulations are in good agreement with previous calculations and experiments. We show that the results are sensitive to the biochemical and fluid dynamical properties of the sediment, which are very difficult to determine experimentally. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AA.00003: Decay of a Passive Tracer in Two-Dimensional Turbulence-Application to Infinitely Fast Chemistry Farid Ait Chaalal, Peter Bartello, Michel Bourqui We investigate the effect of diffusion ($\kappa$) on chemical production in a 2D turbulent flow using ensembles of direct numerical simulations (DNS). Assuming an infinitely fast chemistry between two initially unmixed reactants, the problem simplifies to studying the mean absolute value of the passive tracer $\phi$ defined as the difference between the concentrations of the two reactants. The reaction speed is dictated by the diffusive flux across an isoline of $\phi$. The DNS show that production scales like $\kappa^{p(t)}$ where $p(t)$ is a positive decreasing function of time, during an initial transient period characterized by the exponential lengthening of the contact line. We show theoretically that this behavior of $p(t)$ is determined by the initial gradients along the contact line, with sharp gradients decreasing the effect of diffusion. The long time decay of the first moment of $\phi$ is exponential. At large Peclet number, the decay rate converges and the reaction speed is merely determined by the mixing. At small Peclet number ($<5000$), the decay rate scales like $\kappa^{1/2}$, reflecting a purely diffusive behaviour. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AA.00004: DNS of mixing in spatially developing shear layers Antonio Attili, Fabrizio Bisetti The majority of models proposed for turbulent combustion rely on the knowledge of the mixture fraction $Z$, its pdf, the scalar dissipation, and, in some cases, a mixing time scale. Turbulent mixing in technical devices is almost always inhomogeneous and the flow regime is transitional so that spatially developing mixing layers are highly relevant. DNS have been performed for a Reynolds number up to $2 \times 10^4$, enough to identify mixing transition. All relevant statistics and their dependence on Reynolds number have been analyzed. Previous results show that the scalar pdf can be: {\it non-marching}, when the most probable value of the mixture fraction of mixed fluid is invariant across the width of the layer; {\it marching}, for which the most probable value is the mean mixture fraction at the location considered. The pdf behavior is discussed in several previous works and it has been found to be related to the Reynolds number and the level of turbulence development. The DNS results we present clearly show that the pdf evolves from {\it non-marching} to {\it marching} during the streamwise evolution of the scalar field. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AA.00005: Dynamic model for the joint scalar probability in multi-species turbulent mixing J.R. Ristorcelli, J. Bakosi We present a probability density function (PDF) model for multi-species scalar mixing in turbulent flows. In the proposed model the scalars are governed by a system of stochastic differential equations, discretized and integrated in a Monte-Carlo fashion. The model is local in composition space, accounts for different scalar mixing rates and Schmidt numbers and can represent a variety of PDF shapes, including a multiple-delta in the unmixed and a joint (bounded) Gaussian in the fully mixed states. The method is intended for passive, active or reactive scalars in shear-driven and/or variable-density pressure-gradient-driven turbulence. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AA.00006: Stochastic shell mixing model for scalars in homogeneous turbulence Yanjun XIa, Lance Collins We present a stochastic shell mixing model (SSMM) based on the eddy damped quasi-normal Markovian (EDQNM) theory to study turbulent mixing of scalars. The formulation combines the strengths of spectral model and the probability density function (PDF) method. The model advances the scalar concentration of notional particles that is distributed across spectral shells using a Monte Carlo algorithm. The variance of the concentration within each shell reproduces the scalar spectrum at the corresponding wavenumber, and the sum of the concentrations over the shell for each particle provides the fine-grained joint PDF. Monte Carlo schemes in general have difficult satisfying the bounds imposed on the scalar by the initial and boundary conditions. We present a special procedure for keeping the particle concentrations within the bounds. Due to the inherent full description of length and time scales, the SSMM model is capable of capturing the dependence of mixing on the molecular diffusivity, and hence the effects of differential diffusion. Results are compared with direct numerical simulations (DNS) and are in good agreement. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AA.00007: Modeling Scalar variance from Direct Numerical Simulations of a turbulent mixing layer Baptiste Ravinel, Guillaume Blanquart Many studies have focused on analyzing and predicting the mixing of a scalar such as fuel concentration in turbulent flows. However, the subfilter scalar variance in Large Eddy Simulations (LES) still requires additional considerations. The present work aims at obtaining results for the turbulent mixture of a scalar in configurations relevant to reactive flows, i.e. in the presence of mean velocity/scalar gradients. A Direct Numerical Simulation (DNS) of a turbulent mixing layer has been performed by initially combining two boundary layers. The high order conservative finite difference low Mach number NGA code was used together with the BQuick scheme for the transport of mixture fraction. The self-similar nature of the flow and energy spectra have been considered to analyze the turbulent flow field. High order velocity schemes (4th order) were found to play an important role in capturing accurately the mixing of fuel and air. The scalar variance has been calculated by filtering the solution and has been compared to various models usually used in LES. Following an earlier study by Balarac et al. [Phys. Fluids 20 (2008)], the concept of optimal estimators has been considered to identify the set of parameters most suitable to express the subfilter variance. Finally, the quality of the standard dynamic approach has been assessed. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AA.00008: Numerical diffusivity of scalar transport schemes in high Schmidt number flows Siddhartha Verma, Guillaume Blanquart Accurate simulation of scalar transport in high Schmidt number turbulent flows is essential to studying pollutant dispersion, weather, and several oceanic phenomena. Experiments over such large scales are difficult to conduct and simulations often provide a more practical alternative. Scalar transport in turbulent flows is governed by Batchelor's theory which requires further validation for high Schmidt numbers and high Reynolds numbers. The present study focuses on the impact of numerical diffusivity of various commonly used schemes in the turbulent transport of high Schmidt number scalars. This analysis is performed first in laminar flows, including advection of a Gaussian distribution in steady flow field and scalar stretching in Taylor vortex configuration. Then, the study focuses on the ability of the transport schemes to reproduce faithfully turbulent flow fields with high accuracy and low dissipation while maintaining physical boundedness. This analysis is conducted for isotropic, homogeneous forced turbulence and a mixing layer. Finally, a semi-Lagrangian scheme is considered and its performance is analyzed in the limit of zero diffusion. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AA.00009: Entanglement rules for random mixtures Emmanuel Villermaux, J\'er\^ome Duplat We discuss how two subparts of a randomly stirred scalar mixture interact to form the overall concentration distribution. We derive in particular the appropriate composition laws in absence, and in the presence of strong correlation between the fields. The resulting concentration distributions compare favorably with several distinct experiments, illustrating the two limits (Phys. Rev. Letters {\bf 105}, 034504, (2010)). The initial relative spatial position of the subparts plays a crucial role on the nature of their subsequent entanglement. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AA.00010: Linking spectra and geometry in scalar fields Mihkel Kree, J\'er\^ome Duplat, Emmanuel Villermaux The spectral signature of a turbulent scalar mixture is usually interpreted in terms of cascades, leading to decaying power laws either in the inertial range (the $-5/3$ slope), or in the viscous subrange (the $-1$ slope). An intermittent scalar field produced by an elongational velocity field, whatever it may be, also gives rise to a $-1$ power law. We study here the mixing and homogenization of a passive scalar injected from a point source by both confining it in a channel, or by letting it disperse freely. The resulting spatial field is a set of scalar blobs and voids, with well defined widths distributions. The corresponding power spectra are not pure power laws, and their shape is evolving in time. We will present a novel method for constructing the shape of the scalar power spectra solely based on the information contained in the size distributions of the contiguous regions scalar (the blobs) and the interlacing free space (the voids). [Preview Abstract] |
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