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 G28: Turbulence: Mixing I |
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Chair: Filippo Coletti, University of Minnesota Room: 2011 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G28.00001: The influence of coherent structures on the turbulent dispersion of a passive scalar plume Christina Vanderwel, Stavros Tavoularis We investigated the influence of coherent structures on the dispersion of a passive scalar by studying instantaneous measurements of a plume of dye released in uniformly sheared flow generated in a water tunnel. Measurements were performed using simultaneous stereo particle image velocimetry and planar laser-induced fluorescence to obtain instantaneous concentration and velocity maps in cross-sections normal to the flow direction. Coherent vortices were observed to effectuate scalar transport by inducing motions which displaced dyed fluid. Dye was observed to preferentially congregate within vortex cores and far away from vortices, whereas regions adjacent to vortices were less likely to contain dye. A conditional eddy analysis demonstrated that counter-rotating vortex pairs associated with hairpin vortices were responsible for both large Reynolds stress events and large scalar flux events. This observation was supported by the fact that the Reynolds stress was found to be correlated with the scalar flux. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G28.00002: Turbulent Mixing of Jet in Crossflow with Compound Angle Injection Kevin Ryan, Filippo Coletti, Christopher Elkins, John Eaton A dominant feature governing the development of the jet in crossflow is a pair of longitudinal vortices that originate at the point of injection. These vortices cause a distortion of the jet and promote mixing of the jet and mainstream fluid. The vortex structure is significantly altered for jets with compound angle injection, with respect to jets with no skew relative to the mainstream flow. In skewed geometries, a single dominant vortex controls the development of the jet and mixing of the jet fluid with the mainstream. The 3D velocity and concentration fields were measured for a compound angle jet injected with a skew angle of 30 degrees relative to the incoming flow. Measurements were conducted using magnetic resonance imaging (MRI) techniques using water as the working fluid. The development of the vorticity was investigated at the point of injection. The effect of the single dominant vortex on the turbulent mixing of the jet fluid with the mainstream was evaluated using the scalar concentration field. Spreading of the jet due to turbulent mixing is shown to be highly asymmetric. Results obtained for the skewed jet were compared to an angled jet in crossflow with no skew. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G28.00003: Turbulent Scalar Flux Modeling for an Inclined Jet in Crossflow: An Analysis of the Error Incurred by Various Modeling Assumptions Julia Ling, Kevin Ryan, John Eaton In order to use Reynolds-Averaged Navier Stokes (RANS) solvers to determine a passive scalar concentration distribution, it is necessary to model the turbulent scalar fluxes. Various models have been proposed for these turbulent scalar fluxes, each of which relies on a different set of basic assumptions. The gradient diffusion hypothesis assumes that the turbulent scalar fluxes can be modeled as diffusive fluxes with an isotropic diffusivity. A fixed turbulent Schmidt number model relies on the Reynolds analogy between momentum and scalar transport. Higher order algebraic closures, such as the Generalized Gradient Diffusion Hypothesis (GGDH), break away from the isotropic assumption inherent in a fixed turbulent Schmidt number model. The error from each of these modeling assumptions was isolated and evaluated through the analysis of a high-fidelity Large Eddy Simulation (LES) for an inclined jet in crossflow configuration. The LES velocity field and Reynolds stresses were fed into a RANS solver and the scalar distribution was calculated using various turbulent scalar flux models. This methodology removed the compounding effect of errors in the velocity field and Reynolds stresses, enabling the direct evaluation of the isotropic assumption and the Reynolds analogy. It was shown that neither of these assumptions were appropriate for this flow. However, it was also demonstrated that the largest source of error in the scalar flux modeling was due to poorly tuned model coefficients, not to any particular modeling assumption. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G28.00004: Flow and turbulence structure in a shallow mixing layer developing over a flat surface at high Reynolds numbers Gokhan Kirkil Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a flat surface at a high Reynolds number (Re$=$250,000). The study discusses the influence of Reynolds number on the development of the mixing layer as well as the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale coherent structures with the distance from the splitter plate. Mixing layer growth rate and its change in the vertical direction are compared with experiments and a simulation at Re$=$16,000. Power spectra of the horizontal velocity components are examined for the presence of a -3 and -5/3 subranges at streamwise locations away from the splitter plate. Passive scalar is introduced at the tip of the splitter plate close to the free surface to estimate the size of the mixing structures based on mass transport. The effect of the Reynolds number on the shift of the centerline of the mixing layer is quantified. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G28.00005: Effect of background turbulence on the mixing of a passive scalar within a turbulent jet Alejandro P\'{e}rez Alvarado, Susan Gaskin, Laurent Mydlarski The vast majority of the research on turbulent jets has studied those emitted into quiescent (or laminar) backgrounds. Yet, most jets in environmental or engineering applications are emitted into turbulent backgrounds. It is therefore imperative to fully understand the underlying physics of jets emitted into turbulent environments. The present investigation builds on previous work (Khorsandi, Gaskin and Mydlarski, \emph{J. Fluid Mech.}, 2013, which studied the effect of background turbulence on the velocity field of a turbulent jet) and examines the mixing of a (high-Schmidt-number) passive scalar within a turbulent jet that is emitted into a turbulent environment. A quasi-homogeneous and isotropic, zero-mean-flow turbulent background was generated by means of a random jet array. The concentration field within the turbulent jet was measured using planar laser induced fluorescence. We will present results pertaining to the evolution of the statistical moments of the scalar field, as well as its characteristic length, relative that of a jet emitted into a quiescent background, and as a function of the intensity of the background turbulence. The role of background turbulence on the jet entrainment mechanism will also be addressed. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G28.00006: Mixing efficiency in two-dimensional turbulent and chaotic flows Benjamin Kadoch, Wouter Bos, Kai Schneider We investigate the mixing in a flow generated by a circular rod, describing a figure-eight-shaped motion in a two-dimensional circular vessel. The vessel, the moving rod, and the equations of motion are modeled using a volume penalization method imbedded in a classical Fourier pseudo-spectral code as described in [1]. The influence of the Peclet number on the mixing efficiency is measured for different Stokes and turbulent regimes. Here, the mixing efficiency is measured by evaluating the decay of passive scalar fluctuations for a given energy injection rate. The Stokes regime shows results similar to the ones obtained in [2] for chaotic mixing. For instance, the passive scalar variance decays following a powerlaw, related to the presence of unmixed fluid near the fixed walls, which acts as a reservoir for the mixing away from the wall. For the turbulent regimes, however, the detachment of vorticity in the boundary layer more efficiently injects the unmixed fluid into the domain.\\[4pt] [1] B. Kadoch, D. Kolomenskiy, K. Schneider and P. Angot. J. Comput. Phys., 231, 4365-4383, 2012.\\[0pt] [2] E. Gouillart, O. Dauchot, B. Dubrulle, S S. Roux, and J.-L. Thiffeault. Phys. Rev. E 78, 026211, 2008 [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G28.00007: Statistics and Scaling Laws of Turbulent Mixing at High Reynolds Numbers Michael Gauding, Markus Hempel, Christian Hasse We examine the turbulent mixing of passive scalars with imposed mean gradient and varying diffusivities by means of direct numerical simulation. The transport mechanism within the turbulent cascade is altered when differential diffusion is present. In order to analyze this effect, we derive from first principles an equation in correlation space that quantifies differential diffusion. This equation captures the balance between inter-scale transport, diffusive transport, scalar dissipation, as well as a transport that originates from unequal diffusivities between the involved scalars. This equation is not closed but each term can be analyzed by means of direct numerical simulation. To this end, direct numerical simulations have been conducted with Taylor based Reynolds number varying between 88 and 529. The Schmidt number is varied between 1/8 and 1. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G28.00008: Intermittency and universality of small scales of passive scalar in turbulence Toshiyuki Gotoh, Takeshi Watanabe Recent experiments and Direct Numerical Simulations (DNSs) suggest that the small scale statistics of passive scalar may not be as ``universal'' as in the velocity case. To address this problem, we study the moments of scalar increment in steady turbulence at $R_\lambda > 800$ by using DNS up to the grid points of $4096^3$. In order for the scalar and turbulent flow to be as faithful as possible to the assumptions that would be made in theories, Scalar 1 and Scalar 2 are simultaneously convected by the identical isotropic turbulent flow but excited by two different methods. Scalar 1 is excited by the random scalar injection which is isotropic, Gaussian and white in time at low wavenumber band, while Scalar 2 is excited by the uniform mean scalar gradient. The moments of two scalars as functions of the separation vector are expanded in terms of the Legendre polynomials to extract the scaling exponents of the moments up to the 4th anisotropic sector for Scalar 2. It is found that the exponents of the isotropic sectors seem to have the same values at separation distances in the narrow range over which the 4/3 law holds simultaneously for two scalars. The exponents of the anisotropic sectors and the cumulants of the moments will also be reported. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G28.00009: Optimal mixing of a passive scalar by supercritical 3D plane Poiseuille flow Lukas Vermach, C.P. Caulfield We consider a passive zero-mean scalar field organised into two layers of different concentration, in a 3D plane channel subjected to a constant along-stream pressure gradient. We employ a fully nonlinear adjoint-looping approach to identify the optimal initial perturbation of the velocity field with given initial energy which yields maximal mixing by a target time horizon, in the sense of minimisation of the spatially-integrated variance of the concentration field. Foures \emph{et.al.} (JFM, 2014) considered 2D plane Poiseuille flow at a sufficiently low (subcritical) $Re \sim 500$ to not be subject to flow instabilities, and demonstrated that the initial perturbation which maximizes the time-averaged energy gain of the flow leads to weak mixing, and is qualitatively different from the optimal initial ``mixing'' perturbation which exploits classical Taylor dispersion. We generalise this study to the optimisation of mixing three-dimensional flows at a range of significantly higher (supercritical) Reynolds numbers, showing how the potential triggering of ``strong'' flow instabilities modifies the structure of the optimal initial mixing perturbation qualitatively. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G28.00010: Normalizations of High Taylor Reynolds Number Power Spectra Alejandro Puga, Timothy Koster, John C. LaRue The velocity power spectrum provides insight in how the turbulent kinetic energy is transferred from larger to smaller scales. Wind tunnel experiments are conducted where high intensity turbulence is generated by means of an active turbulence grid modeled after Makita's 1991 design (Makita, 1991) as implemented by Mydlarski and Warhaft (M\&W, 1998). The goal of this study is to document the evolution of the scaling region and assess the relative collapse of several proposed normalizations over a range of $R_{\lambda}$ from $185$ to $997$. As predicted by Kolmogorov (1963), an asymptotic approach of the slope ($n$) of the inertial subrange to $-5/3$ with increasing $R_{\lambda}$ is observed. There are three velocity power spectrum normalizations as presented by Kolmogorov (1963), Von Karman and Howarth (1938) and George (1992). Results show that the Von Karman and Howarth normalization does not collapse the velocity power spectrum as well as the Kolmogorov and George normalizations. The Kolmogorov normalization does a good job of collapsing the velocity power spectrum in the normalized high wavenumber range of $0.0002\leq\kappa\lambda\leq0.4$ while the George normalization does a better job in the normalized mid-wavenumber range of $15\leq\kappa\lambda\leq25$. [Preview Abstract] |
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