Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G22: Turbulent Mixing I: Scalar Mixing |
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Chair: Rodney Fox, Iowa State University Room: 317 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G22.00001: Measurements of the relative diffusion of a passive scalar plume in sheared turbulence Christina Vanderwel, Stavros Tavoularis A neutrally buoyant aqueous solution of Rhodamine 6G fluorescent dye was injected isokinetically from a thin tube into fully developed, uniformly sheared, nearly homogeneous, turbulent flow in a water tunnel. The turbulent Reynolds number was $Re_\lambda \approx 150$. Instantaneous dye concentration variations in several planes normal to the flow were mapped by means of planar laser-induced fluorescence. Mean relative concentration maps were determined by ensemble averaging the instantaneous maps, each shifted to a common centre of mass. These maps could be fitted well by a 2D Gaussian function. The distance-neighbour function was also determined in each plane from mean planar autocorrelation maps and its shape was found to be comparable to analytical expressions by Richardson and by Batchelor. The relative plume width, defined as the standard deviation of the mean relative concentration map, was approximately equal to $1/\sqrt{2}$ times the standard deviation of the distance-neighbour function. The relative plume width remained within the inertial subrange and its streamwise growth rate was consistent with Richardson's 4/3 Law. The estimated value of Richardson's constant was roughly 0.1. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G22.00002: Endwall Vortex Effects on Turbulent Dispersion of Film Coolant in a Turbine Vane Cascade Sayuri D. Yapa, Christopher J. Elkins, John K. Eaton Turbine flows include strong secondary flows due to flow turning. The dominant flow feature is the passage vortex, located in the corner between the endwall and the suction surface of the airfoil. This vortex may have a strong effect on scalar transport in the turbine wake. Experiments were conducted to examine the dispersion of coolant emitted along the trailing edge of the airfoil. 3D velocity and concentration measurements were made using magnetic resonance imaging to study turbulent mixing in a realistic film-cooled nozzle vane cascade. The passage vortex has large effects on the flow features in the vane wake and on coolant mixing. A shear layer is created on the vane's suction side and interacts with the passage vortex after shedding from the trailing edge. The resulting vortex pattern forces the coolant jet into a highly distorted shape. A key question is how this distortion affects the turbulent diffusion of coolant. The 3D MRI-based velocity and concentration measurements allows for estimation of turbulent diffusivity. Control volumes are defined using a streamtube that is defined beginning just downstream of the trailing edge. The turbulent diffusivity is determined by integrating the Reynolds-averaged advection-diffusion equation over these control volumes. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G22.00003: Transfer of passive scalar variance in decaying grid turbulence with a mean scalar gradient Luminita Danaila, Laurent Mydlarski The present work focuses on the mixing of a passive scalar for $Sc=0.7$ ($Sc$ is the Schmidt number) in decaying, homogeneous isotropic turbulence, where the scalar fluctuations are produced via a large-scale, mean scalar gradient. The overall philosophy is \emph{to understand and predict the scalar behaviour, when the velocity field is known}. Of particular interest is the transfer of scalar variance and its comparison with that of the kinetic energy. The experimental evidence suggests that the scalar variance transfer is closer to the asymptotic value of $4/3$ than its kinetic energy counterpart. This behaviour is explained analytically by modelling the scalar variance transfer as a function of the scalar variance at scale $r$, and a characteristic time resulting from the strain effected by a range of scales of sizes slightly larger than $r$, up to $r$ itself. This model is consistent with the experimental data, measured in grid turbulence with a mean scalar gradient, over a relatively wide range of Reynolds numbers ($R_\lambda$ up to $600$). We highlight the fact that the cascade mechanism of the scalar variance appears to be insensitive to whether the scalar field is dominated by decay or by production. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G22.00004: Turbulent transport and mixing of a passive scalar in a confined liquid wake James Hill, Katrine Nilsen, Bo Kong, Rodney Fox, Michael Olsen Turbulent mixing and transport of a passive scalar has been studied in a confined rectangular liquid wake at Reynolds number 37,500. Large-eddy simulations (LES) of transport and mixing of the passive scalar were performed and the results were compared to velocity and concentration data from simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements. Single-point statistics of velocity and concentration from the LES were validated by the experimental data. Two-point spatial correlations of turbulent velocity, passive scalar, and joint velocity-scalar fields were computed from both simulation and experimental data and the results were compared. In this way information was obtained about LES' ability to predict the important coherent structures of the flow and their contribution to the scalar transport. The simultaneous PIV/PLIF data also provided the opportunity to evaluate turbulent fluxes, turbulent Schmidt number and two components of the turbulent diffusivity tensor. Comparison of these quantities between simulation and experiment provides important insight into the turbulent transport processes of the wake and how LES performs in predicting these. The simulation results showed overall good agreement with the experimental data. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G22.00005: Statistical Investigation of Turbulent Mixing by Means of Turbulent Line Segments Michael Gauding, Norbert Peters We examine the turbulent mixing of a passive scalar with imposed mean gradient. The Taylor microscale based Reynolds number varies between 85 and 530. A straight line through the turbulent field of a passive scalar $\phi$ is decomposed into piece-wise monotonously increasing or decreasing segments. These so called turbulent line segments (TLS) start at a local minimum point and end at a local maximum point or vice versa and are parameterized by the distance $\ell$ between the extreme points and by the corresponding scalar difference $\Delta \phi$. The implication is that TLS, whose mean length is about ten times the Kolmogorov length, characterize the dynamic process of scalar-energy dissipation. Firstly, we examine the joint distribution function of $\Delta\phi$ and $\ell$ and define the gradient $\Delta\phi/\ell$ of TLS. This helps to understand cliff-ramp structures as we can show at which length scale large gradients arise. Based on a statistical approach we can further relate the mean gradient to the local gradient and can examine the scaling of the kurtosis of the local gradient with the Reynolds number. Secondly, we define a structure function based on TLS, that relates the extreme points and calculate the scaling exponents. The result is compared with the KOC-theory. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G22.00006: Turbulent generation of scalar covariance between two initially distant scalars: implications for enhanced mixing and reaction Michael Soltys, Farrokh Shoaei, John Crimaldi Mixing and reaction between two scalars initially separated by scalar-free ambient fluid is important in problems ranging from ecology to engineering, but is relatively unstudied compared to the more common topology where the two scalars initially share a material interface. We use a two-channel PLIF system in a laboratory flume to quantify the instantaneous spatial structure of two independent scalars released from laterally separate locations in homogeneous grid turbulence. Local reaction rates in the low-Damkohler limit can then be computed. We demonstrate that the two passive scalars selectively aggregate in attracting regions of the turbulent flow, as quantified by streamwise development of positive scalar covariance. A decomposition of the total reaction into mean and instantaneous contributions reveals that the relative contributions depend strongly on streamwise location. Our results demonstrate that over 80\% of the downstream reaction is associated with the scalar covariance produced by instantaneous flow processes, such that the total reaction greatly exceeds that predicted my mean processes alone. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G22.00007: Statistical and Visual Analysis of Conserved Scalar Mixing Dynamics in Turbulent Jets Using kHz-Rate Imaging Michael Papageorge, Frederik Fuest, Jeffret Sutton The objective of this work is to examine the space-time dynamics of conserved scalar transport and mixing in gas-phase, turbulent jets utilizing kHz-rate, planar laser diagnostics. This research is facilitated by the High-Energy Pulse-Burst Laser System (HEPBLS) at Ohio State, which is capable of delivering high-energy ($\sim$ 1 J) pulses at 532 nm at repetition rates of 10 kHz and higher. With this system, time-resolved 2-D scalar mixing fields are acquired with high signal-to-noise ratios. In this study Rayleigh scattering from a propane jet issuing into a low-speed air co-flow was used to measure mixture fraction at Re $=$ 10,000 to 30,000 at axial locations of x/D $=$ 10 to 40. Single- and multi-point time statistics are employed to gain a better understanding of the dynamics of large-scale features. Single point auto-correlations are used to calculate the integral time scale as a function of axial and radial location and Reynolds number with unprecedented spatial resolution. Multi-point time correlations are then used to examine the nature of scalar advection and spreading rate across the width of the jet. In addition to the statistical representation, both time scale and spreading rate are examined visually to gain an improved qualitative understanding of scalar mixing. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G22.00008: How a scalar puff that is written in turbulence disperses: theory and experiment Willem van de Water, Enrico Calzavarini, Mehrnoosh Mirzaei, Bruno Eckhardt, Federico Toschi, Nico Dam When a blob of passive scalar is released in turbulence, it will spread due to the combined action of turbulence and molecular diffusion. It is a still unresolved question whether molecular diffusion helps or suppresses the spreading of the blob. We write a scalar puff in a strongly turbulent flow of air using molecular tagging with two crossed UV laser beams. The puff is made by fusing N$_2$ and O$_2$ molecules to NO, which is then used as a tracer. The dispersion of the puff is followed using laser--induced fluorescence. When the blob is small (size $\approx 10 \: \eta$), the evolution of its Gaussian parameters $\Gamma$ satisfies a simple linearized equation driven by the gradients of the turbulent velocity field [1]. It was computed using the velocity field of a direct numerical simulation (Re$_{\lambda} = 400$). At short times we find striking agreement between experiment and numerical simulation. A question is whether the strongly anomalous statistics of the gradients will endow the fluctuations of $\Gamma$ with special properties. A preliminary conclusion is that this is not the case, with the fluctuations being close to log--normal. [1] H. Tennekes and J.~L. Lumley, {\em A First Course in Turbulence}. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G22.00009: On the interaction of two scalar plumes in a turbulent flow Bing-Chen Wang, Shahin Oskouie, Eugene Yee Direct numerical simulation is used to study the interaction of two plumes released by two point sources in the context of a turbulent open channel flow. This study is inspired by the classical experiment in quantum physics, Young's double-slit interference experiment on light and energy. The results of the first-order concentration statistics show that in the convective range, the two ground plumes mix faster in the spanwise direction, however, in the turbulent diffusion range, the elevated plumes spread and mix faster. It is observed that streamwise evolution of the second-order correlation function at the midpoint between the two plumes exhibits four distinct mixing stages for both ground and elevated sources. The second-order correlation function demonstrates that the degree of mixing is minimum at the midpoint between the two plumes and is maximum at plume fringes. In general, the elevated plumes exhibit higher degree of mixing in comparison with the ground plumes due to the meandering effects. The scatterplots prove that the higher order statistics of the concentration can be predicted by the knowledge of the first and second-order statistics. [Preview Abstract] |
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