Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G22: Turbulence Mixing I: Mixing in Jets |
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Chair: Godfrey Mungal, Stanford University Room: 30C |
Monday, November 19, 2012 8:00AM - 8:13AM |
G22.00001: Density and Velocity Ratios Effects on the Structure of Transverse Jets in Supersonic Crossflow Mirko Gamba, Victor A. Miller, M. Godfrey Mungal It is generally accepted that the jet-to-crossflow momentum flux ratio, $J$, is the primary parameter describing the structure, penetration and mixing properties of transverse jets. The interplay between density and velocity ratios, that combined define $J$, on these properties is, however, seldom considered nor fully understood. The current experimental work explores this interplay on transverse underexpanded sonic jets in a supersonic nitrogen crossflow ($M=2.25$, $T=480\ K$). A single-excitation, dual-band detection PLIF imaging scheme of toluene seeded into the crossflow is used to mark the crossflow fluid mixing into the transverse jet fluid and to determine the local fluid temperature. Different values of density and velocity ratios, while maintaining a constant value of $J$ (equal to 2.4), are investigated by injecting gases with different molecular weights. Notwithstanding the fact that all cases have the same value of $J$ and some similarity on the penetration characteristics exists, the emerging picture of the instantaneous turbulent structure of the flow indicates that the dynamics of entrainment and local mixing might be altered by low (high) values of the density (velocity) ratio compared to the corresponding case at high (low) values. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G22.00002: Numerical Study on Cryogenic Coflowing Jets under Transcritical Conditions Hiroumi Tani, Susumu Teramoto, Koji Okamoto, Nobuhiro Yamanishi A numerical and experimental study is presented on cryogenic coflowing jets under transcritical conditions for a better understanding of the propellant mixing in supercritical-pressure rocket engines. The major concerns are dominant flow structures in the mixing of cryogenic coflowing jets under transcritical conditions. Experimentally, in advance of detailed numerical simulations, cryogenic nitrogen/gaseous nitrogen coaxial jets were visualized by the backlighting photography technique. It was observed that a dense nitrogen core has a shear-layer instability near the injector exit and eventually breaks up into large lumps which dissolve and fade away downstream. In numerical simulations, LES technique was employed for more detailed discussion on the flow structures. LES of a cryogenic nitrogen/gaseous nitrogen coflowing plane jet was conducted with the same density and velocity ratios of inner/outer jets as the experiments. As observed in the experiments, the shear-layer instability in the inner mixing layers is predominant near the injector exit. After roll-up and paring, the shear-layer instability waves become large-scale vortices. They cause coherent vortex structures which become dominant in the downstream and break the dense core into lumps. Strouhal numbers of the shear-layer instability and the dense lump shedding in the numerical simulations were comparable to those measured in the experiments, respectively. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G22.00003: Large-eddy-simulation and measurements of turbulent transport and mixing in a confined rectangular jet James Hill, Katrine Nilsen, Bo Kong, Michael Olsen, Rodney Fox Large-eddy simulations of transport and mixing of a passive scalar were performed for a confined rectangular liquid jet (Re = 20,000) and compared to results of simultaneous particle image velocimetry and planar laser induced fluorescence measurements. Reasonably good agreement was obtained for single-point statistics as well as for two-point correlation functions of the turbulent velocity, scalar, and joint velocity-scalar fields. Of particular interest was the determination of the diagonal and off-diagonal components of the turbulent diffusivity tensor, the mis-alignment of turbulent fluxes and mean gradients, and determination of the turbulent Schmidt number, with reasonably good agreement between the simulations and experiments. Some of the results are consistent with the measurements of Tavoularis {\&} Corrsin (JFM 104, 331-367 (1981)) for a homogeneous turbulent shear flow with a uniform mean temperature gradient. In our case we find the ratio of diffusivities to range from 1 to 2 (the latter value in agreement with Tavoularis), Sc$_{t}$ generally between 0.5 and 1, and the angle between turbulent flux and mean scalar gradient from 120 to 150 degrees. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G22.00004: Gradient trajectory analysis of the scalar superlayer in a jet flow Markus Gampert, Philip Schaefer, Norbert Peters Based on planar high-speed Rayleigh scattering measurements of the mass fraction of propane discharging from a turbulent round jet into co-flowing carbon dioxide at nozzle based Reynolds numbers $Re_0$=$3,000$-$8,600$, we investigate the \textit{scalar superlayer}. The latter is located between the fully turbulent part of the jet and the outer flow and has the so called turbulent/non-turbulent interface embedded within it. It is termed in analogy to the laminar superlayer introduced by Corrsin and Kistler (NACA Report 1244, 1955). Using scalar gradient trajectories, we partition the turbulent scalar field into the afore mentioned three regions according to an approach developed by Mellado et al. (J. Fluid Mech. 626:333-365, 2009) based on which we in a next step investigate conditioned zonal statistics of the scalar pdf as well as the scalar difference along the trajectory and its mean scalar value. Finally, we relate our results for the scalar superlayer on the one hand to the findings made in other experimental and numerical studies of the turbulent/non-turbulent interface and discuss them on the other hand in the context of the flamelet approach in turbulent non-premixed combustion. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G22.00005: Variable Density Turbulent Jet Mixing Sergiy Gerashchenko, Kathy Prestridge Variable density mixing arises due to differences in molecular weights of the mixing fluids, or due to compressibility effects. A detailed understanding of the mixing processes has important consequences for many scientific and engineering systems such as inertial confinement fusion, atmospheric flows and oceans, or supernovae explosions. A new experiment has been developed at LANL to study the fundamental statistical properties of variable density turbulence that decays in time in subsonic incompressible flows. Initial experimental results are presented of a heavy fluid (sulfur hexafluoride gas) turbulent jet injected into coflowing air of lower turbulence. Buoyancy-mediated mixing is investigated at two Atwood numbers: 0 and 0.6, for a range of Reynolds numbers. The velocity and density fields are measured with simultaneous Particle Image Velocimetry and Planar Laser Induced Fluorescence. The fundamental statistical characteristics of the mixing important for modeling these flows such as spreading rate, mass flux, density self-correlation, kinetic energy flux, and turbulence decay rate are examined. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G22.00006: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 9:18AM - 9:31AM |
G22.00007: Large eddy simulation study of mixing in stratified jets Niranjan Ghaisas, Dinesh Shetty, Steven Frankel The structure and dynamics of a horizontally injected round turbulent buoyant jet are studied using Large Eddy Simulation (LES). A high-order accurate numerical procedure and different constant coefficient and dynamic eddy-viscosity sub-grid scale (SGS) models are used. The numerical procedure and SGS models are validated by conducting simulations at previous experimental flow conditions (characterized by Reynolds number, Re, and Richardson number, Ri), and comparing with the existing experimental results. The experimental results are then supplemented by simulations at different Re and Ri. Previous studies have shown that interaction between buoyancy and turbulence in this configuration leads to suppressed mixing in stably stratified regions and enhanced turbulence and mixing in unstably stratified regions. The ability of different SGS models to capture this phenomenon is studied by examining the jet trajectory, decay of the center-line velocity, radial spread and turbulent kinetic energy budgets at different axial locations. Schmidt number dependence of the results is also analyzed. Finally, the existence of secondary flows which lead to a plume-like vertical motion in addition to the primary horizontal injection is discussed and quantified. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G22.00008: Rapid Confined Mixing with Transverse Jets Part 1: Single Jet David Salazar, David Forliti Transverse jets have been studied extensively due to their relevance and efficiency in fluid mixing applications. Gas turbine burners, film cooling, and chemical reactors are some examples of rapid transverse jet mixing. Motivated by a lack of universal scaling laws for confined and unconfined transverse jets, a newly developed momentum transfer parameter was found to improve correlation of literature data. Jet column drag and entrainment arguments for momentum transfer are made to derive the parameter. A liquid-phase mixing study was conducted to investigate confined mixing for a low number of jets. Planar laser induced fluorescence was implemented to measure mixture fraction for a single confined transverse jet. Time-averaged cross-sectional images were taken with a light sheet located three diameters downstream of transverse injection. A mixture of water and sodium fluorescein was used to distinguish jet fluid from main flow fluid for the test section images. Image data suggest regimes for under- and overpenetration of jet fluid into the main flow. The scaling parameter is found to correlate optimum unmixedness for multiple diameter ratios at a parameter value of 0.75. Distribution A: Public Release, Public Affairs Clearance Number: 12655. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G22.00009: Rapid Confined Mixing Using Transverse Jets Part 2: Multiple Jets David Forliti, David Salazar An experimental study has been conducted at the Air Force Research Laboratory at Edwards Air Force Base to investigate the properties of confined mixing devices that employ transverse jets. The experiment considers the mixing of water with a mixture of water and fluorescein, and planar laser induced fluorescence was used to measure instantaneous mixture fraction distributions in the cross section view. Part one of this study presents the scaling law development and results for a single confined transverse jet. Part two will describe the results of configurations including multiple transverse jets. The different regimes of mixing behavior, ranging from under to overpenetration of the transverse jets, are characterized in terms of a new scaling law parameter presented in part one. The level of unmixedness, a primary metric for mixing device performance, is quantified for different jet diameters, number of jets, and relative flow rates. It is apparent that the addition of a second transverse jet provides enhanced scalar uniformity in the main pipe flow cross section compared to a single jet. Three and six jet configurations also provide highly uniform scalar distributions. Turbulent scalar fluctuation intensities, spectral features, and spatial eigenfunctions using the proper orthogonal decomposition will be presented. Distribution A: Public Release, Public Affairs Clearance Number: 12656. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G22.00010: Two-point statistics for turbulent relative dispersion in quasi-two-dimensional turbulent jets Julien R. Landel, C.P. Caulfield, Andrew W. Woods The study of turbulent jets in relatively enclosed geometries is relevant to rivers flowing into lakes. In the event of a spillage of pollutants into a river, it is critical to understand how these agents disperse with the flow in order to assess damage to the environment. To measure turbulent relative dispersion in the streamwise and cross-stream directions of quasi-two-dimensional jets, we propose a Lagrangian-particle-tracking technique which we name virtual particle tracking (VPT). We seed virtual massless passive tracer particles in the velocity field of a flow measured experimentally using particle image velocimetry. These virtual particles evolve as point passive tracers in the time-dependent velocity field and can be tracked in time and space. After presenting the VPT technique, we show the time-evolution of two-point statistics, such as the distance between two virtual particles, measured from the evolution of large virtual particle clusters seeded in the flow of quasi-two-dimensional jets. We also compare the results given by VPT with a variety of other techniques. We find that the dispersion properties differ significantly between the large scale eddies and the high-speed sinuous core observed in the flow of quasi-two-dimensional jets. As a result, we observe large streamwise dispersion and a significant amount of tracers can be transported faster than the speed predicted by a simple top-hat advection model in the jet. [Preview Abstract] |
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