Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R25: Instability in Jets and Wakes II |
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Chair: Wilfried Coenen, Universidad Carlos III Room: 328 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R25.00001: Mixing in Long Cylinder by a Stratified Jet: Laboratory Modeling and Theory C. Nath, S.I. Voropayev, H.J.S. Fernando The evolution of buoyant turbulent jets released into a low aspect ratio (width/height) cavity filled with a homogeneous fluid was investigated experimentally. The motivation was to understand mixing process in U.S. Strategic Petroleum Reserves (SPR), where crude oil is stored in salt caverns of aspect ratio approximately 0.1. During maintenance, degassed oil is introduced as a jet from the top of the caverns while denser gas-laden crude oil is pumped out from the bottom. The focus was on mixing, formation and development of density layer as well as the time for replenishing oil in the container to an acceptable level of vapor pressure (gas concentration). Basing on the results of experiments a theoretical model was advanced which permits to calculate the vertical density distributions in cavern as a function of time and other external parameters. Satisfactory agreement between theory and measurements was demonstrated. The results obtained could be extrapolated to SPR flow mixing situations and help to improve the efficiency of expensive oil cavern refilling. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R25.00002: The visualization of the acoustic feedback loop in impinging underexpanded supersonic jet flows using ultra-high frame rate Schlieren Daniel Mitchell, Damon Honnery, Julio Soria An acoustic feedback model for supersonic jet impingement has been proposed in past literature. Due to the inherent difficulty in measuring highly transient phenomena, models of the feedback process have mostly relied on inference, and comparison to similar subsonic flows. Through the use of ultra-high speed cameras operating at one million frames per second, it has been possible to directly visualize the acoustic feedback loop for the first time. Time resolved Schlieren and shadowgraph image sequences capture the interaction of upstream travelling acoustic waves with the shear layer at the nozzle lip and shock structures within the jet core. The acoustic forcing at the nozzle lip produces a sinusoid like perturbation in the shear layer that is highly transient both temporally and spatially. This perturbation grows rapidly into a Kelvin-Helmholtz like vortex ring. These time resolved measurements offer new insights into the fundamental physical mechanisms governing the acoustic feedback loop in supersonic jet impingement. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R25.00003: The Effect of Orifice Eccentricity on Instability of Liquid Jets Ghobad Amini, Ali Dolatabadi The hydrodynamic instability of inviscid jets issuing from elliptic orifices is studied. A linear stability analysis is presented for liquid jets that includes the effect of the surrounding gas and an explicit dispersion equation is derived for waves on an infinite uniform jet column. Elliptic configuration has two extreme cases; round jet when ratio of minor to major axis is unity and plane sheet when this ratio approaches zero. Dispersion equation of elliptic jet is approximated for large and small aspect ratios considering asymptotic of the dispersion equation. In case of aspect ratio equal to one, the dispersion equation is analogous to one of the circular jets derived by Yang [1]. In case of aspect ratio approaches zero, the behavior of waves is qualitatively similar to that of long waves on a two dimensional liquid jets and the varicose and sinuous modes are predicted [2]. The growth rate of initial disturbances for various azimuthal modes has been presented in a wide range of disturbances. \\[4pt] [1]. Yang, H. Q. 1992 Asymmetric instability of a liquid jet. Phys. Fluids \textbf{4}, 681-689. \\[0pt] [2]. Hagerty, W. W., Shea, J. F. 1955 A study of the stability of plane fluid sheets. J. Appl. Mech. \textbf{22}, 509-514. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R25.00004: The vortex breakdown of a variable property jet with swirling flow. Ahmad Adzlan Fadzli Bin Khairi, Hiroshi Gotoda, Toshihisa Ueda The vortex breakdown of a coaxial variable property jet with swirling flow has been experimentally investigated in this work, focusing on how the swirl of the inner and outer jets affect the formation of a stagnation point in the swirling jet. In the case of the CO$_{2}$ jet, the stagnation point flow is more easily formed compared to the air jet, and the stagnation point location was lower than that of the air jet. Stagnation point flow is also formed easier with the introduction of the swirl of the outer jet, and its location is also lower compared to the nonswirling case. The lowering of the stagnation point location of the swirling inner jet with density and viscosity differences due to the swirl of the inner and outer jets will be physically explained in this presentation by considering the theoretical equation obtained by analytically solving a simplified Navier-Stokes equation, (S. Matsubara, H. Gotoda, A. Adzlan, T. Ueda, Experiments in Fluids, 2011 (In press)) which has not been reported in previous research on fluid dynamics. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R25.00005: Hydrodynamic Instabilities in Round Liquid Jets in Gaseous Crossflow Barry Scharfman, Alexandra Techet, John Bush Water jets in the presence of uniform perpendicular air crossflow were investigated theoretically and experimentally using high speed imaging for gaseous Weber numbers (We) below 30, small liquid jet Ohnesorge numbers, and large liquid and gaseous Reynolds numbers. Previously, a bag instability has been reported for We between 4 and 30. Jets first deform into curved sheets due to aerodynamic drag, followed by the formation of partial bubbles (bags) along the jet streamwise direction that expand and ultimately burst. Single bags were present at each streamwise position along the liquid jets in prior experiments featuring liquid jet nozzle diameters less than the capillary length of water. We have found that at larger nozzle diameters it is possible to observe multiple bags at the same streamwise jet position because single bags of such large sizes would be unstable. Theoretical predictions for individual bag expansion diameter over time agree with experimental measurements. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R25.00006: Scalar and Velocity Field Measurements in Variable Density Jets in Crossflow Daniel Getsinger, Levon Gevorkyan, Cory Hendrickson, Owen Smith, Ann Karagozian This experimental study explores both unforced and acoustically forced behavior of variable density transverse jets via simultaneous acetone PLIF and PIV measurements. Jets composed of mixtures of helium and nitrogen are injected normally from a converging nozzle into an air crossflow. The jet-to-crossflow density ratio $S$ is varied among test cases by changing the proportions of nitrogen and helium as well as the fraction of seeded acetone. A recent study\footnote{Getsinger, et al., AIAA Paper 2011-0040, 2011} determined that transverse jets (of Reynolds number $Re_{j}=1800$) likely transition to global instability in response to sufficient lowering of the jet-to-crossflow density ratio $S$ (below 0.45-0.40) and/or momentum flux ratio $J$ (below 10). This transition is characterized by weak shear layer instabilities that are easily overcome by external forcing for the convectively unstable (high $S$ and $J$) case, and strong pure-tone oscillations resistant to external forcing for the globally unstable (low $S$ and $J$) case. The effect of this instability transition on jet dynamics and mixing is examined here, as are alterations in the velocity field that may be associated with the behavior of the instabilities. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R25.00007: Analysis of nonlinear interactions among instability mechanisms in a jet in crossflow Milo\v{s} Ilak, Philipp Schlatter, Shervin Bagheri, Dan Henningson We undertake an analysis of datasets from direct numerical simulation of a jet in crossflow, using the method of Dynamic Mode Decomposition (DMD). The procedure reveals coherent structures in the flow known as Koopman modes, which oscillate at frequencies that are also computed by the method. Both the crossflow and the jet inflow profile are laminar. As the jet-to-crossflow velocity $R$ is increased, we observe the breakdown of hairpin vortices characteristic for low values of $R$. Near-wall structures corresponding to low-frequency oscillations are revealed above $R=1.5$ by the DMD analysis, and their interaction with modes on the jet trajectory results in complex flow patterns, which however retain spanwise symmetry. At $R=2.5$ and higher, spanwise symmetry is broken, and the flow exhibits the complex dynamics typically observed in literature. Furthermore, we study the effects of crossflow unsteadiness on jets at low $R$, showing that the hairpin vortices are able to persist under noise of moderate amplitude introduced upstream of the jet orifice. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R25.00008: Linear forcing response of subsonic jets Xavier Garnaud, Lutz Lesshafft, Patrick Huerre, Peter Schmid The linear stability of spatially developing subsonic jets is investigated. A parametric base flow model is employed that matches experimental data for turbulent mean flows and that includes a solid nozzle. Temporal eigenmodes are computed using a newly developed ``shift-relax'' method. All eigenmodes are found to be stable in an isothermal setting. While this observation is in agreement with classical local stability results, a stable eigenmode spectrum seems inappropriate for the description of the convective instability dynamics of jets, which are known to be highly receptive to external perturbations. Instead, we propose to characterize jet instability in terms of the linear global flow response to sustained low-level forcing. External perturbations inside the nozzle duct are identified that give rise to the most amplified flow response at a prescribed frequency. Results will be discussed both for incompressible and compressible settings. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R25.00009: Instability of low viscosity elliptic jets with varying aspect ratio Varun Kulkarni In this work an analytical description of capillary instability of liquid elliptic jets with varying aspect ratio is presented. Linear stability analysis in the long wave approximation with negligible gravitational effects is employed. Elliptic cylindrical coordinate system is used and perturbation velocity potential substituted in the Laplace equation to yield Mathieu and Modified Mathieu differential equations. The dispersion relation for elliptical orifices of any aspect ratio is derived and validated for axisymmetric disturbances with $m = $0, in the limit of aspect ratio, $\mu = 1$, i.e. the case of a circular jet. As Mathieu functions and Modified Mathieu function solutions converge to Bessel's functions in this limit the Rayleigh-Plateau instability criterion is met. Also, stability of solutions corresponding to asymmetric disturbances for the kink mode, $m = 1$ and flute modes corresponding to $m \geq 2$ is discussed. Experimental data from earlier works is used to compare observations made for elliptical orifices with $\mu \ne 1$. This novel approach aims at generalizing the results pertaining to cylindrical jets with circular cross section leading to better understanding of breakup in liquid jets of various geometries. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R25.00010: Local absolute instability in the near field of hot and light round jets Wilfried Coenen, Alejandro Sevilla We present a numerical investigation of the viscous spatiotemporal stability properties of low-density round jets emerging from circular nozzles or tubes. The two particular cases typically studied in experiments, namely a hot gas jet discharging into a quiescent cold ambient of the same species, and an isothermal jet consisting of a mixture between two gases with different molecular weight, discharging into a stagnant ambient of the heavier species, are treated separately. We use a realistic representation of the base velocity and density profiles based on boundary layer theory, taking into account the effect of variable transport properties. Our results show significant quantitative differences with respect to previous studies that use parametric presumed-shape base profiles, and reveal that hot jets are generically more unstable than light jets. In addition, the downstream evolution of the local stability properties of the jet is analyzed, revealing that, whereas a localized pocket of absolute instability can take place in the jet, for the combination of jet-to-ambient density ratio, Reynolds number, and initial momentum thickness used in experiments available in the literature, the absolutely unstable region in the jet is bounded by the jet outlet. The global transition observed in these experiments is demonstrated to take place when the absolutely unstable domain becomes sufficiently large. [Preview Abstract] |
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