Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session BR: Instability: Jets & Wakes I |
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Chair: Paul Strykowski, University of Minnesota Room: 200F |
Sunday, November 22, 2009 10:30AM - 10:43AM |
BR.00001: Stability and adjoint-based control of a confined impinging jet Jean-Marc Chomaz, Philippe Meliga We investigate numerically the dynamics of a laminar plane jet impinging on a flat plate in a channel. A global stability analysis is carried out and shows that, for a strong confinement, the two-dimensional steady flow is unstable to three-dimensional steady perturbations. We then use adjoint methods and sensitivity analyses to assess the efficiency of a 3-D harmonic or 2-D steady control to stabilize the leading 3-D global mode, by means of either bulk or wall forcing. This allows to identify flow regions of particular interest, and suggests different locations of the actuator depending on the control method. These concepts will be applied to two open-loop control strategies in which we introduce into the flow a small control device chosen as a cylinder or a flat-plate airfoil, modeled by the drag or lift force it exerts on the flow. A physical interpretation for the stabilizing effect of such control methods will be proposed, based on the competition between production and advection of disturbances. [Preview Abstract] |
Sunday, November 22, 2009 10:43AM - 10:56AM |
BR.00002: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 10:56AM - 11:09AM |
BR.00003: Shear Layer Instabilities in Low Density Transverse Jets Daniel Getsinger, Kevin Canzonieri, Cory Hendrickson, Owen Smith, Ann Karagozian Shear layer instabilities associated with the gaseous, isodensity jet in crossflow have been explored in detail in recent experiments\footnote{Megerian, et al., {\bf JFM}, 593, pp. 93-129, 2007}, indicating that the jet shear layer is globally unstable when the jet-to-crossflow velocity ratio, $R$, is less than 3.2 for a flush injected jet. Low density jets in quiescent surroundings are also known to become globally unstable for jet-to-ambient density ratios below approximately 0.6-0.7. It is thus of interest to explore the nature of changes in the character of shear layer instabilities for the low density jet in crossflow, with special focus on the influence of jet-to-crossflow momentum flux ratios at which instabilities are altered. A specially designed mixing device is utilized for exploration of helium and nitrogen jet mixtures. Calibration of the mixing device is accomplished using an acoustic waveguide capable of exploring alterations of standing wave frequencies with different gas mixtures. A range of flow conditions are explored, and alterations in the jet's spectral character suggesting transition to absolute instability are quantified. [Preview Abstract] |
Sunday, November 22, 2009 11:09AM - 11:22AM |
BR.00004: Closed- and Open-Loop Control of Isodensity Jets in Crossflow Cory Hendrickson, Daniel Getsinger, Juliett Davitian, Robert M'Closkey, Ann Karagozian Recent experiments have explored the behavior of a gaseous, isodensity jet in crossflow, focusing in part on the jet's response to controlled, strategic acoustic forcing\footnote{J. Davitian, PhD dissertation, UCLA, 2008}. While it is possible for strong sinusoidal jet excitation to overtake the natural shear layer instability when the flow is globally unstable (for jet-to-crossflow velocity ratios $R < 3.2$), square wave excitation of the jet fluid is observed to have a more profound effect on jet penetration and spread. Although creating precise square wave excitation for a globally unstable jet is challenging, open-loop control is observed to have some success. Yet at very small values of $R$, open-loop control is less capable of overcoming the instabilities and hence closed-loop control, whereby the forcing conditions continuously adjust to track a desired output square wave, must be used. The closed-loop controller, employed especially for $R < 1.25$, is observed to eliminate much of the distortion seen in the open-loop generated square wave, more closely matching the ideal square wave given bandwidth limitations of the actuation system, and providing robust control of the flow. [Preview Abstract] |
Sunday, November 22, 2009 11:22AM - 11:35AM |
BR.00005: Linear Stability Analysis of Round and Serrated Jets Kristjan Gudmundsson, Tim Colonius We investigate the velocity and pressure fluctuations of turbulent jets produced by round, and serrated nozzles. We model these fluctuations via the normal modes of the linearized equations and derive a generalized Rayleigh-equation for mean-flows composed of an arbitrary number of azimuthal harmonics, allowing fast solution of both the temporal and spatial stability problems. Using ensamble-averaged turbulent mean-flows from stereoscopic PIV data, we solve the generalized Rayleigh-equation and compare our predictions to instantaneous PIV measurements as well as near-field microphone measurements. Using the proper orthogonal decomposition to filter out uncorrelated fluctuations in data, we find good agreement between data and theory. [Preview Abstract] |
Sunday, November 22, 2009 11:35AM - 11:48AM |
BR.00006: Experimental study of two opposing round jets Hind Alkandry, Koen Stegeman, Hiroshi Higuchi The interaction between two opposing jets submerged in water is studied using particle image velocimetry. The research under present canonical configuration compliments an ongoing personal ventilation project conducted at Syracuse University. The study is carried out using two 9mm opposed round jets at two different angles separated by 25 diameters, one at zero degrees (directly opposing) and another at ten degrees. The Reynolds number based on the jet diameter is 9200, and a fully developed pipe flow exists at the exit. The mean velocity profile near the impact region shows a clear saddle point for both angles and radial jet emanating from that region. However, the instantaneous velocity field of the radial jet is highly unsteady. Three-dimensional characteristics, in particular at ten degrees, of the flow are examined closely. [Preview Abstract] |
Sunday, November 22, 2009 11:48AM - 12:01PM |
BR.00007: On the stability of a recirculation bubble and its application in compact combustion Matt Anderson, Paul Strykowski A 2D channel flow expands asymmetrically via a sudden expansion splitter plate, the Reynolds number based on the channel height and mean velocity is 1.47 x 10$^{4}$. A recirculation bubble generated by a momenturm-driven counter flowing secondary stream located downstream of the sudden expansion is experimentally investigated by means of hot-wire anemometry and PIV. It is conjectured that the fluid field created is one of a separated region of locally absolutely unstable flow. This separated region is the result of both the partial stagnation of the main flow due to the spreading of the second counter-current flow as well as the entrainment of the secondary jet. The secondary stream augments the initial shear layer that has been created after the expansion and a separation bubble appears. This secondary jet creates a control mechanism for the fluid field. The low-velocity zone downstream of the expansion that has been created is necessary for flame anchoring, and the large turbulence levels recorded (total turbulence levels exceeding 100{\%} of the inlet velocity) dramatically increase mixing and may lead to more efficient compact combustion in backward-facing step-combustors. [Preview Abstract] |
Sunday, November 22, 2009 12:01PM - 12:14PM |
BR.00008: Spectral analysis of nonlinear flows Clarence Rowley, Igor Mezic, Shervin Bagheri, Philipp Schlatter, Dan Henningson We present a technique for describing the global behavior of complex, nonlinear flows, by decomposing the flow into modes determined from spectral analysis of the Koopman operator, an infinite-dimensional linear operator associated with the full nonlinear system. These modes, referred to as Koopman modes, are associated with a particular observable, and may be determined directly from data (either numerical or experimental) using a standard Arnoldi algorithm. They have an associated temporal frequency and growth rate and may be viewed as a nonlinear generalization of global eigenmodes of a linearized system. They provide an alternative to Proper Orthogonal Decomposition, and in the case of periodic data the Koopman modes reduce to a discrete temporal Fourier transform. We illustrate the method on an example of a jet in crossflow, and show that the method captures the dominant frequencies and elucidates the associated spatial structures. [Preview Abstract] |
Sunday, November 22, 2009 12:14PM - 12:27PM |
BR.00009: Turbulence and Internal Waves in a Stratified Jet Hieu Pham, Sutanu Sarkar Interaction between a stably stratified jet and internal gravity waves from an adjacent shear layer with mild stratification is investigated using 3D Direct Numerical Simulation. The streamwise velocity mimicks the Equatorial Undercurrents where a mixed shear layer situates on top of a strongly stratified jet. Despite the strong stratification, enhanced dissipation is observed in the jet. The evolution of the shear layer includes shear instability, Kelvin-Helmholtz rollers and subsequent breakdown to turbulence. Internal waves with wavelength larger than that of the rollers are found in and below the jet. The characteristics of the wave field follows linear theory. Analysis of the fluctuating energy budget indicates a balance mostly between the production and the transport for the wave field in the jet. The rate of change in the fluctuating kinetic energy in the wave field below the jet is balanced by the transport and the buoyancy flux. Hot fluid from the shear layer is entrained into the upper-flank of the jet, initiates turbulence and disrupts the internal wave field. The dissipation in the coherent patches of turbulence inside the stably stratified jet is strong, comparable to one inside the shear layer and up to three orders of magnitude stronger than that in the propagating wave field. [Preview Abstract] |
Sunday, November 22, 2009 12:27PM - 12:40PM |
BR.00010: Local concentration and velocity distribution for buoyancy driven mixing flows in long tubes at different tilt angles J. Znaien, F. Moisy, D. Salin, J.-P. Hulin, E.J. Hinch The buoyancy driven mixing of two fluids of different densities (Atwood numbers $10^{-3} \le At \le 10^{-2}$) interpenetrating each other has been studied at the local scale in a $20 \mathrm{mm}$ diameter tube tilted at an angle $15^{\circ} \le \theta \le 60^{\circ}$ from vertical. The velocity and concentration maps are measured by means of PIV and LIF techniques in a vertical diametral plane. At large angles ($\theta = 45-60^{\circ}$) and low density contrast ($At = 10^{-3}$) the flow is laminar with three layers of different densities stabilized by transverse gravity. At high $At$ ($4 \times 10^{-3} - 10^{-2}$) and low $\theta$ ($15^{\circ} - 30^{\circ}$), there is a turbulent shear mixing region with linear velocity and concentration transverse profiles in the middle of the tube section and with two channels of less mixed fluids at the top and bottom. The size of the channels increases at lower $At$ and higher $\theta$. The local viscous and turbulent momentum fluxes have been determined and their variation in the section and their dependence on $\theta$ will be discussed. At intermediate $At$ and $\theta$ values, isolated or periodic turbulent bursts between which the flow returns to laminar are observed and both the velocity and concentration spatial correlations have been determined. [Preview Abstract] |
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