Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session EG: Instability: Jets and Wakes III |
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Chair: Roger Arndt, University of Minnesota Room: Salt Palace Convention Center 250 A |
Sunday, November 18, 2007 4:10PM - 4:23PM |
EG.00001: Dynamic Flow Modeling Using Double POD and ANN-ARX System Identification Stefan Siegel, J\"urgen Seidel, Kelly Cohen, Selin Aradag, Thomas McLaughlin Double Proper Orthogonal Decomposition (DPOD), a modification of conventional POD, is a powerful tool for modeling of transient flow field spatial features, in particular, a 2D cylinder wake at a Reynolds number of 100. To develop a model for control design, the interaction of DPOD mode amplitudes with open-loop control inputs needs to be captured. Traditionally, Galerkin projection onto the Navier Stokes equations has been used for that purpose. Given the stability problems as well as issues in correctly modeling actuation input, we propose a different approach. We demonstrate that the ARX (Auto Regressive eXternal input) system identification method in connection with an Artificial Neural Network (ANN) nonlinear structure leads to a model that captures the dynamic behavior of the unforced and transient forced open loop data used for model development. Moreover, we also show that the model is valid at different Reynolds numbers, for different open loop forcing parameters, as well as for closed loop flow states with excellent accuracy. Thus, we present with this DPOD-ANN-ARX model a paradigm shift for laminar circular cylinder wake modeling that is proven valid for feedback flow controller development. [Preview Abstract] |
Sunday, November 18, 2007 4:23PM - 4:36PM |
EG.00002: Experiments on the global instability of axisymmetric low-density buoyant jets subjected to acoustic forcing Larry Li, Matthew Juniper Results are presented from an experimental study into the effect of acoustic forcing on the near-field development of an axisymmetric, low-density (helium) jet issuing into ambient air. The Reynolds number, Richardson number, and density ratio of the jet were respectively 630, 0.18, and 0.14. The Richardson number is sufficiently large that buoyancy cannot be ignored. Without forcing, hot-wire and pressure measurements revealed self-excited, periodic oscillations at a fundamental frequency of $f_{o} \quad \approx $ 36 Hz, corresponding to a Strouhal number of 0.31. High-speed Schlieren videos showed that this spectral component corresponded to a varicose motion. This varicose motion enhanced local entrainment of ambient fluid into the jet core, leading to the formation of large-scale toroidal vortices. When forcing was applied around the fundamental mode and at low amplitudes, beating was observed until, at a critical amplitude, the oscillations locked into the forcing frequency and completely supressed the jet's natural spectral components. For forcing frequency deviations up to $\Delta f/f_{o} \quad \approx $ 0.24, the critical amplitude increased linearly with frequency deviation although lock-in occured more readily when the forcing frequency was larger than the fundamental frequency. The self-excited, pure-tone nature of the jet oscillations and their relative insensitivity to external perturbations are strong evidence of a global mode. [Preview Abstract] |
Sunday, November 18, 2007 4:36PM - 4:49PM |
EG.00003: Time-Resolved 3-component velocity measurements of an array of heated jets Jack Stepan, Barton Smith Time-Resolved Stereo Particle Image Velocimetry measurements of an array of round, heated jets are presented. A total of 37 jets are generated from a single plenum. The parameter space measured consists of a single Reynolds number (600) and a range of Achimedes numbers, which is a measure of the effects of buoyancy compared to inertia. The PIV cameras look down on the exit plane and measure in a plane normal to the jet exits. The cross-stream extent of the measurement domain contains seven of the jets at the jet exit. Measurements are made at the exit as well as at several downstream locations. With no heating, the flow at the exit of the jets is nearly uniform, and the flow is periodic. However, under even small amounts of heating, convection effects in the plenum lead to three-dimensional motions at the exit. The interaction between the jets is greatly enhanced by heating, and temporal periodicity appears to develop. Any spatial periodicity is destroyed. The distance downstream of the merge points of the jets moves upstream significantly under heating. [Preview Abstract] |
Sunday, November 18, 2007 4:49PM - 5:02PM |
EG.00004: Strategic Forcing of Jets in Crossflow Juliett Davitian, Edson Rodriguez, Robert M'Closkey, Ann Karagozian This presentation will describe recent experiments that explore the response of a gaseous jet in crossflow to alternative methods of acoustic forcing. Building on an extensive prior exploration of transverse jet upstream shear layer instabilities\footnote{Megerian, Davitian, Alves, and Karagozian, {\bf JFM}, to appear.}, the present studies employ alternative temporal waveforms for jet forcing, depending on the jet-to- crossflow velocity ratio $R$, through which transverse jet behavior may be strategically controlled. Smoke visualization is used to observe jet response to forcing. Jets that are injected both flush and elevated with respect to the wind tunnel surface are considered. For values of $ R > 3.5$, prior studies show that the jet's shear layer instabilities appear convective in nature, and hence low to moderate sinusoidal jet forcing can impact jet shear layer response and spread. In contrast, for relatively low values of $R$ (below 3.5 for the flush jet and below 1.2 for the elevated jet), strong natural shear layer instabilities are generated, hence even high amplitude sinusoidal forcing has little effect. At these low $R$ conditions, much higher amplitude square wave forcing is required to produce a jet response, necessitating the introduction of a characteristic time scale associated with vorticity generation. [Preview Abstract] |
Sunday, November 18, 2007 5:02PM - 5:15PM |
EG.00005: Simultaneous global modes in axisymmetric jets Michael Hallberg, Paul Strykowski Axisymmetric jets of varying velocity and/or density ratios (R {\&} S respectively) have been shown to support one of two distinct global modes depending on the combination of these control knobs. Two regimes in the S-R plane have yet to be investigated, the heavy jet with a coflowing secondary stream and a light jet with a counterflowing secondary stream. Theory suggests the latter may support two separate global modes -- a jet column mode and a shear layer mode. It is known that a single global mode dramatically alters flow field dynamic (eg vortex shedding); the possibility of two separate simultaneous global modes is particularly compelling. Results will be presented from a facility designed to probe the counterflowing low-density jet regime. [Preview Abstract] |
Sunday, November 18, 2007 5:15PM - 5:28PM |
EG.00006: A study of wakes trailing a cavitating and ventilated NACA 0015 hydrofoil Rune W. Time, Morten Kjeldsen, Roger E. Arndt In certain types of pumping applications oscillations are induced by operation with liquids containing immiscible gas in solution. To understand the physics of this process, a series of tests with a ventilated 2D NACA 0015 hydrofoil were performed in the water tunnel at the St. Anthony Falls Laboratory of the University of Minnesota. A relation between the cavitation index for both ventilated and naturally cavitating flows has been established. This allows direct comparison between the two types of two-phase flows. A good correspondence is found when comparing drag, lift break-down and lift/drag oscillation amplitudes for a properly defined cavitation index. A careful analysis is required to compare the spectral content of oscillatory flow. This study concentrates on analyzing wake structure for selected ventilation rates and cavitation numbers. The wake structure is mapped by conventional PIV techniques. Results for high cavitation/ventilation number flows are presented. Since conventional PIV represents snap shots of a time evolving phenomenon special care needs to be taken for the analysis. [Preview Abstract] |
Sunday, November 18, 2007 5:28PM - 5:41PM |
EG.00007: Generation of local vortical structures in electromagnetically forced flow under a localized time-dependent magnetic field. Alberto Beltran, Eduardo Ramos, Sergio Cuevas We report experimental observations of an electromagnetically forced flow in a thin layer of an electrolyte (0.4 cm deep, 30 cm wide, 30 cm long), produced by the interaction of an imposed D.C. Current (0.4 A) and a localized time-dependent magnetic field. The field, generated by a permanent magnet (0.95 cm diameter) placed underneath the electrolyte layer, oscillates harmonically along a direction parallel to the injected electric current with a frequency and amplitude of 0.3 - 2 Hz and 0.15 cm, respectively. In the absence of oscillations, the localized Lorentz force produces a vortex dipole with a jet-like flow along the symmetry line in the direction of the force, perpendicular to both the injected current and the normal magnetic field. When the magnet is set in oscillation two different behaviors are observed. If the oscillation frequency, f, is low (0.3 $<$ f $<$ 0.45 Hz) the flow generated in the neighborhood of the magnet forms local vortical structures that are swept away periodically. When the oscillation frequency is high enough (0.45 Hz $<$ f $<$ 2 Hz), the vortex dipole structure remains practically unperturbed. Results point to the existence of a characteristic frequency that intensifies the local generation of vortices and enhances the scalar transport. [Preview Abstract] |
Sunday, November 18, 2007 5:41PM - 5:54PM |
EG.00008: Modeling and Experimentation on a Two-dimensional Synthetic jet Yunfei Wang, Kamran Mohseni Hotwire anemometry is employed in order to investigate the spatial development of a two-dimensional synthetic jet. Flow velocity at various locations downstream from a slit is measured. A self similar behavior in the measured velocity is observed. An analytical model for a steady synthetic jet is developed that accurately matches the experimental data. As observed by other groups, the two-dimensional synthetic jet spreads at a rate higher than a continuous jet. This rate is accurately predicted by our model. It is identified that the main difference between a continuous jet and a synthetic jet is the higher value of the virtual viscosity (eddy viscosity) in a synthetic jet. This is attributed to the pulsate nature of a synthetic jet that makes it more susceptible to turbulence. [Preview Abstract] |
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