Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session GN: Jet and Wake Instabilities II |
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Chair: Lou Cattafesta, University of Florida Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 9 |
Monday, November 20, 2006 10:30AM - 10:43AM |
GN.00001: The ``real'' fully low-dimensional characteristics of a subsonic jet flow Charles Tinney, Mark Glauser, Lawrence Ukeiley, Peter Jordan An experimental investigation concerning the large scale turbulent features of the flow exiting from an axisymmetric converging nozzle at Mach 0.85 (cold) is discussed using PIV techniques. The PIV system allows for all 3 components of the velocity field to be captured along the (r-theta) plane of the jet between x/D=3 and 8. The full kernel matrix is constructed comprising all nine normal and shear stress terms from which the symmetry assumptions of the azimuthal correlations are addressed. A Fourier-azimuthal and Proper Orthogonal Decomposition is performed using scalar and vector forms of the technique whereby the scalar decomposition of the axial velocity is shown to agree with previous investigations with a peak in azimuthal mode 5 at x/D=3, and a shift to mode 2 by x/D=8. The scalar solutions to the radial and azimuthal velocity components are shown to possess most of their energy in the first few azimuthal modes (0, 1 and 2), with very little change along the streamwise direction. The solution to the 3 component vector decomposition shows a peak in azimuthal mode 5 at x/D=3 with a gradual shift to mode 2 at x/D=8, similar to the scalar solution to the axial velocity. [Preview Abstract] |
Monday, November 20, 2006 10:43AM - 10:56AM |
GN.00002: Exploration and Control of Transverse Jet Shear Layer Instabilities Juliett Davitian, Robert M'Closkey, Ann Karagozian This experimental study explores transverse jet shear layer instabilities and the mechanisms by which these instabilities can be used to strategically control jet behavior via forcing. Jets injected from either flush or elevated nozzles demonstrate a strengthening of the instabilities with increasing crossflow magnitude, with initial modes occurring closer to the jet exit as the jet-to-crossflow velocity ratio $R$ is reduced, and with evidence of unusual mode shifting as one moves along the upstream shear layer. These instabilities appear convective in nature when $R > 3.5$, and the shear layer in this regime is affected by even low level sinusoidal jet forcing. The flush-injected transverse jet undergoes a significant transition in the shear layer instability as $R$ is reduced below 3.5, with the development of strong, distinct fundamental and harmonic modes that are not influenced by low level sinusoidal forcing. In contrast, the elevated jet's instabilities are weakened in the range $4 > R > 1.25$, largely as a result of coflow exterior to the jet nozzle, but once the transverse jet has deflected significantly, for $R < 1.25$, the instabilities for the elevated jet resemble those for the flush-injected transverse jet. At these low $R$ conditions, much higher amplitude forcing is required to produce a jet response, necessitating the introduction of a characteristic time scale associated with vorticity generation via square wave excitation. [Preview Abstract] |
Monday, November 20, 2006 10:56AM - 11:09AM |
GN.00003: MHD-LBM Simulations of Magnetic Field Effects on Axis-Switching and Instabilities in Rectangular Jets Benjamin Riley, Sharath Girimaji, Jacques Richard Our objective is to assess the potential for flow control of plasma jets for space propulsion and high-altitude external flow applications. Toward this end, we study the effect of an externally applied magnetic field on rectangular plasma jets using a Magneto- hydrodynamic (MHD) Lattice Boltzmann flow solver. The Lattice Boltzmann Method (LBM) has been shown to be able to accurately capture axis-switching and instability onset in rectangular jets. We investigate the effects of varying magnetic field strengths and magnetic Reynolds numbers on entrainment, centerline velocity, instability onset, and the axis-switching phenomenon. Preliminary results indicate that the magnetic field has the following effects: (i) delays the instability onset; and (ii) inhibits the axis-switching phenomena. This study also seeks to develop more insight into the magnetic-velocity field interactions and investigate possible strategies for MHD flow control. [Preview Abstract] |
Monday, November 20, 2006 11:09AM - 11:22AM |
GN.00004: Effect of liquid property and flow conditions on entrainment and mean droplet diameter in case of two-phase air/ viscous liquid free jets Said Al Rabadi, Lutz Friedel Measurements by using two-dimensional Phase Doppler Anemometer in free jet flows with several nozzle conditions show that the entrainment process is enhanced when increasing only the liquid viscosity, if any effect due to the in parallel raise of the static surface tension is disregarded. The so called two-phase entrainment coefficient starting from zero develops in the region of flow establishment and approaches a self constituting value in the similarity region of the jet. With higher liquid viscosity, the Ohnesorge number substantially increases due to the reducing of the size of primary and satellite droplets. The physical reason is that the higher viscous forces act on the interface in the sense to damp the rate of droplet distortions and to stabilize the surface tension perturbations. Hence, the increment of the jet breakup length and eventually the droplets formation becomes more pronounced. The mean droplet diameter decreases as well with rising air to liquid phase mass flow ratio for equal total mass flow rates, which is attributed to the higher gas stream impact on the liquid phase. Essentially, the surface tension force interferes with the droplet inertia during the massive interactions between the liquid surface with the gas stream and among the droplets themselves. In horizontal flow, a pertinent gravity effect on the droplets is observed owing to the higher liquid density than that of water. [Preview Abstract] |
Monday, November 20, 2006 11:22AM - 11:35AM |
GN.00005: Coupled Structural Behavior-Flow Characteristics of a Synthetic Jet M. Strassburg, K. Mohseni Synthetic jet actuators are zero net mass flux jets, which consist of a cavity with an oscillating wall opposite an orifice through which fluid is ingested and expelled in a periodic fashion. In this study, a piezoelectric bender and an aluminum cavity are employed in order to characterize thrust generation in synthetic jets. Simultaneous measurements of the jet velocity and diaphragm movements are carried out. These measurements are taken using hot wire anemometry and laser nano-sensing, respectively. Many actuators with various cavity and orifice dimensions are fabricated and tested. Data presented contains oscillating frequencies ranging from 500 to 2000 Hz, and a range of different voltages, which allows for different expelled fluid volumes. For different orifice diameters, the exit fluid volume is correlated to center stream velocity. Thrust generation at various formation numbers is investigated. It is found that the rate of thrust enhancement is decreased after a formation number around 3. [Preview Abstract] |
Monday, November 20, 2006 11:35AM - 11:48AM |
GN.00006: Shock wave excited liquid micro-jets Claus-Dieter Ohl, Daan Martens, Aaldert Zijlstra, Michel Versluis, Detlef Lohse, Nico de Jong The meniscus of the gas-water interface in a thin hydrophilic capillary of 1mm and less has some similarities to a ``shaped charge'' used to penetrated armored vehicles. In this presentation we show high-speed recordings of the interface dynamics after the reflection from a shock wave: the interface flattens and shapes into a microscopic needle-like jet, which accelerates to velocities of 100m/s and more. Further the dependencies of the pressure amplitude, capillary diameter, and interface curvature on the jet velocity are presented. [Preview Abstract] |
Monday, November 20, 2006 11:48AM - 12:01PM |
GN.00007: Stability of confined conflowing jet at low Reynolds number. Theory and Experiments in microfluidic devices. Annie Colin, Pierre Guillot, Armand Ajdari We adress the question of the stability of a confined coflowing jet at low Reynolds number in various geometry. Our study is motivated by recent experiments in microfluidic devices. When immiscible fluids flow in microchannels, monodisperse droplets or parallel flows are obtained depending upon the flow rate values of the aqueous phase and the oil phase. In these experiments, the confining and the shape of the geometry play a fundamental role. Analysing the stability of the jet in the framework of the lubrication at low Reynolds number, we link the transition between the droplets regime and the jet regime to the absolute/convective transition of the Rayleigh instability. The effect of the shape of the geometry, of the position of the jet in the microfluidic devices are discussed. Analytical solutions are presented in circular geometry and numerical computations are performed for others geometry. A very good agreement between our model and the experiments is found without any adjustable parameters, for fluids with various viscosity and various surface tension. [Preview Abstract] |
Monday, November 20, 2006 12:01PM - 12:14PM |
GN.00008: New findings on the minimum flow rate in Flow Focusing Benjamin Bluth, Alfonso M. Ganan-Calvo Recently published works on spatial-temporal instability analyses have dealt with flow-focused micro-jets. Those works have revealed that the minimum rate of fluid that can be focused is, in many cases, linked to an absolute/convective instability transition of the focused fluid micro-jet. Further to this, a series of very recent experiments indicate that the minimum flow rate may result very plausibly from the breakdown of a stability ``chain'', a link of it --and possibly not the weakest one- being the micro jet. The dripping-jetting transition curves in the Reynolds-Weber parametrical space show a conspicuous, consistent and interesting turning point. Whether this behaviour reflects a possible cross-over between the instability of the jet and that of the cusp-like meniscus is here discussed. Indeed, these new results are now set in perspective along with those recently published ones. [Preview Abstract] |
Monday, November 20, 2006 12:14PM - 12:27PM |
GN.00009: Electro-flow focusing. The high conductivity, low viscosity limit Alfonso M. Ganan-Calvo, Jos\'e M. Lopez-Herrera Electro-flow focusing, a technique combining the features of electrospray (ES) and flow focusing (FF), provides a reliable tool to reach parametrical micro-jetting ranges not attainable by ES or FF alone under specific operational regimes (liquid properties and flow rate). In this work, we provide not only a closed theoretical model predicting the diameter of a high electrical conductivity electro-flow focused liquid micro-jet, but also its convective/absolute instability, linked to the jetting/dripping transition and the minimum liquid flow rate that can be issued in steady jetting regime. Our predictions are compared to experiments with good accord. [Preview Abstract] |
Monday, November 20, 2006 12:27PM - 12:40PM |
GN.00010: Non-parallel spatial stability of Batchelor vortex Carlos del Pino, Luis Parras, Ramon Fernandez-Feria We analyze the spatial stability of the so-called Batchelor vortex taking into account the non-parallel effects associated to the axial variation of this self-similar vortex. To that end we integrate the Parabolized Stability Equations (PSE) along the axis of the vortex starting from the local ``parallel'' stability results (eigenvalues and eigenfunctions) at a given axial location. We first compare these initial eigenvalues with those obtained at the same conditions from the parallel version of the Batchelor vortex, sometimes also called q-vortex, which is the standard version of the Batchelor vortex used in previous stability analysis, and find that they differ substantially. The differences are shown to be due to a term in the self-similar solution which is neglected in the q-vortex version. This term becomes increasingly important as the swirl parameter $q$ grows. Then we fully characterize the non-parallel stability properties of Batchelor vortex along the axis for several cases of interest in trailing vortices, particularly in the far wake behind large commercial aircrafts. [Preview Abstract] |
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