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 GP: Instability: Jets & Wakes III |
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Chair: Dietmar Rempfer, Illinois Institute of Technology Room: 200D |
Monday, November 23, 2009 8:00AM - 8:13AM |
GP.00001: Visualization of Pulsating Low-Speed Flows from a Basic Annular Jet A. Santiago Padron, John Baker Results of a study involving pulsating low-speed free jets issuing from an annular orifice into a quiescent medium are discussed. Transient flow behavior associated with pulsating jets is known to affect entrainment, mixing, and spread rate characteristics. Also, annular jet flows often provide a better description of the flow associated with nozzles used in engineering applications. However, the flow phenomena related to pulsating annular jets is still not fully understood. In this study, flow in the initial region of a pulsating low-speed annular water jet issuing into a quiescent water reservoir was visualized by means of a dye. The blocking ratio was fixed at 0.7. The Reynolds number was varied from 59 to 155 and the Strouhal number from 0.133 to 1.90. For the experimental conditions considered, two different flow regimes were observed. At high pulse frequencies, the flow field resembled that of the steady annular jet. As the frequency decreased, the flow transitions into a structure composed of a train of toroidal vortices, i.e. vortex rings. The frequency at which transition occurred was proportional to the Reynolds number. [Preview Abstract] |
Monday, November 23, 2009 8:13AM - 8:26AM |
GP.00002: The stability of multiple tip vortices Jim Denier, James Whitehead The talk will present some new results on the viscous stability of multiple tip vortices. Using a BiGlobal stability approach we predict the critical Reynolds number for a variety of vortex configurations. Some discussion of the inviscid instability of these multiple vortex configurations will also be given. [Preview Abstract] |
Monday, November 23, 2009 8:26AM - 8:39AM |
GP.00003: Evolution of turbulent jets in low aspect ratio containers S. Pol, C. Nath, D. Gest, S. Voropayev, H.J.S. Fernando, S. Webb The evolution of homogeneous and buoyant turbulent jets released into a low aspect ratio (width/height) container was investigated experimentally using PIV, MSCT probing and digital imaging. The motivation was to understand mixing process occurring in U.S. Strategic Petroleum Reserves (SPR), where crude oil is stored in salt caverns of low aspect ratio. During maintenance or filling, oil is introduced as a jet from the top of the caverns. This study is focussed on mean and turbulent flow characteristics as well as global flow instability and periodic oscillations intrinsic to jets in low aspect ratio containers. Scaling arguments were advanced for salient flow parameters, which included the characteristic length (container width $D)$ and velocity (for homogeneous jets, $J^{1/2}D,$ where $J$ is the momentum flux at the jet exit) scales. For buoyant jets, the buoyancy flux $B$ needs to be introduced as an additional parameter. Such jet flows do not reach a steady state, but bifurcate periodically with a frequency scale $J^{1/2}/ D^{2}$ while enhancing global mixing. [Preview Abstract] |
Monday, November 23, 2009 8:39AM - 8:52AM |
GP.00004: Controlling a liquid jet inside the regular breakup regime by applying a composite disturbance to the actuator Mina Rohani, Derek Dunn-Rankin, Faryar Jabbari In this work, we control the breakup characteristics of a liquid jet by manipulating a piezoelectric actuator and thus the disturbance applied to the jet. We are thereby able to provide desirable droplet size patterns over a wide frequency range. The regular breakup regime refers to the frequency range where the breakup characteristics are repeatable. Thus, although the droplets may not be uniform in size, they pinch off the stream at a constant rate. The regular breakup regime for different jet velocities and diameters has been specified experimentally. The experiments show formation of secondary droplets between main droplets, mostly around the lower frequency range of the regular breakup regime. We remove these secondary droplets by sending a composite disturbance comprising of a fundamental disturbance at the principal driving frequency and another harmonic mode. The choice of the additional harmonics depends on the desired droplets size pattern. It is thus possible to relate initial input disturbance waveform to the droplet formation pattern. [Preview Abstract] |
Monday, November 23, 2009 8:52AM - 9:05AM |
GP.00005: Three-dimensional radiative instabilities in a stratified plane jet Julien Candelier, Christophe Millet, St\'ephane Le Diz\`es We investigate the three-dimensional stability of a stratified plane Bickley jet in the Boussinesq approximation framework. The angle $\theta$ between the shear plane and the direction of stratification and the Froud number $Fr$ are considered as a control parameters. Following the parallel flow approximation, the instability wave solution is sought in the form of a normal mode in two directions. We draw attention to a mechanism whereby Kelvin-Helmholtz mode may have a radiative structure and more generally how internal waves (or gravity waves) associated with unstable radiative modes may be spontaneously generated. [Preview Abstract] |
Monday, November 23, 2009 9:05AM - 9:18AM |
GP.00006: Microphone-array measurements of the surface-pressure field produced by oblique and normal impinging jets A.M. Naguib, W. Jiang, K. Zhang, M. El-Anwar, A.M. Abouel-Fotouh The sptio-temporal, wall-pressure fluctuation generated by an axi-symmetric jet impinging on a flat wall is measured using a 30-microphone array. The focus of the study is the influence of the impingement angle on the strength, spatial distribution and space-time characteristics of the unsteady wall-pressure field. The investigation is conducted at jet Reynolds number of approximately 13000, based on jet diameter and three impingement angles: 0, 15 and 30 degrees. The results show that the impingement angle has strong influence on the level of pressure fluctuations, leading to large increase on the side where the flow experiences less turning (relative to normal impingement), and vice versa. Substantial influences are also found on the spatial characteristics and convection velocity of the pressure-generating flow structures. These effects and others will be presented and discussed in this talk. [Preview Abstract] |
Monday, November 23, 2009 9:18AM - 9:31AM |
GP.00007: Primary Breakup of a High Speed Liquid Jet Wurigen Bo, Xingtao Liu, James Glimm The primary breakup of a high speed jet is studied numerically in 2D and 3D using the front tracking method. We introduce an improved, robust, locally grid based method for reconstruction of tangled interfaces. This method improves the handling of topological change of the surface mesh in the 3D simulations, and is essential for the success of the simulations presented here. From the 2D axisymmetric simulations, we find agreement with experiment in regard to the tip velocity of the jet and its overall degree of breakup or spreading. Due to resolution restrictions, we observe in 3D breakup primarily in the jet tip region and somewhat larger droplets than expected from theory. [Preview Abstract] |
Monday, November 23, 2009 9:31AM - 9:44AM |
GP.00008: Forming a fine jet in inkjet printing E.Q. Li, J.Y.H. Fuh, Y.S. Wong, S.T. Thoroddsen The formation of fine jets during the piezoelectric drop-on-demand inkjet printing has been investigated using ultra-high-speed video imaging. The speed of the jet can exceed 80 m/s, which is much higher than the general drop velocity during inkjet printing. The diameters of the thinnest jets are of the order of a few microns. The generation of such fine jets has been studied over a wide range of viscosities, using 7 different concentrations of water-glycerin solutions. This jetting is associated with the collapse of an air-pocket which is sucked into the nozzle during the printing. This occurs for longer expansion times for the piezo-element. We have characterized the relationship between the speed of the fine-jet and other parameters like the diameter of the jet and the physical properties of the liquid. [Preview Abstract] |
Monday, November 23, 2009 9:44AM - 9:57AM |
GP.00009: Saturation of the Magnetorotational Instability at Large Elssaser Number Keith Julien, Benjamin Jamroz, Edgar Knobloch The MRI is believed to play an important role in accretion disk physics in extracting angular momentum from the disk and allowing accretion to take place. The instability is investigated within the shearing box approximation under conditions of fundamental importance to astrophysical accretion disk theory. The shear is taken to be the dominant source of energy, but the instability itself requires the presence of a weaker vertical magnetic field. Dissipative effects are suffiently weak that the Elsasser number is large. Thus dissipative forces do not play a role in the leading order linear instability mechanism. However, they are sufficiently large to permit a nonlinear feedback mechanism whereby the turbulent stresses generated by the MRI act on and modify the local background shear in the angular velocity profile. To date this response has been omitted in shearing box simulations and is captured by a reduced pde model derived from the global MHD fluid equations using multiscale asymptotic perturbation theory. Results from simulations of the model indicate a linear phase of exponential growth followed by a nonlinear adjustment to algebraic growth and decay in the fluctuating quantities. Remarkably, the velocity and magnetic field correlations associated with these growth and decay laws conspire to achieve saturation of angular momentum transport. [Preview Abstract] |
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