Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G34: Drops VIII: Fragmentation |
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Chair: Nikolai Priezjev, Michigan State University Room: 405 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G34.00001: Explosive fragmentation Alexandre Vledouts, Jos\'e Gra\~na-Otero, Joel Quinard, Nicolas Vandenberghe, Emmanuel Villermaux We report on an experiment consisting in forcing the fast radial expansion of a spherical liquid shell. The shell is formed by the capillary pinch off of a water thin annular jet surrounding a jet of reactive gaseous mixture at ambient pressure. The encapsulated gas in the resulting water bubble is a mixture of Hydrogen and Oxygen in controlled relative proportions, which is ignited by a laser plasma aimed at the center of the bubble. The strongly exothermic combustion of the mixture induces the expansion of the hot burnt gas, pushing the shell radially outwards in a violently accelerated motion. That motion triggers the instability of the shell, developing thickness modulations ultimately piercing it in a number of holes. The capillary retraction of the holes concentrates the liquid constitutive of the shell into a web of ligaments, whose breakup leads to stable drops. We document the overall process, from the kinematics of the shell initial expansion, to the final drops size distribution as a function of the composition of the gas mixture and bubble shell thickness. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G34.00002: Drop Size Distributions of Aerated Liquid Jets injected in Subsonic Crossflow Adegboyega Adebayo, Khaled Sallam, Kuo-Cheng Lin, Campbell Carter An Experimental investigation of the breakup of aerated liquid jets in subsonic crossflow is described. Test conditions include crossflow Mach numbers of 0.3 and 0.6, Gas-to-liquid ratio of 0{\%}, 4{\%}, and 8{\%}. Double pulsed digital holography was used to investigate the spray characteristics at downstream distances of 25, 50, and 100 jet diameters. The holograms are analyzed using image-processing algorithms to yield information about the drop sizes, drop velocities, and mass fluxes. Different drop size distributions are tested and compared including Rosin-Rammler distribution, log-normal distribution, and Simmons' universal root-normal distribution. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G34.00003: An ultrasonic scrubber: enhanced removal of particles by water sprays via ultrasonic excitation J.R. Saylor, Weiyu Ran, R. Glynn Holt Sprays are commonly used to remove pollutant particles in smokestacks, to reduce coal dust levels in mines, and in dust abatement applications. For typical conditions, sprays work poorly on particles having a diameter on the order of a micron, which is also the particle size most deleterious to the human lung. The acoustic radiation force can be used to move particles and drops, and we hypothesized that by forcing a particle laden flow and a spray into an ultrasonic standing wave field, particles and drops would be concentrated, thereby increasing the effectiveness of particle removal by sprays. Experimental data is presented in the form of scavenging coefficients for micron scale particles that supports this hypothesis. Also discussed is whether increased scavenging by ultrasonics is due to particle/drop interactions particle/particle interactions, or both. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G34.00004: Atomization in Sparkling Fireworks Chihiro Inoue, Mitsuo Koshi, Hiroshi Terashima, Takehiro Himeno, Toshinori Watanabe The physics behind the beauty of sparkling fireworks has not been clarified yet due to a lack of coherent visualization results. In the present study, atomization process in sparkling fireworks is elucidated by using a high-speed video camera. In the first-half sequence of the fireworks, the fireball repeatedly expands, bursts, and shrinks due to the high pressure gas inside the fireball. In contrast, in the last-half sequence, the bubbly fireball slightly deforms, and small bubbles burst on the fireball. A scenario of droplets generation is as follows: a liquid thread extends from the bottom of the bursting fireball, and fragments into droplets. Thus the droplets originate from inside the fireball rather than from its surface. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G34.00005: Free-fall of Water Drops Generated in the Laboratory for Rainfall Simulations Firat Testik, Nasim Chowdhury, Mathew Hornack, Abdul Khan The shape and fall velocity evolution of water drops that are falling freely were studied using high-speed imaging. Digital image processing techniques were utilized to measure drop characteristics. Water drops of three target diameters were generated using needles placed at 12 different stations (approximately 1 m apart vertically for a fall distance of 12 m) of a stairwell and sequential high-speed drop images were obtained at the bottom floor. Our experimental observations indicated that generated drops underwent three distinct regions during free-fall. The first region, where different oscillation modes prevail, is influenced by the source conditions. The drop oscillations are absent in the second region and the shapes of the drops transition towards equilibrium shapes. Through the third region, the drops experience free-fall at equilibrium shapes and terminal velocities and possess characteristics similar to those of raindrops in an actual rain event. The ranges of the different fall regions were delineated based upon our experimental observations. The results of this study are useful in (i) determining the fall distance required for laboratory simulation of rainfall and (ii) studying the shape - fall velocity coupling of raindrops in the laboratory. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G34.00006: Interfacial Instabilities on a Droplet Maziyar Jalaal, Kian Mehravaran The fragmentation of droplets is an essential stage of several natural and industrial applications such as fuel atomization and rain phenomena. In spite of its relatively long history, the mechanism of fragmentation is not clear yet. This is mainly due to small length and time scales as well as the non-linearity of the process. In the present study, two and three-dimensional numerical simulations have been performed to understand the early stages of the fragmentation of an initially spherical droplet. Simulations are performed for high Reynolds and a range of relatively high Weber numbers (shear breakup). To resolve the small-scale instabilities generated over the droplet, a second-order adaptive finite volume/volume of fluids (FV/VOF) method is employed, where the grid resolution is increased with the curvature of the gas-liquid interface as well as the vorticity magnitude. The study is focused on the onset and growth of interfacial instabilities. The role of Kelvin-Helmholtz instability (in surface wave formation) and Rayleigh-Taylor instability (in azimuthal transverse modulation) are shown and the obtained results are compared with the linear instability theories for zero and non-zero vorticity layers. Moreover, the analogy between the fragmentation of a single drop and a co-axial liquid jet is discussed. The current results can be used for the further development of the current secondary atomization models. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G34.00007: Local stability of a fluid interface near a zero-vorticity point Yu-Hau Tseng, Andrea Prosperetti There are many examples of fluid interfaces which give rise to small-scale structures in uniform or nearly uniform flows: ``skirted'' bubbles, filaments trailing rising drops and bubbles or forming at the tip of coaxial jets and others. It is argued that these phenomena are due to a peculiar instability in the neighborhood of a zero-vorticity point (or line). A local stability analysis supports this conjecture and is further illustrated by some numerical results. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G34.00008: Ligament breakup without surface tension Lionel Vincent, Laurent Duchemin, St\'ephane Le Diz\`es, Emmanuel Villermaux We study the breakup of an axisymmetric low viscosity liquid volume (ethanol and water), held by surface tension on supporting rods, when subject to a violent axial stretching. One of the rods is promptly set into a fast motion, either with constant acceleration, or constant velocity. In both cases, a thin ligament is withdrawn from the initial liquid volume, which eventually breaks-up at time $t_b$, leaving a liquid mass $m$ attached to the moving rod. We find that the breakup time and entrained mass are related by $t_b\sim\sqrt{m/\sigma}$, where $\sigma$ is the liquid surface tension. For a constant acceleration $\gamma$, and although the overall process is driven by surface tension, $t_b$ is surprisingly found to be independent of $\sigma$, while $m$ is inversely proportional to $\gamma$. The case with constant velocity will be considered too. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G34.00009: Influence of Geometry on Instability: Breakup of fluid strips with square-wave perturbations Kyle Mahady, Shahriar Afkhami, Lou Kondic Recent work\footnote{Roberts, N., Fowlkes, J., Mahady, K., Afkhami, S., Kondic, L. and Rack, P. \textit{ACS Applied Materials and Interfaces} \textbf{2013}, 5, 4450.} has demonstrated experimentally and computationally that an originally flat structure with an imposed nonlinear square-wave perturbation applied to the edges could break up in a variety of ways. In this talk we will report on the results of a computational study that centers on solving the Navier-Stokes equations using a volume of fluid approach. We focus on exploring the details of the breakup mechanism and on the influence of the initial fluid shape on the instability development. One unexpected result is the finding that the initial geometry may strongly influence the outcome, and in particular lead to closely spaced array of drops. The size and spacing of the drops is found to be strongly influenced by nonlinear stages of the evolution, and cannot be predicted based on the Rayleigh-Plateau instability mechanism. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G34.00010: The viscous Savart sheet Emmanuel Villermaux, Violaine Pistre, Henri Lhuissier We study the viscous version of the planar Savart sheet problem, using an impacting liquid jet up to 300 times more viscous than water. Two surprising observations are made, contrasting with the traditional case introduced by Savart where viscosity plays no role: First, if the radius of a viscous sheet is typically reduced compared to the water case for a given jet radius and impacting velocity, the smooth/flapping transition is delayed, allowing for smooth sheet radii substantially bigger than those permitted with water at large impacting Weber number. Second, the drop size distribution is bimodal, with a substantial fraction of the drops having a very small, well defined diameter. We understand these two facts in terms of an additional model experiment, and simple physical arguments. [Preview Abstract] |
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