Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HC: General Fluid Dynamics II |
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Chair: Oleg Schilling, Lawrence Livermore National Laboratory Room: Long Beach Convention Center 102A |
Monday, November 22, 2010 10:30AM - 10:43AM |
HC.00001: A regularization by stratification of the Stokes flow divergences by translating spheres Roberto Camassa, Claudia Falcon, Joyce Lin, Richard McLaughlin, Anna Miller, Kathryn Valchar Stokes flow solutions of fluid motions in the presence of a moving sphere notoriously suffer unphysical divergences in quantities such as the dragged volume of fluid which have been traditionally regularized by far-field Oseen inertial corrections. This talk will consider an alternative regularization mechanism related to the presence of stable stratification under gravity in the fluid. A first principle theory will be outlined and results with falling spheres in sharp stratifications will illustrate the mechanism. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HC.00002: Super free fall for a container composed of diverging flat plates A. Medina, A. Torres, S. Peralta, P.D. Weidman We have analyzed experimentally and theoretically the characteristics of the upper free surface of a liquid column released from rest in a vertical container whose cross-section opens slowly in the downward direction. In distinction with the work of Villermaux and Pomeau (2010) for a conical container, we consider a container composed of slightly inclined flat surfaces. At small times for which viscous effects can be neglected, the free surface moves downward with an acceleration larger than gravity. The existence of a nipple centered on the upper free surface with amplitude an increasing function of time is observed. A one-dimensional model of the initial acceleration for flat, slightly expanding walls reproduces the observed super free fall experiments fairly well. Details of the nipple development will be presented. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HC.00003: Ratcheting Fluid using Geometric Anisotropy Benjamin Thiria, Jun Zhang We discuss a new type of pump that can effectively transport fluids using vibrational motion imposed onto shapes with anisotropy. In our experiment, two asymmetric, sawtooth-like structures are placed facing each other and form a fluid channel. This channel is then forced to open and close periodically. Under symmetric, reciprocal motion, fluid fills in the gap during the expansion phase of the channel and is then forced out during contraction. Since the fluid experiences different impedence when it takes different directions in the gap, the stagnation point that separates flows of two directions changes within one driving period. As a result, fluid is transported or pumped from one end of the gap to the other. This ratcheting effect of fluid is demonstrated through our measurements and its working principle is discussed in some detail. We also discuss the potential applications of this vibratory fluid pump. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HC.00004: Swelling-Induced Dynamic Responses of Soft Materials: Bending, Buckling, and Twisting Tarun Sinha, Douglas Holmes, Matthieu Roche, Arnaud Saint-Jalmes, George Maurdev, Howard Stone Soft materials (e.g. tissue, gels) undergo volume changes and instabilities when subjected to external stimuli. We present the dynamic instabilities that occur by straining an elastomer anisotropically. We examine how thin elastic plates can undergo rapid bending and buckling instabilities after exposure of the crosslinked, elastic network to a favorable solvent that causes it to swell. The shape of the swollen material is determined by the minimization of the system's bending energy in conjunction with any external forces, or constraints on the geometry; here we focus on dynamics. An unconstrained beam bends along its length, while a circular disc bends and buckles with multiple curvatures that rotate azimuthally around the disc. Theoretical interpretations motivated by the complementary thermal expansion problem of transient shape changes triggered by time-dependent heating will be presented. Developing a quantitative understanding of this phenomena will not only further explain the dynamics of morphogenesis in growing soft tissues, but also will lead to the creation of advanced elastic materials that can adapt to stimuli to change shape, and possibly direct and control fluid flow. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HC.00005: Study of Thermo-Acoustic Instabilities in a Rijke Tube Without and With Porous Inert Medium Cody Osmer, Ajay Agrawal Porous inert medium has been used in the past to reduce combustion noise in atmospheric pressure systems. It is envisioned that this same approach could be used to mitigate combustion instabilities encountered in gas turbines using lean premixed combustion concept. In this study, a simple Rijke tube is used to investigate the effect of porous inert media on thermo-acoustic instabilities. The Rijke tube set up utilizes a simple tube with a localized heat source. Sound pressure level measurements are taken at the tube exit to determine the sound power spectra in decibels. Such measurements were taken without and with porous medium inserted within the Rijke tube. The porous media is a 2.54 cm thick ceramic disk with 10 to 20 pores per inch. Results show that the Rijke tube behavior without the porous media follows the trends observed by previous investigators. The porous media was effective in reducing the sound pressure level, and thus, it offers the potential to mitigate thermo-acoustic instabilities through proper geometric design. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HC.00006: Effect of Leading Edge Tubercles on Marine Tidal Turbine Blades Mark Murray, Timothy Gruber, David Fredriksson This project investigated the impact that the addition of leading edge protuberances (tubercles) have on the effectiveness of marine tidal turbine blades, especially at lower flow speeds. The addition of leading edge tubercles to lifting foils has been shown, in previous research, to delay the onset of stall without significant hydrodynamic costs. The experimental results obtained utilizing three different blade designs (baseline and two tubercle modified) are compared. All blades were designed in SolidWorks and manufactured utilizing rapid prototype techniques. All tests were conducted in the 120 ft tow tank at the U.S. Naval Academy using a specifically designed experimental apparatus. Results for power coefficients are presented for a range of tip speed ratios. Cut-in velocity is also compared between the blade designs. For all test criteria, the tubercle modified blades significantly outperformed the smooth leading edge baseline design blades. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HC.00007: Evaporation and impact of water droplet on superhydrophobic surfaces Peichun Tsai, Maurice Hendrix, Remco Dijkstra, Rob Lammertink, Matthias Wessling, Detlef Lohse We examine both quasi-static and dynamic effects of water droplets upon hydrophobic microstructred surfaces, which possesses a high contact angle $\sim$ 150 $^{\circ}$ for the droplet size of $\sim$ 1 mm in radius. First, a milli-meter sized water droplet sitting on microstructures under a natural evaporation can undergoes a transition from a heterogenous (Cassie-Baxter) to a homogenous (Wenzel) wetting state, when the droplet size is reduced to about a couple hundred microns. The contact angle changes during the evaporation. With the evolution of the contact angle, a model based on global surface energies was developed to predict the transition points, which agree well with the experimental data. Secondly, water droplet impinging on the superhydrophobic surface can completely rebound off the surface when the impacting kinetic energy is comparable with the surface energy. As an increase of kinetic energy is about a few hundred times larger than the surface tension, a splash--emitting satellite droplets--occurs during the advancing phase of the lamella. We will discuss the influence of the geometric patterns on the splash. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HC.00008: Spray From a Rolling Tire: Mechanics of Droplet Formation Dennis Plocher, Fred Browand The spray pattern immediately behind a single-groove tire rolling on a wet surface is produced in the laboratory using a specially designed tire spray simulator. The spray development is examined using high speed video. Water from the groove forms a liquid sheet as the tire-tread lifts away from the surface. The sheet is not of uniform thickness, but it remains attached to the tread. The thinner portions of the sheet become even thinner as the tire rotates, and eventually break to produce holes near the tire surface. The holes grow as the sheet margins surrounding the holes retract into the thicker portions of the sheet which become roughly cylindrical ``ligaments'' aligned at right angles to the direction of spray motion. The ligaments break into large droplets \textit{via} a Rayleigh instability. The smallest droplets form when the margins of two holes collide. As Weber number, \textit{We = $\rho $U}$^{2}$\textit{w/2$\sigma $} , based on tire groove half width, w/2, varies by a factor of 25, the sheet-ligament structure persists, but ligaments become less organized, and more small droplets appear in the pattern. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HC.00009: Experiments on Spray from a Rolling Tire Charles Radovich, Fred Browand A novel laboratory apparatus has been built to understand the mechanisms and statistics of droplet production for spray emerging from a rolling tire. Using high-speed imaging, water passing through a single circumferential groove was observed to leave the tire contact patch in the form of a liquid sheet of non-uniform thickness. The sheet breaks into droplets as a result of several, organized instabilities. Measurements for the breakup length of the liquid sheet showed a dependence on Weber number proportional to We$^{-1/6}$, for Weber numbers of 2700, 10900 and 24400. A technique to identify and size water droplets was developed and the distribution of droplet sizes was determined as a function of Weber number. At We = 2700, droplet sizes between 80 and 9000$\mu m$ were detected, with a mean diameter near 800$\mu m$. Both the range of droplet sizes and the mean diameter were found to decrease with increasing Weber number as (approximately) We$^{-1/2}$. Correlation Image Velocimetry (CIV) was used to estimate the distribution of droplet velocities as a function of droplet size. The spread of droplet velocities about the tire peripheral speed is strongly correlated with droplet size. The spread can be estimated by a simple physical model incorporating rigid droplets subject to gravity and drag. [Preview Abstract] |
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