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 AG: Free Surface Flows I |
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Chair: Alexandra Techet, Massachusetts Institute of Technology Room: 101G |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AG.00001: Resolving the unsteady deceleration and forces after water entry by low mass-ratio spheres B.P. Epps, T.T. Truscott, R.R. LaFoy, A.H. Techet The dynamics of water entry are significantly affected by surface coating and mass ratio, over a range of moderate impact velocities and sphere diameters. A hydrophobic sphere creates a sub-surface vapor cavity whereas a hydrophilic sphere does not, and the forces acting on the sphere after entry depend upon whether or not this cavity is formed. Using high speed-video, sub-pixel-accurate image processing techniques, and a smoothing spline method to find the derivatives of position data, we find the unsteady forces acting on spheres during the water entry event. Our data reveals that as mass ratio decreases from O(10 to 1), the sphere's deceleration becomes highly non-linear, since inertial forces cease to dominate over hydrodynamic forces. The unsteady deceleration is affected by vortex shedding in the no-cavity case, whereas in the cavity forming case, forces are affected by cavity growth and collapse, and vortex shedding is inhibited until after pinch-off. PIV sequences taken in both cavity-forming and non-cavity-forming impact cases highlight the vortex shedding onset and can be used to gain further insight into the sphere dynamics. Ultimately, this work emphasizes the need to accurately account for unsteady effects in modeling the post impact dynamics of spheres, especially as mass ratios approach unity. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AG.00002: The water exit of buoyant spheres Tadd Truscott Results of a combined experimental and theoretical investigation of a buoyant sphere as it passes through the free surface are presented. When a buoyant sphere ascends through a fluid column various behaviors are exhibited (e.g. vortex induced oscillations, etc.) and are a function of release depth and mass ratio. Using high speed dye visualization and particle image velocimetry to reveal the flow behavior, the conditions of release for which maximum height occurs and the governing parameters for optimum water exit are examined. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AG.00003: Wave resistance Marie Le Merrer, Frederic Chevy, Elie Raphael, Christophe Clanet, David Quere The more viscous, the more slippery! This is what happens when millimetric liquid nitrogen drops are thrown at the surface of water or viscous oil. Because these drops float on a cushion of vapor, the resistance to the motion mainly arises from the formation of waves, which mostly occurs on liquids of low viscosity. The wave resistance is very low, of the order of ten micronewtons. However, we could measure it from the slow deceleration of the drops as they move along the surface. We were able to show that this force increases very strongly (in a quasi-discontinuous fashion) when the velocity becomes higher than 23 cm/s, that is, when a stationary wake can exist. We also studied the effect of the bath viscosity, which damps the waves and hence reduces the drag. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AG.00004: Laser-produced microjets Sigurdur Thoroddsen, K. Takehara, T.G. Etoh, C.-D. Ohl We use ultra-high-speed imaging to characterize the formation of a micro-jet when a laser-produced shock hits a bubble sitting under a free surface. The bubble is formed inside a sessile drop, sitting on a glass slide and buoyancy drives it to its top. The jetting is forced by an Nd:YAG-laser pulse of about 30 mJ, focused by a microscope objective sitting under the glass plate. The jet is initiated when the shock hits the curved bottom of the bubble. It emerges out of a bottom crown and has a very regular shape. For water the jets are a few microns in size and can emerge at over 200 m/s. In intermediate viscosity liquids the jetting can be even faster and can emerge at over 500 m/s, depending on the depth of the laser focus. Jets can even be produced in pure glycerin where they emerge at about 100 m/s. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AG.00005: Drag-out of bubbles by a plate withdrawn from a liquid bath Justin Kao, Andrea Blakemore, Anette Hosoi We report work on a new aspect of the classic Landau--Levich problem of liquid drag-out by a moving plate, namely, interaction of bubbles with the coating flow. Due to the cheerios effect, bubbles present in a liquid bath may gather at menisci such as those caused by a partially-submerged plate. Our experiments show that a critical withdrawal speed exists. Below this speed, bubbles are in stable equilibrium and remain stationary in the lab frame as the plate is withdrawn. Above the critical speed, no stable equilibrium exists and bubbles are drawn up onto the plate, with obvious consequences for the uniformity of the resulting coating. We examine the dependence of the critical speed on fluid properties. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AG.00006: Laminar circular hydraulic jumps without separation Ratul Dasgupta, Gaurav Tomar, Rama Govindarajan The traditional inviscid criterion for the occurrence of a planar, standing hydraulic jump is to have the Froude number decrease downstream and go through a value of 1 at some location. Here, upstream propagating, small-amplitude, long, non-dispersive gravity waves are trapped, and non-linear steepening is said to result in a near-discontinuous height profile, but it is not clear how. Such a condition on the Froude number is shown in the present axisymmetric Navier-Stokes computations to hold for a circular jump as well. The relevance of non-linear steepening to a circular jump is therefore a question we wish to answer. In circular jumps, moreover, a region of recirculation is usually observed underneath the jump, underlining the importance of viscosity in this process. This led Tani (J. Phys. Soc. Japan, 1949) to hypothesise that boundary-layer separation was the cause of the circular jump. This hypothesis has been debated extensively and the possibility of circular jumps without separation hinted at. In our simulations, we are able to obtain circular hydraulic jumps without any flow separation. This, and the necessity or otherwise of viscosity in jump formation will be discussed. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AG.00007: Study of Air Entrainment by a Horizontal Plunging Liquid Jet Mario Trujillo, Suraj Deshpande, Xiongjun Wu, Georges Chahine The process of air entrainment following the impact of an initially horizontal circular water jet on a pool of water has been studied computationally and experimentally. It has been found that the entrainment of air cavities in the near field region is periodic, not continuous as reported in earlier studies. The simulations are based on a Volume-of-Fluid methodology with interfacial compression using a modified version of the open source utilities, OpenFoam. Close agreement with experiments is reported on the creation of cavities in the near field, where air entrainment occurs. The period of entrainment is found to be proportional to g, and a simplified closed-form solution for this periodic event is presented. An overall physical picture of the mechanisms leading to bubble formation is given. The far field, which is characterized by the presence of small bubbles is only partially resolved computationally. Comparisons against velocity data are performed in this region leading to adequate qualitative agreement. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AG.00008: Viscous rivulet flow over trenches Thomas Ward, G.M. Homsy The dynamic interfacial behavior of an advancing rivulet encountering a trench of square cross-section is studied experimentally at low capillary and Reynolds numbers. Trench depths vary from slightly smaller than to slightly larger than the capillary length. The fluids are a glycerol/water mixture and a silicone oil, representing a partially wetting and a nearly complete wetting fluid, respectively, and the rivulet interface is observed using low speed CCD imaging. A rich variety of phenomenon is observed in this range of depths suggesting that trench wetting is greatly affected by a combination of geometry and dynamic contact angle. The dynamics are characterized by measuring the local film height as a function of time and are compared with the theory of Gramlich et al. (Phys. Fluid, 2004). In spite of the fact that the theory is 2D while the experiments are 3D, remarkably good qualitative agreement is observed for large trench depths and partially wetting fluids. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AG.00009: Temporal statistics of a meandering rivulet Peter Vorobieff, Keith Mertens, Vakhtang Putkaradze Meandering of a rivulet on an inclined, partially wetting surface can be triggered or suppressed by introducing small flow-rate fluctuations or, correspondingly, by reverting to a constant flow rate. Here we study a rivulet continuously meandering with flow-rate fluctuations present. Image sequences of the plane of the meander reveal a spatially and temporally resolved picture of the process, covering spatial scales from millimeters to meters and time scales from seconds to hours. For a given coordinate in the direction downstream from the origin of the rivulet, we construct time histories of the location of the rivulet centerline. Statistics of these time histories show an interesting correspondence with the spatial meandering statistics known from earlier work (such as the absence of a dominant wavelength), while also possibly manifesting previously unobserved trends at short time scales. It is also noteworthy that even a modest volume fraction of solid particles in the flow can radically alter the behavior of the rivulet, producing a stationary (pinned) meandering pattern. [Preview Abstract] |
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