Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P8: Focus Session: Jets, Shocks & Splashes |
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Sponsoring Units: DFD GSNP Chair: Wendy Zhang, University of Chicago Room: Baltimore Convention Center 314 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P8.00001: The secret of splashing: interplay of air and roughness Invited Speaker: We studied splashing on both smooth and rough dry surfaces using high speed photography. For smooth substrates, a striking phenomenon is observed: splashing can be completely suppressed by decreasing the pressure of the surrounding gas. The threshold pressure where a splash first occurs is measured as a function of the impact velocity and found to depend on the molecular weight of the gas and the viscosity of the liquid. Both experimental scaling relations support a model in which the compressibility of the gas is responsible for creating the splash[1]. For the case of rough substrates, we systematically varied both the surface roughness and the pressure of the surrounding gas and found two distinct contributions to a splash. One is caused by air and has the same characteristics as the ``coronal'' splash observed on smooth substrates. A second, ``prompt'' splash, contribution is caused by surface roughness. We have also measured the size distribution of the droplets emitted from a splash. For a smooth surface, a broad distribution of droplet sizes is found at high gas pressures. As the gas pressure is lowered towards the splash/no-splash transition the distribution gets more and more peaked at a characteristic size. For a rough surface, the distribution is strongly correlated with the surface roughness. [1] L Xu, W W Zhang and S R Nagel, Phys. Rev. Lett. 94, 184505 (2005) [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P8.00002: Statistical Mechanics of a Geophysical Jet Emily Conover, J.B. Marston We investigate the equal-time statistics of an equatorial jet in a two-dimensional quasi-geostrophic model of a planetary atmosphere on a rotating sphere\footnote{R. S. Lindzen, A. J. Rosenthal, and R. Farrell, J. Atmos. Sci. {\bf 40}, 1029 (1983).}. Potential vorticity is advected by the barotropic flow and at the same time relaxes towards the zonal shear flow of an underlying equatorial jet. A transition to turbulence occurs at sufficiently slow relaxation rates. Statistics accumulated by direct numerical simulation\footnote{Akio Arakawa, J. Comp. Phys. {\bf 1}, 119 (1966).} are compared to those obtained by a simple cumulant expansion. We study rigorous upper bounds on the instability size\footnote{T. G. Shepherd, J. Fluid. Mech. {\bf 196}, 291 (1988).} and discuss the limitations of the cumulant expansion. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P8.00003: High-Speed X-ray Investigation of Granular Jets John Royer, Eric Corwin, Andrew Flior, Bryan Conyers, Maria-Luisa Cordero, Mark Rivers, Peter Eng, Heinrich Jaeger When a heavy sphere is dropped onto a bed of loose, fine sand, a large, focused jet of sand shoots upward.\footnote{Thoroddsen, S. T. and Shen, A. Q. Phys. Fluids {\bf13}, 4-6 (2001).} \footnote{Lohse, D. et al. Phys. Rev. Lett. {\bf93} (2004).} Experiments at reduced air pressure reveal that the jet in fact consists of two components: a wispy, thin jet that varies little with pressure followed by a thick air-pressure-driven jet \footnote{Royer, J. et al. Nature Physics, December 2005.} . To observe the initial stages of jet formation inside the granular bed, we employed x-ray radiography using the high-intensity beams available at the Advanced Photon Source. This technique allowed us to image the motion of the sphere and the evolution of the void left behind it at frame rates up to 6600 frames per second. The x-ray movies reveal that gravity-driven collapse produces the initial, thin jet, while the compression of an air pocket trapped below the surface drives up the thick jet. We also find that the interstitial air alters the compressibility of the sand bed. In vacuum a visible compaction front precedes the ball, while at atmospheric pressure the sand flows out of the way of the ball, behaving more like an incompressible fluid. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P8.00004: Measurement of Stopping Force in Low Velocity Impact Cratering Joseph Amato, Michael Nitzberg The time dependent stopping force on a ball dropped into a granular medium has been measured using an accelerometer embedded within the ball. The velocity dependence of the force shows two distinct behaviors: (1) for impacts with large (200 $\mu $m) irregularly shaped sand particles, $F(v)\propto v^{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}$; for impacts with 100 $\mu $m spherical glass beads, $F(v)\propto \left( {v-v_0 } \right)$. The accelerator apparatus yields reproducible, low noise data that reveals peculiar features such as a downward acceleration pulse just before the ball comes to rest. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 1:03PM |
P8.00005: Giant bubble-pinchoff Invited Speaker: Self-similarity has been the paradigmatic picture for the pinch-off of a drop. Here we will show through high-speed imaging and boundary integral simulations that the inverse problem, the pinch-off of an air bubble in water, does not obey self-similarity (of the first kind): A disk is quickly pulled through a water surface, leading to a giant, cylindrical void, which at collapse creates an upward and a downward jet. The neck radius h(tau) of the void does NOT scale with the inertial power law exponent 1/2 (i.e., does not obey ``Rayleigh-scaling''). This is due to a second length-scale, the inverse curvature of the void,which follows a power-law scaling with a different exponent. Only for infinite Froude numbers the scaling exponent 1/2 is recovered. In all cases we find the void-profile to be symmetric around the minimal void radius up to the time the airflow in the neck deforms the interface. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P8.00006: Nano-liquid bridges in ambient conditions Wei Kang, Uzi Landman The dynamics of nano-liquid bridges in an ambient gaseous environment is studied using molecular dynamics simulations. Under these conditions new behavior close to break-up is found, compared to the behavior in vacuum. The probability for appearance of a long-thread structure close to pinch-off, versus the appearance of a double-cone profile in vacuum, depends on the density of the ambient gas. The stochastic lubrication equation that has been introduced by Moseler and Landman [1] for the case of break-up in vacuum is modified to include an additional term representing the effect of the ambient gas. Numerical integration of the modified stochastic lubrication equation shows good agreement with the molecular dynamics simulations. [1] M. Moseler and U. Landman, Science 289, 1165(2000). [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P8.00007: Splashing on dry, smooth inclined surfaces James Bird, David Weitz, Howard Stone, Michael Brenner We investigate splashing of drops on dry, smooth inclined surfaces. The asymmetry of the impact leads to an azimuthal variation of the ejected rim. We show that under certain conditions only part of the rim splashes. A model for the azimuthal splash threshold is compared both with the data and with existing splash criteria. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P8.00008: Levitation of Falling Spheres in Stratified Fluids Richard McLaughlin, Roberto Camassa, Byron Huff, Richard Parker The motion of sphere's falling under the influence of gravity is a classical problem dating back to Galileo and earlier. How a falling body additionally interacts with its environment is an equally challenging problem and involves strong coupling between the body and fluid via hydrodynamic drag. We present new phenomena$^2$ concerning the motion of a sphere falling through a sharply stratified (two layer) fluid in which the falling heavy body stops and reverses its direction (bounces) before ultimately returning to descent. Shadowgraph imaging shows the physics responsible for this surprising motion is a coupling between the body and the ambient boundary layer fluid, which is endowed with a negative potential energy as it is drawn into the lower layer, forming a rising turbulent plume. The hydrodynamic coupling between the sphere and this plume temporarily arrests the motion, even causing the bead to rise back through the transition layer. We present measurements of this trapping phenomena, and report the long residence times in which the sphere is trapped within the transition layer as a function of the bottom layer fluid density field for an array of different sized spheres. $^2$ N. Abaid, D. Adalsteinsson, Akua Agyapong, and R. M. McLaughlin, ``An Internal Splash: Falling Spheres in Stratified Fluids,'' Physics of Fluids, 16, no. 5, 1567-1580, 2004. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P8.00009: Sodium luminescence from laser-induced bubbles in salt water Sonny Vo, Tim Hsieh, Han-Ching Chu, Gary Williams Luminescence from collapsing laser-induced bubbles in salt water (up to 1M NaCl) has been studied. We find a new emission pulse from the 589 nm sodium line that arrives about 50 ns prior to the main blackbody luminescence pulse. This may be related to the dynamics of the compressional heating process in the bubble. We have also noticed in the salt water that the time duration of the blackbody pulse is reduced by up to 30\% from the duration in pure water, and this has been observed in several other alkali salt solutions. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P8.00010: The dynamics of a flexible loop in a high-speed flow Sunghwan Jung, Kathleen Mareck, Michael Shelley, Jun Zhang We study the behavior of an elastic loop in a fast-flowing soap film. The loop is wetted into the film and is held fixed at a single point against the oncoming flow. We interpret this system as a 2D closed flexible body moving in a quasi-2D flow. The loop is deformed by the flow, and this coupled fluid-structure system shows bi-stability: stationary and oscillatory. In its stationary state, the loop essentially remains motionless and its wake is a von K\'{a}rm\'{a}n vortex street. In its oscillatory state, the loop sheds two vortex dipoles within each oscillation period. The frequency of oscillation of the loop is linearly proportional to the flow velocity. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P8.00011: Superfluid-like shock waves in nonlinear optics Wenjie Wan, Jason W. Fleischer It is well-known, but often underappreciated, that condensate dynamics has analogies with nonlinear light propagation in optics. In both cases, a single, macroscopic wavefunction describes the coherent wave behavior of interest. Here, we take advantage of this correspondence and examine superfluid-like spatial shock waves by propagating coherent light through a nonlinear crystal. We report the observation of both 1D and 2D shock waves, their nonlinear behavior as a function of intensity, and double-shock wave collisions. Analytical calculations and numerical simulations show excellent agreement with the experimental results. The fine structures and features observed here match similar observations in previous shock studies using superfluids and BEC, obtained in this case in a table-top apparatus, without the need for vacuum isolation, ultracold temperatures, etc. Moreover, the inherent optical advantages of easy control of the wavefunction input and direct imaging of the output make us optimistic that the nonlinear photonic systems described here will lay the foundation for a whole series of condensate-inspired experiments, many of which would be difficult (if not impossible) to perform in the corresponding condensed matter environments. [Preview Abstract] |
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