Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R3: Bubbles IV |
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Chair: Steve Ceccio, University of Michigan Room: 303 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R3.00001: Velocimetry in cavitating flows by X-ray imaging Olivier Coutier-Delgosha, Marko Hocevar, Ilyass Khlifa, Sylvie Fuzier, Alexandre Vabre, Kamel Fezzaa, Wah-Keat Lee A promising method to measure velocity fields in a cavitating flow is presented. Dynamics of the liquid phase and of the bubbles are both investigated. The measurements are based on ultra fast X-ray imaging performed at the APS (Advanced Photon Source) of the Argonne National Laboratory. The experimental device consists of a millimetric Venturi test section associated with a transportable hydraulic loop. Various configurations of velocity, pressure, and temperature have been investigated. The slip velocity between vapor and liquid is calculated everywhere both velocities can be obtained. Reynolds stresses are also calculated, and compared with the ones obtained in non-cavitating conditions. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R3.00002: Influence of Vortex Cavitation on Vortex Breakdown Harish Ganesh, Andre Pinheiro, Bu Guen Paik, Steven L. Ceccio Cavitation inception and development was observed in the vortices created by an $\Lambda $ = 70$^{\circ}$ delta wing over a range of attack angles and cavitation numbers. The location of cavitation inception, bubble size, growth rate, and the effect of cavitation on the location of vortex breakdown were studied. From the observations a rationale that governs the observed inception location, growth of incepted nucleus into a given shape is proposed. It is based on the alterations of the vortical core flow that an elongating bubble will cause. The minimizing potential theory of \textit{Rusak et al}\footnote{``The evolution of a perturbed vortex in a pipe to axisymmetric vortex breakdown,'' Rusak, Z., Wang. S., Whiting. C. H., \textit{Journal of fluid mechanics}, 1998, Vol 336, pp 211-237} is used to identify the constraints on the bubble growth which in turn helps us to understand the bubble shape and growth rate. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R3.00003: High-speed microjet generation using laser-induced vapor bubbles Nikolai Oudalov, Yoshiyuki Tagawa, Ivo Peters, Claas-Willem Visser, Devaraj van der Meer, Andrea Prosperetti, Chao Sun, Detlef Lohse The generation and evolution of microjets are studied both experimentally and numerically. The jets are generated by focusing a laser pulse into a microscopic capillary tube ($\sim$50 $\mu$m) filled with water-based red dye. A vapor bubble is created instantly after shooting the laser ($<$1 $\mu$s), sending out a shockwave towards the curved free surface at which the high-speed microjet forms. The process of jet formation is captured using high-speed recordings at 1.0 $\times$ $10^6$ fps. The velocity of the microjets can reach speeds of $\sim$850 m/s while maintaining a very sharp geometry. The high-speed recordings enable us to study the effect of several parameters on the jet velocity, e.g. the absorbed energy and the distance between the laser spot and the free surface.The results show a clear dependence on these variables, even for supersonic speeds. Comparisons with numerical simulations confirm the nature of these dependencies. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R3.00004: Bubbles in drops: from cavitation to exploding stars Philippe Kobel, Danail Obreschkow, Nicolas Dorsaz, Aurele de Bosset, Marc Tinguely, Mohamed Farhat We performed an experiment to generate single cavitation bubbles inside centimetric quasi-spherical water drops. To produce such drops, our experiment was realized under microgravity conditions (42nd ESA parabolic flight campaign). The ultra-fast recording of the bubble collapse and ensuing dynamics revealed consequences of the unique geometry of the drop's free surface. We obtained the first visualizations of a jet pair escaping the drop after the collapse of eccentrically-placed bubbles. The high quality of the images also disclosed some features of the inner drop dynamics. Due to their confinement within the isolated drop volume, shock waves emitted at the bubble collapse bounce back and forth thereby exciting gas nuclei into sub-millimetric bubbles. When located beneath the free surface, the collapse of these bubbles gives rise to narrow ``hair-like'' jets on the surface. Here we briefly describe the physics underlying these observations while discussing possible analogies with various astrophysical processes from the Sun (spicules) to asymmetric supernovae. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R3.00005: Homogeneous cavitation in microfluidics: a record high dynamic tensile threshold Keita Ando, Ai-Qun Liu, Claus-Dieter Ohl An experimental technique is presented which allows one to measure the rupture strength of water using a microfluidic approach. A transparent microfluidic channel is filled with clean water, partially leaving an air-water interface. A focused infrared laser pulse within the liquid creates a spherical shock wave near the interface. The shock reflects, due to acoustic impedance mismatch, as a strong tension wave with high negative pressures. The liquid becomes stretched and at the homogeneous cavitation threshold ruptures with the nucleation of vapor bubbles. These bubbles are captured using an optical delay and very short exposure times. Reproducible observations of the bubble nucleation are obtained, supporting our claim of homogeneous bubble nucleation. Multicomponent Euler flow simulation estimates a tensile stress threshold of -60 MPa, which is the largest reported tension for dynamic measurements. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R3.00006: Understanding Cavitation Intensity through Pitting and Pressure Pulse Analysis A. Jayaprakash, S. Singh, J-K. Choi, G. Chahine Cavitation erosion is of interest to the designers of ship propulsion devices because of its detrimental effects. One of the difficulties of predicting cavitation erosion is that the intensity of cavitation is not well predicted or defined. In this work we attempt to define the intensity of a cavitation erosion field through analysis of cavitation induced erosion pits and pressure pulses. In the pitting tests, material samples were subjected to cavitation field for a short duration of time selected within the test sample's incubation period, so that the test sample undergoes plastic deformation only. The sample material reacts to these cavitation events by undergoing localized permanent deformation, called pits. The resulting pitted sample surfaces were then optically scanned and analyzed. The pressure signals under cavitating jets and ultrasonic horns, for different conditions, were experimentally recorded using high frequency response pressure transducers. From the analysis of the pitting data and recorded pressure signals, we propose a model that describes the statistics, which in the future can be used to define the cavitation field intensity. Support for this work was provided by Office of Naval Research (ONR) under contract number N00014-08-C-0450, monitored by Dr. Ki-Han Kim. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R3.00007: Vapor bubble evolution on a heated surface containing open microchannels Christopher J. Forster, Ari Glezer, Marc K. Smith Power electronics require cooling technologies capable of high heat fluxes at or below the operating temperatures of these devices. Boiling heat transfer is an effective choice for such cooling, but it is limited by the critical heat flux (CHF), which is typically near 125 W/cm$^2$ for pool boiling of water on a flat plate at standard pressure and gravity. One method of increasing CHF is to incorporate an array of microchannels into the heated surface. Microchannels have been experimentally shown to improve CHF, and the goal of this study is to determine the primary mechanisms associated with the microchannels that allow for the increased CHF. While the use of various microstructures is not new, the emphasis of previous work has been on heat transfer aspects, as opposed to the fluid dynamics inside and in the vicinity of the microchannels. This work considers the non-isothermal fluid motion during bubble growth and departure by varying channel geometry, spacing, and heat flux input using a level-set method including vaporization and condensation. These results and the study of the underlying mechanisms will aid in the design optimization of microchannel-based cooling devices. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R3.00008: Acoustically enhanced boiling heat transfer on a heated surface containing open microchannels Thomas R. Boziuk, Marc K. Smith, Ari Glezer Acoustic actuation is used to enhance boiling heat transfer on a submerged heated surface containing an array of open microchannels by controlling the formation and evolution of vapor bubbles and inhibiting the instability that leads to film boiling at the critical heat flux. The effect of actuation at millimeter and micrometer scales is investigated with emphasis on the behavior of bubble nucleation, growth, contact-line motion, condensation, and detachment. The results show that microchannels control the location of boiling and reduce the mean surface superheat. In addition, acoustic actuation increases the heat flux at a given surface temperature and leads to a significant increase in the critical heat flux, a reduction of the vapor mass above the surface, and the breakup of low-frequency vapor slug formation. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R3.00009: The most spherical cavitation bubble Marc Tinguely, Danail Obreschkow, Philippe Kobel, Nicolas Dorsaz, Aurele de Bosset, Mohamed Farhat Focusing a high energy pulsed laser into water is a widely used method to generate a single cavitation bubble. Such a bubble is generally assumed to collapse spherically. However, the precision of the focusing system and the effect of gravity, most often neglected, break the spherical symmetry of the collapse. In the experiment presented here, we generated the ``most spherical'' cavitation bubbles in a series of experiments by (1) running the experiments in microgravity (during the 52nd ESA Parabolic Flight Campaign), and (2) using an optimized laser focusing system. The collapse and the rebound of the bubbles were then investigated to experimentally determine the fraction of potential energy of the bubble transformed into the rebound bubble and into the shock wave at the collapse. We found that the transfer of the energy into both channels can be well predicted using a single non-dimensional parameter. A theoretical model is developed and shows good agreement with the experimental results. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R3.00010: Drops levitated by an air cushion: asymptotic analysis and stability Jacco H. Snoeijer, Phillippe Brunet, Jens Eggers Liquid drops can be kept from touching a plane solid surface by a gas stream entering from underneath, as observed for example for Leidenfrost drops. We discuss the limit of small flow rates, for which the gap between the drop and the substrate becomes very small, to obtain a full analytical description of the drop shapes and their stability. It is found that above a critical drop radius no stationary drops can exist and that unstable drops develop a gas ``chimney'' that breaks the drop in its middle. We point out similarities with the breakup of drops in a microfluidic T- junctions. [Preview Abstract] |
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