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 MJ: Acoustics III: Oscillatory Excitations |
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Chair: Daniel Bodony, University of Illinois at Urbana-Champaign Room: Long Beach Convention Center 201A |
Tuesday, November 23, 2010 8:00AM - 8:13AM |
MJ.00001: The Effect of Shape Mode Oscillations on the Particle Scavenging Efficiency of Water Droplets Using Acoustic Levitation J. Kizzee, J.R. Saylor The effect of drop oscillations on the scavenging of solid particles is studied using an ultrasonic transducer to levitate a water droplet in an airflow of particles. Shape mode oscillations are induced in the drop by modulating the acoustic field used for levitation. The effect of oscillation frequency, the oscillation amplitude, and the drop diameter on the scavenging of particles is presented. The particle diameters are on the order of 1$\mu$m and the drop diameters are on the order of 1mm. Although single droplets are studied here, the application of interest is improved scavenging of particles by spray drops. Specifically, improving the elimination of coal dust particles from mines using waters sprays excited ultrasonically is of interest. [Preview Abstract] |
Tuesday, November 23, 2010 8:13AM - 8:26AM |
MJ.00002: Droplet actuation by surface acoustic waves: an interplay between acoustic streaming and radiation pressure Philippe Brunet, Michael Baudoin, Olivier Bou Matar, Farzam Zoueshtiagh Surface acoustic waves (SAW) are known to be a versatile technique for the actuation of sessile drops. Droplet displacement, internal mixing or drop splitting, are amongst the elementary operations that SAW can achieve, which are useful on lab-on-chip microfluidics benches. On the purpose to understand the underlying physical mechanisms involved during these operations, we study experimentally the droplet dynamics varying different physical parameters. Here in particular, the influence of liquid viscosity and acoustic frequency is investigated: it is indeed predicted that both quantities should play a role in the acoustic-hydrodynamic coupling involved in the dynamics. The key point is to compare the relative magnitude of the attenuation length, i.e. the scale within which the acoustic wave decays in the fluid, and the size of the drop. This relative magnitude governs the relative importance of acoustic streaming and acoustic radiation pressure, which are both involved in the droplet dynamics. [Preview Abstract] |
Tuesday, November 23, 2010 8:26AM - 8:39AM |
MJ.00003: Finite-Element Modeling of Forced Axisymmetric Sessile Drop Oscillation Using a Moving Mesh Chris Forster, Marc Smith, Ari Glezer The behavior of a sessile drop undergoing axisymmetric oscillations is studied using the finite element method with a moving mesh to track the fluid interface. The drop is modeled in two different ways. In the first model, the surrounding fluid is assumed to have negligible viscosity and inertia. The drop motion is computed using a single fluid domain for the drop and solving the incompressible continuity and Navier-Stokes equations with appropriate interfacial boundary conditions. In the second model, the surrounding fluid is explicitly modeled. Here, two fluid domains are used, each having their own respective set of continuity and Navier-Stokes equations and these are fully coupled with the appropriate interfacial boundary conditions. The behavior of the drop is investigated for both pinned and dynamic contact line boundary conditions. A comparison of results from the two models and to the results from previous work is offered. The focus of this work is on extending this simulation to higher modes of oscillation where the effects of viscous damping from both the primary and surrounding fluids are more important. [Preview Abstract] |
Tuesday, November 23, 2010 8:39AM - 8:52AM |
MJ.00004: Mechanically-Excited Sessile Drops Chun-Ti Chang, Joshua Bostwick, Susan Daniel, Paul Steen The volume and contact-line mobility of a sessile drop determine the frequency response of the drop to mechanical excitation. A useful signature of the drop is its response to a sweep of frequency. At particular frequencies the drop exhibits standing wave patterns of different mode numbers and/or azimuthal, spinning motion. We report observations of the spectrum of standing wave patterns and compare to predictions of a linear stability theory. On the side of application, the results suggest how to tune the pinning-unpinning of a sessile drop in order to maximize its translation. [Preview Abstract] |
Tuesday, November 23, 2010 8:52AM - 9:05AM |
MJ.00005: Compressibility and Related Thermal and Diffusional Effects in Acoustics Streaming Satwindar Sadhal, Alexey Rednikov The effects of air compressibility have been analyzed for acoustic streaming due to acoustic fields interacting with solid boundaries. These include possible influence of the thermal boundary layer and, if the surrounding gas is a multicomponent mixture, of the diffusional boundary layer. It is well known that acoustic streaming originates in the viscous boundary layer at the surface of a particle, where flow is genuinely rotational and thus, a steady flow component is generated as soon as nonlinearities take effect. While this crucial role of the viscous boundary layer has been widely recognized, little attention has been paid to the role of the thermal boundary. The latter one, besides being of interest to purely thermoacoustics, may also affect the streaming itself. Since density is a function of temperature (as well as gas composition), a sharp temperature variation in the boundary layer gives rise to the corresponding density variation, influencing the flow field (continuity equation), and globally affecting streaming intensity. [Preview Abstract] |
Tuesday, November 23, 2010 9:05AM - 9:18AM |
MJ.00006: Numerical simulation of particle dispersion in an acoustic field J. Cleckler, F. Liu, S. Elghobashi Particles with small relaxation time, $\tau_p$, subjected to sound waves for many acoustic periods execute both periodic motion and mean drift. Particle acceleration in an acoustic flow field is often modeled via a linearized Stokes drag law. This simple model can predict the oscillatory particle velocity amplitude for large particle-to-fluid density ratios, $\rho_p/\rho_f$, and small velocity-amplitude acoustic waves. However, this model is not accurate for other conditions and does not predict particle drift velocities. We present the results of two-dimensional numerical simulations in which the particle trajectories are obtained via the complete Lagrangian particle motion equation which includes the forces due to non-linear Stokes drag, Basset's unsteady viscous drag, pressure gradient, virtual mass and gravity. Particle behavior is found to depend on three non-dimensional parameters: $(\rho_p/\rho_f)$, $( \omega \tau_p) $, where $\omega$ is the acoustic frequency, and the Mach number, $M$, which is the ratio of the acoustic wave velocity amplitude to the speed of sound. Results for large $\rho_p/\rho_f$ are in good agreement with the experimental results of Gonzalez et al. (2000) for the range of frequencies tested. Results for other conditions agree with a perturbation solution of the Lagrangian particle motion equation for moderate strength acoustic waves. Particle model simplifications are recommended for important ranges of the three parameters, $(\rho_p/\rho_f)$, $ ( \omega \tau_p )$ and $M$. [Preview Abstract] |
Tuesday, November 23, 2010 9:18AM - 9:31AM |
MJ.00007: Vortex structures and heat transfer in acoustic streaming flows In Mei Sou, John Allen, Christopher Layman, Chittaranjan Ray The velocity and heating in an acoustically induced streaming flow are investigated using simultaneous particle image velocimetry (PIV) and infrared thermography. This study is motivated by the increasing applications of ultrasound-based processing of substances in various fields such as wastewater treatment and biotechnology. The characterization of the acoustic streaming field is an important step in the overall design for sonochemical reactors used in these treatment processes. Results of the coherent structures and heating are obtained from an experimental study of acoustic streaming in a clear acrylic tank. The PIV resolved velocity fields show a jet-like flow along the centerline of the horn and a main vortex pair propagating in the direction of the flow. The coherent vortex structures are examined in terms of the swirling strength and Lagrangian coherent structures (LCS). The swirling strength is used to visualize the vortices in the Eulerian reference frame while the LCS approach is used to reveal the underlying flow structures for the unsteady case. The swirling strength is defined as the imaginary part of the complex eigenvalue of the local velocity gradient tensor. The LCS is defined as the local maxima of the finite-time Lyapunov exponent (FTLE). We present the evolution of the temperature fields together with the corresponding swirling strength and the LCS calculations. [Preview Abstract] |
Tuesday, November 23, 2010 9:31AM - 9:44AM |
MJ.00008: Radiative heating of acoustically levitated nano-silica droplets: Internal flow pattern leading to ring or bowl shaped structure Abhishek Saha, Erick Tijerino, Ranganathan Kumar, Saptarshi Basu An experimental setup using radiative heating has been used to understand the thermophysical phenomena inside acoustically levitated droplets. In this transformation process, through IR thermography and high speed imaging, events such as vaporization, precipitation have been recorded at high temporal resolution; leading to bowl or ring shaped structures. High solute loading is seen to form high concentration precipitate near the surface with a weak center linkage which results in a horizontal ring formation initially. Droplet recirculation is more effective at lower concentrations, inducing a bridge formation near the center leading to a bowl formation. With non-uniform particle distribution, these structures can experience rupture which modifies the droplet rotational speed with preferential orientation. PIV on sub millimeter sized droplets shows presence of strong single core vortex around droplet center. Study with droplet diameter and viscosity of the liquid leads to the conclusion that the strength of the vortex is dependent on these parameters. Further investigation with LIF confirms preferential accumulation of particles at the bottom of the droplet. [Preview Abstract] |
Tuesday, November 23, 2010 9:44AM - 9:57AM |
MJ.00009: Effectiveness of Thin Urethane Coatings for Attenuating Flow Noise in SONAR Arrays William Keith, Ian Cook, Alia Foley, Kimberly Cipolla The wall pressure fluctuations exerted on the wall beneath a turbulent boundary layer introduce flow noise which limits the performance of SONAR arrays. One method for mitigating this flow noise is to increase the standoff distance between the fluid-solid interface and the sensors in the array. A parametric study was conducted comparing the energy spectra of the wall pressure fluctuations measured by wall pressure sensors under .025'' and .05'' coatings of urethane beneath an aqueous, flat plate turbulent boundary layer. Measurements made with the sensors flush mounted in a flat plate, directly under the boundary layer (no coating) serve as a baseline for comparison. Reynolds number effects are considered, as is the effectiveness of boundary layer variables for collapsing the data (i.e. scaling relationships). [Preview Abstract] |
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