Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session AK: Waves |
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Chair: James Rottman, University of California, San Diego Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 4 |
Sunday, November 19, 2006 8:00AM - 8:13AM |
AK.00001: An Experimental Investigation of Wind- and Mechanically Generated Short Wavelength Spilling Breakers J.D. Diorio, X. Liu, J.H. Duncan Short wavelength spilling breakers are studied in a wind wave tank that is 12.8 m long and 1.15 m wide and 0.91 m deep. The crest profile histories during breaking are measured with a photographic technique that employs a high-speed digital movie camera, a laser light sheet, and fluorescent dye. The photographic system is mounted on an instrument carriage that is set to move along the tank in phase with the crests of the breaking waves. In the first step in the experiment, breakers generated by the wind are measured at three wind speeds and three fetches at each wind speed. In the second step in the experiment, a mechanical wave maker is used without wind to generate a wave train consisting of a dominant wave and two unstable sidebands. The amplitudes and frequencies of these wave components are adjusted to create breakers at the various fetches and dominant wave frequencies found in the wind wave experiments. The similarities and differences between the ripple patterns at the crest during breaking between the wind wave and mechanical wave cases are discussed. [Preview Abstract] |
Sunday, November 19, 2006 8:13AM - 8:26AM |
AK.00002: The Effects of Surfactants on Breaking Wind- and Mechanically Generated Waves X. Liu, J.D. Diorio, J.H. Duncan, G.M. Korenowski The effects of surfactants on breaking waves that are generated by both wind and a mechanical wave maker are studied experimentally. The waves are generated in a tank that is 11.8 m long, 1.1 m wide and 1.8 m high (1.0 m of water, wind speeds up to 10 m/s). The wave maker, which resides at the upwind end of the tank, can be used alone or to superimpose a wavetrain with frequencies of up to 3 Hz on a wind wave system. Wave profiles along the center plane of the tank are measured with an LIF technique that utilizes a high-speed digital movie camera. An anamorphic lens system is used to expand the vertical resolution of the images in order to more precisely measure the small scale features of the free surface. The measurement system is mounted on an instrument carriage that can be set to move along the tank with the speed of the breaking crests. Measurements are performed with water mixed with various concentrations of Triton X-100, a soluble surfactant. Surface dynamic properties are characterized with a Langmuir trough combined with a Whilhelmy plate and longitudinal wave device. Surface concentrations of surfactants are measured with a second harmonic generation technique. The effect of the surfactant on the frequency of occurrence of small-scale breaking events is studied and correlated with the measured surface dynamic properties. [Preview Abstract] |
Sunday, November 19, 2006 8:26AM - 8:39AM |
AK.00003: Flow structure in a turbulent stationary breaker Javier Rodriguez-Rodriguez, Alberto Aliseda, Juan Lasheras The flow in the air-entraining region of a deep-water stationary breaker has been studied using a combination of high-speed visualizations and feature tracking techniques (FTT) similar to those used in PIV. Instantaneous velocity measurements obtained with the FTT clearly show the existence of large coherent structures that develop similarly to those observed in stratified mixing layers. The dynamics of these large coherent structures has been characterized for a number of experimental conditions. This has allowed us to relate the mean convective velocity and growth rate of the structures with the relevant parameters of the bore, namely the free stream velocity of the unperturbed fluid and the jump between the upstream and downstream heights. Moreover, when the velocity field is averaged in time, a recirculation bubble generally referred as the `'roller'' in the literature, has been found. The average velocity profiles in this recirculating region exhibit a self-similar behavior. Supported by ONR grant N00014-05-1-0121. [Preview Abstract] |
Sunday, November 19, 2006 8:39AM - 8:52AM |
AK.00004: On the Geometrical Characteristics of Jets Produced in Plunging Breaking Bow Waves M. Shakeri, M. Tavakolinejad, M. Mayer, P. Kang, J.H. Duncan Simulated breaking bow waves were generated using a 2D+T wave maker in a tank that is 14.8 m long, 1.15 m wide and 2.1 m deep (water depth of 1.83 m). In the 2D+T simulation, the sequence of shapes of the flexible surface (wave board) of the wave maker reproduce the time varying intersection of one side of the ship hull with a vertical plane oriented normal to the ship’s track as the ship moves in calm water at constant speed. The profile histories of the breaking bow waves created by the wave maker were measured with an LIF system that employs a high-speed digital movie camera recording at 250 frames per second. The optics and camera are mounted on an instrument carriage that is set to move with the top portion of the wave board. For equivalent full scale ship speeds greater than about 20 knots, a large plunging breaker is formed. In this talk, measurements of the geometrical characteristics and motion of the plunging jet of the breaker are presented. The impact of the jet with the smooth surface of the forward face of the wave creates a large splash ahead of the impact point. The relationship between the jet characteristics and the geometrical characteristics of the splash zone are discussed. [Preview Abstract] |
Sunday, November 19, 2006 8:52AM - 9:05AM |
AK.00005: Effects of Porous Barriers on Coastal Wave Propagation S.P. Samarawickrama, H.J.S. Fernando, S.S.L. Hettiarachchi Although corals and mangroves are believed to provide shoreline protection against waves, currents and storm surges, solid evidence for such through scientific studies is remarkably little and has been mostly accidental or circumstantial. Despite the obvious general buffering capacity, not much information available on how the removal of natural barriers effects the wave forcing on the shoreline, especially for the cases where the removal is confined to the local pockets with regulatory supervision. The paper presents the changes in velocities for the conditions of: (i) no structure, (ii) structure without an opening, (iii) structure with a narrow opening and (iv) structure with a wide opening. With the introduction of the structure there is a considerable reduction in velocities in the water column apart from the surface velocity. [Preview Abstract] |
Sunday, November 19, 2006 9:05AM - 9:18AM |
AK.00006: The Propagation of Nonlinear Pressure Waves Through Regions of Non-Uniform Temperature Nicholas DiZinno, George Vradis, Volkan Otugen A numerical study of wave propagation through gases with non-uniform temperature distributions will be presented. The aim of this study is to determine the impact of temperature gradients on high-intensity pressure waves of various initial wave forms. Emphasis is paid to wave reflection and transmission. Ultimately, the performance of thermal barriers in attenuating nonlinear waves is evaluated. The concept of using regions of hot gas inside an ambient environment has potential in aeroacoustic applications, such as jet screech mitigation. This analysis considers the one-dimensional compressible unsteady Euler's equations with an ideal gas state equation. The domain is composed of two regions with uniform and equal gas properties separated by a third region with higher gas temperature (lower density). Pressure is uniform throughout the domain. We introduce various high-intensity wave forms into this medium. Our investigation studies how the shape and extent of the thermal zone affect transmission and reflection of the wave. This is done for a range of wave and thermal field parameters. A Fourier analysis will study the frequency content of the incident, transmitted and reflected waves. These results will help determine the effectiveness of using thermal barriers for nonlinear wave attenuation. [Preview Abstract] |
Sunday, November 19, 2006 9:18AM - 9:31AM |
AK.00007: A Model for the Internal Wave field Produced by a Horizontally Moving Body and its Wake in a Stratified Fluid James Rottman, Dave Broutman, Geoff Spedding, Peter Diamessis A linear theory is derived to describe the evolution of the internal wavefield generated by a horizontally moving body in a stratified fluid. The theory involves a ray solution in wavenumber coordinates that is mapped into a spatial solution by inverse Fourier transform. This is a more practical approach than calculating the ray solution directly in the spatial domain, and it is general enough to treat background flows with depth dependent shear and stratification. In this talk a novel method is described by which the ray calculations can be initialized using data, from either experimental measurements or direct numerical simulations, of the wave flow just outside the turbulent wake of the body. Here comparisons are made of the theory with tank experiments and direct numerical simulations of a towed sphere in a uniformly stratified background. [Preview Abstract] |
Sunday, November 19, 2006 9:31AM - 9:44AM |
AK.00008: Numerical simulations of stratified fluid flow over topography near resonance Harmony Brown, James Rottman, Keiko Nomura We use a high-resolution spectral numerical scheme to solve the two-dimensional equations of motion for the flow of a uniformly stratified Boussinesq fluid over isolated bottom topography in a channel of finite depth. The focus is on conditions such that the flow is near linear resonance. The results are compared with the existing theories: nonresonant steady hydrostatic theory, resonant and nonresonant time-dependent long-wave theory, and resonant fully nonlinear, weakly dispersive theory. We also analyze the approach to breaking that occurs downstream of the topopgraphy near resonance. [Preview Abstract] |
Sunday, November 19, 2006 9:44AM - 9:57AM |
AK.00009: Edge waves in a rotating stratified fluid Stefan Llewellyn Smith, Alexander Adamou, Richard Craster We examine trapped edge waves on a fluid layer overlying general one-dimensional topography and study the effects of rotation and stratification on the trapping. The problem is inherently of interest in oceanography, but it is also a canonical example of wave trapping that can also arise in elasticity, acoustics and quantum waveguides. The underlying eigenvalue problem is non-trivial to solve numerically as the eigenvalue arises in both the boundary conditions and the governing equations, and so we develop an accurate and efficient scheme to overcome the difficulties. The numerical solutions are complemented by an asymptotic study in which a modified WKBJ approach is developed that is uniform not just at caustics and shorelines, but also uniform in range. This general approach can be used to reduce much of the algebra usually associated with uniform asymptotics and should be of widespread utility. [Preview Abstract] |
Sunday, November 19, 2006 9:57AM - 10:10AM |
AK.00010: Resonant long--short wave interaction in an unbounded stratified fluid Ali Tabaei, T.R. Akylas A finite-amplitude theory is developed for the interaction of `flat' internal wavetrains with induced mean-flow disturbances in a uniformly stratified Boussinesq fluid. Flat wavetrains, which feature predominantly vertical modulations, interact resonantly with the induced mean flow since the modulation scales are compatible with those of gravity--inertial waves in the weak-rotation limit. This long--short wave interaction is described by a coupled set of evolution equations for the vertical wavenumber, the wave envelope and the induced mean flow. In contrast to wavepackets equally modulated in the vertical and horizontal directions, which are non-resonant so the mean flow is slaved to the packet, flat wavetrains in the course of their evolution leave a wake of gravity--inertial waves behind. It turns out that finite-amplitude wavetrains can develop `caustics' as a result of their interaction with the induced mean flow, consistent with fully numerical simulations. Modulational instability of a uniform wavetrain interacting with the induced mean flow is also addressed. [Preview Abstract] |
Sunday, November 19, 2006 10:10AM - 10:23AM |
AK.00011: Evolution of disturbances on separate pycnoclines M. Nitsche, P. Weidman, R. Grimshaw Motivated by the experiments on leapfrogging internal solitary waves reported in Weidman and Johnson (J. Fluid Mech., 122, 1985), we have developed a numerical code to follow the long-time inviscid evolution of isolated mode-two-like disturbances on two separate pycnoclines in a three-layer stratified fluid bounded by rigid horizontal top and bottom walls. We study the dependence of the solution on the input parameters, namely the three fluid densities and the two interface thicknesses, for fixed initial disturbance amplitude. For most parameter values, the two disturbances separate immediately and travel as solitary waves with distinct speed. In a certain parameter regime however, the two waves pair up and oscillate for some time in a leapfrog fashion with equal average speed. The motion is slowly varying as each wave loses energy into a dispersive tail, which causes the oscillation period and magnitude to increase, until the waves eventually separate. We record the separation time, oscillation period and magnitude, and final speed and magnitude as a function of the input parameters. [Preview Abstract] |
Sunday, November 19, 2006 10:23AM - 10:36AM |
AK.00012: Simulation of acoustic wave propagation in the sun Thomas Hartlep, Nagi Mansour, Mark Miesch By analyzing the oscillations on the surface of the sun, helioseimology is able to infer information about the internal structure, composition, flow structures and magnetic activity in the solar interior. One of the profound achievement of helioseismic inversions has been for instance the mapping of the differential rotation profile of the sun as a function of latitude and radius. Helioseismology is also able to detect active regions on the far-side of the sun - the side facing away from us - before they are directly visible from earth. However, the inversion techniques are in general based on simplified models of solar oscillations which in most cases have not been fully tested. We have develop numerical methods to directly simulate the propagation the acoustic waves in the full solar sphere. In these simulations, we are studying the effects of localized variations in the speed of sound and/or magnetic fields on the acoustic oscillations. We are also performing direct tests of some of the assumptions employed by helioseismology. [Preview Abstract] |
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