Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M5: Waves II: Internal and Interfacial Waves |
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Chair: Bruce Sutherland, University of Alberta Room: 3008 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M5.00001: Generation of Long Internal Waves by Vertically Propagating Compact Internal Wavepackets Bruce Sutherland, Ton van den Bremer The divergence of the horizontal flux of horizontal momentum associated with surface gravity wavepackets results in a horizontal flow that turns out to be identical to the Stokes drift. This ``divergent-flux induced flow'' is itself divergent and so induces a deep response flow whose momentum is equal and opposite to the momentum associated with the Stokes drift. Thus the total momentum is zero. By contrast there is momentum associated with internal wavepackets. Like surface gravity wavepackets, the divergent-flux induced flow of horizontally localized internal waves is itself divergent. However, because the ambient is stratified, and so inhibits vertical motion, there can be no deep return flow. Different from the approach of Bretherton (1969), we follow a physically intuitive but mathematically rigorous quasi-monochromatic wavepacket analysis complemented by fully nonlinear numerical simulations to show that the dominant response is an induced horizontally long internal wave that extends laterally well to either side of the wavepacket. This suggests a new mechanism for efficient energy and momentum transfer from local to long and slow time-scale disturbances that does not involve irreversible deposition through wave breaking. Weakly nonlinear effects are discussed. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M5.00002: On 3D internal wave beams and induced large-scale mean flows T.R. Akylas, Takeshi Kataoka A theoretical model is developed for the 3D propagation of internal gravity wave beams in a uniformly stratified Boussinesq fluid, assuming that variations in the along-beam and transverse directions are of long lengthscale in comparison with the beam width. This situation applies, for instance, to the far-field behavior of a wave beam generated by a horizontal line source with weak transverse dependence. In the 2D case, where only along-beam variations are present, it is known that nonlinear effects are minor, even for beams with finite steepness. By contrast, in 3D, nonlinear interactions can cause transfer of energy to a circulating horizontal mean flow far from the vicinity of the beam. For a small-steepness beam, this process is described by two coupled equations, which govern the 3D beam evolution along with the induced mean flow. This asymptotic model is applied to the experimental setup of Bordes et al. (2012) and qualitative agreement with their observations is found. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M5.00003: Can internal waves descend a double-diffusive staircase? Sasan Ghaemsaidi, Hayley Dosser, Luc Rainville, Thomas Peacock Due to the rapid loss of ice cover, internal waves are expected to play an increasingly important role in the Arctic Ocean. As such, we present the results of a theoretical study investigating the role of double-diffusive layering, characteristic of the Arctic Ocean, on the fate of internal waves. We begin by considering the transmission properties of a single double-diffusive layer, from which we progress to consider multiple layers, and conclude with a realistic stratification. We investigate the possibility that double-diffusive layer structures can be efficient internal wave inhibitors, shielding the deep ocean from the transmission of momentum and energy flux associated with near inertial waves generated by passing storms. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M5.00004: Assessing the importance of internal tide scattering in the deep ocean Maha Haji, Thomas Peacock, Glenn Carter, T.M. Shaun Johnston Tides are one of the main sources of energy input to the deep ocean, and the pathways of energy transfer from barotropic tides to turbulent mixing scales via internal tides are not well understood. Large-scale (low-mode) internal tides account for the bulk of energy extracted from barotropic tides and have been observed to propagate over 1000 km from their generation sites. We seek to examine the fate of these large-scale internal tides and the processes by which their energy is transferred, or ``scattered,'' to small-scale (high-mode) internal tides, which dissipate locally and are responsible for internal tide driven mixing. The EXperiment on Internal Tide Scattering (EXITS) field study conducted in 2010-2011 sought to examine the role of topographic scattering at the Line Islands Ridge. The scattering process was examined via data from three moorings equipped with moored profilers, spanning total depths of 3000-5000 m. The results of our field data analysis are rationalized via comparison to data from two- and three-dimensional numerical models and a two-dimensional analytical model based on Green function theory. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M5.00005: Internal wave penetration into an evanescent layer via parametric subharmonic instability Sasan Ghaemsaidi, Thierry Dauxois, Sylvain Joubaud, Philippe Odier, Thomas Peacock The effect of parametric subharmonic instability (PSI) on the transmission properties of a boundary forced, two-layer density stratification is experimentally studied. In regimes where linear theory simply predicts evanescent decay in the lower layer, PSI creates two daughter waves that are capable of penetrating deep into the stratification in opposing horizontal directions. PSI is shown to be a reasonable mechanism for the injection of energy flux and momentum into an otherwise forbidden lower layer by means of the creation of ``burrowing'' daughter waves of the primary, forced mother wave. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M5.00006: Sensitivity of Rogue Waves Predictions to the Oceanic Stratification Qiuchen Guo, Mohammad-Reza Alam Oceanic rogue waves are short-lived very large amplitude waves (a giant crest typically followed or preceded by a deep trough) that appear and~disappear suddenly in the ocean causing damages to ships and offshore structures. Assuming that the state of the ocean at the present time is perfectly known, then the upcoming rogue waves can be predicted via numerically solving the equations that govern the evolution of the waves. The state of the art radar technology can now provide accurate wave height measurement over large spatial domains and when combined with advanced wave-field reconstruction techniques together render deterministic details of the current state of the ocean (i.e. surface elevation and velocity field) at any given moment of the time with a very high accuracy. The ocean water density is, however, stratified (mainly due to the salinity and temperature differences). This density stratification, with today's technology, is very difficult to be measured accurately. As a result in most predictive schemes these density variations are neglected. While the overall effect of the stratification on the average state of the ocean may not be significant, here we show that these density variations can strongly affect the prediction of oceanic rogue waves. Specifically, we consider a broadband oceanic spectrum in a two-layer density stratified fluid, and study via extensive statistical analysis the effects of strength of the stratification (difference between densities) and the depth of the thermocline on the prediction of upcoming rogue waves. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M5.00007: Internal waves incident upon an interface John McHugh Recent results have shown that a vertical packet of internal waves that are horizontally periodic will develop a discontinuous mean flow at an interface, depending on wave reflection. Here we consider a similar configuration where the waves are not horizontally periodic, but instead exist within a wave packet that is limited both horizontally and vertically. The basic state has constant stability $N$ in two layers without a shear flow. The horizontal limit of the wavepacket results in a much different wave-induced mean flow than the periodic case, as the mean flow is confined to the wavepacket and therefore must have approximately a zero net flow across any vertical surface. The net effect is that gradients of the mean flow at the interface are stronger than the periodic case. Waves are treated with the nonlinear Schrodinger equations that are solved numerically. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M5.00008: Shoaling Large Amplitude Internal Solitary Waves in a Laboratory Tank Michael Allshouse, Conner LaRue, Harry Swinney The shoaling of internal solitary waves onto the continental shelf can change both the wave dynamics and the state of the environment. Previous observations have demonstrated that these waves can trap fluid and transport it over long distances. Through the use of a camshaft-based wavemaker, we produce large amplitude shoaling waves in a stratified fluid in a laboratory tank. Simulations of solitary waves are used to guide the tuning of the wave generator to approximate solitary waves; thus nonlinear waves can be produced within the 4m long tank. PIV and synthetic schlieren measurements are made to study the transport of fluid by the wave as it moves up a sloping boundary. The results are then compared to numerical simulations and analyzed using finite time Lyapunov exponent calculations. This Lagrangian analysis provides an objective measure of barriers surrounding trapped regions in the flow. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M5.00009: Sound propagation through internal gravity wave fields in a laboratory tank Likun Zhang, Harry L. Swinney, Ying-Tsing Lin We conduct laboratory experiments and numerical simulations for sound propagation through an internal gravity wave field. The goal is to improve the understanding of the effect of internal gravity waves on acoustic propagation in the oceans. The laboratory tank is filled with a fluid whose density decreases linearly from the bottom to the top of the tank; the resultant buoyancy frequency is 0.15 Hz. A 1 MHz sound wave is generated and received by 12.5 mm diameter transducers, which are positioned 0.2 m apart on a horizontal acoustic axis that is perpendicular to the internal wave beam. The fluid velocity field, measured by Particle Image Velocimetry (PIV), agrees well with results from simulations made using a Navier-Stokes spectral code. The sound intensity at the receiver is computed numerically for different measured and simulated frozen density fields. Fluctuations in the sound speed and intensity are determined as a function of the location of the receiver and the frequency and phase of the internal waves. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M5.00010: Internal waves patterns in the wake of a 3D body towed in a two-layer fluid Laurent Lacaze, Matthieu Mercier, Olivier Thual, Alexandre Paci Stratified flows over obstacles are important features in meteorology and oceanography. The characterization of these flows is crucial in order to propose models of geophysical processes such as mixing and ocean circulation or orographic drag in the atmosphere. For some specific stratification profiles, the energy of internal waves generated by the obstacle can be trapped at a given depth, at the base of the oceanic mixing layer or at the top of the atmospheric boundary layer for instance. This scenario can be modelled by a two-layer stratified fluid for which gravity waves spread at the interface between the two layers. The work presented here focuses on a two-layer flow over a 3D obstacle, or equivalently, an obstacle towed in a fluid at rest. Experiments performed both in the large-scale flume of CNRM-GAME Toulouse (METEO-FRANCE \& CNRS) and in a smaller tank apparatus, are presented with a specific attention on the measurement of the 3D wave patterns. A non-hydrostatic linear analysis is used to describe the observed wave patterns. The experiments highlight the strong influence of the Froude number on the generated waves. More specifically, we investigate the nature of the wake angle obtained from the wave pattern, and discuss a transition from Kelvin to Mach angle. [Preview Abstract] |
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