Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H01: Internal and Interfacial Waves |
Hide Abstracts |
Chair: Morris Flynn, University of Alberta Room: 2A |
Monday, November 25, 2019 8:00AM - 8:13AM |
H01.00001: Dominant Resonance in Parametric Subharmonic Instability of Internal Waves Reza Alam, Yong Liang, Louis-Alexandre Couston, Qiuchen Guo Parametric Subharmonic Instability (PSI) is one of the most important mechanisms that transfer energy from tidally-generated long internal waves to short steep waves. Breaking of short waves results in diapycnal mixing through which oceanic abyssal stratification is maintained. It has long been believed that PSI is strongest between a primary internal wave and perturbative waves of half the frequency of the primary wave. Here, we show that this is not the case. Specifically, we show that neither the initial growth rate nor the maximum long-term amplification occur at the half frequency, and demonstrate that the dominant subharmonic waves have much longer wavelengths than previously thought. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H01.00002: Resonance and transmission of axisymmetric internal wave modes Philippe Odier, Samuel Boury, Thomas Peacock To date, axisymmetric internal wave fields, relevant to atmospheric waves generated by storm cells and oceanic near-inertial waves produced~by surface perturbations, have been experimentally produced using an oscillating sphere or torus as the source. Here, we use a wave generator configuration capable of exciting axisymmetric internal wave modes of arbitrary radial form. The efficiency of the wave generator is measured through careful estimation of the wave amplitude based upon group velocity arguments, and the effect of vertical confinement is considered to induce resonance, identifying a series of experimental resonant peaks agreeing well with theoretical predictions. In the vicinity of resonance, the waves undergo a transition to nonlinear behavior. In a second step, we investigate transmission of these modes in~nonlinear~stratifications. Two configurations are studied: in the case of a free incident wave, a transmission maximum is found in the vicinity of evanescent frequencies. In the case of a confined incident wave, resonant effects lead to enhanced transmission rates from upper to lower layer. We consider the oceanographic relevance of these results by applying them to an example oceanic stratification~from the Arctic, finding that there can be real-world implications. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H01.00003: Numerical Simulations of the Internal Waves Produced by a Submerged Body in a Stratified Fluid Laura Brandt, Devin Conroy, James Rottman We attempt to gain some insight into the modeling of internal waves produced by a submerged body traveling horizontally at high Reynolds number in a strongly stratified fluid by comparing numerical simulations with linear theory. Two types of internal waves are generated by the horizontal motion of a body in a stratified fluid: lee waves, which are steady in a reference frame moving with the body and are generated by the motion of the body itself, and wake waves, which are unsteady and generated by the turbulent wake. Traditionally the lee waves have been represented in linear theories by a source singularity in the continuity equation, but recently it has been argued that a body force needs to be added to the momentum equations in order to accurately represent the lee waves. We test this latter hypothesis directly by comparing linear theory with our numerical simulations in which either a free-slip or a no-slip boundary condition at the surface of the body is imposed. The free-slip boundary condition represents a body with no downstream recirculation region, so the source singularity should be accurate, and the no-slip boundary condition would have a downstream recirculation region, so that the body force should be necessary. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H01.00004: Linear and nonlinear fate of an axisymmetric inertial wave attractor Samuel Boury, Thierry Dauxois, Sylvain Joubaud, Philippe Odier, Evgeny Ermanyuk, Ilias Sibgatullin For a few decades now, numerous studies have been devoted to the properties of inertia-gravity wave reflection. Since the angle of propagation of these waves is set by the ratio of their frequency to the buoyancy or rotation frequency, the reflection on a wall does not follow the usual Snell-Descartes law. In particular, in a confined trapezoidal domain, the wave beam experiences a focusing effect and eventually ends on a limit trajectory called attractor. Experimental and numerical studies have shown evidence of this structure for internal waves in 2D geometry. Due to the local energy focusing, nonlinear triadic cascades occur in the branches of the attractor, leading to energy transfer between scales. More recently, geometric and 3D aspects of internal wave attractors have been explored using Direct Numerical Simulations of inertial waves. The DNS pictured an axisymmetric inertial wave attractor, in a trapezoidal cylindrical domain, with focusing and defocusing effects caused by wave reflections and by the radial geometry itself. Wave instability occurs while forcing the attractor and leads to an azimuthal symmetry breakdown. Using an apparatus relevant for axisymmetric wave generation, we produce an inertial wave attractor in a cylindrical domain and we explore its properties. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H01.00005: Modal decomposition of polychromatic internal wave fields in arbitrary stratifications Morris Flynn, Alexis Kaminski Internal waves e.g.~those produced by tidal sloshing over bathymetry play a crucial role in the energetics of the oceanic overturning circulation. Understanding their spatial and temporal structure, which depend on both the details of the forcing topography and the forcing frequency, is essential in predicting where mixing may occur, details of which remain poorly understood. Past work has largely focused on the case of a monochromatic wave-field; however, tides are composed of multiple frequency constituents. Here we present an approach by which the modal structure of a polychromatic internal wave-field may be computed from velocity data without any {\it a-priori} knowledge of the details of the forcing topography. We consider wave-fields in both uniform and vertically-varying stratification, and show using synthetic data that our approach is able to accurately reconstruct the vertical mode strengths. The sensitivity of our approach to noise and vertical resolution is also examined. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H01.00006: Instabilities of finite-amplitude locally confined internal wave beams: theory and experiment Boyu Fan, Triantaphyllos Akylas The instabilities of spatially monochromatic internal waves have been well-studied in the past decades, owing to their fundamental nature and relevance to geophysical processes. On the other hand, internal wave beams with locally confined spatial profile have only recently gained appreciation for their distinct properties and instability mechanisms. Such wave beams provide a more realistic setting for the study of internal wave instabilities in the ocean as they naturally arise, for instance, from the interaction of the barotropic tide with bottom topography. Owing to their additional complexity, internal wave beam instabilities have thus far been analyzed primarily in weakly nonlinear contexts, with the most famous being the parametric subharmonic instability. Here, we instead investigate the stability of finite-amplitude internal wave beams to two-dimensional perturbations using Floquet theory. We then compare these theoretical results with novel experimental observations. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H01.00007: Experimental results on the long-time spatial and temporal development of Triadic Resonance Instability Katherine Grayson, Stuart Dalziel, Andrew Lawrie Various mechanisms have been cited as an explanation of how internal waves transfer energy across the wavenumber and frequency spectra and eventually contribute to small-scale turbulence and mixing. Triadic Resonance Instability (TRI) has become increasingly recognised as one of these methods. This talk will focus on new experimental investigation into the temporal and spatial evolution of this instability. Experiments have been conducted using a new generation of wave maker, featuring a flexible horizontal boundary condition which is driven by an array of independently controlled actuators. This allows for varying amplitude, frequency and wavelength in both the spatial and temporal domain. We present results utilising this ability to vary the forcing parameters during the course of an experiment and investigate what effects this has on the evolution of TRI. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H01.00008: Large internal waves in deep water: models, numerics and experiments Roberto Camassa, Richard McLaughlin, Pierre-Yves Passaggia, Colin Thomson We investigate the propagation of large solitary internal waves in continuously salt-stratified water close to homogeneous two-layer configurations, when the lower, denser layer is much deeper than the one at the top. Experiments are performed in the UNC wave tank, with data collected via PIV and LIF, from both fixed locations and by a cart moving with the waves. Predictions from an optimized two-layer fully nonlinear model are compared with the experiments and direct numerical simulations of Euler equations in two dimensions. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H01.00009: Joint effects of topography and rotation in internal solitary waves Karl Helfrich, Lev Ostrovsky An asymptotic, adiabatic theory for the evolution of an internal solitary wave governed by the variable-coefficient, rotation-modified Gardner equation (Korteweg-de Vries with cubic nonlinearity) is developed and used to explore the joint effects of variable topography and rotation on wave evolution. In particular, we explore the interplay between different singularities: terminal damping of the solitary wave due to radiation of inertia-gravity waves, the disappearance of quadratic nonlinearity, and, in the case of a two-layer stratification, the propagation toward and ``internal” beach (zero lower layer depth). Examples of the adiabatic evolution of a single solitary wave are compared to full numerical solutions of the rotating-Gardner equation. These results are also compared to those from an earlier study of the rotating-KdV equation (Ostrovsky and Helfrich 2019, {\it JPO} {\bf 49}). The effects of quadratic bottom drag will also be discussed. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H01.00010: Examining the initial development of convective instability in a three-dimensional shoaling internal solitary wave of depression in over gentle slopes Gustavo Rivera-Rosario, Peter Diamessis A convectively unstable internal solitary wave (ISW) of depression, shoaling over gentle slopes (\textless 3{\%}), is examined through fully nonlinear and non-hydrostatic simulations. These simulations are based on a high resolution/accuracy deformed spectral multidomain penalty method flow solver. The convective instability occurs due to a sudden decrease in the propagation speed, below the maximum horizontal wave induced velocity; the wave retains its nearly symmetric shape as it shoals. Subsequently, an unstable region develops characterized by the entrapment of heavier-over-light fluid, in the form of a recirculating, or trapped, core. The preceding convective instability is attributed to the stretching of the near-surface vorticity layer of the baroclinic background current from the propagating ISW. According to recent field observations in the South China Sea, this region may persist for more than 10 km and drive turbulent-induced mixing, estimated to be up to four times larger than that in the open ocean. Motivated by such observations, emphasis in this presentation is placed on the onset of the 3D convective instability as the ISW shoals. The ISW propagates in the normal-to-isobath direction. The initial 3D instability is visualized via a transitional structure that develops in the lateral direction. The evolution of the lateral instability is compared with the convective overturn of the core. As such, a preliminary understanding of the formation of recirculating cores in ISWs is obtained. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H01.00011: Propagation and Breaking of Three-Dimensional Boussinesq Wave Packets without and with Rotation Alain Gervais, Quinlan Ede, Gordon Swaters, Ton Van Den Bremer, Bruce Sutherland Internal gravity waves (IGWs) propagate vertically and horizontally within stably stratified fluids. As IGWs propagate vertically, nonlinear processes lead to instabilities that may cause a wave to overturn and eventually break, thus irreversibly depositing momentum to the background flow. Even before breaking, moderately large amplitude IGWs induce a mean flow that interacts nonlinearly with the waves, Doppler-shifting their frequency and altering the height at which the waves would have otherwise overturned. Here we derive explicit formulae for the induced flows of localised wavepackets influenced by Coriolis forces. Numerical simulations are initialised with quasi-monochromatic wave packets with the predicted induced flow superimposed. Simulations with small amplitude wave packets confirm that the prediction captures the induced flow. In simulations with larger amplitude waves, the nonlinear interactions between the waves and the flows they induce result in much lower overturning heights than predicted by linear theory. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H01.00012: Wave focusing and related multiple dispersion transi- tions in plane Poiseuille flows Federico Fraternale, Gabrielle Nastro, Isae Supaero, Daniela Tordella Motivated by the recent find- ing of a dispersive-to-nondispersive transition for linear waves in shear fluid flows, we accurately explored the wavenumber-Reynolds number parameters space in the limit of long waves. We discovered the ex- istence of regions having different dispersion and propagation features than their surroundings. These regions look like niches tilted by 45◦ in the log-log space and are nested in the dispersive, low-wavenumber, part of the map. This complex dispersion-propagation structure allows to quantitatively explain the focusing of different components of a wave- packet in sub-regions of the physical space and, as a consequence, the morphology of the wave-packet. In particular, the arrowed shape and the spatial spreading rates are described. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2020 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700