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 G23: Geophysical Fluid Dynamics: Internal Waves |
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Chair: Scott Wunsch, John Hopkins University Room: 2001 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G23.00001: Internal Waves Generated by Unsteady Impulsive Forcing - Laboratory Experiments Kara Shipley, Alan Brandt, Matthew Paoletti Internal waves are generated in laboratory experiments using impulsive forcing to further the understanding of unsteady source mechanisms. Impulsive forcing events, unlike steady or periodic forcing, are both transient and broadband, and have been the focus of only a limited number of fundamental studies. The experiments presented here examine the dynamics of the release of a homogeneous heavy fluid into a fluid with a density-stratified layer above a region of constant density. The miscible forcing volume is visualized utilizing fluorescent dye, which allows for measurements of the plume energy flux, while the internal wave field is characterized by measuring the density fluctuations with an array of conductivity probes. In all cases, the uniformly stratified region has a buoyancy frequency of \textit{N} = 1 rad/s, the depth of which varied 12-50\% of the total fluid depth. The descending plume entrains ambient fluid and subsequently rebounds to an equilibrium level. The effects of varying the density and volume of the forcing fluid on the energy flux of the radiated internal waves are presented. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G23.00002: Internal waves generated by unsteady impulsive forcing - numerical simulations Matthew Paoletti, Kara Shipley, Alan Brandt Numerical simulations of the generation of internal waves by an unsteady impulse are presented. While extensive work has examined the generation of internal waves by steady flow, such as winds over mountains, or periodic flow, an example being tidal flow over bathymetry, internal waves can also be generated by transient events like those produced by local instabilities. The studies presented here focus on the generation of internal waves by the release of a patch of miscible fluid of constant density into a stably stratified water column. The fluid descends owing to its initial momentum, spreads in the lateral direction, and vertically displaces the isopycnals, leading to the generation of internal waves. The transfer of energy from the impulse to the internal wave field is characterized by the energy flux of the radiated internal waves. While the impulse is initially axisymmetric, the effects of the three-dimensional nature of the turbulent evolution are examined by comparing the results of two-dimensional and three-dimensional numerical simulations. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G23.00003: Internal Wave Generation in Evanescent Regions Allison Lee, Julie Crockett Internal waves are well known to be generated by flow over topography in regions where the excitation frequency is less than the local buoyancy frequency of the fluid. Linear theory indicates that if the excitation frequency is greater than the buoyancy frequency, then internal waves will be evanescent. These evanescent waves' amplitudes decrease exponentially away from the region of generation. However, under certain conditions, the wave stress generated by flow over topography in a region that is weakly stratified can be transported by evanescent waves into a region of higher stratification, far from the generation region, and internal waves will begin to propagate away. Further exploration of this theory is undertaken with physical experiments performed with varying stratifications and multiple types of topography. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G23.00004: Parametric subharmonic instability (PSI): from internal plane waves to realistic beams Thierry Dauxois, Baptiste Bourget, Sylvain Joubaud, Philippe Odier, H\'el\`ene Scolan We study experimentally the parametric subharmonic instability, which corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. We show how, using a time-frequency analysis and a Hilbert transform, one can characterize precisely the instability. Moreover, we present results showing the crucial importance of the finite width when considering beams. Experiments and numerical results will be discussed in relation with a new theoretical approach. The latter brings new insights on energy transfers in the ocean where internal waves with finite size beams are dominant. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G23.00005: PSI and turbulence during the interaction of the internal wave beam with upper ocean pycnocline Bishakhdatta Gayen, Sutanu Sarkar Three-dimensional numerical simulations are performed to investigate the interaction of a semidiurnal internal wave (IW) beam with the nonuniform stratification of an upper ocean pycnocline. During the initial stage of the interaction, higher harmonics originate after reflection of the IW beam at the caustic and are trapped in the pycnocline, while at later time the incoming beam undergoes a parametric subharmonic instability (PSI) inside the pycnocline, that exhibit exponential growth with a rate of 2/3 day$^{-1}$. During PSI small vertical waves form resulting in wave steepening and produce convective overturns. Convective instability initiates transition to turbulence while shear production maintains it. Turbulence is characterized by examining the temporal evolution of its production and dissipation. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G23.00006: Internal waves incident on a sheared ocean pycnocline Scott Wunsch, Hwar Ku Internal waves are commonplace in the oceans. Near the surface, they interact with a sharply increasing density gradient (pycnocline) as well as near-surface currents. Here, fully nonlinear numerical simulations are used to study internal waves incident on a sheared pycnocline. Linear analysis of the unstable modes of a sheared pycnocline above a stably stratified fluid reveals that two types of instabilities may occur. One is the well-known Holmboe instability, while the other is a longer wavelength Kelvin-Helmholtz mode which couples more strongly to incident internal waves. Both types of instabilities are seen in the simulations, and the nonlinear evolution of each is explored. Possible implications of these results for oceanic internal waves are considered. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G23.00007: Energetics of nonlinear harmonic generation during the incidence of an internal wave beam on a model oceanic pycnocline Anil Aksu, Diamessis Peter, Scott Wunsch An energetic analysis of the interaction of a numerically simulated IWB with a model ocean pycnocline is presented. The focus is on the nonlinear generation of harmonics. The analysis consists of a) monitoring the transfer of the primary beam's energy into higher harmonics along the beam path and b) evaluating how any energy trapped inside the pycnocline is distributed across different wave frequencies propagating within it. The majority of the analysis is performed on a dataset spanning a wide range of pycnocline strengths and thicknesses restricted to an IWB propagating at 45$^{\circ}$ from the horizontal. For such an angle, internal wave refraction is the primary driver of nonlinear harmonic generation. Moreover, all resulting harmonics remain trapped within the pycnocline. Preliminary results from additional simulations with shallower angles of IWB incidence are also analyzed. When the incidence angle is less than 30 degrees, IWB reflection is an additional important mechanism of harmonic generation and lower harmonics are able to radiate back out of the pycnocline. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G23.00008: Harmonic Generation of Internal Waves Reflected from a Slope Bruce Rodenborn, Daniel Kiefer, Hepeng Zhang, Harry L. Swinney Internal wave reflection from a uniform sloping boundary is often analyzed using linear or a weakly nonlinear inviscid theory.\footnote{T. Dauxois and W.R. Young, J. Fluid Mech. {\bf390}, 271-295 (1999)} Under these assumptions for a linearly stratified fluid, Thorpe\footnote{S. A. Thorpe, J. Fluid Mech., {\bf178}, 279-302 (1987)} and Tabaei et al.\footnote{A. Tabaei, T. R. Akylas and K. G. Lamb, J. Fluid Mech. {\bf526}, 217-243 (2005)} derived predictions for the boundary angle where second harmonic generation should be most intense. We previously conducted experiments and simulations that found the angle that maximizes second harmonic generation is given instead by an empirical geometric relationship between the wave beam and boundary angles.\footnote{B. E. Rodenborn, D. Kiefer, H. P. Zhang, and H. L. Swinney. Phys. Fluids, 23(2), 2011.} We used integrated kinetic energy as a measure of beam intensity, but the method of Lee et al.\footnote{Frank M. Lee, M. S. Paoletti, H. L. Swinney, P. J. Morrison. arXiv:1401.2484.} determines the energy flux of the second harmonic wave in the experiments. We compare results using this new method to weakly nonlinear theories and our empirical prediction. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G23.00009: Suppression of tidal conversion by virtual seafloor Harry L. Swinney, Likun Zhang We examine in numerical simulations how the conversion of tidal energy into internal gravity wave energy is suppressed by wave interference between adjacent ridges of steep topography [L.K. Zhang and H.L. Swinney, Phys. Rev. Lett. 112, 104502 (2014)]. Simulations for both periodic and random steep topographies reveal that the time-averaged wave energy radiated upwards arises only from the portion of the ridges above an elevated ``virtual seafloor.'' We find that the average radiated wave power can be predicted by linear theory for weak topography by replacing the actual floor with the virtual floor. The virtual floor concept is used to extend linear theory to predict the energy conversion rate for steep topography. This nonlocal modification of linear theory should be useful in estimating the energy flux generated by tidal flow over the global seafloor. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G23.00010: Determination of internal wave power from synthetic schlieren data Frank M. Lee, Michael Allshouse, P.J. Morrison, Harry L. Swinney Internal waves are generated in the ocean by tidal flow over bottom topography, and they are of considerable interest because of their significant contribution to the energy budget of the ocean. One way of measuring internal waves produced in the laboratory setting is by a technique called ``synthetic schlieren,'' whereby the perturbation density field is obtained from the change in index of refraction in the fluid. However, the usual computation of power requires the velocity and pressure, or under certain assumptions, the stream function [Lee et al., ``Experimental determination of radiated internal wave power without pressure field data,'' Phys. Fluids 26, 046606, (2014)]. We present a method for computing the radiated internal wave power that uses only the perturbation density field, assuming the flow is sufficiently 2-dimensional, and we demonstrate the method using data from simulations and experiments. [Preview Abstract] |
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