Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G1: Geophysical: Oceanographic IV |
Hide Abstracts |
Chair: Matthew Paoletti, University of Texas at Austin Room: 323 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G1.00001: Nonlinear generation of harmonics by an internal wave beam incident on a model oceanic pycnocline: numerical study Peter Diamessis, Scott Wunsch, Ian Delwiche The interaction of an internal wave beam with an idealized oceanic pycnocline is examined using 2-D fully nonlinear direct numerical simulation. These simulations vary the normalized pycnocline thickness and the ratio of peak pycnocline buoyancy frequency to that of the stratified lower layer. The angle of the beam with respect to the horizontal is held fixed at 45 degrees. Harmonics at the point of beam entry into the pycnocline increase in amplitude and number with the maximum gradient of the buoyancy frequency, suggesting refraction as an important factor in harmonic generation. For thin pynoclines, harmonics trapped within the pycnocline have maximum amplitude when their frequency and wavenumber match those of a natural pycnocline interfacial wave mode. For thicker pycnoclines, whose thickness is equal to the incident beam thickness, beam refraction results in harmonic generation at multiple locations in addition to pycnocline entry, giving rise to complex flow structure inside the pycnocline. These results are consistent with weakly nonlinear theory and with recent laboratory experiments. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G1.00002: Nonlinear generation of harmonics by an internal wave beam incident on a model oceanic pycnocline: laboratory experiments Scott Wunsch, Ian Delwiche, Peter Diamessis The interaction of an internal wave beam with an idealized oceanic pycnocline is examined using laboratory experiments. Laboratory results are compared to weakly nonlinear theory for a thin pycnocline, which succesfully predicts their qualitative features. The data show that harmonic modes with multiples of the incident frequency and wavenumber are generated near the point of pycnocline entry. For incidence angles exceeding 30 degrees, all harmonic modes are trapped within the pycnocline. Trapped harmonics appear to be strongest when their frequency and wavenumber match those of a natural pycnocline interfacial wave mode. For smaller incidence angles, the first harmonic radiates away from the pycnocline. These conclusions are supported by recent 2D numerical simlations. They also may be relevant to internal wave harmonic generation recently observed in the South China Sea and to the local generation of pycnocline internal solitary waves observed in the Bay of Biscay and elsewhere. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G1.00003: Mixing by internal gravity waves that break at sloping topography Vamsi Chalamalla, Sutanu Sarkar Direct and large eddy simulations are performed to study the near-bottom mixing that occurs during the interaction of internal waves with a critical slope. The pathway from the input wave energy to the irreversible mixing of density field is explored. Diagnostics such as the turbulent kinetic energy budget and the density variance budget are discussed to explain the phasing of turbulence and associated mixing. Background and available potential energies are utilized to differentiate irreversible mixing from the reversible buoyancy flux. Mixing efficiency in all the simulated cases is found to be much higher than the frequently used value of 0.2 especially during large convective overturns. The ratio of Ozmidov and Thorpe length scales averaged over various sections of a wave cycle is investigated to assess inferences of turbulent dissipation rate from the Thorpe length scale. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G1.00004: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 8:52AM - 9:05AM |
G1.00005: Resonant boundary currents from tidal flow over topography need not generate intense internal waves Harry Swinney, A. Dettner, M.S. Paoletti The relationship between boundary currents generated by tidal flow over topography and the radiated internal wave power is examined in two-dimensional numerical simulations for a uniformly stratified fluid. The radiated power and the kinetic energy density of the boundary currents are computed as a function of the internal wave slope and the criticality parameter (ratio of the maximum topographic slope to the internal wave slope). We consider cases where the hydrostatic approximation is valid as well as test theoretical predictions for models of the deep ocean where the beam slope diverges and thus the hydrostatic approximation fails. We confirm that resonant boundary currents with large kinetic energy densities form over critical topography. However, this resonance phenomenon does not extend to the power radiated by the internal waves that propagate away from the topography. The conclusion is that the kinetic energy density in the boundary currents cannot be used as a proxy to characterize the conversion of tidal energy to radiated internal wave power. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G1.00006: Internal wave generation by tidal flow over topography in the deep ocean Matthew S. Paoletti, Matthew C. Drake, Harry L. Swinney We present experimental and numerical studies of internal wave generation by tidal flow of an exponentially stratified fluid over bottom topography in a model of the deep ocean. King et al. (J. Geophys. Res. 117, C04008 (2012)) recently found many locations in the deep ocean where the stratification becomes so weak that the buoyancy frequency (proportional to the square root of the density gradient) becomes less than the tidal frequency; below such turning depths internal waves decay exponentially. Prior studies predict that topography beneath a turning depth would be unable to extract power from tidal motions and convert it to internal waves.~ However, we find that tidal motions over topography beneath a turning depth radiate internal waves, although the power is greatly diminished compared to cases of stronger stratification.~ We recover prior predictions of the radiated power~by averaging the nonuniform stratification over an effective height. In the absence of a turning depth, the effective height is given by the actual topographic height, but for weak stratification where there is a turning depth, the effective height monotonically increases with turning depth height until it encompasses the entire fluid depth for very weak stratification.~ [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G1.00007: Determination of internal wave energy fluxes without pressure Frank M. Lee, M.S. Paoletti, Harry L. Swinney, P.J. Morrison 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. However, the determination of the energy flux from real world data is difficult because knowledge of both the perturbation velocity and pressure fields is required. While the velocity perturbation field can be measured, e.g., by Particle Image Velocimetry, it is difficult to obtain simultaneous precision measurements of the pressure perturbation field. We propose a straightforward computational method to circumvent this difficulty for flows that are sufficiently 2-dimensional: the energy flux can be determined using only velocity data in the complete absence of pressure field knowledge. We demonstrate our method using laboratory and simulation data, and find that the conversion rate calculated from the flux field determined by the proposed method can be accurate to within 3 percent. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G1.00008: Effect on $Ex$ on internal waves created by tidal flow over near-critical topographic features Masoud Jalali B., Narsimha R. Rapaka, Sutanu Sarkar Topographic bumps with small horizontal length under energetic surface tides with large velocity lead to internal tide generation in a regime with O(1) values of the excursion number, $Ex$, the ratio of fluid tidal advection to the topographic length scale. DNS is performed for a smoothed triangular ridge to study how internal gravity waves and turbulence change when $Ex$ is varied from a low to O(1) values, keeping the Reynolds number constant. The near-field internal wave field looses it beam like character with increasing values of $Ex$. Analysis of the baroclinic energy shows significant reduction in the radiated wave flux higher $Ex$ cases owing to a substantial rise in advection and baroclinic dissipation. There is small change in energy conversion consistent with the linear approximation. Turbulence changes qualitatively with increasing $Ex$. When $Ex \sim 0.1$, turbulence is intensified at the near-critical regions of the slope, and is also significant in the beams adjacent to the top of the ridge. However, at $Ex \sim 1$, turbulence is confined a narrow boundary region spanning the ridge and the adjacent flat bottom. The size of the turbulent overturns increases with increasing $Ex$ until $Ex \sim 0.5$, followed by a substantial decrease. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G1.00009: Energy Flux of Transmitted and Reflected Internal Waves Prajvala Kurtakoti, James Munroe As a preliminary step towards understanding how internal waves reflect off sloping topography such as the continental shelf, we performed a series of laboratory experiments to study the energy flux of propagating and reflecting internal waves in a continuously stratified salt water fluid. The internal waves are generated by a wave generator that is capable of producing monochromatic, vertically trapped waves. These internal waves propagate along the length of the tank ($\sim$5m) and reflect. The structure and amplitude of the internal waves are measured using a technique called ``synthetic schlieren'' that also enables us to measure the energy in the wave. We examine how the vertical displacement amplitude and energy flux change as we change the frequency of the wave generator and the stratification of the fluid. Using Hilbert transform we have separated the waves traveling right to the waves traveling left enabling us to understand how much energy from the incoming waves is present in the reflected internal waves. The analysis of the energy flux of the internal waves during propagation and reflection using the Hilbert transform is helpful as it brings insight into phenomena that are difficult to observe during field studies. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G1.00010: Subharmonic instability of locally confined internal wave beams Hussain Karimi, T.R. Akylas Oceanic internal wave beams are central to tidal conversion---the transfer of tidal energy to internal waves by the interaction of the barotropic tide with sea-floor topography---a process believed to be important in deep-ocean mixing. There is evidence from recent experiments and numerical simulations that instability of internal wave beams is triggered by nonlinear interactions with fine-scale subharmonic disturbances. Motivated by these findings, we study analytically resonant triad interactions of a locally confined wave beam with small-amplitude, fine-scale, subharmonic disturbances for the purpose of discovering the conditions under which wave beams become unstable. A primary concern is to understand the dependence of the instability growth rates on beam amplitude and width. Furthermore, the effect of the Earth's rotation is investigated and possible resonant configurations are identified. [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. |
© 2024 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
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700