Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session AG: GFD: Oceanography I |
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Chair: Julie Crockett Vanderhoff, Brigham Young University Room: Long Beach Convention Center 103B |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AG.00001: Internal wave emission by a stratified turbulent wake with non-zero net momentum Ammar Abdilghanie, Peter Diamessis The internal waves emitted by the stratified turbulent wake of a towed sphere are simulated using a 3D fully nonlinear spectral code in a parallel computing environment at two Reynolds numbers: 5,000 and 100,000 and three Froude numbers: 4, 16, and 64. The 2D Arc wavelet is used to extract the resonant horizontal scales from the horizontal divergence field on horizontal planes above the wake center line. Wave packets with length scales comparable to the sphere diameter are emitted from the wake with a decay rate increasing with both Froude and Reynolds numbers. The length scales increase with increasing Froude numbers and decreasing Reynolds numbers. Azimuth angles obtained from the Morlet2D wavelet are highly concentrated around 60 deg. Analysis of time series using 1D wavelet transforms reveals nearly constant frequencies (corresponding to polar angle 30$\pm$2 deg.) at the low Reynolds number simulations. At the high Reynolds number the polar angles are much higher (45-60 deg.) and slowly decay over time. Finally, wave steepness increases with both Reynolds and Froude numbers. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AG.00002: Internal gravity wave absorption and reflection in a non-uniformly stratified Boussinesq fluid and subcritical Richardson number Michael Richter, Ammar Abdilghanie, Peter Diamessis 2-D numerical simulations of the reflection and absorption of internal gravity waves in a non-uniformly stratified Boussinesq fluid are reported. The stratification profile combines a surface mixed layer separated from a uniformly stratified bottom layer (where waves are generated) by a sharp hyperbolic tangent pycnocline. The role of the incident wave steepness, the ratio of the vertical wavelength to the pycnocline thickness, and the peak pycnocline stratification strength in the reflection and absorption of the internal waves is studied. A hyperbolic tangent velocity profile collocated with the stratification profile is then introduced. The shear profile is such that a critical level exists inside the pycnocline and the gradient Richardson number is subcritical. Finally the influence of the stability of the shear flow on the wave reflection and absorption is appraised. Simulations are performed with and without an externally destabilized shear layer. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AG.00003: The fluid mechanics of oil released into deep water Andrew W. Woods We present a series of models which describe the processes controlling the transport of oil through the water column from a deep-water release, accounting for the presence of dispersant which may cause the oil to break up into small droplets. We compare the model predictions with recent observations from the Gulf of Mexico in order to provide insights and constraints on the migration of oil through deep-water towards the surface. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AG.00004: Non-invasive turbulent mixing across a density interface in a turbulent Taylor-Couette flow C.P. Caulfield, Andrew W. Woods, J.R. Landel, A. Kuesters We present experimental measurements of the turbulent transport of salt across an interface between two layers of fluid of different salinities, confined to a cylindrical annulus with gap $L$ where the inner cylinder rotates to produce an approximately irrotational mean azimuthal flow, with narrow boundary layers. We focus on the limit of high Richardson number flow, defined as $Ri=g \Delta \rho H /(\rho_0 u_{rms}^2)$ where $\rho_0$ is a reference density, $\Delta \rho$ is the time-dependent difference of the layers' mean densities, $u_{rms}$ is the rms of the turbulent velocity fluctuations and $H$ is the layer depth. The mean flow has $Re \sim 10^4-10^5$, and the turbulent fluctuations in the azimuthal and radial directions have rms speed of order $10\%$ of the mean azimuthal flow. The interface between the two layers remains sharp, each layer remains well-mixed, and the vertical flux of salt between the layers, ${F}_s \sim (1.15 \pm 0.15) Ri^{-1}{\cal A}(H/L) u_{rms} \Delta S$, where $\Delta S$ is the spatially-averaged time-dependent salinity difference between the layers and ${\cal A}(H/L)$ is a function of the aspect ratio. The salt transport appears to be caused by turbulent eddies scouring and sharpening the interface and implies a constant rate of conversion of the turbulent KE to PE, independent of the density contrast between the layers. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AG.00005: Ocean Circulation Modeling Using Adaptive Wavelet Collocation Method Shanon Reckinger, Oleg Vasilyev The adaptive wavelet collocation method is applied to basin-scale, wind-driven ocean circulation models. This method solves the governing equations on temporally and spatially varying meshes, which allows higher effective resolution to be obtained with less computational cost. The grid adaptation is achieved by using the ability of wavelet multiresolution analysis to identify and isolate localized dynamically dominant flow structures, e.g., vortices, and to track these structures on adaptive computational meshes. In addition to studying how various ocean models behave on non-uniform, time varying grids, this work also sets out to improve the representation of continental topology and bottom bathymetry through an extension of the Brinkman penalization method. Due to the complicated geometry inherent in ocean boundaries, the stair-step representation used in the majority of current global ocean circulation models causes accuracy and stability problems. Brinkman penalization is a numerical technique used to enforce no slip boundary conditions through the addition of a term to the governing equations. When coupled with the adaptive wavelet collocation method, the flow near the boundary can be well defined. This is especially useful for simulation of boundary currents. Therefore, the Gulf stream and western boundary currents have been the focus of the work presented here. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AG.00006: Turning points for semidiurnal (M2) internal tides in the deep ocean Benjamin King, Mark Stone, Hepeng Zhang, Michael Marder, Harry Swinney, Robert Scott Previous work has mentioned the possibility of ``turning points'' in the deep ocean, depths at which the local buoyancy frequency $N(z)$ becomes smaller than the lunar semidiurnal (M2) tidal frequency: $N(z)<\omega_{M2}$) [W. Munk, Evolution of Physical Oceanography, MIT Press (1981)]. At these hypothetical locations, incident M2 internal tides would reflect from the turning points, resulting in regions in the deep ocean that are off limits to M2 internal tides. We have conducted the first systematic search for turning points by analyzing CTD (conductivity, temperature, depth) data obtained at 18,000 locations as a part of the World Ocean Circulation Experiment (WOCE), to determine $N(z)$ on a global scale. We have found that turning points are common in the deep ocean. We also present numerical simulations of internal wave beam interactions with turning points, and solutions of the vertical mode eigenvalue problem to determine what effects turning points might have on both internal wave beams and vertical modes. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AG.00007: Langmuir circulation in the presence of lateral density gradients Greg Chini, Ke Li Comparably little is known about the impact of lateral density gradients (associated with, e.g., submesoscale fronts) on Langmuir circulation in the ocean surface mixed layer. Here, 2D pseudospectral numerical simulations of the laterally stratified Craik--Leibovich (CL) equations are performed to elucidate the effect of an imposed horizontal density gradient on Langmuir cells. The dominant mode of instability consists of counter-rotating cells with up- and downwelling jets inclined to the vertical. Linear stability analysis confirms that although no instability occurs in the absence of the CL vortex torque, the dominant instability mode exhibits growth rates exceeding those realized in a constant density fluid. An energy budget is used to gain insight into the physics of this cooperative instability. The fully nonlinear simulations reveal a secondary instability, in which the tilted cells are laterally sheared, and a subsequent energy cascade to fine scales. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AG.00008: Directional spreading of surface waves by wind-wave interaction Sang Soo Lee, David Wundrow A transition process from long-crested to short-crested wind- driven surface waves was analyzed using a first-principles- based asymptotic method. It is shown that a nonlinear interaction between wind and a surface wave, that initially grows linearly, can generate higher spanwise harmonics whose spanwise wave numbers are integer multiples of the primary wave. The amplitudes of the nonlinearly generated spanwise harmonics are of the same order as the primary fluctuation in the air and can be as large as the primary wave in the water. The mean wind is two-dimensional and there is no mean current. The primary wave can start as a single wave that propagates obliquely to the wind direction. The spanwise harmonics are generated by the nonlinear interaction in the air critical layer. They then induce corresponding perturbations in the water. Even though the magnitude of the primary surface wave is small, it generates spanwise harmonics of equal amplitude which lead to multi-directional water wave field. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AG.00009: Internal tide scattering (and generation) by arbitrary two-dimensional topography in arbitrary stratifications Thomas Peacock, Manikandan Mathur, Glenn Carter The generation and scattering of internal tides plays an important role in the energetics of the ocean. We have advanced the analytical Green function method to handle generation and scattering of internal tides by arbitrary two-dimensional topography in arbitrary stratifications. This provides a very useful tool for both fundamental studies of internal tide processes and for making reasonable predictions at important geophysical locations, such as the Hawaiian Ridge. Here, we give an overview of the method and present some fundamental and geophysical results obtained using it. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AG.00010: ABSTRACT WITHDRAWN |
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