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 LG: GFD: Oceanography III |
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Chair: Darek Bogucki, Texas A&M Corpus Christi Room: Long Beach Convention Center 103B |
Monday, November 22, 2010 3:35PM - 3:48PM |
LG.00001: Cross-equatorial flow of Antarctic Bottom Water and the complete Coriolis force Andrew Stewart , Paul Dellar Conservation of potential vorticity strongly constrains large-scale flows in the oceans. It resists fluid crossing the equator, because a large relative vorticity is needed to balance the change in the sign of the planetary vorticity between hemispheres. However, the Antarctic Bottom Water (AABW) successfully crosses the equator deep in the Atlantic off the coast of Brazil. Our theoretical and numerical study of the AABW uses multilayer shallow water equations that include the complete Coriolis force due to the horizontal and vertical components of the Earth's rotation vector. The widely neglected horizontal component is most prominent in the weakly-stratified abyssal ocean at the equator. The horizontal component combines with topography to create an extra term in the potential vorticity that offsets changes in the planetary vorticity with latitude. The observed topography between 33$^\circ$W and 36$^\circ$W is remarkably close to an ideal profile that exactly cancels the change in planetary vorticity. Analytical solutions for steady currents show 50\% increases in transport due to the complete Coriolis force, as confirmed by numerical simulations of unsteady flows using an energy and enstrophy preserving scheme. [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LG.00002: Generation of a jet by a sphere descending in stratified fluids Hideshi Hanazaki , Hiroyasu Yoshikawa , Tohru Okamura Flow around a sphere descending vertically at constant speeds in uniformly stratified fluids is investigated by numerical simulations. As observed in recent salt-stratified tank experiments for strongly stratified fluids at high Schmidt number, vertical thin round jets and ``bell-shaped'' structures along the jets were observed in the lee of the sphere. The bell-shaped structure was found to be the consequence of steady internal waves in the lee of the obstacle, and it was actually generated where the downward velocity was the maximum. Temporal density variation on the rear stagnation point of the sphere and on the vertical symmetry axis of the flow in the lee of the sphere give some hints on the generation mechanisms of the jet, including the significant effects of molecular diffusion of salt. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LG.00003: Boundary Layer Effects on Internal Wave Generation in a Stably Stratified Fluid Lauren Eberly , Julie Vanderhoff Through a series of laboratory experiments we attempt to quantify internal wave generation due to flow over the rough topography of a continental slope. Although significant progress has been made in flow over rough topography, few experimental studies have been done where the topography is oriented at an angle to both the isobaths and flow. Laboratory investigation is critical as linear theory is not completely accurate in describing generated internal waves. The disparity between linear theory and physical observation is greatest when the wave amplitudes reach a critical level or when boundary layer separation occurs. Previous experimental work on bottom topography suggests that linear theory over predicts the amplitude of generated lee waves as it does not account for effects due to boundary layer separation. This study employs a series of experiments to analyze an approximately two-dimensional, stably stratified fluid undergoing tidal flow over a topographically rough, sloped shelf. The laboratory set up utilizes a corrugated slope towed through the fluid as the forcing mechanism behind internal wave generation. The waves are visualized using the Synthetic Schlieren technique. Results show decreased internal wave amplitude from that predicted by linear theory. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LG.00004: Direct numerical simulations of flow over ridges in presence of waves and current Long Chau , Kiran Bhaganagar In this talk we demonstrate Direct numerical simulation (DNS) as a robust and a valid tool to study fundamental physics for coastal problems. We focus on turbulent pulsatile flow over 3-D ridged surfaces, which are relevant for oceanographic problems. We consider different morphological surfaces to explore the differences in turbulence production, dissipative and transport mechanisms. The influence of ridge shape and the pulse frequency on the scaling of the drag is explored. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LG.00005: Turbidity Currents in Meandering Channels Mohamad Nasr-Azadani , Michael Zoellner , Eckart Meiburg We consider continuous, particle-laden gravity currents flowing along sinusoidal submarine channels bounded by levees, with special emphasis on the sediment transport. We investigate these flows via highly resolved three-dimensional direct Navier-Stokes simulations, based on an immersed boundary representation of the channel topography. Results are reported from a parametric study that focuses on shear stress profiles along the channel bed, secondary flow structures in channel cross-sections, lateral overflow over the levees, and sediment deposition, as functions of the channel geometry, the flow parameters, and the particle settling velocity. [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LG.00006: Double-Diffusive and Gravitational Instabilities in Particle-laden River Outflows Peter Burns , Eckart Meiburg When a sediment-laden river flows into the salty ocean, various instabilities may arise. In an initially static environment, these instabilities can be due to either double-diffusive or gravitational effects. As a function of the governing Peclet numbers and the particle settling velocity, we investigate via linear stability analysis under which conditions each instability mode dominates, and when the modes coexist. We find that the settling velocity has a non-monotonic effect on the temporal instability growth rates. While small settling velocities can serve to increase the growth rate of the instability, larger settling velocities are found to reduce the growth rate. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LG.00007: Exact solutions for scattering by a patch of finite amplitude periodic bottom topography Jie Yu , Guangfu Zheng Scattering of water waves by undulating bottom topography commonly occurs in coastal oceans, influencing many processes such as sediment transport and underwater acoustic propagation. In this study, we examine the scattering of water waves by a patch of periodic bottom corrugations with large amplitude, using an exact theory (Howard and Yu, {\em J. Fluid Mech}., vol. {\bf 593}, 2007, pp.209-234). Issues of matching the solutions at the boundaries of corrugated and flat bottom will be discussed. Of particular interest is Bragg scattering, or Bragg resonance, which occurs when the spacing of corrugations is close to an integer multiple $m$ of half a water wavelength. The primary ($m=1$) scattering has been studied for well over two decades, studies of those higher order ($m>1$) cases, however, are few. This will be discussed and the results presented. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LG.00008: Investigation of internal wave amplitude estimates through phase-space ray-tracing Julie Vanderhoff An effective way to track the propagation and refraction of internal waves is by applying the ray, or WKBJ, approximation and performing numerical ray-tracing. Wave amplitudes are easily obtained in a phase-space formulation of the ray-tracing, which also avoids the caustics that are typically present in both the spatial and spectral formulations. However, the resulting phase-space solution must be projected onto the spatial and spectral domains to obtain solutions in those domains. Initial estimates are made to asses the accuracy of these projections using a simple background that varies in one spatial dimension and time. Implications for the short oceanic internal waves, and for cases of higher dimensions, are discussed. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LG.00009: Numerical Simulations of an Asymptotically Reduced Model of Anisotropic Langmuir Turbulence Zhexuan Zhang , Greg Chini , Keith Julien , Edgar Knobloch ``Langmuir turbulence'' is a wind and surface-wave driven flow that is thought to dominate vertical transport and mixing in the ocean surface boundary layer. The characteristic occurrence of quasi-coherent counter-rotating vortical structures elongated in the wind direction renders Langmuir turbulence strongly anisotropic. Recently, an asymptotically exact reduced model of this flow was derived using multiscale analysis (Chini, Julien \& Knobloch, GAFD 2009). The reduced PDEs go beyond strictly 2D (downwind invariant) formulations of the governing Craik--Leibovich (CL) equations by consistently incorporating the dominant 3D physical processes while continuing to average or filter certain fast, fine downwind-scale flow features. Here, pseudospectral numerical simulations of the reduced PDEs are performed to explore the dynamics and bifurcation structure of the reduced model. [Preview Abstract] |
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