Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KL: Geophysical: Oceanographic I |
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Chair: Julie Vanderhoff, Brigham Young University Room: Salt Palace Convention Center 250 F |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KL.00001: Ocean Modeling Based on Low-Dimensional Concepts Battalgazi Yildirim, Chryssostomos Chryssostomidis, George Karniadakis Proper Orthogonal Decomposition (POD) was applied to the outputs of three different regional ocean models (HOPS, ROMS and FVCOM) for Massachusetts Bay, New Jersey coast, and gulf of Maine, respectively, for both short (several days) and medium range (several months) simulations. The POD energy spectra showed that the simulated dynamics of all regional ocean models is (very) low-dimensional. Furthermore, the optimal sensor placement in the ocean was investigated within the framework of Gappy POD; it was found that the extrema of spatial POD modes are best sensor locations. With this knowledge, the ocean's properties based on the POD modes with few measurements was reconstructed with an error of about 1{\%} for a 3-day forecast and an error of about 10{\%} for a 47-day forecast. Finally, it was shown that the larger magnitude of POD modes and large uncertainty values are related. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KL.00002: A laboratory model of vertical ocean circulation driven by mixing John Whitehead A model of deep ocean circulation driven by turbulent mixing is produced in a rectangular laboratory tank. Salinity difference substitutes for thermal difference of surface forcing. Fresh water gently flows in at the top of one end, dense water enters at the other end, and an overflow in between removes the same amount of water. A mixing rod, extending from top to bottom of the tank, mixes near the fresh water source by traveling back and forth at constant speed (Reynolds number $>$500). A stratified upper layer deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (``deep-ocean overflow'') from the dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper layer deepening. Recirculation from plume entrainment has a volume flux greater than the sources. Over a wide range of parameters there is approximate agreement with a scaling theory. Also, the rate of potential energy increase from mixing equals potential energy rate of decrease from the plume. Many of these features are found in our ocean. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KL.00003: Transport routes in the North-Western Mediterranean Sea: a dynamical system perspective Ana Maria Mancho, Stephen Wiggins, Emilio Hernandez-Garcia, Vicente Fernandez Vortices are a well studied ocean structure. Frequently they are long lived, and water trapped inside can maintain its properties for long time, being transported with the vortex. Jets and strong currents are also important ocean features. They can be rather persistent and, as it is difficult for particles to cross them, water at both sides can keep different physical properties. In this presentation these two relevant eulerian structures are identified at the surface velocity field of a realistic model of the Western Mediterranean Sea. We show that tools coming from the dynamical systems theory such as hyperbolic trajectories, stable and unstable manifolds and lobe dynamics are also at work in this non idealized context and supply detailed information by locating volumes of water particles that evolve in time escaping from the interior of the eddy or crossing the current or doing both things one after the other. A close link between abstract concepts such as lobes and transported scalar quantities such as temperature or salt is found. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KL.00004: Distinguished trajectories in time-dependent flows Jos\'e Antonio Jim\'enez Madrid, Ana Maria Mancho The theory of dynamical systems has provided recently a good framework to describe transport in time dependent aperiodic flows. It was first applied to Lagrangian transport in the context of 2D time-periodic flows and stationary 3D flows. Recently these techniques have been extended to describe aperiodic flows. Mathematical theory for aperiodic time dependent flows is far from being completely developed. In the context of stationary flows the idea of {\em fixed point} is a keystone to describe geometrically the solutions. It is extended to time periodic flows, as periodic orbits become fixed points on the Poincar\'e map. Recent articles by Ide {\em et al.} and Ju {\em et al.} provide an important step-forwards to extend the concept of hyperbolic fixed point to aperiodic dynamical systems. Following these ideas, we propose a new formal definition of {\em Distinguished trajectory} (DT) in aperiodic flows. We numerically test this definition in forced Duffing type flows with known exact distinguished trajectories. The definition accurately locates these trajectories. We also check the defintion for examples of aperiodic flows in oceanographic contexts and we find that it overcomes some technical difficulties of former approaches. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KL.00005: Numerical and observational investigation of small-scale wave interactions with time-dependent shears in the ocean. Julie Vanderhoff, James Rottman, Keiko Nomura, Rob Pinkel We study the interactions of internal waves in a realistic ocean environment using ray theory and numerical simulations by following an initial spectrum of short waves as they propagate through near inertial waves. We also analyze observational data taken on the stationary Floating Instrument Platform over Kaena Ridge, Hawaii as a part of the Hawaiian Ocean Mixing Experiment. We are looking for signs that an interaction is occurring between small-scale, high-frequency waves and the inertial shear. Then we relate the observational conclusions to the results of the ray theory and numerical simulations. A strong coherence between the inertial shear and the strain rate field is found in all three methods of analysis, showing the short waves are being affected by the inertial wave. An analysis of the triple product of the Reynold's stress and inertial shear shows the short waves tend to have a net transfer of energy to the inertial shear. Calculating short wave overturning shows that when the short waves strongly refract in the inertial wave they may take enough energy from the inertial wave to break. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KL.00006: High Reynolds number simulations of a stratified bottom Ekman layer John Taylor, Sutanu Sarkar Large eddy simulations of a stratified benthic Ekman layer over a non-sloping, rough seafloor have been carried out at a realistic Reynolds number ($Re_*=10^6$) and Richardson numbers ($Ri_*=N^2/f^2=0-5625$). In order to achieve high Reynolds number simulations, a near-wall model has been used with a novel adaptive stochastic forcing component added to compensate for known deficiencies of the sub-grid scale model. The primary influence of the outer layer stratification is to limit the thickness of the bottom mixed layer and increase Ekman veering, while the drag coefficient only increases slightly. When the outer layer is stratified the mean velocity profile deviates from the unstratified log law, even in the well-mixed layer near the wall; the consequences for how bottom stress and dissipation rates are inferred from field data will be discussed. Above the pycnocline, a region with local shear instabilities and enhanced mixing is observed. Turbulence-generated internal waves radiate away from the boundary layer and their properties will be examined. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KL.00007: Coupled fluid-sediment model of wave bottom boundary layer Joseph Calantoni, Donald Slinn, Todd Holland The desire to develop predictive models for nearshore bathymetric evolution necessitates a better understanding of the physics of the fluid-sediment wave bottom boundary layer (WBBL). For example, an incomplete description of the effect of fluid-sediment interactions on near bed turbulence hinders our ability to improve parameterized models for both bedload and suspended load transport. We have developed a small-scale model that performs a coupled, direct numerical simulation of both the fluid and sediment in a two-phase WBBL. The fluid phase solves a modified version of the Navier Stokes equation on a fixed grid that accounts for the presence of particles through mass conservation and momentum exchange. The sediment phase is a DEM that resolves the Lagrangian motions of every sediment particle in the simulation domain. Coupling allows preliminary simulations to focus on the role of turbulent suspension in particle saltation dynamics. Direct comparisons between model predictions and existing measurements from a laboratory U-tube for sediment concentration and velocity within the active particle layer will be presented. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KL.00008: Channel formation by turbidity currents: Navier-Stokes based linear stability analysis Brendon Hall, Eckart Meiburg, Ben Kneller The linear stability of an erodible sediment bed beneath a turbidity current is analyzed, in order to identify mechanisms for the formation of longitudinal channels. Based on the Navier-Stokes equations, the analysis accounts for the coupled interaction of the three-dimensional fluid and sediment motion with the erodible bed. For instability to occur, the suspended sediment base concentration profile needs to decay more slowly away from the sediment bed than the base flow shear stress. This destabilizing effect of the base flow is modulated by the stabilizing perturbation of the suspended sediment concentration, and by the shear stress due to a secondary flow in the form of counterrotating streamwise vortices. These are stabilizing for small Reynolds numbers, and destabilizing for large values. For a current height of 10m, we obtain a most amplified wavelength of about 250m, which is consistent with field observations. In contrast to previous analyses based on depth-averaged equations, the instability mechanism identified here does not require any assumptions about sub- or supercritical flow, nor does it require the presence of a slope. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KL.00009: Gulf-Stream Separation and the Modeling of Subgrid Scales B.T. Nadiga The formation of the cyclonic Northern Recirculation Gyre (in the absence of direct wind forcing) north of the Gulf-Stream is essential to the separation of the Gulf-Stream at Cape Hatteras. A poor representation of this process in ocean models is a reason for why the models have a difficult time getting Gulf-Stream separation right. We consider regularization of resolved small scales as a method for modeling the effects of subgrid scales and study its effect on Gulf-Stream separation. An explanation of the improved representation of the dynamics in the model is offered. [Preview Abstract] |
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