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 EG: GFD: Oceanography II |
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Chair: Jie Yu, North Carolina State University Room: Long Beach Convention Center 103B |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EG.00001: Downwelling in Boundary Currents Subject to Buoyancy Loss Claudia Cenedese Recent observational, theoretical, and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries. This is fundamentally different from the traditional view that downwelling takes place in regions of deep convection. Theoretical understanding of what controls the downwelling near boundaries (its magnitude, length scales) is weak and relies on parameterizations of poorly known turbulent mixing processes. In this study, laboratory experiments were carried out to explicitly resolve the turbulent mixing due to convective plumes and identify where downwelling takes place. The dependence of the downwelling, circulation, and free surface temperature on the non-dimensional parameters that describe the boundary current and surface forcing will be discussed. The laboratory results are compared to previous theories and numerical models for downwelling boundary layers, to determine whether the details of these small-scale turbulent processes need to be explicitly resolved in order to represent their influence on the larger-scale circulation. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EG.00002: Diffusion-limited settling of porous particles in a stratified fluid Kolja Kindler, Bo Liu, Roman Stocker, Arzhang Khalili Marine particles settle at low Reynolds numbers, are often highly porous, and are frequently observed to accumulate at pycnoclines. We present the first study of the settling of porous particles in a stratified fluid, by combining laboratory experiments and Lattice-Boltzmann simulations. We find that porosity markedly affects settling by causing retention of particles at pycnoclines. The excess density of highly porous particles is largely determined by the density of the interstitial fluid. The latter is adaptive in a stratified ambient, through exchange with the surrounding fluid. For low-permeability particles at sharp density interfaces, we observed the retention time to scale quadratically with particle size, as predicted based on a purely diffusive exchange of interstitial fluid. The simulations reveal that the interstitial fluid exchange, and thus the settling velocity, is modulated by a wake of lighter fluid that the particle entrains from upper layers. Both porosity and wake entrainment contribute to increase drag. These findings will affect estimates of carbon export rates from the upper ocean to the deep sea. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EG.00003: LES of turbulent stratified flows on shallow continental shelves Guillaume Martinat, Andres Tejada-Mart\'Inez, Chester Grosch Turbulent shear flow on shallow continental shelves (here shallow means that the interaction with the solid, no slip bottom is important) are of great importance because tides and wind driven flow on the shelf are drivers of the transfer of momentum, heat, and mass (gas) across the air-sea interface. We use Large Eddy simulation to study and quantify the impact of a stable stratification on the dynamic of shear driven and pressure gradient driven turbulent flows. These computations are compared to the corresponding unstratified flows to provide a better understanding of the physics governing the interaction between stratification and turbulent flows. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EG.00004: Turbulent Mixing Efficiency in Stratified Couette Taylor Flow Bruce Rodenborn, Guenther Ebert, Harry L. Swinney Ocean mixing is critical to sustaining the meridional overturning circulation. Global ocean and climate models must parameterize ocean mixing because it occurs at scales well below the resolution of the models. The current understanding of ocean mixing requires about 20\% of the kinetic energy in a turbulent flow to be converted into a change in the fluid's gravitational potential energy. Laboratory work on mixing efficiency has used towed grids and other means to create turbulence, but this turbulence is not sustained and its relation to the turbulent patches observed in the oceans is not known. We study mixing in a linearly stratified fluid contained between two counter-rotating cylinders. An initial density variation of up to 200\% over the height of our system is achieved using sodium polytungstate salt. Measurements are made for laminar to fully turbulent flows - Reynolds numbers from a few hundred to 10,000. The flow pattern is visualized using Kalliroscope, and the characteristic vertical length scale is determined from spatial fourier transforms of images. The power input is determined by measuring the torque and rotation rate of both cylinders. The fluid's gravitational potential energy is determined by measuring density as a function of height. We find that mixing efficiency is strongly dependent on the total Reynolds number and the total initial density variation. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EG.00005: Multiple-Scale Asymptotics for Oceanic Fluid Dynamics: Coupled Planetary- and Quasi-Geostrophic Equations Ian Grooms, Keith Julien, Baylor Fox-Kemper The planetary geostrophic (PG) equations for large-scale oceanic flow are linked to the quasigeostrophic (QG) equations for mesoscale flow in a multiple-scales asymptotic expansion. The model describes the coupling of planetary-scale and mesoscale dynamics: eddy kinetic energy is generated by baroclinic instability of the planetary flow, and the resulting eddy buoyancy fluxes feed back on the planetary flow. Anisotropy of the planetary flow is seen to play a key role in allowing the two-way coupling. The resulting equations are amenable to theoretical and computational investigation of the interaction of mesoscale and planetary scale dynamics. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EG.00006: Evaluation of turbulent Prandtl (Schmidt) number parameterizations for stably stratified turbulent flows Zachary Elliott, Subhas Venayagamoorthy In this study, we evaluate four different formulations of the turbulent Prandtl (Schmidt) number $Pr_t=\nu_t/\Gamma_t$ where $\nu_t$ is the eddy viscosity and $\Gamma_t$ is the scalar eddy diffusivity, for stably stratified flows. All four formulations of $Pr_t$ are strictly functions of the gradient Richardson number $Ri$ which is a measure of the strength of the stratification. A zero equation turbulence model for the eddy viscosity $\nu_t$ in a one-dimensional, turbulent channel flow is considered to evaluate the behavior of the different formulations of $Pr_t$. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems such as atmospheric flows and tidally-driven estuarine flows, to quantify the behavior of each of the four formulations of $Pr_t$. We discuss which of the models of $Pr_t$ allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of density stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to highlight the importance of choosing the appropriate parameterization of $Pr_t$, given a model for $\nu_t$ in for stably stratified flows. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EG.00007: Response of a stable stratified jet to surface wind and buoyancy forcing Hieu Pham, Sutanu Sarkar The fine-scale response of a subsurface linearly-stable stratified jet to the forcing of surface wind stress and surface cooling (downward buoyancy flux) is investigated using Direct Numerical Simulation. The simulation involves a symmetric jet situated below a laminar surface layer driven by a constant windstress. The surface layer is well mixed while the jet is stably stratified such that the gradient Richardson number inside the jet is larger than the critical value for linear shear instability. The simulation setup follows the background conditions in the Equatorial Undercurrents (EUC) and, similar to the EUC, internal waves and intermittent patches of intense dissipation are observed in spite of nominally stable conditions. However, the wave momentum flux is significantly smaller than the Reynolds turbulent stress extracted from the background velocity in the surface layer. The wave energy flux is also smaller than the turbulent production. Intermittent patches of intense turbulence are observed inside the jet upper-flank where the background gradient Richardson number is larger than $0.25$. The dissipation rate inside the patches is at least three orders of magnitude larger than the ambient value. The patches are the results of ejections of fluid parcels into gravitationally unstable regions. The ejections are observed to direct both upward and downward and are driven by the formation of vortex tubes. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EG.00008: Bottom turbulence during resonant generation of Internal waves at a critical slope Bishakhdatta Gayen, Sutanu Sarkar A numerical study based on direct numerical and large eddy simulation is performed to investigate internal tide generation that occurs when a barotropic tide oscillates over a sloping bottom. An intense boundary flow is generated in the near-critical case with slope angle equal to the natural internal wave propagation angle. The intensification of upslope and downslope flow increase with the length of the near-critical region of the topography. Nonlinear processes become important in the vicinity of the slope. The propagating internal tide has higher harmonics, subharmonics and inter harmonics. The resonant wave undergoes both convective and shear instability and promote strong turbulence over the entire slope and afterward it effects on the intensification rate. The maximum turbulent kinetic energy, dissipation and production lag with respect to the peak external velocity depending on the height above the bottom. The baroclinic energy flux, turbulence production and turbulent dissipation rate increase with increasing slope length. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EG.00009: Drift in sheared shallow water waves William R.C. Phillips, Albert Dai, Kuan Tjan The drift in an $O(\epsilon)$ monochromatic wave field on a shear flow whose characteristic velocity is $O(\epsilon)$ smaller than the phase velocity of the waves is considered. It is found that shear plays an increasingly important role as the depth decreases. Details of the shear flow likewise affect the drift. Two temporal cases common in coastal waters are studied: wind driven shear and current driven shear. In the former, the magnitude of the drift (maximum minus minimum) in shallow water waves is increased significantly above the Stokes drift. In the latter, on the other hand, the magnitude decreases. However, while the drift at the free surface is oriented always in the direction of wave propagation in stress driven shear, that is not always the case in current driven shear, especially in long waves as the boundary layer grows to fill the layer. This later finding is of particular interest vis a vis Langmuir circulation, which arise through an instability that requires differential drift and shear of the same sign. This means that while Langmuir circulation form near the surface and grow downwards (top down), perhaps to fill the layer, in stress driven shear, their counterparts in current driven flows grow from the sea floor upwards (bottom up) but can never fill the layer. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EG.00010: Coherent structures in a stratified and rotating shear layer with horizontal shear Eric Arobone, Sutanu Sarkar One of the least understood scales of the ocean is the submesoscale. Here, rotation is important but does not necessarily control the dynamics, instabilities and nonlinear cascades are possible, and stable stratification affects the flow. Previous work by the authors has revealed the zigzag instability, coherent structures, and strong vertical mixing for turbulent horizontal shear flow in a stratified medium. Prior investigations into the rotating shear layer have shown the formation of longitudinal vortices and destabilization for weak anticyclonic rotation and two-dimensionalization for cyclonic and strong anticyclonic rotation. We will perform numerical experiments to examine the effect of both rotation and stratification on coherent dynamics with environmental parameters appropriate for submesoscale flows. Competition between the increased vertical correlations associated with rotation and decreased vertical correlations associated with stratification will be assessed. Additionally the statistical evolution and physical mechanisms driving the flow evolution will be discussed. [Preview Abstract] |
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