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 NL: Geophysical: Oceanographic II |
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Chair: Mariano Cantero, University of Illinois Room: Salt Palace Convention Center 250 F |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NL.00001: ABSTRACT HAS BEEN MOVED TO BL.00010 |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NL.00002: Generation of Vortices from Internal Gravity Waves Chung-Hsiang Jiang, Philip Marcus Internal gravity waves are ubiquitous in both the ocean and in protoplanetary disks around forming stars. Numerical simulations of disks suggest that these waves can ``break'' and produce intense vortices aligned with the axis of rotation of the disk. Here we report numerical simulations in which vortices are generated by this previously unreported mechanism. The simulated waves and vortices occur in an anelastic, salt-stratified (with local Brunt-Vaisala frequency $N(z)$, where $z$ is the vertical coordinate), rotating fluid (with Coriolis parameter $f$) that could be realized in a laboratory experiment. The stratification is chosen so that $N(z)$ increases with height. Internal gravity waves exist only if their frequency $\omega$ satisfies $N < \omega < f$ or $f <\omega < N$. In these simulated experiments the waves satisfy the first condition. However, as the internal waves propagate upward from their source, they reach a height $z$ where the local $N(z)$ becomes greater than $\omega$. Rather than refract or reflect at this height, the waves breakdown and produce intense vortices. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NL.00003: Turbulent structures and vortex dynamics on gravity currents Mariano Cantero, S. Balachandar, Gary Parker, Marcelo Garcia Highly resolved three-dimensional simulations are presented for planar and cylindrical gravity currents. The Navier-Stoke equations are solved for Reynolds numbers $Re=15000$ and $Re=8950$ with 131 and 1340 millions grid-point resolution for the planar and cylindrical cases, respectively. The work is oriented to visualize and describe the turbulence structures present in the flow. The near-front region is composed of a boundary layer at the bottom and a shear layer at the top. The shear layer undergoes instabilities and the formation of Kelvin-Helmholtz vortices which decay to smaller scale turbulence populating the rear part of the front of the current with hairpin vortices oriented in the direction opposite to the flow. On the other hand, the near-bottom boundary-layer-like region is populated with hairpin vortices oriented in the direction of the flow. The body of the current is characterized by the interaction of these turbulent structures. The decay of Kelvin-Helmholtz vortices and their complex dynamics induce large pressure recovery by homogenization of the flow which modifies substantially the front velocity, which shows the importance of three-dimensional effect on the spreading rate. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NL.00004: Asymptotically Reduced Equations for Weakly Three-Dimensional Langmuir Turbulence Greg Chini, Keith Julien Ocean observations, numerical simulations and theoretical considerations all suggest that Langmuir circulation (LC) is characterized by counter-rotating vortical structures elongated in the wind direction. We identify the source of this downwind invariance by exploring the Craik--Leibovich (CL) equations modeling LC in the physically-relevant large $Re_s$ limit; here, the Stokes Reynolds number $Re_s$ is based on the Stokes drift velocity of the surface waves and on the depth of the mixed layer. Inspection of the CL equations reveals that the CL vortex force dominates the flow physics when $Re_s\gg 1$, and vortices aligned with the wind direction are preferred. Using multiscale asymptotics, we leverage this limit to derive a reduced set of PDEs governing weakly 3D Langmuir turbulence. Linear and secondary (i.e. nonlinear) stability studies show that the reduced equations economically capture the key 3D instabilities. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NL.00005: The LANS-alpha turbulence model in primitive equation ocean modeling Mark Petersen, Matthew Hecht, Darryl Holm, Beth Wingate The POP primitive equation ocean model is widely used by the climate modeling community. Like all numerical models, computational time limits the spatial resolution at which POP can operate; standard climate simulations use grids of 0.5 to 1 degree in latitude and longitude. This resolution does not capture the motion of eddies at the Rossby radius of deformation, and thus lacks the correct energy cascade and heat transport at these scales. Simulations using the Lagrangian-averaged Navier Stokes-alpha (LANS-alpha) turbulence parameterization in the POP ocean model resemble higher resolution simulations of standard POP in statistics like kinetic energy, eddy kinetic energy, and potential temperature fields. The LANS-alpha model accomplishes this improvement through an additional nonlinear term and a smoothed advecting velocity. I will discuss my implementation of LANS-alpha in the POP ocean model, and results using an idealized channel domain that invokes the baroclinic instability. Results from a North Atlantic simulations show that LANS-alpha can achieve higher eddy kinetic energy in realistic domains as well. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NL.00006: Scaling and Flow Structure of Stratified Turbulent Wakes at High Reynolds Numbers Peter Diamessis Direct Numerical Simulations of the stratified turbulent wake of a towed sphere have been performed over a range of Reynolds and internal Froude numbers, varied by a factor of 20. At higher Reynolds numbers, the duration of the non-equilibrium (NEQ) regime is prolonged, as secondary Kelvin-Helmholtz instabilities and turbulence emerge through the action of the intensified vertical shear. This is the first observation of such secondary events in a controlled stratified flow with turbulent initial conditions. The robust agreement with inviscid scaling arguments proposed by a number of researchers suggests that the secondary instabilities are a manifestation of the ``stratified turbulence'' phenomenon proposed by Lilly (1983). Significant reconsideration of the life-cycle of a turbulent patch in the ocean may therefore be necessary. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NL.00007: Characterization of turbulent mixing in an Oceanic Overflow Facility Philippe Odier, Jun Chen, Michael Rivera, Robert Ecke The mixing and entrainment process in oceanic overflows, e.g. Denmark Strait overflow (DSO), affects the global thermohaline circulation. Due to limited spatial resolution in global climate prediction simulations, the small-scale dynamics of oceanic mixing must be properly modeled. A laboratory Oceanic Overflow Facility is used to investigate the fine structure of the entrainment and mixing. Inside a water tank, lighter, turbulent fluid with Taylor Reynolds number R(lambda)=150 is introduced along an inclined plate into a denser environment. Simultaneous PIV and PLIF measurements are conducted to visualize and quantify the flow structure. The data are used to characterize the turbulent mixing. Eddy diffusivities are measured for the transport of momentum and buoyancy. A mixing length model is applied, yielding an estimate of the mixing length in the flow, for various values of the gradient Richardson number. The results are compared to the values measured in the ocean and to the values used in climate models. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NL.00008: Laboratory Investigation of Turbulent Mixing in a Stratified Environment at Different Richardson Numbers Jun Chen, Philippe Odier, Michael Rivera, Robert Ecke The mixing processes in stratified environments have a broad application in geophysical flows, e.g. oceanic overflows. A laboratory apparatus is built to investigate the small-scale flow structures and the dynamics of mixing in a stratified environment. A gravity current is generated inside a water tank, moving along an inclined plate into a denser environment. Velocity and density are measured using simultaneous PIV and PLIF measurements. The dynamics of mixing in the non-stratified case and for different Richardson numbers is evaluated. The results are used to explain the different flow patterns observed. The effects of turbulence decay and stratification are also investigated by studying various terms in the energy budget. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NL.00009: Turbulence generation by tide-driven resonant internal waves on a continental slope Hepeng Zhang, Ben King, Harry L. Swinney Turbulent flow over a continental slope is spatially heterogeneous. Near so-called ``critical regions'', where the topographic slope coincides with the propagation slope of semidiurnal internal waves, turbulence can be orders of magnitude stronger than other places. Formation of these localized energetic regions is conventionally explained by critical reflection from the continental shelf of remotely generated internal waves. We have conducted laboratory experiments on tidal flow past a sloping bottom boundary in a tank of stratified fluid. The experiments reveal that at resonance, the tidal forcing generates a narrow and intense internal wave beam parallel to the slope. When the Richardson number ($\frac{N^{2}}{(dU/dz)^{2}}$), where $N$ is the buoyancy frequency, is less than 1/4, the strong shear produced by the intense internal wave beam destabilizes the beam, and turbulence is generated through the formation and breaking of Kelvin-Helmholtz billows. [Preview Abstract] |
Tuesday, November 20, 2007 1:32PM - 1:45PM |
NL.00010: Particle settling in a stratified fluid Roman Stocker, King-Yeung Yick, Thomas Peacock, Carlos Torres Particles settling in a stratified fluid experience an increased drag caused by the vertical displacement of isopycnals. Using a combination of microscale Synthetic Schlieren experiments and numerical simulations, we have quantified the extra drag due to stratification at low and moderate Reynolds numbers for particles as small as 157 $\mu$m. Our results suggest that the extra drag depends on a combination of the Reynolds, Froude and Prandtl numbers through a single dimensionless parameter which we called the Stratification number. [Preview Abstract] |
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