Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L1: Geophysical Flows: Oceanography IV |
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Chair: Tony Dalrymple, Johns Hopkins University Room: 301 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L1.00001: Quantifying Parametric Uncertainty in Ocean General Circulation Models: A Sparse Quadrature Approach Justin Winokur, Alen Alexanderian, Ihab Sraj, Mohamed Iskandarani, Ashwanth Srinivasan, Carlisle Thacker, Omar Knio We use Polynomial Chaos (PC) expansions to quantify propagation of parametric uncertainties in Ocean General Circulation Models (OGCMs). We focus on short-time, high-resolution simulations in Gulf of Mexico with wind stresses corresponding to hurricane Ivan. A non-intrusive sparse quadrature approach is used to determine the PC coefficients providing a detailed representation of the stochastic model response. The quality of the PC representation is examined through a systematic refinement of the number of resolution levels. The resulting PC representation is then utilized in computing distributions of model variables and analyzing local and global sensitivity of the solution to uncertain parameters. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L1.00002: Large-eddy simulation of large-scale convection cells in unstably stratified open channel flow Andres Tejada-Martinez, Guillaume Martinat, Rachel Walker, Chester Grosch Results are presented from large-eddy simulation of unstably stratified open channel flow driven by a pressure gradient with zero surface shear stress and a no-slip bottom. Unstable stratification is imposed by a constant cooling flux at the surface and an adiabatic bottom wall. Under neutrally stratified conditions, the flow is characterized by weak full-depth streamwise cells similar to Couette cells in plane Couette flow. Surface cooling leads to stronger full-depth convection cells of larger spanwise scale. Surface cooling increases vertical and spanwise velocity fluctuations in the upper half of the channel, while increasing mixing throughout the water column. Similarities and differences between the flow with full-depth convection cells and a second flow with full-depth Langmuir cells generated via surface wave-current interaction will be highlighted. Comparison between flows is based on visualizations and diagnostics including (i) profiles of mean velocity, (ii) profiles of resolved Reynolds stress components, (iii) invariants of the resolved Reynolds stress anisotropy tensor and (v) balances of the transport equations for mean resolved turbulent kinetic energy. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L1.00003: Approximate deconvolution large eddy simulation of a barotropic ocean circulation model Anne Staples, Omer San We investigate a new large eddy simulation closure modeling strategy for two-dimensional turbulent geophysical flows. This closure modeling approach utilizes approximate deconvolution, which is based solely on mathematical approximations and does not employ additional phenomenological arguments. The new approximate deconvolution model is tested in the numerical simulation of the wind-driven circulation in a shallow ocean basin, a standard prototype of more realistic ocean dynamics. The model employs the barotropic vorticity equation driven by a symmetric double-gyre wind forcing, which yields a four-gyre circulation in the time mean. The approximate deconvolution model yields the correct four-gyre circulation structure predicted by a direct numerical simulation, but on a coarser mesh and at a fraction of the computational cost. This first step in the numerical assessment of the new model shows that approximate deconvolution could be a viable tool for under-resolved computations in the large eddy simulation of more realistic turbulent geophysical flows. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L1.00004: Mesoscale- and submesoscale-resolving simulations with an anisotropic Smagorinsky subgrid model Sanjiv Ramachandran, Amit Tandon, Amala Mahadevan The oceanic submesoscales are motions of scale O(100m-1km) that lie between the large, O(100km), mesoscales and the smaller, O(10m), three dimensional eddies. Past studies showed submesoscales are critical to the evolution of fronts in the ocean and to the ecological cycle. This study discusses results from submesoscale-resolving simulations with a nonhydrostatic ocean model employing an anisotropic Smagorinsky subgrid model. Our simulated domain is O(100km) in the horizontal and O(100m) in the vertical with highly anisotropic grid resolutions, O(500-1000m) and O(10m), respectively. The domain resolves both mesoscale and submesoscale eddies. Past studies with similar domains have achieved horizontal subgrid mixing either through constant eddy-viscosities or implicitly through the underlying numerical algorithm. We present results from simulations with and without surface winds. Intense submesoscale activity occurs near the fronts and is associated with O(1) Rossby numbers. With winds, the subgrid dissipation of resolved kinetic energy is strongest in regions with negative potential vorticity and the vertical buoyancy flux is enhanced in the presence of baroclinicity below the mixed layer. Without winds, the subgrid dissi pation and vertical velocity are smaller than in the wind-driven case. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L1.00005: Turbulence, mixing, and blooms at ocean fronts John Taylor, Raffaele Ferrari Regions with large horizontal density gradients, or fronts, are ubiquitous features of the upper ocean. As locations where density surfaces outcrop from the ocean interior, fronts serve as conduits for transport of fluid properties, linking the deep ocean and the atmosphere. Although fronts are under-resolved in most global ocean models, recent work has shown that they strongly affect the large-scale circulation and biology of the ocean. This talk will describe results from recent studies based on large-eddy simulations (LES), which find that turbulent mixing is strongly affected by fronts and is subject to two competing effects: turbulence is generated from the available potential energy associated with the front, but vertical mixing is inhibited by the stable stratification that develops as the front slumps. By coupling a simple biological model with the LES, we find that reduced vertical mixing at fronts can trigger phytoplankton blooms in light-limited conditions. These results help explain satellite-based observations of unexpected mid-ocean blooms at high latitudes. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L1.00006: A Variable Resolution Global Ocean Model Mark Petersen The Model for Prediction Across Scales (MPAS) is a new software framework for the rapid development of climate model components on unstructured grids. The grids may be quasi-uniform or variable density, on a sphere or rectangular domain, and may use quadrilateral cells, triangle cells, or Voronoi tessellations. MPAS variable density grids are particularly well suited to regional climate simulations. MPAS is developed cooperatively by NCAR MMM and the LANL COSIM team. The MPAS-Ocean component now includes most of the features of a full ocean-climate model. High resolution global simulations with full bathymetry have been run for hundreds of simulated years and produce realistic currents and eddying behavior. MPAS-Ocean may be run with z-level or isopycnal vertical coordinates, and includes high-order horizontal and vertical advection and implicit vertical mixing. Current efforts focus on split barotropic/baroclinic timestepping and improving performance. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L1.00007: Evaluation and Improvement of RANS models for Stably Stratified Turbulence Farid Karimpour, Subhas Venayagamoorthy The focus of this study is to account for the effects of buoyancy forces in RANS models for stratified turbulence. To this end, changes to the stratification parameters that account for buoyancy effects in RANS models are proposed. DNS data of stably stratified turbulence are used to study the parameters in two equation turbulence models such as the buoyancy parameter $C_{\epsilon3}$, and the turbulent Prandtl number $Pr_t$ in the $k$-$\epsilon$ model. Both the gradient Richardson number $Ri = N^2/S^2$, where $N$ is the buoyancy frequency and $S=d\overline{u}/dz$ is the mean shear rate, and the turbulent Froude number $Fr_k=\epsilon/(Nk)$, are used as correlating parameters to characterize stratification in the $k$-$\epsilon$ model. We show that it may be more appropriate to use $Fr_k$ as the parameter of choice for modeling the stratification parameters in the $k$-$\epsilon$ model since it is based on the local properties of the turbulence as opposed to $Ri$, which is a mean property of the flow. The proposed modifications were implemented in a 1-D water column model called the General Ocean Turbulence Model (GOTM) and used to simulate stably stratified channel flows. The results from numerical simulations using the modified $k$-$\epsilon$ model are compared to DNS data of stably stratified channel flow to assess its efficacy. This modified formulation is also compared with other stability functions in GOTM. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L1.00008: Energy Cascade in the Regime of Realistic Ocean Circulation B.T. Nadiga, W.R. Casper Ocean circulation is forced at the large scales and the instability of the resulting large-scale flow gives rise to intermediate-scale eddies. The large-scale flow and the resultant eddies are both in approximate geostrophic balance; such balance results in an inverse cascade of energy. Consequently, small scale dissipation becomes ineffective and dissipation is limited to interactions of the larger and intermediate scale flow structures with solid boundaries. We consider the modification of this asymptotic behavior in the presence of a range of scales over which unbalanced motions are possible. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L1.00009: Examination of the Turbulent Kinetic Energy Budgets in the Mid-Water Column of the Chesapeake Bay Estuary Luksa Luznik, Louise Wallendorf A local turbulent kinetic energy (TKE) balance is examined from measurements obtained during two-week long field experiment in the Chesapeake Bay near Kent Island, MD, under low to moderate wind conditions. Velocity data were collected with two vertically separated Acoustic Doppler Velocimeters and an upward looking pulse coherent profiler covering 0.8 to 2.5 m above bottom in approximately 5 m of water. Additionally, upward oriented AWAC mounted on a separate frame was used to obtain water column tidal current vertical profiles and overlying wave directional system. Stratification is monitored using two vertically separated CT sondes and estimated Ozmidov scale ranges from very small values up to 5m. Occurrences of large TKE dissipation rates coincide with times of maximum bottom boundary layer shear production with consistently larger values during ebb flows. In general dissipation rate exceeds shear production minus buoyancy flux in the mid-water column. Potential sources of enhanced levels of dissipation are examined including vertical turbulent transport, wind driven shear production and presence of surface gravity waves. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L1.00010: ABSTRACT WITHDRAWN |
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