Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D10: Geophysical: Oceanographic II |
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Chair: Sutanu Sarkar, University of California, San Diego Room: 334 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D10.00001: Dynamics of SQG Vortices and Passive Scalar Transport Cecily Keppel, Stefan Llewellyn Smith The surface quasi-geostrophic (SQG) equations are a model for low-Rossby number geophysical flows in which the dynamics are governed by potential temperature dynamics on the boundary. We examine the dynamics of SQG vortices and the resulting flow in the entire fluid including at first order in Rossby number ($O(Ro)$). This requires solving an extension to the usual QG equation to compute the velocity corrections, and we demonstrate this mathematical procedure. As we show, it is simple to obtain the vertical velocity, but difficult to find the $O(Ro)$ horizontal corrections. We then consider specific cases of interactions of vortices and examine the tracer transport properties in the interior of the fluid. We show various diagnostics for examining the effect of the vertical transport. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D10.00002: An efficient coarse grid projection method for quasigeostrophic models of large-scale ocean circulation Anne Staples, Omer San We present a coarse grid projection (CGP) multiscale method to accelerate computations of quasigeostrophic (QG) models for large scale ocean circulation. These models require solving an elliptic sub-problem at each time step, which takes the bulk of the computational time. The method we propose here is a modular approach that facilitates data transfer with simple interpolations and uses black-box solvers for solving the elliptic sub-problem and potential vorticity equations in the QG flow solvers. After solving the elliptic sub-problem on a coarsened grid, an interpolation scheme is used to obtain the fine data for subsequent time stepping on the full grid. The potential vorticity field is then updated on the fine grid with savings in computational time due to the reduced number of grid points for the elliptic solver. The method is applied to both single layer barotropic and two-layer stratified QG ocean models for mid-latitude oceanic basins in the beta plane. The method is found to accelerate these computations (at a linear rate) while retaining the same level of accuracy in the fine-resolution field. In addition, numerical oscillations due to lower grid resolutions are effectively eliminated with CGP method. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D10.00003: Nonlinear Scale Interactions and Energy Pathways in the Ocean Hussein Aluie, Matthew Hecht, Geoffrey Vallis Large-scale currents and eddies pervade the ocean and play a prime role in the general circulation and climate. The coupling between scales ranging from $O(10^4)$ km down to $O(1)$ mm presents a major difficulty in understanding, modeling, and predicting oceanic circulation and mixing, where the energy budget is uncertain within a factor possibly as large as ten. Identifying the energy sources and sinks at various scales can reduce such uncertainty and yield insight into new parameterizations. To this end, we refine a novel coarse-graining framework to directly analyze the coupling between scales. The approach is very general, allows for probing the dynamics simultaneously in scale and in space, and is not restricted by usual assumptions of homogeneity or isotropy. We apply these tools to study the energy pathways from high-resolution ocean simulations using LANL's Parallel Ocean Program. We examine the extent to which the traditional paradigm for such pathways is valid at various locations such as in western boundary currents, near the equator, and in the deep ocean. We investigate the contribution of various nonlinear mechanisms to the transfer of energy across scales such as baroclinic and barotropic instabilities. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D10.00004: Exploring the dynamics of turbulence suppression due to dispersed phase in various geophysical flows Mrugesh Shringarpure, Mariano Cantero, Tian-Jian Hsu, Balachandar S. Geophysical flows that are characterized as multiphase and turbulent, the feedback of dispersed phase tends to alter the carrier phase turbulence and degree of isotropy. In turbidity currents, suspended sediments sustain turbulence by driving the flow and at the same time can kill the flow by suppressing turbulence through stratification effects. Similarly in hurricanes, the intensity of wind can be modulated by the stratification effects of water droplets that are injected into it by sea-sprays. To study such flows, we implemented models with dilute suspensions and performed DNS at moderate Reynolds numbers. These studies show that there are three governing parameters: Reynolds number, Richardson number and size of the dispersed phase particles. Parametric groupings that quantify turbulence suppression were identified for these flows. We have also looked into the interaction of wave induced turbulence and its ability to transport fine sediments. Here we will present the mechanism that is responsible for modulating the near wall vortical structures that can potentially explain the loss of turbulence in turbidity currents, fluctuations in the carry capacity of wave induced sediment transport and drag saturation of hurricane intensity winds due to sea-sprays. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D10.00005: LES of full-depth Langmuir circulation in a crosswind tidal current Andres Tejada-Martinez, Nityanand Sinha, Chester Grosch, Guillaume Martinat We report on the impact of a crosswind tidal current on full-depth Langmuir circulation (LC) in shallow water computed via large-eddy simulations (LES). LC consists of parallel counter rotating vortices that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear current with the Stokes drift velocity induced by surface gravity waves. During times of weak tidal current, full-depth LC disrupts the classical log-layer dynamics occurring at the bottom of the water column. For example, in terms of mean velocity, the mixing due to LC induces a large wake region eroding the classical log-law profile within the range 90 \textless\ z$+$ \textless\ 200. However, during times of strong tidal current, bottom-generated turbulence induced by the tide is able to break-up the full-depth LC giving rise to smaller scale LC characterized by different turbulent structure. The LC turbulent structure during strong and weak tidal currents is consistent with field measurements during episodes of full-depth LC. Statistics of the turbulence associated with LC during strong and weak tides will be contrasted. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D10.00006: A K-profile parameterization of Langmuir turbulence in shallow water Nityanand Sinha, Andres E. Tejada-Martinez, Chester E. Grosch, Guillaume Martinat Langmuir turbulence in shallow water is often characterized by full-depth Langmuir circulation (lc) generated by the interaction between the wind-driven shear current and the stokes drift velocity induced by surface gravity waves. Large-eddy simulations (LES) of full-depth LC in a wind-driven shear current have revealed that mixing due to LC erodes the bottom log-law velocity profile inducing a profile resembling a wake law. Meanwhile, near the surface, stokes drift shear serves intensify small scale eddies leading enhanced mixing and disruption of the surface log-law. A k-profile parameterization (KPP) comprised of local and nonlocal components is introduced capturing these basic mechanisms by which full-depth LC and associated turbulence impact the mean flow. Single water column Reynolds-averaged Navier-Stokes (RANS) simulations with the new parameterization are presented showing good agreement with les in terms of mean velocity profiles. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D10.00007: Numerical Simulations of the Reduced Craik--Leibovich Equations in Spatially-Extended Domains Zhexuan Zhang, Gregory Chini, Keith Julien Large-eddy simulations of the Craik--Leibovich (CL) equations, a surface-wave filtered version of the Navier--Stokes equations, have been used extensively over the last decade to investigate Langmuir turbulence in the ocean surface boundary layer (BL). However, the simulations are generally restricted to moderate-sized domains, several hundreds of meters in lateral scale; in contrast, spatially-extended arrays of quasi-coherent vortical structures in Langmuir turbulence routinely span lateral scales from 1--10 km. To facilitate simulations of Langmuir turbulence in spatially-extended domains, Chini et al. (GAFD 2009) derived an asymptotically reduced model that consistently filters streamwise variability on scales comparable to the BL depth or less and temporal fluctuations associated with rapid-distortion transients. Preliminary simulations were performed in a moderate-sized domain (fitting only 1 or 2 pairs of vortical flow structures) to verify that the model is well posed and retains the essential dynamics of Langmuir circulation. Here, a more comprehensive set of simulations in a spatially-extended domain is performed to investigate the physics and the computational efficiency of the reduced model. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D10.00008: The Craik-Leibovich Vortex Force as a Skin Effect Ziemowit Malecha, Gregory Chini, Keith Julien The Craik–Leibovich (CL) equations are a surface-wave filtered version of the instantaneous Navier–Stokes equations in which the rectified effects of the surface waves are captured through a so-called ``vortex force'' term: the cross-product of the Stokes, or Lagrangian, mass drift associated with the filtered surface waves and the filtered vorticity vector. For locally generated wind waves, the Stokes drift is very strongly surface confined. In this scenario, the induced body force may be represented as a surface, or skin, effect. Using matched asymptotic analysis in this limit, we derive effective boundary conditions (BCs) for the flow beneath the Stokes drift layer (i.e. in the bulk of the mixed layer). We establish the regime of validity of the resulting formulation by performing linear stability analyses and numerical simulations of both the asymptotic model and the full CL equations for a variety of vertical Stokes drift profiles. The effective BC formulation offers both theoretical and computational advantages, and should be particularly useful for LES of Langmuir turbulence for which the need to resolve very small scale near-surface flow structures imposes severe computational constraints. [Preview Abstract] |
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