Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session LN: Geophysical Flows IV |
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Chair: Gregory King, University of Warwick Room: Hilton Chicago PDR 2 |
Tuesday, November 22, 2005 8:00AM - 8:13AM |
LN.00001: Characteristics of turbulence in the bottom boundary layer of the coastal ocean Roi Gurka, Luksa Luznik, Erin Hackett, Joseph Katz, Thomas Osborn PIV measurements performed in the bottom boundary layer of the U.S. Southeastern continental shelf provide seven data sets, each containing 4000 pairs of 2-D vector maps. Each set consisted of two velocity distributions obtained at two magnifications, one with a field of view of 35 cm, and the other with 11.5 cm, with a gap of 83 cm between their centers. This arrangement enables measurements of spectra and structure functions covering two orders of magnitude of length scales, the smallest being close to the Kolmogorov scale. By combining the time series of data, the spectral range can be extended by additional two orders of magnitude, but this time with wave contamination. These data are collected within 3 m above the bottom, at Taylor microscale Reynolds numbers of 150-200. The flow consists of tidal currents and a strong oscillatory wave-induced motion. Spectral analysis reveals anisotropy at all scales. Vertical profiles of Reynolds stresses, dissipation and production rates are obtained. The dissipation estimates by fitting a -5/3 slope to the energy spectra, and by fitting the dissipation spectra to the universal plots for oceanic flows, are compared to values calculated directly from the measured velocity gradients. The results show good agreement. However, unlike isotropic turbulence, the dissipation is higher than that obtained from estimates based on the measured component of enstrophy. [Preview Abstract] |
Tuesday, November 22, 2005 8:13AM - 8:26AM |
LN.00002: Navier-Stokes based linear stability analysis of submarine channel formation by turbidity currents Brendon Hall, Eckart Meiburg Submarine channels represent common features on the continental shelf. They are formed, and in turn affect, sediment transport from shallow to deeper waters. We perform a Navier-Stokes based linear stability analysis, based on the Boussinesq approximation, to assess turbidity currents as a potential mechanism for the initiation of such channels. A one-dimensional base state is assumed for the streamwise velocity and the particle loading, and the fluid/substrate interface evolves according to a balance of erosion and sedimentation. The stability analysis demonstrates that a perturbation of this balance results in local variations of particle concentration, which in turn lead to the formation of counterrotating, streamwise vortex pairs. These modify the local balance of erosion and sedimentation such as to amplify the initial perturbation. Dispersion relationships are presented in order to evaluate the influence of the governing dimensionless parameters. The ratio of the particle concentration and velocity boundary layer thicknesses is seen to be important. [Preview Abstract] |
Tuesday, November 22, 2005 8:26AM - 8:39AM |
LN.00003: Basic Wave Dynamics in a Cloudy Vortex David Schecter, Michael Montgomery This paper presents a basic model for small-amplitude wave dynamics in a cloudy vortex. The moist wave equations are identical to the linearized dry hydrostatic primitive equations, with reduced buoyancy. An explicit thermodynamic formula is provided for the buoyancy reduction factor. The stability of hurricane-like vortices is then reexamined in the context of the moist model. Preliminary computations show that pseudo-barotropic eyewall instabilities can critically slow down as internal moisture brings the vortex toward slantwise convective neutrality. Furthermore, it is shown that internal moisture can damp vortex Rossby waves and inhibit spiral inertia-gravity wave radiation. [Preview Abstract] |
Tuesday, November 22, 2005 8:39AM - 8:52AM |
LN.00004: On modeling volcanic eruptions with open-ended shock tubes Susan W. Kieffer, Andreas Haselbacher, S. Balachandar The flow from open-ended shock tubes may be viewed as a simple model for vertical Plinian eruptions and lateral eruptions such as the blast at Mount St. Helens, 1980. In such models, the volcanic chamber is represented by the driver section and the atmosphere by the open driven section. Previous models have placed the diaphragm at the exit plane; however, although not much is known about conditions at volcanoes, we do know that this is an oversimplification because craters develop rapidly in response to high stresses. We modeled this by assuming that the diaphragm was not exactly at the exit plane. In examining the effects of pressure ratio and gas properties, we have discovered an effect not previously considered: that in the start-up, unsteady phase of discharge, choked conditions may be approached from either subsonic or supersonic conditions. For a given gas, at high pressure ratios the flow behind the contact discontinuity is supersonic. Consequently, the expansion fan not only propagates into the chamber, but also into the atmosphere, introducing an unsteadiness as the flow adjusts to sonic conditions. [Preview Abstract] |
Tuesday, November 22, 2005 8:52AM - 9:05AM |
LN.00005: Quantifying transport properties by exchange matrix method Tatyana Krasnopolskaya, Vyacheslav Meleshko The exchange matrix method is described to study of transport properties in chaotic geophysical flows. This study is important for applying in problems of pollutants transport (such as petroleum patches) in tidal flows and others. In order to construct this special exchange matrix\textbf{ (}first suggested by Spencer {\&} Wiley) we use an approximation of such flows made by Zimmerman, who adopted the idea of chaotic advection, first put forward by Aref. Then for a quantitative estimation of the transport properties we explore a coarse-grained density description introduced by Gibbs and Welander. Such coarse-grained representations over an investigation area, show a ``residence place'' for the pollutant material at any instant. The orbit expansion method, exploited an assumption that the contributions of tidal and residual currents are of different orders (the tidal is much stronger), does not give answers in many real situations. The exchange matrix can show transport of patches or particles from any place in the area under consideration to an arbitrary location in the tidal sea and time if it happens. [Preview Abstract] |
Tuesday, November 22, 2005 9:05AM - 9:18AM |
LN.00006: Spectral regimes in sea-surface winds Robert M. Kerr, Greg King, Kuang C. Pien Second and third-order structure function analysis of the sea-surface wind over the Pacific Ocean from 40S to 40N finds spectral regimes that change slope and possibly direction of energy transfer depending upon the latitude or longitudinal bands chosen. Slopes tend to $r^{2/3}$ for all zonal (E-W) 2nd-order structure functions for $r\leq 500km$, with larger slopes for $r>1000km$ between mid-latitude bands $30N/S-40N/S$ and smaller slopes for $r>1000km$ from $10S-10N$.Meridional (N-S) structure functions are much steeper and have smooth changes from one side of the Pacific to the other.3rd-order structure functions are consistent with a downscale flow of energy for $r<500km$ in all bands. For $30N/S-40N/S$ there could be downscale energy flow for all $r$, but for $10S-10N$ there could be an upscale flow of energy for $r>500km$, pointing to an energy source in the equatorial regions for $r\approx 500km$. [Preview Abstract] |
Tuesday, November 22, 2005 9:18AM - 9:31AM |
LN.00007: Large Eddy Simulation of Langmuir Circulation in Shallow Water Andres Tejada-Martinez, Chester Grosch We report results of large-eddy simulation (LES) of Langmuir circulation (LC) in a wind-driven shear current in bottom- resolved shallow water. After the introduction and a brief description of the governing equations and the numerical method, we focus on major differences in the dynamics between wind shear- driven Couette flow and the same flow with LC. This comparison relies on flow visualizations and diagnostics including 1) profiles of mean velocity, 2) profiles of resolved Reynolds stress components, 3) invariants of the resolved Reynolds stress anisotropy tensor and 4) balances of the transport equations for resolved mean turbulent kinetic energy and resolved Reynolds stress tensor. In addition to comparisons between Couette flow with and without LC, comparisons are made with recent observations recorded in the southern coast of New Jersey. [Preview Abstract] |
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