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 H1: Geophysical Flows: Oceanography III 
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Chair: Chester Grosch, Old Dominion University Room: 301 
Monday, November 21, 2011 10:30AM  10:43AM 
H1.00001: The Influence of Crosswind Tidal Currents on Langmuir Circulation in a Shallow Ocean Tobias Kukulka, Albert Plueddemann, John Trowbridge, Peter Sullivan Langmuir circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. On the coastal shelves the largest scale LC span the whole water column and thus couple the surface and bottom boundary layers and enhance turbulent mixing. Observations and large eddy simulations (LES) of a shallow coastal ocean demonstrate that these relatively large scale Langmuir cells are strongly influenced by crosswind tidal currents. Two mechanisms by which crosswind tidal shear may distort and disrupt Langmuir cells are proposed. The first mechanism involves cell shearing due to differential advection across the whole cell. For the second mechanism, middepth vertical LC currents advect sheared mean crosswind current, leading to the attraction of up and downwelling regions, so that LC cells are unsustainable when both regions overlap. Scaling arguments indicate that LC cells are more susceptible to crosswind shear distortion for smaller LC surface velocity convergence and greater cell aspect ratio (vertical to horizontal LC scale), which is consistent with the results obtained from the observations and LES. [Preview Abstract] 
Monday, November 21, 2011 10:43AM  10:56AM 
H1.00002: Multiscale asymptotic analysis and numerical simulation of the winddriven ocean surface boundary layer Ziemowit Malecha, Greg Chini, Keith Julien A primary challenge in physical oceanography is to understand the interaction between smallscale vertical mixing processes in the upper ocean, such as Langmuir circulation (LC), and submesoscale eddies, fronts, and their associated instabilities. This problem is computationally challenging because LC is strongly nonhydrostatic, only weakly affected by the Earth's rotation and density stratification, and has length scales commensurate with the ocean surface boundary layer, O(50) m. In contrast, submesoscale flows are approximately hydrostatic, strongly affected by Coriolis accelerations and density stratification, and have O(10) km lateral scales. In this investigation, we take a first step toward developing a physically consistent and computationally efficient model of this interscale coupling using multiscale asymptotic analysis and multiscale pseudospectral numerical simulations. We have achieved over an order of magnitude acceleration of our computations relative to bruteforce simulations using a singlescale algorithm. [Preview Abstract] 
Monday, November 21, 2011 10:56AM  11:09AM 
H1.00003: Beyond meanfield theory: An asymptoticallyinspired singlemode model of Langmuir circulation Zhexuan Zhang, Greg Chini, Keith Julien Meanfield theory for convecting flows dates back to the pioneering work of Spiegel (1962), Herring (1963, 1964, 1966), and others. In the classical approach, all fields are decomposed into horizontallyaveraged and fluctuating components. The approximation involves retaining only those fluctuatingfluctuating nonlinearities in the governing equations that modify the mean fields. In laterally periodic geometries, the resulting meanfield PDEs admit single horizontal (Fourier) mode solutions, reducing the problem to a set of PDEs in the vertical (boundarynormal) coordinate and time. In this investigation, we develop a single horizontalmode reduction of the CraikLeibovich (CL) equations governing Langmuir circulation (LC). Unlike classical meanfield theory, however, we retain important but spatially localized fluctuatingfluctuating nonlinearities in our reduced model. The spatial structure of the basis functions and the choice of modes on which to project the dynamics are guided by the matched asymptotic analysis of fully nonlinear LC by Chini (2008). As in meanfield theory, our singlemode reduction yields a set of 1D PDEs, which are readily solved numerically. We demonstrate the efficacy of our reduced model via quantitative comparisons with full pseudospectral numerical simulations of the downwindinvariant CL equations. [Preview Abstract] 
Monday, November 21, 2011 11:09AM  11:22AM 
H1.00004: Influence of a crossed tidal current on a wind shear driven flow in shallow water with and without wave forcing by means of LES. Guillaume Martinat, Andres TejadaMartinez, Chester E. Grosch Turbulent generated by tidal current and wind shear is important because it controls heat and mass transfer at the airsea interface. Understanding the turbulence dynamics of this class of flows presents complications because of the presence of a free surface, the necessity of including the flow interaction with a solid bottom.The intent of the LES discussed here is to simulate the turbulent flow on shallow shelves having turbulence scales of O(100) m in the horizontal and depths in the range of 10 to 50 m. On these scales the turbulent flow is homogenous in the horizontal. The purpose of our calculations is to elucidate the structure of the turbulence dynamics as driven by pressure gradient (tidal) flow crossed with a surface stress driven flow both imposing an equal stress on the solid bottom, with and without wave forcing. [Preview Abstract] 
Monday, November 21, 2011 11:22AM  11:35AM 
H1.00005: LES of scalar transport in wave and winddriven flows with largescale structures Cigdem Akan, Andres E. TejadaMartinez, Chester E. Grosch Surface scalar (mass) transport results from largeeddy simulation (LES) of winddriven flow with Langmuir circulation are presented. Wavecurrent interaction gives rise to Langmuir turbulence characterized by Langmuir circulation (LC) consisting of a spectrum of scales of counter rotating vortices roughly aligned in the direction of the wind. The typical crosswind length scale of the smallest observed vortices is on the order of several centimeters when the wind begins to blow over a quiescent interface and short capillary waves first appear. The crosswind length scale of the largest vortices reaches up to tens of meters under sustained winds and longer waves. Two different types of flows will be analyzed. The first type is characterized by small scale (centimeter scale) LC and the second type is characterized by large scale LC in shallow water where the cells span the entire water column. Both types of flows exhibit increases in surface mass transfer velocity (efficiency) due to the presence of LC. Statistical analysis of LES variables will be presented highlighting the differences between smallscale and largescale LC structures and their impact on nearsurface scalar transport. [Preview Abstract] 
Monday, November 21, 2011 11:35AM  11:48AM 
H1.00006: Lateral mixing of passive scalars around porous obstacles in tidal flows Hyeyun Ku, Subhas Venayagamoorthy In this study, we use highresolution twodimensional numerical simulations to study mixing and transport of passive scalars around porous cylindrical obstacles in tidal flows. A submerged obstacle blocks the flow (partially or fully depending on its porosity) causing deceleration of the flow, shedding of vortices and the formation of a downstream wake. The lateral distribution of the passive scalar is quantitatively examined by calculating the lateral mixing coefficient as a function of three nondimensional parameters namely; the nondimensional drag coefficient, $C_{d}$ (imparted by the obstacle); the ratio of the tidal to mean flow velocity amplitudes, $\eta = U_{T}/U_{M}$; and an tidal excursion parameter, $K = 2U_{T}/{\omega}D$, where $\omega$ is the forcing frequency and $D$ is the diameter of an obstacle. The simulation results show that the lateral mixing is substantially enhanced due the combined effect of the drag exerted by the obstacle and the oscillatory flow compared to the classical dispersion in a unidirectional flow without drag. The results also highlight the complex dispersion patterns around submerged obstacles in oscillatory flows. [Preview Abstract] 
Monday, November 21, 2011 11:48AM  12:01PM 
H1.00007: Numerical simulations of internal tides at a model ridge Narsimha Rapaka, Bishakhdatta Gayen, Sutanu Sarkar Direct and large eddy simulations are performed to study tidal flow over a model ridge. The NavierStokes equations are solved on a collocated grid using a mixed spectralfinite difference algorithm with a RK3ADI time integration scheme. The effect of criticality parameter on the tidal conversion is studied in the laminar flow regime and compared with existing inviscid linear theory. Nonlinear processes become important at critical slope of the topography even for low excursion numbers. The effect of turbulence on tidal conversion is assessed under nearcritical flow conditions by increasing the barotropic forcing amplitude. Phase dependence of turbulence statistics over a tidal cycle is analyzed at different locations on the ridge. Turbulence near the topography results in up to $25 \%$ reduction in the radiated tidal energy. [Preview Abstract] 

H1.00008: ABSTRACT WITHDRAWN 
Monday, November 21, 2011 12:14PM  12:27PM 
H1.00009: Turbulent dynamics over evolving threedimensional sandy rippled beds Allison Penko, Joseph Calantoni Ripples on the seafloor affect the development of the wave bottom boundary layer impacting wave energy dissipation and sediment transport. Using a threedimensional numerical model, SedMix3D, we simulate the complex turbulent flow of a fluidsediment mixture over a rippled bed and the subsequent threedimensional bed evolution. In general, results from SedMix3D are in excellent agreement with laboratory observations of timedependent vorticity and swirling strength fields, ripple geometry characteristics, and statistically averaged flow quantities. We present simulations of evolving rippled beds of varying initial topographies with domains covering areas of the seafloor up to 12 cm x 24 cm with a vertical extent up to 24 cm. A significant difference between the spatial distribution of turbulence over threedimensional ripple structures and twodimensional parallel ripples is observed in the simulations. We also examine the effect of the turbulence on the maintaining and evolution of the two and threedimensional ripple structures. [Preview Abstract] 
Monday, November 21, 2011 12:27PM  12:40PM 
H1.00010: Transport and Mixing of the Oglio River Inflow into Lake Iseo Charlie Hogg, Herbert Huppert, Jorg Imberger The fate of river water entering a lake remains an uncertainty in many important limnological questions. These questions include how to improve standard water management practices and how lake ecosystems will change in future climate scenarios. This paper describes a field campaign carried out to understand the transport and mixing of a river inlet into Lake Iseo, a subalpine lake in Italy. We observed the low Froude number inflow to fall laterally after entering the lake. We suggest that this is caused by baroclinic acceleration. This laterally falling regime has not, to our knowledge, previously been described in the literature. In addition, measurements of a range of tracers were taken to find the dilution of the river after it had started to intrude into the lake. The tracers used were temperature, salinity, dissolved oxygen, coloured dissolved organic matter and turbidity. Our results found selfconsistent mixing rates from the available natural tracers. These findings contribute added evidence and improve the understanding of what mechanisms cause mixing of river inflows. [Preview Abstract] 
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