Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session HV: Geophysical Flows: Coherent Structure and Transportation |
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Chair: George Haller, Massachusetts Institute of Technology Room: Ballroom B |
Monday, November 24, 2008 10:30AM - 10:43AM |
HV.00001: Finding Lagrangian Coherent Structures from Spatially Limited Flow Data Wenbo Tang, George Haller, P.W. Chan Extracting Lagrangian Coherent Structures (LCS) from a flow is challenging when a velocity field is only observed or computed over a spatially limited domain. In such cases, Lagrangian particle tracking is typically stopped on the boundaries of the domain, which turns the boundaries into artificial attractors for the particle motion. In addition, trajectories heading towards, and then stopped at, different boundaries tend to generate artificial shear-type LCS inside the domain. We present a technique borrowed from invariant manifold theory that eliminates artificial LCS and enhances real LCS in the Lagrangian flow analysis of flows on limited domains. We apply this technique to extract LCS from a flow fitted to LIDAR observations near Hong Kong Airport. Our improved LCS extraction method eliminates noise from the results and enhances LCS strongly correlated with topography. [Preview Abstract] |
Monday, November 24, 2008 10:43AM - 10:56AM |
HV.00002: An LES study of pollen dispersal from isolated populations: Effects of source size and boundary-layer scaling Marcelo Chamecki, Charles Meneveau, Marc B. Parlange A framework to simulate pollen dispersal in the atmospheric boundary layer based on the large eddy simulation technique is developed. Pollen is represented by a continuum concentration field and is evolved following an advection-diffusion equation including a gravitational settling term. The approach is validated against classical data on point-source releases and our own field data for a natural ragweed field. The LES is further used as a tool to investigate the effect of source size on the patterns of pollen ground deposition, an issue of fundamental importance in the development of policies for genetically modified crops. The cross-wind integrated deposition is shown to scale with the pollen boundary-layer height at the trailing edge of the field and a simple practical expression based on the development of the pollen boundary layer is proposed to scale results from small test fields to realistic agricultural conditions. [Preview Abstract] |
Monday, November 24, 2008 10:56AM - 11:09AM |
HV.00003: LES study of polluted dust dispersion in a Trieste densely populated area Valentina Stocca, Francesco Montanari, Fulvio Stel, Vincenzo Armenio, Katepalli R. Sreenivasan Large eddy simulations are employed to study dispersion of pollutants in urban areas. We have developed the model LES-AIR to accurately predict under critical wind conditions the dispersion of dust from polluting steelworks. The model is applied to the study of a suburban area of Trieste, where the coexistence of an obsolete steel plant and civil habitation within few square kilometers has led to serious health risks. LES-AIR is a curvilinear, second order accurate, finite difference, large eddy simulation solver. The LES-AIR sub-grid model, whose validity has been tested against previous numerical studies of the atmospheric boundary layer, is a mixed model composed of a Smagorinsky and a scale-similar part. We have simulated a domain extending 1.5 km x 1 km in horizontal directions, and 0.6 km in the vertical. The combined usage of a terrain following curvilinear coordinate system, which uses a refined grid in the near ground region and the immersed boundary technique, allows us to realistically reproduce the shapes of buildings. The model validation as well as results from the LES-AIR Trieste case study will be presented. [Preview Abstract] |
Monday, November 24, 2008 11:09AM - 11:22AM |
HV.00004: Measuring droplet coalescence rates in turbulent Gregory Bewley, Kelken Chang, Eberhard Bodenschatz An outstanding problem in the understanding of droplet suspensions is the accurate prediction of the rate of coalescence of the droplets in a turbulent flow. A notable application is in cloud physics, where it is the coalescence of droplets that forms rain. We present results from measurements of droplet coalescence rates in a turbulent airflow as a function of Stokes and Reynolds number. We observe the dynamics of water droplets between 5 and 20 microns in diameter in isotropic and anisotropic turbulence with $R_\lambda$ up to 350. The data complement a long history of numerical and theoretical work. [Preview Abstract] |
Monday, November 24, 2008 11:22AM - 11:35AM |
HV.00005: Coherent Structure Identification Techniques applied to Stereoscopic PIV Measurements in an Urban-type Boundary Layer Candace Wark, Bruno Monnier, Brian Neiswander An experimental investigation of the flow through an urban-type boundary layer (4 rows of 3 cuboid Plexiglas blocks) in an experimentally modeled atmospheric boundary layer will be presented. This work focuses on the effect of the incidence angle of the approaching boundary layer as well as the streamwise spacing between adjacent rows covering two different flow regimes, the wake interference and skimming flow regimes. This study utilizes Stereo PIV measurements: a 2D traverse system carrying the entire SPIV system allows us to precisely position the measurement plane at hundreds of positions within the domain. The spacing between adjacent planes is chosen in order to resolve details close to the edges of the blocks. Various existing coherent structure identification tools are used and compared: isosurfaces of vorticity, lambda 2, swirling strength, second invariant of the velocity gradient tensor (Q) and normalized angular momentum. [Preview Abstract] |
Monday, November 24, 2008 11:35AM - 11:48AM |
HV.00006: On the transition between saltation and suspension on Earth and on Mars David Korda, Robert Sullivan, Donald Banfield, Lance Collins If the wind blowing along a bed of sand grains exceeds the threshold friction speed, it will induce the grains to move either by saltation (grains bumping along the surface) or suspension (grains swept by the turbulent eddies). The transition between these two modes of motion on Earth is governed by the Rouse number, defined as the ratio of the terminal velocity to the threshold friction speed. On Earth the transition from suspension to saltation occurs at a Rouse number of unity. Images taken by the Mars Exploration Rover (MER) show evidence of saltation at Rouse numbers considerably below unity. Our study re-examines the physics of the transition from suspension to saltation using direct numerical simulations (DNS) of channel flow. Tracking the motion of particles at conditions that mimic the atmosphere on Earth and on Mars, we have identified a second parameter, the particle Stokes number, that also influences the transition. The trends we observe are consistent with the MER images that show a shift in the transition Rouse number to lower values. [Preview Abstract] |
Monday, November 24, 2008 11:48AM - 12:01PM |
HV.00007: Flushing of a buoyant pollutant from an urban canyon Nigel Kaye, Gary Hunt, Konstantinos Syrios We examine the wind driven flushing of a negatively buoyant
pollutant from an urban canyon. We examine the rate at which a
fluid of buoyancy $g'$ is removed from a two dimensional urban
canyon of width $W$ formed by two buildings of height $H$ and
square cross section. The flushing is driven by a wind flow, of
mean velocity $U$, normal to the axis of the canyon. A previous
study of flushing of a neutrally buoyant fluid by Caton {\it{et
al.}} (2003) showed that the mean concentration decayed
exponentially with time indicating a steady exchange of fluid
between the canyon and the flow above and a uniformly well mixed
canyon. However, for the case of a negatively buoyant pollutant,
the fluid buoyancy will tend to suppress fluid exchange as
additional work is required to lift the dense fluid up and out of
the canyon. One would therefore expect that the flushing rate
would be a function of the Froude number $Fr=U/\sqrt{g'H}$ as
well as the aspect ratio of the canyon $H/W$. In our experiments
the canyon does not remain well mixed, but rather the buoyant
fluid is skimmed off the top and the pollutant is etched away
over time. We present experimental results for $H/W=0.5$ and $1$
and $0.4 |
Monday, November 24, 2008 12:01PM - 12:14PM |
HV.00008: Generalized fixed points and the Lagrangian structure of time dependent geophysical flows Ana M. Mancho, Jose A. Jimenez Madrid Aperiodic geophysical flows are poorly understood as theory which is well established in autonomous or periodic flows is not directly applicable to them. In stationary flows the idea of {\it fixed point} is a keystone to describe geometrically the solutions of the dynamical system. The concept of fixed point is extended to time periodic flows by means of the Poincar\'e map, as periodic orbits with $T$ period become fixed points on the Poincar\'e map. To gain insight on the geometrical structure of aperiodic flows typically are used concepts such as Lyapunov exponents and its finite time versions (FSLE and FTLE). In this presentation we propose to this end a generalisation of the concept of fixed point to aperiodic dynamical systems: the distinguished trajectory. In the context of highly aperiodic realistic flows our definition characterizes trajectories and states that they hold the property of being distinguished in a finite time interval. Previous works by Ide {\em et al.} and Ju {\em et al.} have addressed the existence of {\it distinguished hyperbolic trajectories} but our new definition shows that non-hyperbolic orbits may also fall within this category. This type of trajectories might be of special interest for their applications in oceanography as they are related to eddies or vortices. [Preview Abstract] |
Monday, November 24, 2008 12:14PM - 12:27PM |
HV.00009: Numerical simulations of aquaculture dissolved waste transport in a coastal embayment Subhas Venayagamoorthy, Oliver Fringer, Jeffrey Koseff, Rosamond Naylor The present study focuses on understanding the transport and fate of dissolved wastes from aquaculture pens in near-coastal environments using the hydrodynamics code SUNTANS (Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier- Stokes Simulator), which employs unstructured grids to compute flows in the coastal ocean at very high resolution. Simulations of a pollutant concentration field (in time and space) as a function of the local environment (bathymetry, rotation), flow conditions (tides, wind-induced currents and wind stress), and the location of the pens were performed to study their effects on the evolution of the waste plume. The presence of the fish farm pens causes partial blockage of the flow, leading to the deceleration of the approaching flow and formation of downstream wakes. Results of both the near-field area (area within 10 to 20 pen diameters of the fish-pen site) as well as far-field behavior of the pollutant field are presented. These results highlight for the first time the importance of the wake vortex dynamics on the evolution of the near-field plume as well as the rotation of the earth on the far-field plume. The results provide an understanding of the impact of aquaculture fish-pens on coastal water quality. [Preview Abstract] |
Monday, November 24, 2008 12:27PM - 12:40PM |
HV.00010: Sediment wave formation by turbidity currents: a Navier-Stokes based linear stability analysis Brendon Hall, Lutz Lesshafft, Eckart Meiburg, Ben Kneller We explore the formation of sediment waves by turbidity currents, based on a linear stability analysis of the bottom boundary layer in a turbidity current. The analysis employs the 2D Navier-Stokes equations for the fluid, and it accounts for the coupled interaction of fluid and suspended particle motion with the erodible bed below. Wavy perturbations of the bottom topography may either be amplified or leveled out under the competing effects of sediment deposition and erosion. The destabilizing effect of the base flow on the stability of the bedform is modulated by the perturbation eigenmodes of sediment deposition and of erosive shear stress. The phase relation between these two perturbation fields determines the total growth rate and phase velocity of the sediment wave. Upstream-traveling waves are dominantly caused by preferred erosion of sediment into the flow along the downstream side of the interface wave, in qualitative agreement with existing experimental and numerical investigations. Results indicate that both short- and long-wavelength modes are amplified. The associated short-wavelength eigenmodes travel at negative phase velocities over parameter regimes that are typical of turbidity currents. [Preview Abstract] |
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