Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session AS: Geophysical: Atmospheric I |
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Chair: Zellman Warhaft, Cornell University Room: 200G |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AS.00001: Effects of Release Characteristics on Urban Contaminant Dispersal A.J. Wachtor, F.F. Grinstein, H.J. Catrakis The release of a chemical, biological, or radioactive contaminant in an urban environment is of particular interest due to the high population densities in urban areas. The wind flow that transports the contaminant through the urban setting is highly complex and exhibits a wide range of multi-scale phenomena. Studies of urban flows can provide information that can be of critical importance to city, state, and federal officials for creating risk management plans. Classical field experiments measuring the dispersion of scalars in urban environments provide only rather limited results. Computational experiments have the advantage of being able to offer greater insight and knowledge about the three-dimensional flow physics than field experiments are able to provide. Implicit Large Eddy Simulation (ILES) is currently a promising computational method to obtain reasonable results of urban flows. ILES resolves the large scale flow features and relies on inherent numerical dissipation to model energy transfer from the resolved scales to the sub-grid scales. Since it is the large scale dispersion of the contaminant that is of key interest, ILES is particularly well suited for this application. NRL's FAST3D-CT model based on ILES is used to simulate scalar contaminant transport in a complex urban setting. We present a study of the effects that location and associated potential temperature of the scalar contaminant release have on the subsequent dispersion of that scalar within the specified urban geometry. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AS.00002: Temporal behavior of topographic wave-breaking Olivier Eiff, Nicolas Boulanger, Karine Leroux, Alexandre Paci At low Froude numbers, the internal waves generated by flow over an obstacle or mountain will overturn and break. In the atmosphere, this results in high altitude clear air turbulence but also affects the flow field below, the most commonly known effect being the acceleration the downslope winds. Surprisingly litte is known, however, of the dynamics of the wave breaking itself. Afanasyev and Peltier (JAS, vol. 55, 1998) investigated the wave breaking region via LES and Eiff et al. (DAO, vol. 40, 2005) via PIV measurements, but both presumed a statistically stationary wave-breaking process after the initial wave overturning. Here, we propose to take a closer look at this assumption by closely analyzing the spatio-temporal structure of internal wave breaking region and the surrounding flow. The analysis is based on Hovm\"{o}ller diagrams and spatial correlations obtained from 2D-PIV measurements of flows generated in uniform stratified flow over 2D and quasi-2D obstacles in salt-stratified hydraulic channels at different Reynolds numbers ranging from laminar to turbulent. The results reveal low frequency variations throughout the flow field, in and outside the wave-breaking region. This characteristic frequency can be related to be due to a sequence of growth and decay of wave-breaking. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AS.00003: Satellite observations of atmospheric water vapor distributions Kyle Pressel, William Collins The Intergovernmental Panel on Climate Change Fourth Assessment Report identified cloud feedback as the largest source of uncertainty in Global Climate Model (GCM) estimates of climate sensitivity. Cloud feedback is resultant from the sensitivity of clouds to the thermodynamic structure of the atmosphere which is in turn modified by the clouds themselves. Prognostic statistical cloud schemes have been developed to account for subgrid-scale cloud variability in a more physically consistent manner. Statistical cloud schemes assume a distributional form for some measure of water substance concentration and then determine cloud cover and properties based on a particular parameterization of that distribution. As the majority of atmospheric water substance exists in a vapor state, we will report preliminary results of a characterization of water vapor distributions based on retrievals from the Advanced Infrared Sounder (AIRS) onboard NASA's Aqua satellite. We will report on the vertical variation of distributional forms with height and comment on the physical mechanisms maintaining these distributions. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AS.00004: Experimental study of the effect of turbulence on the dynamics of sedimenting inertial particles Colin Bateson, Alberto Molina, Alberto Aliseda, Hossein Parishani, Lian Ping Wang, Wojciech Grabowski Understanding the dynamics and mutual interactions of droplets in turbulent flows is important to many engineering and environmental problems including fuel injector sprays, warm rain formation, and the mass and energy transfer between the ocean and the atmosphere. Specifically, the collision and coalescence in turbulent flows is considered a key element for the growth of condensation droplets into a size range where gravitational settling mechanism can take over to produce rain drops. We study experimentally the effect of turbulence on the collision-coalescence of water droplets over a parameter range relevant to rain formation. Droplets in a size range between 1 and 40 microns are injected inside a low speed wind tunnel through an array of atomizers located at the nodes of a turbulence-inducing grid that covers the tunnel's cross section with uniform spacing. The evolution of the droplet size distribution, concentration and settling velocity is measured along the wind tunnel's test section. We will present a comparison between experimental measurements of the one and two dimensional droplet radial distribution functions and collision statistics against equivalent quantities computed from a three dimensional numerical simulation performed and presented here by Wang et al. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AS.00005: Statistics of Small-Scale Velocity Fluctuations and Internal Intermittency in Stratocumulus Clouds Raymond Shaw, Holger Siebert, Zellman Warhaft Clouds are known to be turbulent but the details of their internal turbulent structure have been largely unexplored. Measurements of turbulent velocities in stratocumulus clouds reveal an intermittent structure consistent with that observed in classic homogeneous isotropic turbulence. The measurements were taken using a hotwire anemometer on the helicopter-borne ACTOS measurement system. Hotwire signal artifacts resulting from droplet impacts are removed without significantly degrading the signal, such that high-order velocity structure functions can be evaluated. The structure function analysis for orders 2 through 8 show statistically significant departures from the Kolmogorov 1941 scaling, yielding scaling exponents consistent with the Kolmogorov-Obukhov refined similarity hypothesis with an intermittency exponent of 0.25. We find no evidence of any departure from the large body of knowledge obtained from the laboratory on the fine scale turbulence structure. This suggests that processes depending on the fine-scale structure of turbulence that cannot presently be measured in clouds can be explored in the laboratory setting. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AS.00006: Remote flow sensing of complex systems: steps towards spatio-temporal~prediction of flow patterns Bruno Monnier, Paritosh Mokhasi, Dietmar Rempfer, Candace Wark Prediction of the spatial and temporal phenomena of wind flow patterns~through urban areas is investigated. Typically sparse measurements~are used in wind forecasting models for updating and prediction via a~method~called variational data assimilation. To improve upon this method, an~experimental investigation combining various measurement tools (Hot Wire~Anemometry,~Stereoscopic Particle Image Velocimetry SPIV), static pressure~measurements and Laser Doppler Velocimetry(LDV)) is carried out to study~the~airflow around wall mounted obstacles in a turbulent boundary layer.~The method of Proper Orthogonal Decomposition (POD) is used to~decompose the flow field into a finite set of POD coefficients which~vary only in time associated with a corresponding set of POD basis~functions which vary only in space. Direct measurement models utilizing~the~measurements from SPIV and LDV, along with indirect measurement models~using~sparse measurements from microphones are investigated and may ultimately~be~combined with state-space models to obtain more robust dynamical models. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AS.00007: Wind speed and direction measurements using the sphere anemometer Hendrik Heisselmann, Michael Hoelling, Joachim Peinke In times of growing energy demand, the importance of wind energy is rapidly increasing and so is the need for accurate wind speed and direction measurements. The widely spread cup anemometers show significant over-speeding under turbulent wind conditions as inherent in atmospherical flows while being solely capable of detecting the wind speed. Therefore, we propose the newly developed sphere anemometer as a simple an robust sensor for direction and velocity measurements. The sphere anemometer exploits the velocity-dependent deflection of a tube, which is the order of $\mu$m and can be detected by means of a light pointer as used in atomic force microscopes. In comparative measurements under laboratory conditions the sphere anemometer showed a significantly higher temporal resolution then cup anemometers while it does not exhibit any over-speeding. Additionally, results of atmospherical wind measurements with the sphere anemometer and state-of-the-art cup anemometry are presented. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AS.00008: Experimental study of \emph{starting plumes} simulating cumulus cloud flows in the atmosphere Duvvuri Subrahmanyam, K.R. Sreenivas, G.S. Bhat, S.S. Diwan, Roddam Narasimha Turbulent jets and plumes subjected to off-source volumetric heating have been studied experimentally and numerically by Narasimha and co-workers and others over the past two decades. The off-source heating attempts to simulate the latent heat release that occurs in cumulus clouds on condensation of water vapour. This heat release plays a crucial role in determining the overall cloud shape among other things. Previous studies investigated steady state jets and plumes that had attained similarity upstream of heat injection. A better understanding and appreciation of the fluid dynamics of cumulus clouds should be possible by study of \emph{starting plumes}. Experiments have been set up at JNCASR (Bangalore) using experimental techniques developed previously but incorporating various improvements. Till date, experiments have been performed on plumes at $Re$ of 1000 and 2250, with three different heating levels in each case. Axial sections of the flow have been studied using standard PLIF techniques. The flow visualization provides us with data on the temporal evolution of the \emph{starting plume}. It is observed that the broad nature of the effect of off-source heating on the \emph{starting plumes} is generally consistent with the results obtained previously on steady state flows. More complete results and a critical discussion will be presented at the upcoming meeting. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AS.00009: Attracting structures in volcanic ash transport Jifeng Peng Volcanic eruptions and ash clouds are a natural hazard that poses direct threats to aviation safety. They may also affect human and ecosystem health. Many transport and dispersion models have been developed to forecast trajectories of volcanic ash clouds, as well as to plan safety measures. Predictions based on these models are heavily dependent on initial parameters of ash clouds, e.g., location, height, particle size and density distribution, water vs. ash content, etc. However, these initial parameters are usually difficult to determine, leading to possible inaccurate predictions of ash clouds trajectories. In this study, a dynamical systems approach is combined with volcanic ash transport models to help improve prediction. A type of attracting structures in volcanic ash transport is identified. These structures act as attractors in volcanic ash transport, and they are independent of initial parameters of specific volcanic eruptions. The attracting structures are associated with hazard zones with high concentrations of volcanic ash. And the prediction in hazard maps can be used to plan flight route diversions and ground evacuations. [Preview Abstract] |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AS.00010: Mathematical Model for the Behavior of Wildfires Kevin DelBene, Donald Drew Wildfires have been a long-standing problem in today's society. In this paper, we derive and solve a fluid dynamics model to study a specific type of wildfire, namely, a two dimensional flow around a concentrated line of fire, resulting in a narrow plume of hot gas rising and entraining the surrounding air. The model assumes that the surrounding air is constant density and irrotational, and uses an unsteady plume model to describe the evolution of the mass, momentum and energy inside the plume, with sources derived to model mixing in the style of Morton, Taylor, and Turner (Proc. Roy. Soc. London, A 234, 1-23, 1956). The sources to the dynamical processes in the plume couple to the motion through the surrounding air through a Biot-Savart integral formulation to solve the equations of motion with a line of singularities along the plume. The singularities model a vortex sheet in the same manner as Alben and Shelley (Phys. Rev. Letters, 100, 074301, 2008), except that we include a sink term in the Biot-Savart integral to couple the entrainment. The results show that this model is capable of capturing a complicated interaction of the plume with the surrounding air. [Preview Abstract] |
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