Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GG: GFD: Atmospheric Flows I |
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Chair: Karan Venayagamoorthy, Colorado State University Room: Long Beach Convention Center 103B |
Monday, November 22, 2010 8:00AM - 8:13AM |
GG.00001: ABSTRACT WITHDRAWN |
Monday, November 22, 2010 8:13AM - 8:26AM |
GG.00002: A dynamic roughness model for LES of flow over multiscale, fractal-like surfaces: application to synthetic and real topography William Anderson, Charles Meneveau The topography of many natural surfaces encountered in geophysical flows is known to be multiscale and fractal-like. We present the so-called dynamic surface roughness (DSR) model, a framework for representation of drag effects imposed on a high- Reynolds number boundary layer by a multiscale rough surface. In developing and testing the DSR model, we consider synthetic fine-grained multiscale surfaces with spectral exponent $\beta_s$ ranging between -3.0 (smoothest) to -1.2 (roughest). The fine-grained surface is spatially filtered at the large-eddy simulation (LES) resolution, resulting in a resolvable and subgrid-scale (SGS) component. The SGS component is represented with an effective roughness length. The effective roughness length is written as the product of local SGS roughness root-mean-square and an unknown roughness index, $\alpha$. We dynamically evaluate $\alpha$ with a self- consistency condition applied to the plane-average of total wall stress resolved at the grid- and a test-filter width. Results for flow over synthetic surfaces indicate strong dependence of $\alpha$ on surface spectral exponent. We also apply the DSR model to LES of flow over a real landscape using digital elevation map data from the USGS. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GG.00003: Applying renormalized numerical simulation to model turbulent flow over a fractal tree canopy Jason Graham, Charles Meneveau Renormalized Numerical Simulation (RNS) is a down-scaling approach that uses drag forces from resolved flow fields to parameterize the drag forces due to unresolved scales (Chester et al., 2007, J. Comp. Phys.). The RNS procedure is analogous to the dynamic sub-grid scale model. In RNS a form drag model is used to parameterize the forces and the drag coefficient, $c_D$, is dynamically evaluated by learning from the large scale problem and recursively feeding back to the small scale problem the renormalized drag forces. In this study a suite of Large Eddy Simulations using RNS are performed to analyze boundary layer flow over a canopy of fractal trees. The fractal trees provide complex boundary-turbulence interactions while maintaining tractable characteristics that can be systematically studied. Resolved branches are represented in the LES using the immersed boundary method. Several RNS implementations are tested and compared: 1) explicit and 2) implicit time formulations, and two spatial treatments for $c_D$: 1) local 2) global definitions. For these set of simulations the time averaged flow field, Reynolds and dispersive stresses, and drag forces of the canopy are computed. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GG.00004: Challenges in large-eddy simulation of cumulus convection Georgios Matheou, Daniel Chung, Louise Nuijens, Bjorn Stevens, Joao Teixeira High-resolution simulation is a vital tool for studying the physical processes in the atmospheric boundary layer. In spite of the numerous encouraging large-eddy simulation (LES) results, prediction of complex turbulent flows continues to present many challenges. The present study considers the impact of various choices pertaining to the numerical solution of the governing equations on the LES prediction and the association of these choices to flow physics. Simulations corresponding to the trade wind precipitating shallow cumulus composite case of the Rain In Cumulus over the Ocean (RICO) field experiment were carried out. Global boundary layer quantities such as cloud cover, surface precipitation rate, power spectra and the overall convection structure were used to compare the effects of different discretization implementations, grid resolution and computational-domain size. The different discretization implementations were found to exert a significant impact on the LES prediction. The observed differences can be attributed to the nonlinear nature of moist convection, especially when precipitation is present, which results in an increased sensitivity of the atmospheric boundary layer statistics to the representation of small-scale turbulence. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GG.00005: Large-eddy Simulation of Turbulent Flows in an Urban Street Canyon Jeong-Min Hwang, Byung-Gu Kim, Changhoon Lee Turbulent flow inside an urban street canyon is studied by means of large-eddy simulation. The simulated site is the 'Teheran Street' in Gangnam district of Seoul in Korea, which is one of the representative street canyon in Korea. The Reynolds number, based on the height of the tallest building in the domain and mean velocity there, is around ten million, The domain size is 600m in each direction, and tested grid size varies from 2m to 12m while typical small buildings are of order of 20 m. A constant Smagorinsky coefficient subgrid-scale (SGS) model is used. Performance of the SGS model for various resolutions is assessed by investigating contribution by the SGS stresses to the total stresses. Also, the statistics of the flow and turbulence is investigated by changing wind direction. Many elements such as wind direction, height, shape, and distribution of buildings are found to be the key factors affecting flow field characteristics. Particularly, tall buildings near the street canyon predominantly generate turbulence, leading to homogenization of the mean flow inside the street canyon, Detailed simulation results will be presented in the conference. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GG.00006: Large-eddy simulation of the stable atmospheric boundary layer with explicit filtering and reconstruction Fotini Chow, Bowen Zhou Large-eddy simulation (LES) of the stably stratified atmospheric boundary layer is performed using an explicit filtering and reconstruction approach with a finite difference method. The dynamic reconstruction model (DRM) is used to represent the resolvable subfilter-scale and subgrid-scale stresses which make up the total turbulent stress. Several surface cooling rates are used, ranging from mildly stable to strongly stable intermittent turbulence cases. A low-level jet develops with associated turbulent kinetic energy (TKE) generated around the top of the boundary layer, in agreement with field observations. The role of filtering on generation of this elevated TKE is explored. The ability of the DRM to represent energy backscatter from small to large scales is examined as a function of surface cooling and grid resolution. A turbulent bursting event is analyzed during intermittent conditions. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GG.00007: Sensitivity Study of Contrail Development: Large Eddy Simulation and Parameterized Model Alexander Naiman, Sanjiva Lele, Mark Jacobson The development of aircraft condensation trails is sensitive to factors including ambient relative humidity, aircraft type, and environmental turbulence. The effect of these parameters on the transition from linear contrails to induced-cirrus clouds is a key uncertainty in estimating the impact of contrails on climate. A sensitivity study of these parameters has been conducted using a three-dimensional Large Eddy Simulation (LES) of the first twenty minutes of contrail development. The LES solves the incompressible Navier-Stokes equations with a Boussinesq approximation. The numerical scheme uses a second-order finite volume spatial discretization and an implicit fractional-step method for time advancement. Lagrangian contrail particles grow according to a model of ice deposition and sublimation. We present results in which turbulence, wind shear, and aircraft type were varied. Additional cases include variations in microphysical processes and in initial conditions. Results from the LES are compared to a simple parameterization of plume dynamics developed to model aircraft emissions in a global climate model. The parameterized model is shown to be valid for the late stages of the LES model results. Additional LES work will be required to validate the parameterized model to time horizons later than twenty minutes, which are relevant for the transition from linear contrail to induced cirrus. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GG.00008: Modeling the Influence of Wind Characteristics and the Atmospheric Stability on Wind Turbine Performances Darko Koracin, Radian Belu The uncertainty of wind turbine performance measurements is closely related to the uncertainty of the wind velocity and other meteorological parameters. An inherent uncertainty in the power curve estimate is by using the wind speed measured at the hub height, as such considerable deviations often occur between the expected and produced power. Wind shear, direction changes, turbulence and atmospheric stability vary with height because of either meteorological and/or terrain conditions. The rotor size combined with the hub height of large turbines implies that turbines are often exposed to highly varying wind conditions (large wind and direction shears, turbulence and atmospheric stability) within the rotor span. These parameters will affect turbine structural safety and production. Velocity, temperature, and turbulence intensity are generated using a model developed from Monin-Obukov similarity theory and the k-$\varepsilon $ turbulence model to resolve the atmospheric parameters (friction velocity, Monin-Obukov length, temperature scale, and roughness length). The resulting nonlinear equations were solved numerically and tested against the observations. The rotor averaged wind speed is evaluated by numerically integrating the resulting velocity profile over the rotor area. Power output estimates were compared with the available data (manufacturers and literature) and are used in the turbine design. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GG.00009: Visualizing the effects due to a wind turbine in a stratified turbulent boundary layer Nicholas Hamilton, Raul Bayoan Cal As sustainable technologies and energy generation become more prolific, the need for larger wind farms becomes highly evident. It has been hypothesized that the behavior of heat and moisture transfer between the air and the ground is altered in the wakes of wind turbines. An experimental study at the Complex Boundary-layer and Wind Energy Based (CoBWEB) wind tunnel in Portland State University is performed to visualize the effects of these rotating structures under stratified conditions, thus modeling environments observed by a wind turbine array. A Schlieren technique is applied to study the interaction between the turbulent thermal boundary layer and the wind turbine. The Schlieren system employed here captures the temperature differences between the heat supplied through the floor of the wind tunnel and the air stream to image focal planes in upstream and downstream positions of a wind turbine. The data collected from this study demonstrates observable differences and effects due to the presence of the wind turbine. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GG.00010: Insight on Turbulence Characteristics of an Urban-type Boundary Layer Bruno Monnier, Jonathan Swanson, Candace Wark 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. Stereoscopic PIV is utilized to obtain 3D flow characteristics of the flow field within this complex geometry. The streamwise spacing of the array is chosen so as to mimic a common flow regime in urban areas, i.e. skimming flow regime. A large number of vertical planes distributed across the streets allows for a very good spatial description of the flow features. Measurements are obtained directly upstream of the model and in each of the middle streets of the 4 by 3 array. Coherent structure identification tools are used to highlight the 3D patterns within each of the streets. A large number of SPIV realizations in the domain provides valuable information about the flow field turbulence statistics as the flow is evolving from one street to the next. The incidence angle of the incoming flow field is also varied to assess the effect of flow channeling within the urban environment. Finally, two mean free stream speeds are studied to investigate the effect of the incoming wind profiles on the flow field turbulence. [Preview Abstract] |
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