Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A1: Geophysical: Atmospheric I |
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Chair: William Layton, University of Pittsburgh Room: 323 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A1.00001: Coupling building-resolving LES with meso-scale NWP: effect of the simulation parameters Kyongmin Yeo The effects of the simulation parameters on the scalar dispersion in an urban area are reported. To study scalar dispersion under realistic meteorological conditions, the building-resolving large-eddy simulations (LES) are driven by downscaling boundary conditions from a numerical weather prediction model (NWP). Here, we focus on the changes in the dispersion characteristics under the different downscaling modes and computational parameters. Two sets of numerical simulations are performed for transient morning and unstable day-time atmospheric boundary layers. It is shown that the scalar dispersion is strongly affected by the downscaling method. The computational domain size also has a significant effect on the scalar dispersion even for the ground release events. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A1.00002: Measurements of Roughness Length and Displacement Heights in Model Urban Canopies Auvi Rahman, Pablo Huq, Fernando Camelli We present the results of roughness length and displacement height based on PIV velocity measurements in a water tunnel experiment of flow over idealized models of urban canopies. Experiments were conducted with large roughness elements of regular arrays of buildings of uniform height with aspect ratios of 1 and 3. Mean velocity profile above the canopy is described by the log law and a simple optimization procedure to compute the roughness length and displacement height has been developed. Laterally averaged values of displacement height d/H increase from 0 to 1 with plan area density $\lambda_{\mathrm{p}}$ of the urban canopy. In contrast, laterally averaged roughness height z$_{\mathrm{0}}$/H increases to a maximum value (as $\lambda _{\mathrm{f}}$ approaches a value of 0.2) and then decreases to zero. We present data for effective roughness heights (z$_{\mathrm{0}}+$d)/H as a function of aspect ratio H/w$_{\mathrm{b}}$ of buildings. This also reveals three categories of values: street canyon, building wake, and laterally averaged values. Measurements taken at the centerline of canyons form a lower bound on the effective roughness height whereas measurements behind building wakes form an upper bound. Laterally averaged values of friction velocity u$_{\mathrm{\ast}}$/U$_{\mathrm{H}}$ varied inversely with the aspect ratio (U$_{\mathrm{H}}$ is the mean velocity at the building height). [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A1.00003: Coupled Convective and Radiative Heat Transfer Simulation for Urban Environments Stefan Gracik, Mostapha Sadeghipour, George Pitchurov, Jiying Liu, Mohammad Heidarinejad, Jelena Srebric A building's surroundings affect its energy use. An analysis of building energy use needs to include the effects of its urban environment, as over half of the world's population now lives in cities. To correctly model the energy flow around buildings, an energy simulation needs to account for both convective and radiative heat transfer. This study develops a new model by coupling OpenFOAM and Radiance, open source packages for simulating computational fluid dynamics (CFD) and solar radiation, respectively. The model currently provides themo-fluid parameters including convective heat transfer coefficients, pressure coefficients, and solar heat fluxes that will be used as inputs for building energy simulations in a follow up study. The model uses Penn State campus buildings immersed in the atmospheric boundary layer flow as a case study to determine the thermo-fluid parameters around buildings. The results of this case study show that shadows can reduce the solar heat flux of a building's surface by eighty percent during a sunny afternoon. Convective heat transfer coefficients can vary by around fifty percent during a windy day. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A1.00004: Modeling and measuring neighborhood scale flow, turbulence, and temperature within Chicago heat island Patrick Conry, Ashish Sharma, Laura Leo, H.J.S. Fernando, Mark Potosnak, Jessica Hellmann The modeling of urban heat island (UHI) requires a multi-scale approach as it involves numerous physical phenomena spanning a range of scales. We have performed a comprehensive study of Chicago's UHI via coupling of mesoscale Weather Research and Forecasting (WRF) and micro-scale ENVI-met models. The application of the latter model to a Lincoln Park neighborhood and a parallel observational campaign will be the primary focus of this presentation. ENVI-met employs a computational fluid dynamics model to represent heterogeneity of urban areas, providing fine resolution output of UHI dynamics. In the field campaign, two stations located on rooftops of DePaul University buildings were each equipped with a sonic anemometer and vertical array of thermocouples, allowing investigations of spatial variability of flow, turbulent fluxes, and temperature profiles in an urban roughness sublayer. One of these was located above a rooftop garden and the other above a conventional rooftop. Downscaled output from the WRF model or a set of observational data served as initial and boundary conditions for the ENVI-met model. The model's predicative capabilities were assessed through comparison with another set of observational data, and dynamical causes for the model's poor behavior were identified. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A1.00005: Quantifying the effect of inflow variability in RANS simulations of the JU2003 field experiment Catherine Gorle, Clara Garcia Sanchez, David Philips, Gianluca Iaccarino Predicting flow and dispersion in realistic urban canopies is challenging because of the high variability in the governing flow parameters, such as atmospheric conditions and street-level geometrical characteristics. As a result, one deterministic prediction for a specific condition is unlikely to provide an adequate representation of the problem and uncertainty quantification is required to determine confidence bounds on the predictions. Assessing the predictive capability of the resulting model requires validation with field measurements that represent the full complexity of the problem. In this study we present a comparison of the JU2003 field measurements with computational results from RANS simulations performed within an uncertainty quantification framework. The variability in the inflow conditions observed during the field experiment is represented in the simulations, and regions in the urban canopy that are particularly sensitive to this variability are identified. The standard deviation in the results is compared to that observed during the field measurements. Three uncertain variables were considered: the velocity magnitude and direction and the aerodynamic roughness used in the log law that defines the incoming boundary layer profile. A sparse grid Clenshaw-Curtis Stochastic Collocation approach was used, and a polynomial chaos representation of the velocity at different field measurement locations was constructed to extract the mean and standard deviations. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A1.00006: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A1.00007: Approximate Deconvolution Large Eddy Simulation of Atmospheric Turbulence in Spectral Space Leila Nasr Azadani, Anne Staples Numerical simulations of geophysical turbulence are best performed by large eddy simulation (LES) in which large geophysical scales are solved numerically and effect of physical and dynamical processes accruing at small scales are modeled. Most LES closure models are based on the forward energy cascade from the large scales to the small scales in three-dimensional turbulence. Since, under most conditions, geophysical flows are considered to be approximately two-dimensional turbulent flows, it is questionable to apply these closure models for LES of geophysical flows. Here we present the approximate deconvolution (AD) closure model as a purely mathematical approach, not based on any physics modeling, for LES of turbulent flows. The AD model employs repeated filtering of the filtered variables to obtain an approximation of the unfiltered variables. We apply the AD closure model in the LES of the bartoropic vorticity equation on the sphere in spectral space. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A1.00008: Direct Statistical Simulation of a Two-Layer Primitive Equation Model Wanming Qi, Brad Marston Low-order statistics of the large-scale circulation of planetary atmospheres may be directly accessed by solving the equations of motion for the equal-time statistics. We implement such Direct Statistical Simulation of a two-layer primitive equation model by systematic expansion in the cumulants. The first cumulant is the zonally averaged vorticity, divergence, and temperature as a function of latitude and level, and the second cumulant contains information about nonlocal teleconnections. At second order (CE2) the expansion retains the eddy -- mean-flow interaction but neglects eddy-eddy interactions and is realizable. Eddy-eddy interactions appear at third (CE3) order, but care must be taken to maintain realizability with a non-negative probability distribution function. The cumulant expansion is conservative, order-by-order, in the total angular momentum, total energy, and mean-squared potential temperature. First and second cumulants accumulated by time-integration of the two-layer primitive equations are compared with those obtained at the fixed points found at CE2 and CE3 levels of approximation. CE2 reproduces qualitative features of the zonal mean general circulation such as the mid-latitude jets. CE3 improves quantitative agreement in the teleconnections. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A1.00009: Radiative instabilities in vertically sheared rotating stratified flows Christophe Millet, Fran\c{c}ois Lott One of the preferential location for the breakdown of balanced dynamics are inertial levels. Across these, balanced disturbances become inertia-gravity waves in the linear approximation. In this work, we analyse how an incident potential vorticity anomaly triggers a baroclinic instability in a rotating stratified fluid with a vertical constant shear. The destabilized character of the wave emission is shown to be associated with the presence of an inertial critical layer that couples a balanced edge wave near the ground and gravity waves aloft. One striking feature of the eigenfunctions is that the gravity wave field appears to have a pronounced asymmetry in meridional wavenumber, with larger amplitudes for horizontal wave vectors pointing toward the warm air. The theoretical predictions for the frequency and growth rate of the normal modes are shown to be in good agreement with the WKB approximation for large Richardson numbers; the latter includes an exponentially small term which captures the radiation feedback in the region below the inertial level. [Preview Abstract] |
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