Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FL: Geophysical: Atmospheric I |
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Chair: Leslie Smith, University of Wisconsin, Madison Room: Salt Palace Convention Center 250 F |
Monday, November 19, 2007 8:00AM - 8:13AM |
FL.00001: Scaling of Turbulent Flow through Model Building Canopies H.J.S. Fernando, D. Zajic Flow and turbulence in building canopies continue to be a topic of profound interest in the context of pedestrian comfort, wind loading, contaminant dispersion and energy usage of populated urban areas. To this end, a new velocity scale of turbulence for flow through urban building canopies is derived in terms of the approach flow variables and canopy morphology. A control volume method is used for the derivation, together with known drag laws for individual building elements. An instrumented mock building cluster made of a regular array of simple man-sized objects (trailers), placed in the atmospheric boundary layer, is used to investigate the efficacy of the turbulent scale derived, and a good agreement is found. The flow adjustment at the leading and trailing edges of the canopy was found to be in general agreement with a formulation by Belcher, Jerram and Hunt (JFM, 488, 2003). The parameterizations resulted from this study have applications to developing simple and fast transport and dispersion models for predicting contaminant distribution in building canopies. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FL.00002: Turbulent flow over different groups of cubical obstacles Stefano Leonardi, Ian Castro, Paolo Orlandi Atmospheric boundary layers flow over a rough surface, composed of buildings, hills, valleys, vegetation. The present investigation extends previous studies by carrying out a set of DNSs of flow over arrays of cubical obstacles with varying the plan densities (1:4, 16:81, 4:25, 16:121, 1:9, 1:25). The Reynolds number based on the bulk velocity and the obstacle height is \textit{Re }= 7000. Boundary conditions are periodic in the streamwise and spanwise directions, and free slip is applied on the upper boundary. The passive scalar diffusion equation is solved with Pr=1 and concentration c=1 at the bottom wall and c=0 at the upper boundary. The effect of the cubes is to increase the wall normal velocity fluctuations therefore the scalar is transported away from the wall most effectively. The drag is almost entirely due to the form drag, the frictional contribution being small. The roughness function and the roughness length scale well with \textit{the wall normal velocity rms at the crests plane}. This confirms that the effect of roughness on the overlying flow is via the wall normal velocity fluctuations. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FL.00003: Stereoscopic PIV Measurements in an Urban-type Boundary Layer Bruno Monnier, Brian Neiswander, Candace Wark, Dietmar Rempfer An experimental investigation of the flow through urban-type boundary layers (4 rows of 3 cuboid Plexiglas blocks) in a modeled atmospheric boundary layer, will be presented. This study utilizes SPIV, hot-wire and oil-film interferometry measurements. Hot-wire measurements provide input on the incoming flow while direct measurements of the wall shear stress are realized using oil-film interferometry. Flow dispersion in urban areas is highly 3-D; therefore, a 2D traverse system carrying the entire SPIV system was designed which allows us to precisely position the measurement plane. All three velocity components are measured in 2-D planes throughout the model. More than 300 data planes in a 102mm by 50mm by 500mm domain corresponding to the middle street of the urban model are presented. The spacing between adjacent planes is chosen in order to resolve details close to the edges of the blocks. 3D streamlines, vorticity contours, isosurfaces of the second invariant of velocity gradient and Reynolds stresses will be presented and serve as a unique database for the numerical model being developed in parallel at IIT (see talk by Kandala, Rempfer, Wark and Fischer). [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FL.00004: Large-Eddy Simulation of an Urban-Type Boundary Layer S. Kandala, D. Rempfer, C. Wark, P. Fischer Large eddy simulations based on the scale-dependent Lagrangian dynamic model (Meneveau, Phys. Fluids (17), 2005) allow for scale dependence. This is particularly relevant when the filter scale approaches the upper limits of the inertial range, which is typically the case when modeling urban boundary layers. Scale-dependent Lagrangian dynamic models are also known to exhibit favorable dissipation characteristics. In this talk we present the results of a numerical simulation of an urban-type boundary layer described in the talk by Monnier, Neiswander, Wark \& Rempfer. The domain consists of 4 rows of 3 cuboids placed in a wind tunnel. The inlet velocity conditions are obtained from hot-wire measurements upstream of the blocks. The flow is simulated using the dynamic Smagorinsky model available with the commercial software FLUENT and the scale-dependent Lagrangian dynamic model available with spectral element code nek5000. These results are compared with the PIV data obtained from the experiments. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FL.00005: Investigation of Reynolds stresses in a 3D idealized urban area using large eddy simulation Akshay Gowardhan, Eric Pardyjak, Inanc Senocak, Michael Brown Large eddy simulation (LES) of neutral flow through an array of cubes has been conducted with periodic boundary conditions in horizontal directions. In this paper, we first describe the model formulation and validate the simulation by comparing the mean flow and turbulence statistics with wind-tunnel experimental data from a cube array of buildings. The LES model is then used to investigate the physical mechanisms that lead to the low turbulent stresses that have been reported in the lower half of the urban canopy layer. To do this, the urban boundary layer is conceptually broken down into three distinct regions: (a) the urban roughness sub-layer, (b) street channels (roads with axis aligned with mean wind direction aloft) and (c) street canyons (roads with axis normal to the mean wind direction aloft). The distribution of the Reynolds stresses differs significantly amongst these regions. In a complex urban area, these regions can be observed intermittently at the same physical location, thus, stresses with opposite signs have the potential to cancel each other and on average yield a low magnitude. In this paper, mean turbulence statistics and spectra from high resolution LES have been analyzed for these scenarios and the results have been interpreted. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FL.00006: Particle Dispersion in the Urban Boundary Layer by Complete and Reduced-Order Flow Fields Retrieved from Lidar Data Quanxin Xia, Ching-Long Lin The four-dimensional variational data assimilation (4DVAR) method is used to retrieve complete 3D wind fields in the urban boundary layer from the Doppler lidar data measured during the Joint Urban 2003 atmospheric dispersion field experiment. The proper orthogonal decomposition technique is then applied to the retrieved wind fields to extract turbulent coherent structures (eigenmodes). It is found that the overall flow features, such as roll vortices, are captured by the first several low-order eigenmodes. These low-order eigenmodes are used to reconstruct the transient 3D reduced-order flow fields. We then study turbulent particle dispersion in the urban setting by those complete and reduced-order velocity fields. The Lagrangian particle dispersion model is first validated by using the large-eddy simulation (LES) data of an idealized convective boundary layer flow. The results show that the reduced-order flow fields could yield similar particle dispersion patterns as the complete flow fields. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FL.00007: The Effect of Geometry on the Wake Structure of a Surface Mounted Obstacle Bhagirath Addepalli, Eric Pardyjak, Michael Brown Experiments were conducted to better understand the flow structure in the wake of a square cylinder as a function of its height and develop a parameterization for tall buildings for the QUIC-URB wind model. The experiments were conducted in an open-circuit wind tunnel in a fully turbulent boundary layer. 2D PIV was used to measure the flow field along the vertical symmetry plane of the model buildings. Numerous experimental cases were run in which the geometry was varied by increasing the wall-normal height H of a square cylinder (where W=L; L is streamwise length and W is spanwise width) from H/L=1 to H/L=3 in increments of 0.3L. Preliminary results indicate that a saddle point appears for heights greater than H/L=1.6. The saddle is accompanied by a significant modification of the wake structure. This change can be attributed to the enhanced flux of momentum around the sidewall into the near-wake as the height of the model building is increased. Future work will include horizontal plane measurements in the wake of the model building to further explore the mechanisms that lead to the change in the flow structure. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FL.00008: Flow structures around a gable-roofed building model in tornado-like winds Zifeng Yang, Vasanth Balaramudu, Fred Haan, Partha Sarkar, Hui Hu Tornadoes are violently rotating columns of air which are considered as nature's most violent storms. In an average year, 800 $\sim $ 1000 tornados would occur in the U.S. alone, and cause about 80 deaths (on average), over 1500 injuries, and {\$}850 million worth of property damage. By using the world-largest tornado simulator of Iowa State University, a comprehensive experimental investigation was conducted to characterize the flow structures around a low-rise, gable-roofed building model in tornado-like winds. While pressure taps and force transducers were used to map the pressure distributions around the building model and measure the aerodynamic forces acting on the building model induced by the tornado-like winds, a high-resolution Particle Image Velocimetry (PIV) system was used to conduct detailed flow velocity field measurements around the gable-roofed building model. The ultimate objective of the present study is to quantify the surface winds generated by tornadoes and flow-structure interactions between tornadoes and built environments to assess wind-induced damage with the purpose of mitigating damage and improving public safety. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FL.00009: Numerical model for atmospheric contaminant cloud rise scenarios Yuliya Kanarska, Ilya Lomov, Tarabay Antoun, Lewis Glenn Our numerical approach includes fluid mechanical model which is the combination of a compressible GEODYN code and a Low Mach code (LMC). The first one is an explicit code and it is intended to simulate early stages of nuclear explosions up to 15 s. The second one is an implicit code based on a pressure projection method and it is intended to simulate subsequent cloud rise events up to few hours. We perform series of cloud rise scenarios ranging from idealized bubble rise problem to realistic air bursts. We analyze effects of compressible dynamics and different turbulent parameterizations on the cloud evolution. It is found that in a realistic configuration interaction of a reflected shock wave from the ground with a fireball affects significantly cloud evolution in contrast to idealized bubble rise simulations. We show that by providing initial source from compressible GEODYN code, later times flow evolution can be successfully simulated with fast and efficient LMC code. Finally, we develop formalism for tracer particles and their fallout and present some preliminary results. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FL.00010: ABSTRACT WITHDRAWN |
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