Consequences of Urban Stability Conditions for Computational Fluid Dynamics Simulations of Urban Dispersion

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The validity of omitting stability considerations when simulating transport and dispersion in the urban environment is explored using observations from the Joint URBAN 2003 field experiment and computational fluid dynamics simulations of that experiment. Four releases of sulfur hexafluoride, during two daytime and two nighttime intensive observing periods, are simulated using the building-resolving computational fluid dynamics model, FEM3MP to solve the Reynolds Averaged Navier-Stokes equations with two options of turbulence parameterizations. One option omits stability effects but has a superior turbulence parameterization using a non-linear eddy viscosity (NEV) approach, while the other considers buoyancy effects with a simple linear eddy ... continued below

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Lundquist, J K & Chan, S T November 30, 2005.

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The validity of omitting stability considerations when simulating transport and dispersion in the urban environment is explored using observations from the Joint URBAN 2003 field experiment and computational fluid dynamics simulations of that experiment. Four releases of sulfur hexafluoride, during two daytime and two nighttime intensive observing periods, are simulated using the building-resolving computational fluid dynamics model, FEM3MP to solve the Reynolds Averaged Navier-Stokes equations with two options of turbulence parameterizations. One option omits stability effects but has a superior turbulence parameterization using a non-linear eddy viscosity (NEV) approach, while the other considers buoyancy effects with a simple linear eddy viscosity (LEV) approach for turbulence parameterization. Model performance metrics are calculated by comparison with observed winds and tracer data in the downtown area, and with observed winds and turbulence kinetic energy (TKE) profiles at a location immediately downwind of the central business district (CBD) in the area we label as the urban shadow. Model predictions of winds, concentrations, profiles of wind speed, wind direction, and friction velocity are generally consistent with and compare reasonably well with the field observations. Simulations using the NEV turbulence parameterization generally exhibit better agreement with observations. To further explore this assumption of a neutrally-stable atmosphere within the urban area, TKE budget profiles slightly downwind of the urban wake region in the 'urban shadow' are examined. Dissipation and shear production are the largest terms which may be calculated directly. The advection of TKE is calculated as a residual; as would be expected downwind of an urban area, the advection of TKE produced within the urban area is a very large term. Buoyancy effects may be neglected in favor of advection, shear production, and dissipation. For three of the IOPs, buoyancy production may be neglected entirely, and for one IOP, buoyancy production contributes approximately 25% of the total TKE at this location. For both nighttime releases, the contribution of buoyancy to the total TKE budget is always negligible though positive. Results from the simulations provide estimates of the average TKE values in the upwind, downtown, downtown shadow, and urban wake zones of the computational domain. These values suggest that building-induced turbulence can cause the average turbulence intensity in the urban area to increase by as much as much as seven times average 'upwind' values, explaining the minimal role of buoyant forcing in the downtown region. The downtown shadow exhibits an exponential decay in average TKE, while the distant downwind wake region approaches the average upwind values. For long-duration releases in downtown and downtown shadow areas, the assumption of neutral stability is valid because building-induced turbulence dominates the budget. However, further downwind in the urban wake region, which we find to be approximately 1500 m beyond the perimeter of downtown Oklahoma City, the levels of building-induced turbulence greatly subside, and therefore the assumption of neutral stability is less valid.

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PDF-file: 47 pages; size: 0.6 Mbytes

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  • Journal Name: Journal of Applied Meteorology and Climatology, vol. 46, N/A, July 1, 2007, pp. 1080-1097; Journal Volume: 46

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  • Report No.: UCRL-JRNL-217489
  • Grant Number: W-7405-ENG-48
  • Office of Scientific & Technical Information Report Number: 936439
  • Archival Resource Key: ark:/67531/metadc897616

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  • November 30, 2005

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  • Sept. 27, 2016, 1:39 a.m.

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  • Nov. 23, 2016, 11:27 a.m.

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Lundquist, J K & Chan, S T. Consequences of Urban Stability Conditions for Computational Fluid Dynamics Simulations of Urban Dispersion, article, November 30, 2005; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc897616/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.