Robert N. Meroney

Study of line source characteristics for 2-D physical modelling of pollutant dispersion in street canyons

Abstract The University of Hamburg initiated a wind tunnel study of car exhaust dispersion from street canyons in an urban environment to investigate how pollution dispersion is affected by street geometry. Particular emphasis at the beginning of this work was put on the design of a line source to represent traffic exhaust. Pollution dispersion was studied in two dimensions (i.e., infinite-length streets were assumed). The case of an isolated street canyon in open country was examined first. The same street canyon geometry was subsequently studied in an urban environment, i.e., with additional canyons of similar geometry upstream and downstream of the test street. The dynamic and dispersion characteristics of the flow in the two cases were quite different. In the canyon amidst open country we observed better canyon ventilation than in the urban roughness case.

Concentration and flow distributions in urban street canyons: wind tunnel and computational data

The goal of this paper is to present bluff body flow and transport from steady point sources of pollutants, or chemical and biological agents in an idealized urban environment This paper includes ventilation behavior in different street canyon configurations. To evaluate dispersion in a model urban street canyon, a series of tests with various street canyon aspect ratios (B/H) are presented. Both open-country roughness and urban roughness cases are considered. The flow and dispersion of gases emitted by a point source located between two buildings inside an urban street canyon were determined by the prognostic model FLUENT using four different RANS turbulent closure approximations and in the model fire dynamics simulator using a large eddy simulation methodology. Calculations are compared against fluid modeling in the Industrial Meteorological Wind Tunnel at Colorado State University. A basic building shape, the Wind Engineering Research Field Laboratory building (WERFL) at Texas Tech University, was used for this study. The urban street canyon was represented by a 1:50 scale WERFL model surrounded by models of similar dimensions. These buildings were arranged in various symmetric configurations with different separation distances and different numbers of up- or downwind buildings. Measurements and calculations reveal the dispersion of gases within the urban environment are essentially unsteady, and they are not always well predicted by the use of steady-state prediction methodologies.

Wind-tunnel and numerical modeling of flow and dispersion about several building shapes

Abstract The flow and dispersion of gases emitted by sources located near different building shapes separately studied in various wind tunnels were determined by the commercial prognostic model FLUENT and FLUENT/UNS using the standard k–e, the RNG k–e, and the Reynolds-stress RSM turbulence closure approximations. Inlet conditions and boundary conditions were specified numerically to the best information available for each fluid modelling simulation. Calculations are compared against the wind-tunnel measurements, but no special effort was made to force-fit agreement between the numerical and experimental data by post adjusting, coefficients, surface roughness, initial conditions, etc., beyond the specifications supplied by the laboratory researchers. The intent of these calculations were to determine if a relatively robust commercial CFD package using “reasonable” boundary and initial conditions could be used to simulate wind engineering situations without massaging the results interactively.

Characteristics of Wind and Turbulence in and above Model Forests

Abstract A model forest canopy was designed to simulate the meteorological characteristics of typical live forests. Measurements were made of velocity, turbulence, drag, and gaseous plume spread within the simulated canopy. The resulting data compares favorably with prototype field measurements in all cases. Several new aspects of the flow at the upwind edge of a forest are displayed.

TURBULENCE STRUCTURE IN A STRATIFIED BOUNDARY LAYER UNDER STABLE CONDITIONS

Turbulence structure in stably stratified boundary layers isexperimentally investigated by using a thermally stratified wind tunnel. Astably stratified flow is created by heating the wind tunnel airflow to atemperature of about 50 °C and by cooling the test-section floor to asurface temperature of about 3 °C. In order to study the effect ofbuoyancy on turbulent boundary layers for a wide range of stability, thevelocity and temperature fluctuations are measured simultaneously at adownwind position of 23.5 m from the tunnel entrance, where the boundarylayer is fully developed. The Reynolds number, Reδ, ranges from 3.14× 104 to 1.27 × 105, and the bulk Richardson number, Riδ,ranges from 0 to 1.33. Stable stratification rapidly suppresses thefluctuations of streamwise velocity and temperature as well as the verticalvelocity fluctuation. Momentum and heat fluxes are also significantlydecreased with increasing stability and become nearly zero in the lowest partof the boundary layer with strong stability. The vertical profiles ofturbulence quantities exhibit different behaviour in three distinct stabilityregimes, the neutral flows, the stratified flows with weak stability(Riδ = 0.12, 0.20) and those with strong stability (Riδ= 0.39,0.47, 1.33). Of these, the two regimes of stratified flows clearly showdifferent vertical profiles of the local gradient Richardson number Ri,separated by the critical Richardson number Ri cr of about 0.25. Moreover,turbulence quantities in stable conditions are well correlated with Ri.

Gas dispersion near a cubical model building. Part I. Mean concentration measurements

The dispersion of effluent plumes emitted on or in the near-wake region (x/H ⪕ 5.0) of a cubical model building has been examined. The model study was performed in a wind tunnel with a simulated neutrally stratified shear layer. Mean concentration measurements were made on the model building for three different roof vent locations and three different building orientations. A full-scale measurement was conducted in the near-wake region for central roof vent release. The concentration level on the lee face of a model building is greatly reduced by the presence of a sharp edge on the model. The optimum location for the intake vent on the building, for equal vent exhaust to vent intake distance, is a position away from the downwind direction and where it cannot “see” the exhaust vent. Orientation of the building at an angle of 45° results in a secondary peak concentration on the building and in the near-wake region.

Turbulent boundary-layer growth over a longitudinally curved surface

Measurements are reported for turbulent boundary-layer growth in a prolonged bend where the additional rates of strain produced by streamline curvature influence the turbulent development. The growth rate of the boundary-layer thickness over the convex side is almost halved and the skin friction coefficient falls to about 0.9 of the value expected on a plane surface. The mixing rate on the concave side is increased to about 1.1 times the plane surface value, and the customary evidence of longitudinal rolls appears. These measurements are the first since those of Schmidbauers (1936) to provide a test of existing curvature correction formulas for curvatures typical of airfoils and turbomachinery without the complications of compressibility. Results have been compared against calculation techniques proposed by Bradshaw (1973), with good agreement.

Car exhaust dispersion in a street canyon. Numerical critique of a wind tunnel experiment

Abstract Due to increasing car traffic in cities, problems related to car induced air pollution in street canyons have become important. Physical modeling in wind tunnels or numerical codes may be used for dispersion simulation when investigating air quality. Rafailidis [in: Annalen der Meteorologie] carried out an extensive set of test runs recently in the BLASIUS wind tunnel at the Meteorological Institute of the University of Hamburg, Germany. In the present study the wind tunnel experiments were simulated numerically using the CFD-code Fluent®. In a first approach, the idealized two-dimensional case was calculated. Several test runs have been carried out to study the effect of emission rate and source design on flow structures and dispersion in the street canyon. It could be shown that alternative emission conditions and the source design might affect the concentration field within a modeled street canyon. A second set of calculations for a simplified three-dimensional simulation of the street canyon setup was performed to investigate the presence of secondary flow patterns found during wind tunnel tests. The lateral flow structure within the street canyon observed during wind tunnel measurements was simulated, and the effect of changing boundary conditions on the secondary flow structure was studied. In the paper the advantages of CFD simulations for planning wind tunnel dispersion tests are discussed.

Numerical and physical modeling of bluff body flow and dispersion in urban street canyons

Abstract To develop reliable computer models for the bluff body flow and transport of pollutants or chemical and biological (CB) agents in urban environments requires accurate measurements of the basic flow fields for carefully controlled, well-known conditions. Fluid modeling in an industrial wind tunnel provides an opportunity to produce accurate simulations of the bluff body flow and transport of urban pollution or of CB agents associated with urban terrorism incidents. A basic building shape, the Wind Engineering Research Field Laboratory building (WERFL) at Texas Tech University, is used for this study. The urban street canyon was represented by a 1:50 scale WERFL model that was surrounded by models of similar dimensions. These buildings were arranged in various symmetric configurations with different separation distances and different numbers of surrounding building. A series of measurements is made over a generic urban street canyon arrangement using flow visualization, anemometry, pressure transducer and gas chromatography. The experimental data include visualization, velocity and turbulence intensity profiles, surface pressure on the building and dispersion of releasing gas. Results are compared to three-dimensional numerical models of the same configuration using the commercial code, FLUENT 5.3. The effects of grid resolution, boundary conditions, source placement and selection of turbulence model (kappa-epsilon, RNG kappa-epsilon, Reynolds stress, etc.) are examined in a series of sensitivity calculations.

Flow visualization of conical vortices on flat roofs with simultaneous surface pressure measurement

Abstract Wind tunnel and full-scale pressure studies of flow over low-rise buildings have repeatedly shown that on the roof, the largest mean and peak suction values are observed for taps beneath the conical “delta-wing type” corner vortices that occur for oblique winds. To better understand the flow mechanism which produces these negative pressure coefficients, a flow visualization study of conical vortex behaviour was performed in the wind tunnels of Colorado State University (CSU) and at full scale at Texas Tech University (TTU). The mean position and size of the vortices as a function of wind direction is presented. Pressures were also simultaneously measured beneath the vortex visualization plane in the wind tunnel for the worst case wind directions. These pressure profiles were correlated with the digitally enhanced images of the vortex flow. The greatest suction was found to follow directly beneath the moving vortex core. For smooth flow, the magnitude of the suction beneath the core was seen to vary inversely with the vortex size, but no relationship between vortex size and suction could be seen for turbulent flow.