Roger S. Thompson

The structure of strongly stratified flow over hills: dividing-streamline concept

In stably stratified flow over a three-dimensional hill, we can define a dividing streamline that separates those streamlines that pass around the hill from those that pass over the hill. The height Hs of this dividing streamline can be estimated by Sheppards simple energy argument; fluid parcels originating far upstream of a hill at an elevation above Hs have sufficient kinetic energy to rise over the top, whereas those below Hs must pass around the sides. This prediction provides the basis for analysing an extensive range of laboratory observations and measurements of stably stratified flow over a variety of shapes and orientations of hills and with different upwind density and velocity profiles. For symmetric hills and small upwind shear, Sheppards expression provides a good estimate for Hs. For highly asymmetric flow and/or in the presence of strong upwind shear, the expression provides a lower limit for Hs. As the hills become more nearly two-dimensional, these experiments become less well defined because steady-state conditions take progressively longer to be established. The results of new studies are presented here of the development of the unsteady flow upwind of two-dimensional hills in a finite-length towing tank. These measurements suggest that a very long tank would be required for steady-state conditions to be established upstream of long ridges with or without small gaps and cast doubt upon the validity of previous laboratory studies.

Experimental and theoretical study of the wake of a block-shaped vehicle in a shear-free boundary flow

Abstract The wake of a moving vehicle was simulated using a specially-constructed wind tunnel with a moving floor. A “block-shaped” model vehicle was fixed in position over the test-section floor while the floor moved at the freestream air speed to produce a uniform, shear-free, approach flow. This simulates an automobile traveling along a straight highway under calm atmospheric conditions. Vertical and lateral profiles of mean and fluctuating velocities and Reynolds stresses in the wake of the vehicle were obtained using a hot-film anemometer with an X-probe. Profiles were taken at distances of 10–80 model heights downwind. A momentum type wake was observed behind the block-shaped vehicle. The wake does not have a simple self-preserving form. However, it is possible to collapse the velocity deficit with one length and one velocity scale. Two new theories for the velocity deficit are compared to the theory of Eskridge and Hunt (1979). A theory which considered a height-dependent eddy viscosity was found to fit the data best. Length and velocity scales were found for the longitudinal variation of the turbulent kinetic energy. The lateral variation is described by a two-dimensional numerical fit of the crosswind variation of the data.

Building amplification factors for sources near buildings: A wind-tunnel study

Abstract The aerodynamics of a building can strongly influence the dispersion of pollutants released from nearby sources. Low releases may be entrained into the buildings highly turbulent flow region and result in high pollutant concentrations on the building surface where building air intakes are located. High releases may result in increased concentrations at ground level downwind of the building as a result of the buildings influence on the mean flow field. High releases at distances far upwind of the building can produce significant concentrations on the building surfaces if the building extends up into the elevated plume. Concentration measurements from a wind-tunnel study for numerous release locations upwind, above and downwind of each of four rectangular buildings are presented and compared with some previous measurements and calculations. Both building surface and ground-level values are presented. The concentrations are used to compute “building amplification factors”, which are defined as the ratios of the maximum concentration from a given source near the building to the maximum concentration from the same source in the absence of the building. This simple measure of the buildings influence showed a significant influence of the building on concentrations from sources far upwind of the building, sources well above the building cavity and sources in the near wake of the building.

Flow and dispersion of pollutants within two-dimensional valleys

Abstract Wind tunnel experiments and a theoretical model concerning the flow structure and pollutant diffusion over two-dimensional valleys of varying aspect ratio are described and compared. Three model valleys were used, having small, medium, and steep slopes. Measurements of mean and turbulent velocity fields were made upstream, within and downwind of each of these valleys. Concentration distributions were measured downwind of tracer sources placed at an array of locations within each of the valleys. The data are displayed as maps of terrain amplification factors, defined as the ratios of maximum ground-level concentrations in the presence of the valleys to the maxima observed from sources of the same height located in flat terrain. Maps are also provided showing the distance to locations of the maximum ground-level concentrations. The concentration patterns are interpreted in terms of the detailed flow structure measured in the valleys. These data were also compared with results of a mathematical model for treating flow and dispersion over two-dimensional complex terrain. This model used the wind tunnel measurements to generate mean flow fields and eddy diffusivities, and these were applied in the numerical solution of the diffusion equation. Measured concentration fields were predicted reasonably well by this model for the valley of small slope and somewhat less well for the valley of medium slope. Because flow separation was observed within the steepest valley, the model was not applied in this case.

TURBULENT DIFFUSION BEHIND VEHICLES: EXPERIMENTALLY DETERMINED INFLUENCE OF VORTEX PAIR IN VEHICLE WAKE

Abstract The wake of a moving vehicle was simulated using a wind tunnel with a moving floor. The vehicle models, both block-shaped and ‘true’ scale models of actual automobiles, were held in a fixed position while the floor moved at the upstream air speed. This simulates an automobile travelling on a straight highway in still ambient air. Vertical and lateral profiles of mean and fluctuating velocities and mean tracer concentration were obtained. Profiles were taken at distances of 15–60 model heights downstream. Two exhaust source positions were used: at the center of the rear of the vehicle and on the side just behind the rear wheel. It was found that the models of true vehicles induce a pair of vortices in the wake that modify the velocity and concentration patterns in a minor way from that of the block-shaped vehicle.

Dense gas removal from a valley by crosswinds

Abstract Wind tunnel experiments were made to determine how rapidly dense gas trapped in a topographic depression could be removed by an entraining crosswind. The two-dimensional outflow volume flux, ν o , was assumed equal to the inflow rate during 92 steady-state experiments with CO 2 continuously supplied into the bottom of two-dimensional, V-shaped valleys. As predicted by theory, at large Reynolds numbers it was found that ν o α U s 3 / g ′ i , where U s is the speed just above the dense gas pool and g ′ i is gravity times the relative density difference. The width of the pool, w , does not affect ν o when the primary Froude number ⩽ 1, except at low Reynolds numbers; in this case the data suggest ν o α ( U s wK ) 1/2 as an asymptote, where K is the molecular diffusivity. A universal relationship is suggested for ν o bridging these two asymptotes Transient experiments were conducted by filling a valley with dense gas, turning it off, then quickly removing a sliding cover; ν o was measured as a function of time with an array of samplers downwind. These experiments essentially confirmed predictions based on the steady-state results, even when SF 6 was substituted for CO 2 . Insertion of a flat floor into the valley had only minor effects on ν o ( t ) until the pool level subsided almost to the floor level. Substantial changes in the removal process were observed for the few tests run at Froude numbers exceeding unity.

Estimation of maximum surface concentrations from sources near complex terrain in neutral flow

Abstract A wind tunnel study was conducted to determine maximum ground-level concentrations for a variety of source positions (locations and heights) both upstream and downstream of two model hills, an axisymmetric hill (maximum slope 24°) and a two-dimensional ridge (maximum slope 16°), immersed in a simulated neutral atmospheric boundary layer. Terrain amplification factors derived from these measurements were used to construct contour plots showing regions or ‘windows’ of enhanced ground-level concentration. These windows of enhanced ground-level concentration are shown to be a useful guide for estimating the effects of complex terrain on pollutant dispersion or, conversely, for determining source locations near complex terrain which minimize the enhancement of ground-level concentration.

Laboratory simulation of the rise of buoyant thermals created by open detonation

Laboratory experiments were conducted in a water tank to investigate the rise through the atmosphere of thermals generated by the detonation of surplus military munitions. The fall of a dense volume of fluid through the water in the tank follows the same governing equations as the rise of a buoyant volume of gas in the atmosphere. By filling the tank with a layer of water and a layer of salt water, an elevated step change in density was obtained, simulating a temperature inversion or jump in the atmosphere. The growth of the linear dimensions of the thermal and its volume were determined and used in the development of a criterion for predicting when a thermal will fully penetrate the inversion. Replacing the second layer of fluid with water of gradually increasing salinity, an elevated constant density gradient was obtained. In these cases, the maximum penetration distance of the thermal was observed and the equilibrium position and vertical spread were determined experimentally. These observations and the empirical relationships determined from them should prove useful in the development and evaluation of air pollution dispersion models for predicting the atmospheric transport and diffusion of material released during such detonations.

Air pollution and terrain aerodynamics: a review of fluid modeling studies at the EPA fluid modeling facility

Abstract Recent fluid modeling studies conducted at the EPA Fluid Modeling Facility of flow and diffusion in complex terrain are reviewed. Ratios of the maximum concentration on a hill surface to the maximum concentration in the absence of the hill are estimated. This ratio may be regarded as a terrain amplification factor and is a function of hill aspect ratio (two-versus three-dimensional), hill slope, atmospheric stability, etc. For upwind sources, terrain amplification factors are typically 1 to 2 for neutral flow over two-dimensional hills and 2 to 4 for three-dimensional hills. Terrain amplification factors as large as 10 or 15 were found for low sources placed downwind of two-dimensional hills of moderate to large slope. For strongly stable flow over three-dimensional hills, it is more useful to compare maximum surface concentrations with those at the centerline of the plume in the absence of the hill. These concentrations have been shown to be essentially equal.

Flushing of Dense Gas from Bottomlands by Wind

Mappings of the areas of occurrence of fatalities in the aftermath of disasters such as those in Bhopal, India and at Lake Nyos, Cameroon, reveal strong influences of topography on channeling and inhibiting diffusion of dense gases. Wind tunnel experiments were carried out at the USEPA Fluid Modeling Facility to determine how fast a crosswind removes dense gas that is otherwise trapped in a topographic depression. V-shaped valleys set into the tunnel floor were used. A series of 92 steady-state experiments, with inflow of CO2 at the valley bottom balanced by outflow due to entrainment, established relationships between outflow rate, height of the dense gas pool in the valley, reference wind speed upwind of the valley, and relative gas density difference times gravity, g. Measurements of wind speed just above pool height, Us, confirmed that the outflow is proportional to Us 3/g’ for Froude number <1 and large Reynolds number, Re; it does not depend on valley width, W. At low Re a weak dependence on W was found; an algebraic expression fitting both large and small Re asymptotes revealed that molecular diffusion can dominate entrainment even at real-world scales if Us < 2 m/s. “Flushing” experiments using CO2 and SF6 with no inflow provided further support for relationships developed from the steady-state experiments.