A video image analysis system for concentration measurements and flow visualization in building wakes

-A video image analysis technique for concentration measurements and flow visualization was developed for the study of diffusion in building wakes and other wind tunnel flows. Smoke injected into the flow was photographed from above with a video camera, and the video signal was digitized in real time during the experiment and during playback with a video image analysis system. The relation between the digitized smoke intensity and the vertically-integrated concentration of smoke particles was obtained from calibration experiments in which the smoke was replaced by a mixture of ethane and air. The time-averaged vertically-integrated concentration of the ethane tracer was measured at discrete points throughout the flow field and correlated with the time-averaged smoke intensity data. After the system was calibrated, the instantaneous and time-averaged fields of vertically-integrated concentration were obtained by simply photographing the smoke plume. Color-contoured images of the plume were created and displayed in real time thus creating a pseudo-color movie that was very useful for flow visualization. This technique was used to study the effects of building geometry and source location on the concentration and flow patterns in building wakes. Key word index: Building wakes, dispersion in building wakes, flow visualization, video image analysis systems, wind tunnel modeling. I. I N T R O D U C T I O N The short-range dispersion of material released into the atmosphere from vents on buildings is dominated by the complex, unsteady flow in the building wake. There are no general theoretical models for diffusion in such flows, and most of our knowledge comes from wind tunnel experiments in which the building is immersed in a simulated, turbulent atmospheric boundary layer. The most important parameters in these simulations are the geometry of the building, the location of the vent, and the direction and vertical variation of the ambient wind. The emphasis in most wind tunnel experiments has been on defining the structure of the flow field by measuring local velocities and turbulence, and on defining concentration patterns by releasing a tracer and measuring point concentrations on the building surface and on the ground downstream of the building. Comprehensive reviews of wind tunnel studies and semi-empirical models * Earth and Environmental Sciences Division, Geoanalysis Group (MS-D466), Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. t Space Science and Technology Division, Data Analysis Group (MS-D440), Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ~/Fluid Modeling Facility (MD-81), U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A. (On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.) based upon this database are presented by Hosker (1984) and Meroney (1982). Image analysis techniques have been used in various fluid dynamics experiments for flow visualization, panicle tracking and concentration measurements; Yang (1985) and Vrret (1987). These techniques use lasers or other high intensity light sources to illuminate a thin sheet of a flow that has been seeded with smoke, small tracer particles or a substance which flouresces in the laser light. The major advantage of these techniques over point measurements is their definition of the spatial structure of the flow field. In some diffusion problems the quantity of primary interest is the concentration integrated along a path through a plume. For example, visibility depends upon the concentration integrated along the line-ofsight of the observer, and many remote sensing instruments measure concentration integrated along a propagation path in the atmosphere. Wind tunnel simulations in which integrated concentrations are measured directly can be useful in these areas. Also, as will be shown in this paper, a surprising amount of information on the horizontal structure of the flow field is contained in the vertically-integrated concentration field. The objectives of this study are to develop a video image analysis technique for measuring the vertically integrated concentration in wind tunnel experiments and to apply this technique to the study of diffusion in building wakes. Similar studies with less emphasis on the quantitative measurement of concentrations were