Volume 5, Number 4 • Winter 2003/2004
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New Experimental Techniques for Measuring and Visualizing MixingPhilip J. W. Roberts, School of Civil Engineering, Georgia Institute of TechnologyMost processes for the treatment of drinking water and preservation of its quality during storage rely critically on turbulent mixing. Typical of this is the estimated 400,000 finished water storage facilities in the United States. In storage tanks, the mixing is caused primarily by the momentum of the inflow. Although jet mixing is well understood in simple unbounded configurations, mixing in reservoirs is much more complicated due to the transient nature of the flows, three-dimensional geometries, and buoyancy forces resulting from temperature differences. Previous research has consisted primarily of scalemodel laboratory experiments and some applications of computational fluid dynamics (CFD). The experiments have been limited by measurement access and the very small number of point measurements (for example, of a tracer mixing in the tank) that have typically been possible. This lack of detailed experimental data for refining CFD models has also limited the usefulness of numerical models. Measurement techniques have substantially advanced in recent years by the advent of Laser-Induced Fluorescence (LIF) that enables non-intrusive simultaneous instantaneous whole-field measurements of tracer concentrations. A new three-dimensional Laser Induced Fluorescence system (3DLIF) has been developed in the Environmental Fluid Mechanics Laboratory at the Georgia Institute of Technology. This system is being used to measure the spatial evolution of the mixing processes induced by inflow momentum and the spatial variation of tracer concentration. Such 3DLIF systems have only recently become feasible due to rapid advances in instrumentation, especially opto-electronics, low-light high-speed cameras, high-speed scanning mirrors, image capture and processing techniques, and fast mass storage devices. The system has been extensively applied to studies of turbulent mixing in buoyant jet flows typical of wastewater discharges.
The experimental configuration is shown in Figure 1. The test tanks are constructed of clear lucite to allow the laser beam to pass through. Two fast scanning mirrors drive a laser beam from an Argon-Ion laser through the flow in a programmed pattern. A small amount of a fluorescent dye is added to the effluent; the laser causes the dye to fluoresce, and the emitted light is captured by a CCD camera. The vertical mirror sweeps the beam up and down to make a laser sheet while the camera is exposing. The horizontal mirror then moves the sheet a small distance, and another image is obtained. This is repeated so that multiple vertical "slices" through the flow are obtained. After computation of tracer concentrations, the data can be converted, by image processing techniques, into three-dimensional images of the flow field. An example result of the system applied to a tank with a vertical jet at the center is shown in Figure 2. This three dimensional instantaneous false-color visualization of the tracer concentrations illustrates the complex flow pattern with a ring-shaped dead zone visible in the perspective view. Such flow structures would be impossible to visualize with conventional measurement techniques.
The 3DLIF system can obtain vastly more information than point-based techniques. With point probes, measurements can be made at only around 10 to maybe 100 points. With the 3DLIF system, we can easily measure at more than 1 million points through the tank, and these measurements can be repeated 10 times each second. This information is leading to great insight into the hydrodynamics of turbulent mixing and is being used to aid in the development of CFD models and the design of more efficient mixing tanks. This article is the third in a regular series of reports on emerging and innovative technologies relevant to the area of the environment and water resources produced by EWRI’s Emerging and Innovative Technologies Council (EITC). EITC’s mission is to advance the development, knowledge, and application of emerging and innovative technologies for the planning and management of water resources and the protection and enhancement of the environment. If you are interested in contributing an article or becoming a member of this Council, please contact Richard Palmer, Chair of EITC, at palmer@u.washington.edu. |
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