Passive-scalar dispersion development and measurements in grid turbulence

We describe experimental measurement and data-processing methodologies based on planar laser-induced fluorescence (PLIF) to investigate the downstream transport and mixing at high Schmidt number, of a miscible passive scalar, released from a point source in grid-turbulence. The resulting scalar plumes were in uniform-density (unstratified) flow formed by a grid towed in a tank under computer control, through otherwise stationary water, also exploring the injection-to-surrounding flow-speed ratio. PLIF image data were processed to calibrate and correct for local variations of pixel data of a custom-designed and fabricated charged-coupled device (CCD) focal-plane image sensor, with data recorded by a custom-designed and built data-acquisition system. Data were processed to correct for shot-to-shot variations of PLIF transect illumination using a quad-head pulsed Nd:YAG laser, at pulse rates up to 200 Hz. The optics generated laser-sheet transects oriented either streamwise or transverse to the flow. Scalar concentration measurements from these transects were processed for quantitative flow visualization, as well as turbulent-plume statistics, such as mean plume profiles, spatial scalar spectra, and other information, over a range of mesh Reynolds numbers and downstream distances from the grid. Preliminary sample data illustrate the application of the experimental method and the measurement dynamic range, as were required to discern far-field downstream behavior.