Wind Tunnel Experiment of Tracer Gas Concentration Fluctuation in Turbulent Boundary Layer. Characteristics of Concentration Fluctuation with Low Concentration Threshold.:Characteristics of Concentration Fluctuation with Low Concentration Threshold

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Wind tunnel experiments were conducted to study concentration fluctuation within a gas plume, considering the characteristics of plume dispersion in the atmosphere. Tracer gas was continuously released from an elevated point source located in a simulated atmospheric boundary layer and concentration fluctuation was measured using fast-response flame ionization detectors. The threshold concentration was determined by noise effects of the concentration measurement.<BR>Almost constant values were observed not only for the conditional ratio of RMS values to mean concentration, estimated using the threshold value within the plume, but also for the skewness and flatness factors of concentration fluctuation. These constant values were obtained at allthe threshold values used in this study. Probability distribution functions of concentration fluctuation were approximated by those of clipped normal distribution at near-by regions, and exponential and lognormaldistributions at regions far from the tracer gas release point. Concentration ratios at maximum 1% values were almost constant within the plume, and showed nearly the same values as those of lognormal and exponential distributions.
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