Experimental Measurements of Concentration Fluctuations and Scales in a Dispersing Plume in the Atmospheric Surface Layer Obtained Using a Very Fast Response Concentration Detector

Abstract

High-frequency fluctuations of concentration in a plume dispersing in the atmospheric surface layer have been measured with high-resolution concentration detectors (approximately 270 Hz at the ?6-dB point) to extract various concentration statistics of the fluctuating concentration field. Crosswind and alongwind variations of amplitude statistics (e.g., the total and conditional fluctuation intensity, skewness, and kurtosis), the intermittency factor, and the shapes of the concentration probability density function (PDF) are presented. The behavior of temporal concentration statistics such as the autocorrelation function; power spectrum; PDF of upcrossing intervals PDF of excursion durations; various concentration timescales, length scales, and microscales (e.g., Taylor microscale, correlation scale, length scale based on the spectral peak, etc.); as well as the velocity-to-concentration timescale ratio are studied. It is shown that all the concentration length scales and microscales (with the exception of the correlation scale) grow with downwind distance in proportion to the mean plume width.