Concentration Fluctuation Measurements in a Plume Dispersing Through a Regular Array of Obstacles

Abstract

We describe a comprehensivestudy of the statistical characteristicsof concentration fluctuations in a neutrallybuoyant tracer plume dispersing through a largearray of building-like obstacles, each of whichmeasured 12.2 m × 2.42 m ×2.54 m. The plumes were released bothupwind and within the obstacle array for a rangeof source heights between 0.15 and 5.2 m.Detailed flow field and instantaneous plume concentrationdata were obtained from a comprehensive seriesof tracer experiments that utilized a large numberof high-resolution concentration detectors, accompaniedby the simultaneous acquisition ofmeteorological and turbulence measurements withsonic anemometer/thermometers.Extensive analyses are performed on the plumeconcentration data, and results are presented fora number of concentration statistics such as themean plume lateral and vertical spreads, meanconcentration, fluctuation intensity, peakconcentration to concentration standard deviationratio, concentration probability density function (pdf),concentration power spectra, and variousconcentration time and length scales of dominant motionsin the array plume (e.g., integral scale,Taylor microscale). For the range of downwind distances from the sourceexamined, the lateral mean concentration profiles arewell approximated by a Gaussian distribution. Thevertical profiles of mean concentration develop ina rather complex manner with downwind distance, withthe result that the reflected Gaussian form isgenerally a less than ideal description of the meanarray plume in the vertical direction. A comparisonof the array plume with an open-terrain plume as afunction of downwind distance indicates that theobstacle array significantly increases the lateraland vertical plume spreads and decreases the magnitudeof the plume centreline mean concentration.The small-scale, high-intensity turbulence generated in the obstaclearray results in a drastic reduction in theconcentration fluctuation level in the array plume compared to anopen-terrain plume under similar conditions. Theevolution of the concentration pdf at a fixed range, butwith decreasing height from above and into the obstaclearray is similar to that obtained at a fixed heightbut with increasing downwind distance from the source.The integral and Taylor microscale time and lengthscales of the plume increase significantly within theobstacle array. Concentration power spectra measuredwithin the array had a greater proportion of the totalconcentration variance in the lower frequencies(energetic subrange), with a correspondingly smallerproportion in the higher frequencies (inertial-convectivesubrange). It is believed that these effects result fromthe rapid and efficient stirring and mixing of plumematerial by the small-scale, high-intensity turbulencewithin the array.