Characteristics of high concentration pulses in turbulent mass diffusion

When a material spreads in turbulent flow, areas with high concentration are formed by the local turbulence. These high concentration islands can lead to heavy damage. In this study, the high concentration areas are extracted as pulses by a conditional sampling technique, and statistical analysis were performed in order to investigate the difference between the pulses and concentration plume. Assuming homogeneous flow, a series of images by Planar Laser Induced Fluorescence and Particle Image Velocimetry techniques are acquired at several downstream positions. For PLIF images, a conditional sampling technique is used to extract the high concentration areas. For simultaneous PIV and PLIF images, wavelet analysis is performed to probe into the relationship between high concentration pulses and velocity fluctuation. Turbulent diffusion coefficient of the high concentration areas is found to increase with the downstream distance whereas that of the concentration plume is constant, which implies that the diffusion process of the pulses differs from that of the plume. Besides by wavelet analysis result, it is indicated that concentration fluctuation in the pulses is negatively correlated with streamwise velocity fluctuation. It is concluded that high concentration pulses and plume have different generative mechanisms.