Abstract
When a mass spreads in a turbulent flow, areas with obviously high concentration of the mass compared with surrounding areas are formed by organized structures of turbulence. In this study, we extract the high concentration areas and investigate their diffusion process. For this purpose, a combination of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques was employed to obtain simultaneously the two fields of the concentration of injected dye and of the velocity in a water turbulent channel flow. With focusing on a quasi-homogeneous turbulence in the channel central region, a series of PLIF and PIV images were acquired at several different downstream positions. We applied a conditional sampling technique to the PLIF images to extract the high concentration areas, or spikes, and calculated the conditional-averaged statistics of the extracted areas such as length scale, mean concentration, and turbulent diffusion coefficient. We found that the averaged length scale was constant with downstream distance from the diffusion source and was smaller than integral scale of the turbulent eddies. The spanwise distribution of the mean concentration was basically Gaussian, and the spanwise width of the spikes increased linearly with downstream distance from the diffusion source. Moreover, the turbulent diffusion coefficient was found to increase in proportion to the spanwise distance from the source. These results reveal aspects different from those of regular mass diffusion and let us conclude that the diffusion process of the spikes differs from that of regular mass diffusion.
Highlights
With the aim of calculating the scalar statistics of high concentration spikes, which relate to the diffusion theory, we experimentally demonstrated the concentration and velocity fields of passive scalar diffusing from a fixed emission point in a quasi-homogeneous turbulent flow with simultaneous PIV (Particle Image Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurement
Areas of the same size were selected based on the recorded PLIF images, and the further spike statistical analysis was done focused on the selected areas
This paper focuses on the high concentration spike formed by turbulent structures, and we investigate their diffusion process from the viewpoint of the difference with the concentration plume
Summary
Diffusion accidents have affected a large number of victims around the world, as in the Chernobyl Nuclear Power Plant disaster in 1986 and the Fukushima Daiichi Nuclear Power Plant in 2011. In such cases, a quick and accurate method of predicting pollutant diffusion is needed in order to prevent damages. There are pioneering studies on the diffusion theory such as Taylor’s diffusion theory [4] and Richardson’s diffusion theory [5] These theories reference the spreading width of a diffusing material released into homogeneous turbulence, revealing that the relationship between the increasing rate of the spreading width and the diffusion time depends on whether the diffusion time is smaller or larger than the time scale of eddies in the flow. Owing to the progress of research on the diffusion theory, the diffusion prediction has become more accurate and speedy
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