When a material spreads in the turbulent flow, its instantaneous concentration distribution becomes not homogenous in space, and areas with high concentration containing sudden change of the concentration are formed by the local intense turbulence. In this study, we extracted such localized high-concentration areas by a conditional sampling technique, and observed the behavior of the areas. In order to realize the scalar diffusion in quasi homogeneous isotropic turbulent flow, a model experiment was performed in a water channel flow. Fluorescent dye was introduced from a nozzle located at the center of the channel, and the concentration images of the dye were obtained at several downstream positions by PLIF measurement. To extract areas of high concentration containing a sudden change of the concentration, three types of analysis techniques including the conditional sampling technique were applied to PLIF images. By the conditional sampling technique, we can extract areas of high concentration, which cannot be identified by the other two kind methods, and the effectiveness of this technique was proved. It was found that the areas extracted by the conditional sampling technique appear as lumps. The spanwise numerical probability distribution of the lumps roughly follows the Gaussian distribution, and the peak of the distribution decreases while its standard variation increases as streamwise distance increases. This result implies that performing extraction of areas with high concentration appropriately would enable to realize the material diffusing process as a Lagrange particle and provide a prediction method about the material diffusion source and the damage of the pollutant.

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.

To evaluate the pollutant dispersion in background turbulent flows, most researches focus on statistical variation of concentrations or its fluctuations. However, those time-averaged quantities may be insufficient for risk assessment, because there emerge many high-intensity pollutant areas in the instantaneous concentration field. In this study, we tried to estimate the frequency of appearance of the high concentration areas in a turbulent flow based on the Probability Density Function (PDF) of concentration. The high concentration area was recognized by two conditions based on the concentration and the concentration gradient values. We considered that the estimation equation for the frequency of appearance of the recognized areas consisted of two terms based on each condition. In order to represent the two terms with physical quantities of velocity and concentration fields, simultaneous PIV (Particle Image Velocimetry) and PLIF (Planar Laser-Induced Fluorescence) measurement and PLIF time-serial measurement were performed in a quasi-homogeneous turbulent flow. According to the experimental results, one of the terms, related to the condition of the concentration, was found to be represented by the concentration PDF, while the other term, by the streamwise mean velocity and the integral length scale of the turbulent flow. Based on the results, we developed an estimation equation including the concentration PDF and the flow features of mean velocity and integral scale of turbulence. In the area where the concentration PDF was a Gaussian one, the difference between the frequencies of appearance estimated by the equation and calculated from the experimental data was within 25%, which showed good accuracy of our proposed estimation equation. Therefore, our proposed equation is feasible for estimating the frequency of appearance of high concentration areas in a limited area in turbulent mass diffusion.