Research on Turbulent Diffusion from Instantaneous Source of Matter and Its Mathematical Modelling

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1 . Development of Measurement System of Instantaneous Concentration FieldA method based on Laser-Induced-Fluorescence (LIF) has been adopted for the measurement of instantaneous concentration field of matter in turbulent flow. A preliminary examination has been conducted to select an appropriate fluorescent dye which emits enough strong luminescent light for the incidence laser beam (YAG laser) to realize a high-precision quantitative measurement of instantaneous concentration of dye. Results of the absorption and luminescence spectra showed that Rhodamine B and Rhodamine 6G are most appropriate for our purposes. Furthermore, a good linear relation between the concentration of dye and the luminescence intensity has been found to exist in the range of the concentration less than 1 mg/l These results suggest that our system could be available enough for the measurement of instantaneous concentration of matter.2. Modelling for the Numerical Prediction of Turbulent Diffusion based on Lagrangian MethodA combined method of Eulerian and Lagrangian techniques has been deve1oped for the calculation of statistics of a fluctuating sca1air field in turbulence. In this model, the Eulerian velocities which are obtained by a DNS method are used to compute the Lagrangian trajectories of material particles. Then, concentration statistics are estimated on the basis of stochastic theory for the Markovian process. This method can be applied into a diffusion field with high Prandtl number and any higherorder statistics can be obtained in principle.The effects of the Prandtl number on the scalar dispersion in isotropic homogeneous turbulence have been investigated. It is found that the decay of the scalar variance has a. strong dependence of the Prandtl number whereas the turbulence/scalar time-scale ratio seems to approach to an asymptotic value which is independent of the Prandtl number .