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

1 ． 瞬态浓度场测量系统的研制采用激光诱导荧光（LIF）方法测量湍流中物质的瞬态浓度场。为实现染料瞬时浓度的高精度定量测量，初步探讨了选择合适的荧光染料，使其能发出足够强的入射激光（YAG激光）。吸收光谱和发光光谱结果表明，罗丹明B和罗丹明6G最适合我们的目的。在小于1mg /l的浓度范围内，染料浓度与发光强度之间存在良好的线性关系，表明该系统可以用于物质瞬时浓度的测量。基于拉格朗日方法的湍流扩散数值预测建模研究了欧拉和拉格朗日技术的结合方法，用于计算湍流中波动尺度场的统计量。在该模型中，利用DNS方法得到的欧拉速度来计算物质粒子的拉格朗日运动轨迹。然后，根据随机理论估计了马尔可夫过程的浓度统计量。该方法可应用于具有高普朗特数的扩散场，原则上可得到任意高阶统计量。研究了各向同性均匀湍流中普朗特数对标量色散的影响。发现标量方差的衰减与普朗特数有很强的依赖性，而湍流/标量时标比似乎接近于一个与普朗特数无关的渐近值。

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 .