Investigation on the Spatial Structure in Passive Scalar Field of High Schmidt Number and Development of Molecular Mixing Model in the PDF Method

本项目的最终目标是开发PDF方法中的闭合模型，用于数值预报具有高施密特数的被动标量的湍流扩散场。为了实现这一目标，必须提供详细的实验数据，包括有关扩散标量的速度和浓度的同步信息。为此，在两年的时间里，为了积累基础数据，进行了几次关于湍流射流的实验。首先，用PLIF方法研究了圆形射流中被动标量在反水流中的截面浓度场。结果表明，与静止环境中的常规射流相比，交叉面内瞬时浓度场的质心存在随机起伏，其幅值在下游增加得更快。这些结果显然与喷流核心的径向振荡有关，这预计是其提高混合效率的原因。其次，对有限水深渠道中平行于偏置底壁的平面淹没湍动水射流进行了实验研究。给出了用PIV方法测量的速度场的详细数据，以考察对向边界对偏转射流的影响，以及对自由边界和对固体边界的偏转对射流发展的影响。为了建立PDF方程中分子混合项的闭合模型，利用实验数据对浓度场的两点关联进行了研究。在此基础上，通过三维直接数值模拟，研究了两粒子间距的增长与湍流细尺度结构的关系。

The final goal of this project is to develop closure models in the PDF method for numerical prediction of the turbulent diffusion field of passive scalar with high Schmidt number. In order to approach this goal, detailed experimental data including simultaneous information on velocities and concentration of the diffusing scalar are essential. Thus, several experiments concerning with turbulent jet were conducted during the project period of two years with the aim of the accumulation of fundamental data.Firstly, cross-sectional concentration field of a passive scalar in a round jet discharged into a counter water stream were investigated using the PLIF method. It was shown that a random fluctuation in the centroid of instantaneous concentration field exists in the cross plane and its amplitude increases downstream more rapidly as compared with a conventional jet in a quiescent ambient. These results are clearly related with a radial oscillation of the jet core which is expected to be responsible for its enhanced mixing efficiency. Secondly, plane submerged turbulent water jet discharged parallel to the offset bottom wall in the channel with a finite water depth was investigated experimentally. Detailed data on the velocity field measured by using the PIV method were given in order to examine the influence of the opposing boundary upon the deflecting jet and the difference of the jet development between the deflection to the free boundary and that to the solid boundary. Third experiment is concerned with the interaction of two intersecting round jets.In order to develop closure models for the molecular mixing term in the PDF equation, two-point correlation of the concentration field has been studied by using experimental data. Furthermore, the relation between a growth of two-particle distance and the fine-scale structures of turbulence has been investigated numerically by means of the three-dimensional direct numerical simulation.