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

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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.