Fine Structure Of Passive Scalar In Homogeneous And Isotropic Turbulence

In turbulence study the passive scalar has no dynamical effect on the fluid motion .Passive scalar behaviour is important in turbulent mixing,combustion,and pollution and provides impetus for the study of turbulence itself. The early conceptual framework of this subject is strongly influenced by the Kolmogorov cascade phenomenology and the passive scalar statistics are just reflection of the turbulence character.Since 1980's,new results of experimental measurements,numerical simulations and theoretic analyses have been reviewed ; the new phenomena and concepts are undergoing a drastic reinterpretation against the classical turbulent theories and modify application models at present.This thesis presents a numerical study of the relations between the small-scale velocity and scalar fields in fully developed isotropic turbulence with an imposed constant mean scalar gradient.In all of the simulations,"Box"turbulent velocity model is adopted. The computational grids are 963 in R =25 Pr=0.3 and 0.7, and 1283 in R =48 Pr=2.0 and 4.0,which is needed to meet the request KmaxηB>1.5 in exactly solving scalar field when Pr>1.0.The Eulerian and Lagrangian statistics show that for all Pr steep scalar gradients are concentrated in intermittently distributed sheet-like structures.We observe that these sheets are preferentially aligned perpendicular to the direction of the mean scalar gradient.The simulations show that the steep cliffs are formed by straining motions by compressing the scalar field along the mean scalar gradient in a very short time period ,about 10 Kolmogorov time scale.The combination of high strain rate and the alignment with the compressive strain results in a large increase of the scalar gradient and therefore in a large scalar dissipation rate. Although vorticity has only a minor direct influence on the formation of scalar gradient, it can restrain the occurrence of intensive scalar gradient with the same direction, by destroying the coupling of scalar gradient with compressive strain.