Direct Numerical Simulation The Dynamic Behavior Of Passive Scalar In Isotropic Turbulence

The passive scalar statistics are important in turbulence theories and engineering problems related to turbulent combustion, pollutant diffusion et al. Since the 1980s, studies of passive scalar have been one of hotspots in turbulence research. Experiments and numerical simulations have provided lots of new phenomena and concepts, which put an impetus to reinterpret the classical turbulent theories and modify application models at present. In this thesis, passive scalar fields with a mean scalar gradient mixing in statistically homogeneous, isotropic turbulence were studied via direct numerical simulation, for Taylor Reynolds numbers, R_λ=25 and 48, Pr numbers from 0.3 to 4.0. In all of these simulations, "Box"turbulent velocity assumption was adopted and the mean scalar gradient in y direction was chosen to equal 1.0. The computational grids are 96³ in R_λ=25, Pr=0.3 and 0.7, and 128³ in R_λ=48, Pr=2.0 and 4.0 respectively, which is needed to meet the request K_maxηB>1.5 in exactly solving scalar field when Pr>1.0. The Navier-Stokes and scalar transport equations were both solved with pseudo-spectral method, and with second-order Runge-Kutta scheme in time-stepping. The basic statistics computed include the probability distribution function (pdf) of scalar and scalar dissipation etc. Because of its key role in combustion models, the dynamics of scalar dissipation was investigated in more detail. The results show that the formation of intense scalar dissipation is mostly due to the preferential coupling between the scalar gradient and the compressive principle of strain tensor. Although vorticity has only a minor influence on the formation of scalar gradient, it can weaken the effect of mean scalar gradient forced on large scale by producing scalar gradient, which has a reverse direction with the mean scalar gradient, and restrain the occurrence of intensive scalar gradient with the same direction, by destroying the coupling of scalar gradient with compressive strain. With the help of invariant theory, we found most regions of intensive scalar dissipation locate in the fourth quadrant with a topology of unstable node/saddle/saddle, where two eigenvalues of the strain tensor are positive and one is negative, and owing to the induction of vortex tubes; meanwhile a few samples of moderate-value scalar dissipation scatter in the second quadrant, which correspond to a topology of stable focus/stretching.