PASSIVE SCALAR FLUCTUATIONS WITH AND WITHOUT A MEAN GRADIENT - A NUMERICAL STUDY

We report a numerical study of a passive scalar advected by a random i ncompressible Gaussian velocity field. The calculations are carried ou t on a two-dimensional square lattice. Depending on the two dimensionl ess parameters in the problem, the scalar fluctuations at the center c an be Gaussian, exponential, or stretched exponential. This is true no t only when the passive scalar is subjected to a uniform mean gradient but also true when a new alternating boundary condition is used which results in a mean scalar profile that has a vanishing gradient in the central region.