The diffuselet concept for scalar mixing

Abstract

The advection–diffusion of a small surface element of scalar in three dimensions (or of a small line element in two dimensions) is solved analytically thanks to the Ranz transform (Ranz, AIChE J., vol. 25, issue 1, 1979, pp. 41–47). As the quantum or elementary brick of any complex mixture, we call this element a diffuselet. Its evolution is computed numerically from the integration of the velocity gradient along the trajectory, as classically done for the Lyapunov exponents. The concentration profile across the diffuselet is obtained from the product of its initial orientation with a dimensionless tensor. Averaging over all initial orientations yields simple formulae for the mean scalar variance and the scalar probability distribution function (p.d.f.). This technique is then applied to two-dimensional and three-dimensional sine flows, in excellent agreement with direct numerical simulations. For these simple flows, the temporal integration is obtained analytically leading to simple integrals for the scalar variance and p.d.f. Statistics of stretching rates are calculated as well. The Lyapunov exponent is close to the value for short-time correlated flows (Kraichnan, J. Fluid Mech., vol. 64, issue 4, 1974, pp. 737–762) in the case of a small displacement during each step; it is close to the value for a simple shear in the case of a large displacement. The p.d.f. of stretching factors are log normal with a ratio between the mean and the variance equal to half the dimension of space for small displacements (in agreement with Kraichnan, J. Fluid Mech., vol. 64, issue 4, 1974, pp. 737–762), but increases strongly for large displacements.