On the kinematics of scalar iso-surfaces in decaying homogeneous, isotropic turbulence

Scalar iso-surfaces have been used to describe many interface problems in turbulence, such as flame surfaces in turbulent reacting flows and turbulent/non-turbulent interfaces. In this paper we report on direct numerical simulations of the behaviour of iso-surfaces of two differently initialised conserved scalars evolving in isotropic turbulence. The terms in the equation for the iso-surface area density, along with the area density itself, are evaluated for a wide range of iso-surface values using a novel numerical method for evaluating surface integrals. Specifically, we quantify the behaviour of the iso-surface area density production term, related to the strain-rate, and the destruction term, related to iso-surface curvature and molecular diffusion. We find that production of iso-surface area density is not affected by the initial conditions, but it does have a small dependence on the iso-surface value. The destruction term depends not only on the iso-surface value, but also on the initial conditions.