The transport mechanisms of substances in natural water bodies are considered. Dissolved substances can be regarded as passive in that they do not affect the velocity field of the water bodies. The particles of substances can be regarded as either passive or active. Transport of substances is caused by convection, and/or molecular and turbulent diffusion. The buoyancy forces thus have some infl...

Though flux freezing is a good approximation frequently assumed for molecular clouds, ambipolar diffusion (AD) is inevitable at certain scales. The scale at which AD sets in can be a crucial parameter for turbulence and the star formation process. However, both observation and simulation of AD are very challenging and our knowledge of it is very limited. We recently proposed (Li and Houde 2008)...

We study material lines and passive vectors in a model of turbulent flow at infiniteReynolds number, the Kraichnan-Kazantsev ensemble of velocities that are white-noise in time and rough (Hölder continuous) in space. It is argued that the phenomenon of “spontaneous stochasticity” generalizes to material lines and that conservation of circulations generalizes to a “martingale property” of the st...

We study of the effect of turbulence on diffusion processes within magnetized medium. While we exemplify our treatment with heat transfer processes, our results are quite general and are applicable to different processes, e.g. diffusion of heavy elements. Our treatment is also applicable to describing the diffusion of cosmic rays arising from magnetic field wandering. In particular, we find tha...

The diffusion of uni-directional magnetic fields by two dimensional turbulent flows in a weakly ionized gas is studied. The fields here are orthogonal to the plane of fluid motion. This simple model arises in the context of the decay of the mean magnetic flux to mass ratio in the interstellar medium. When ions are strongly coupled to neutrals, the transport of a large–scale magnetic field is dr...

Simulated mixing rates of the Rayleigh-Taylor instability for miscible fluids with physical mass diffusion are shown to agree with experiment; for immiscible fluids with physical values of surface tension the numerical data lie in the center of the range of experimental values. The simulations are based on an improved front tracking algorithm to control numerical surface tension and on improved...

The purpose is to study relative turbulent diffusion under controlled, reproducible conditions in the laboratory in order to estimate the constant C in Richardson-Obukhov’s law. We get C ≈ 0.4 – 0.6. We furthermore measure the distance-neighbour function, which is the probability density function of the separation of two fluid particles that are initially close to each other. We find that the m...

Horizontal dispersion in the surface waters of a medium-sized lake was investigated based on four experiments with ensembles of 14 to 17 drifters deployed in Lake Constance during the winter season. The experiments cover length scales between 30 and 3000 m which are typical for the grid length scales employed in 3-D models. Horizontal dispersion coefficients Kdisp were estimated to range from 0...