STUDY OF TRACER DISPERSION IN SELF-AFFINE FRACTURES USING LATTICE-GASAUTOMATA

This paper studies the problem of hydrodynamic dispersion of a tracer in a fluid flowing through a two-dimensional rough channel bounded by self-affine surfaces. Changing the surface roughness exponent H, rough walls having different microstructure are obtained. In order to simul ate hydrodynamics, a lattice-gas automata modified to introduce two di fferent species of particles is used. In the studied range of Peclet n umbers (20-50), the concentration profiles along the channel are well described by Gaussian-type dispersion. A clear enhancement of-the disp ersion due to roughness is observed. For the studied regime of Peclet numbers, a simple approach is proposed which allows us to interpret th e dispersion enhancement in terms of surface roughness. It is shown th at the dispersion enhancement in the rough channel is due to the prese nce of two characteristic lengths, the hydraulic diameter delta(H) whi ch determines the velocity in the channel and the average aperture del ta(av) which determines the transverse diffusion length; next shown is that the dispersion in the rough channel varies as D(parallel to)simi lar to(delta(av)/delta(H))(2). The values of delta(H) Obtained from th e dispersion results are compared with those obtained from permeabilit y measures and a good agreement is observed. In the studied domain of Peclet numbers, the roughness exponent H has only a weak influence on tho dispersion. (C) 1995 American Institute of Physics.