Pore-scale flow and dispersion

Pore-scale simulations of fluid flow and mass transport offer a direct mean s to reproduce and verify laboratory measurements in porous media We have c ompared lattice-Boltzmann (LB) flow simulations with the results of NMR spe ctroscopy from several published flow experiments. Although there is qualit ative agreement, the differences highlight numerical and experimental issue s, including the rate of spatial convergence, and the effect of signal atte nuation near solid surfaces. For the range of Reynolds numbers relevant to groundwater investigations, the normalized distribution of fluid velocities in random sphere packings collapse onto a single curve, when scaled with t he mean velocity. Random-walk particle simulations in the LB flow fields ha ve also been performed to study the dispersion of an ideal tracer. These si mulations show an encouraging degree of quantitative agreement with publish ed NMR measurements of hydrodynamic and molecular dispersion, and the simul ated dispersivities scale in accordance with published experimental and the oretical results for the Peclet number range 1 less than or equal to Pe les s than or equal to 1500. Experience with the random-walk method indicates t hat the mean properties of conservative transport, such as the first and se cond moments of the particle displacement distribution, can be estimated wi th a number of particles comparable to the spatial discretization of the ve locity field. However, the accurate approximation of local concentrations, at a resolution comparable to that of the velocity field, requires signific antly more particles. This requirement presents a significant computational burden and hence a numerical challenge to the simulation of non-conservati ve transport processes.