Reversible and irreversible dispersion in a porous medium

Pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR) has been used to examine the approach to asymptotic dispersion for flow of water in a randomly packed array of 500 mum latex spheres in a cylindrical tube of inner diameter 10 mm. Measurements have been performed using single pulse pair and double pulse pair PGSE NMR in directions both transverse to and co-linear with the flow direction, Peclet numbers (Pe) covering a range from 2.8x10(3) to 8.4x10(3). In the case of longitudinal flow we find evidence for both reversible and irreversible dispersion over observation times of up to 10tau(v) where tau(v) is the correlation time associated with flow across one bead diameter. The fluctuating component of the velocity field appears to obey a simple Ornstein-Uhlenbeck correlation function with a single correlation time in close agreement with tau(v). In transverse dispersion, no stationary velocity distribution is evident and the correlation time for fluctuation is much smaller, over the entire range of Pe values. (C) 2003 American Institute of Physics.