Abstract
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 μm 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, Péclet numbers (Pe) covering a range from 2.8×103 to 8.4×103. In the case of longitudinal flow we find evidence for both reversible and irreversible dispersion over observation times of up to 10τv where τv 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 τv. 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.
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