Flow coherence in a bead pack observed using frequency domain modulated gradient nuclear magnetic resonance

We have used a new nuclear magnetic resonance (NMR) method based on periodic wave form magnetic field gradients to investigate temporal correlations for flow in porous media. The frequency domain modulated field gradient NMR technique directly yields the frequencydependent-dispersion coefficient, i.e., the spectral density of the velocity autocorrelation function. Our measurements of dispersion spectra have been carried out, in the direction transverse to the mean flow, for water flowing in a monodisperse latex bead pack (diameters 50-136 mum) and at Peclet numbers ranging from 10 to 5000. We observe spectral peaks at a frequency corresponding to the inverse time for flow around a bead, an effect we attribute to coherent meandering flow around the bead. This observation is in close agreement with the recent computer simulations of Maier , in which negative velocity autocorrelation function transients are seen. (C) 2001 American Institute of Physics.