Pulsed-field-gradient NMR is used to measure tracer dispersion in flow through a porous medium. Data are presented for water flowing through packs of plastic beads at P\'eclet numbers 0{≤}Pe{≤}150, using strong, fast gradient pulses to measure pore-scale molecular displacements. The transition at Pe{~}1 from tortuosity-reduced diffusion to dispersion is observed. The data are Fourier transformed to provide a measurement of the wave-number- and frequency-dependent nonlocal dispersion coefficient. The experimental results compare favorably with an approximate calculation of Koch and Brady [J. Fluid Mech. 180, 387 (1987); Chem. Eng. Sci. 42, 1377 (1987)]. {} 1996 The American Physical Society.
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Ding et al. (1996) studied this question.
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