Time-dependent velocities in porous media dispersive flow

Pulsed Gradient Spin Echo (PGSE) NMR methods may be used to measure the asy mptotic dispersion coefficient as well as the velocity autocorrelation func tion (VACF) in porous media flow. The VACF can be measured in the frequency domain using repetitive gradient pulse trains, and in the time domain usin g double PGSE encoding. The one dimensional double PGSE method, and the two dimensional velocity exchange experiment (VEXSY) are briefly outlined and their application to flow in monodisperse 0.5 mm diameter beads packs descr ibed, both axial and transverse VACFs being examined. The measured correlat ion times are shown to agree well with calculated values. The asymptotic di spersion coefficients agree with literature Values in the case of transvers e flow while in axial flow it is shown that asymptotic conditions are not a chieved, even for observation times longer than the correlation time for fl ow around a bead. (C) 2001 Elsevier Science Inc. All rights reserved.