CHARACTERIZATION OF TURBULENT FLOWS BY NMR MEASUREMENTS WITH PULSED GRADIENTS

A theoretical description is given of the dependence of the signal from a turbulent flowing liquid in a pulsed-gradient nuclear magnetic resonance measurement. The relationships between the characteristic time scale of velocity fluctuations of the fluid and the timing parameters of the gradients and NMR pulse sequence are made explicit. An experimental approach has been developed by which the time scale of random changes in the velocity can be determined, and this has been used to show that pipe flow at Reynolds numbers of 0-3000, and grid-generated turbulence, are characterized by correlation times that are relatively long on the scale of the NMR measurement. The turbulent intensity has been measured at different spatial locations distal to meshes of different spatial frequency as well as in a pipe with Reynolds numbers of 0-6500. The onset of turbulence at Re approximate to 2300 in plain pipe flow and the spatial variation of turbulent intensity distal to each mesh are in good agreement,vith measurements made by other techniques. NMR may have unique advantages for spatial mapping and characterization of some turbulent flows. (C) 1994 Academic Press, Inc.