Counter-flow phenomena studied by nuclear magnetic resonance (NMR) velocimetry and flow simulations

Abstract

Flow patterns including counter-flow and flow reversal effects have been studied by a combination of nuclear magnetic resonance flow imaging and numerical modeling using the finite volume method in the open-source computational fluid mechanics package OpenFOAM. Two cylindrical geometries have been used: In a concentric double-cylinder system the flow reversal under oscillatory rotation of the inner cylinder has been followed, and the time evolution of the flow reversal has been studied. We find extended periods of counter-rotating flow in the gap where fluid in the inner part of the gap follows the new direction of the rotor, while the outer part takes a longer time until the viscous forces transmit the reverted flow direction outwards. The radial position of the reversal of flow direction has been monitored as a function of the oscillation angle after the turning point. In the second cylindrical geometry, the rotating bob is placed off the center and a counter-rotating vortex is detected in the wider part of the gap. At constant viscosity and eccentricity, the position of the center of the vortex was found to depend on the rotation frequency of the bob. Qualitative and quantitative agreement between experiment and laminar (nonturbulent) flow simulations has been obtained for both steady-state flow using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm and time-dependent flow using the Pressure Implicit with Splitting of Operators (PISO) algorithm.