Critical assessment of rheological tests to characterize polymer support fluids using CFD

Abstract Aqueous solutions of hydrolyzed polyacrylamide polymer (HPAM) are now regularly used to support excavation in underground construction, e.g., trenches and pile bores, because of their high viscosity and shear-thinning nature. To ensure excavation stability, it is crucial to maintain the high fluid viscosity and therefore to be able to measure it accurately on site. In this work, two tests commonly used in civil engineering are assessed, i.e. the Fann viscometer and the Marsh funnel. These tests measure viscosity in different ways; the Fann viscometer uses a torque measurement, while the Marsh funnel measures the discharge time for a fixed flow volume. We employed high-resolution computational fluid dynamics (CFD) techniques to simulate both test processes and assessed the extent to which the test predictions by the standard measurement protocols can capture the rheological parameters. The results suggest that the Fann viscometer is reliable at very low shear rates; however, inertial effects lead to an overestimation of apparent viscosity at high rotational velocities. Though a correction factor can be successfully applied to correct Fann viscometer data in the case of Newtonian fluids, it is unlikely that a single correction factor can be used for HPAM fluids. The Marsh funnel time is very sensitive to the infinite-shear-rate viscosity for HPAM fluids. This presents a challenge to its use as an in-situ quality control measure as the infinite-shear-rate viscosity is similar for HPAM with different concentrations. The critical assessment offered in this work supports the development of new experimental approaches for quality control when polymer fluids are used in ground engineering construction projects.