Procedure to evaluate the shear rate profile and the apparent viscosity of non-Newtonian fluids in the falling cylinder viscometer

The falling cylinder viscometer (FCV) is a reliable instrument to quantify the viscosity of Newtonian fluids. Nevertheless, when non-Newtonian fluids are tested in this device, difficulties appear to determine the apparent viscosity. Thus, conventional rheometric calculations cannot be applied directly to experimental data provided by the FCV in order to obtain the apparent viscosity function, because the knowledge of a rather complex shear rate profile in the annular flow, between the falling cylinder and the container, is required. Consequently, experimental data of the FCV must be processed numerically by including inevitably an appropriate model of the apparent viscosity for the fluid under study. Previous works used the Power Law model, within several small sub-regions of shear rates, selected heuristically, as a reasonable approximation. The present work proposes an algorithm to process the experimental data provided by the FCV for different non-Newtonian fluids. Thus, this generic procedure allows one to perform calculations for any model of the apparent viscosity that includes a set of parameters to be appropriately identified.