Simulation of viscoplastic flow past cylinders in tubes

Drag flow past circular cylinders concentrically placed in a tube filled with viscoplastic fluids, obeying the Herschel-Bulkley model, is analyzed via numerical simulations with the finite element method. The purpose it to find limiting drag values for cessation of motion of the object in steady flow. Different aspect ratios have been Studied ranging from a disk to a long cylinder. For the simulations, the viscoplastic model is used with an appropriate modification proposed by Papanastasiou, which applies everywhere in the flow field in both yielded and practically unyielded regions. The extent and shape of yielded/unyielded regions are determined along with the drag coefficient for a wide range of Bingham numbers. The simulation results are compared with previous experimental values [L. Jossic, A. Magnin, AlChE J. 47 (2001) 2666-2672] for cessation of flow. They show that the values of the drag coefficient are lowest for the disk and highest for the long cylinder. Discrepancies are found and discussed between the simulations and the experiments, with the simulations providing lower values in the limit of very high Bingham numbers. (C) 2008 Elsevier B.V. All rights reserved