Finite-element modelling of simple shear flow in Newtonian and non-Newtonian fluids around a circular rigid particle

The flow perturbation around a circular rigid particle during simple shear deformation has been investigated for both Newtonian and non-Newtonian (power-law) fluids by finite-element modelling. If the particle is rotating under the applied shear couple and no-slip occurs at the particle–fluid interface, a ‘bow-tie-shaped’ streamline pattern results for both Newtonian and power-law fluids and the shape of streamlines does not change noticeably if the stress exponent (n) is changed. In contrast, if the fluid is allowed to separate from the particle, a ‘double-bulge-shaped’ flow develops in the case of Newtonian fluids, and the type of streamline pattern is influenced by n. We suggest that both stair-stepping and non-stair-stepping geometries of porphyroclast tails may be produced in mylonites, depending on the degree of coherence between the porphyroclasts and the embedding matrix. A different behaviour of the fluid–particle interface may occur as the result of changing fluid rheology, owing to the contrasting stress fields developed for Newtonian and non-Newtonian fluids.