Negative wake behind a sphere rising in viscoelastic fluids: a lattice Boltzmann investigation.

We investigate the complex flow field around a sphere rising in a Maxwell fluid by means of the lattice Boltzmann simulation to provide insights into the strange negative wake experimentally observed behind a bubble or particle in non Newtonian fluids. The influence of the rise velocity, sphere diameter, and fluid's rheology is considered through two dimensionless numbers: the Deborah number De and the Reynolds number Re. Our simulation shows that the negative wake appears behind the sphere when De>2 . On the other hand, the shape of the negative wake described by the opening angle theta of the upward flow cone surrounding the negative wake is mainly determined by the Reynolds number Re. These results reveal that the physical origin of the negative wake stems mainly from the competition between the elastic and viscous stresses in the fluid.