Particle rotation in shear flows

For a \textit{linear shear flow} it is known that a particle rotates \textit{slower} than the surrounding flow when inertial effects are included. Experiments performed for a sphere fixed (but free to rotate) in a flow in \textit{solid body rotation} indicate that the rotation rate of a sphere can be \textit {faster} than the rotation rate of the flow. Numerical simulations at moderate $Re$ confirm this observation. To gain understanding of the phenomenon the effects of stream-wise and cross-stream shear on the rotation rate of a fixed sphere in a flow are numerically investigated. Moreover, the change of the flow due to the sphere is recorded. The results indicate that for moderate $Re$ the two types of shear have completely different effects on the particle rotation rate. Moreover, the cross-stream effects appear to be dominant to the stream-wise effects. When the two types of shears are combined to create a strain field, the particle starts to rotate, although the undisturbed flow is non-rotational.