Dynamic structuring and mixing efficiency in rapidly rotating shear layers

Flow evolution in a rotating mixing layer is investigated using direct and large-eddy simulation. The mixing layer rotates about a fixed vertical axis perpendicular to the plane of its mean initial shear. The rotating mixing layer forms oscillatory large-scale columnar structures and rapid horizontal flow-reversals. The frequency of these oscillations varies approximately inversely with the Rossby number, $Ro$. At low $Ro$ vertical mixing of a passive scalar is strongly reduced. This is quantified by investigating the evolution of level-sets of the scalar field. The surface-area of the level-sets remains virtually constant even at modest rotation rates. More localized motions are less affected by rotation and yield comparatively high levels of surface-wrinkling. Rotation effects are accurately predicted in large-eddy simulations that involve the dynamic eddy-viscosity model or the LANS-$\alpha$ or Leray regularization models. The small-scale variability is best preserved when using the LANS-$\alpha$ formulation.