Secondary circulations in rotating-flow boundary layers

The rotating-flow boundary layer is a special case of the more general three-dimensional boundary layer in which the pressure gradient imposed by the outer flow (above the boundary layer) is not in the same direction as the outer flow. The rotating-flow boundary layer thus has motion that is transverse to the streamlines of the outer flow, that is, there is a secondary circulation to the primary circulation of the outer flow. That the secondary circulation can extend far above the boundary layer presents a set of perplexing conceptual problems unlike any encountered in a two-dimensional boundary layer. This paper reviews, critically discusses and presents numerical simulations attempting to supplement existing theory for several canonical problems concerning secondary circulations in rotating-flow boundary layers. Based on the present results, brief comments on atmospheric vortices are made.