Secondary circulations in rotating-flow boundary layers

The rotating-flow boundary layer is a special case of the more general threedimensional 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.