A note on the linear stability of a two-dimensional Stokes layer

The fluid motion generated adjacent to an infinite flat plate undergoing orbital motion in its own plane is a generalization of the classical Stokes-layer profile. We show that the stabilities of these flows can be related to each other via two transformations directly analogous to the well-known Squire transformation. The main result obtained is that, in general, the two-dimensional Stokes layer is more stable than the corresponding unidirectional Stokes layer. A further by-product of the analysis is the construction of a shear flow having identical neutral stability conditions when subject to either two- or three-dimensional disturbances.