Instability of plane parallel shear flow (toward a mechanistic picture of how it works)

This paper reviews work done over the last twelve years on wave overreflection in shear flows, and on the development of a mechanical picture of how overreflection and instability work. It is argued that the wave geometry of a flow configuration is the primary determinant of its stability. It is also argued that the primary mechanism for wave amplification is the Orr mechanism.