Modulational instability of plane waves in a two-dimensional jet and wake

The modulational instability problem is reformulated by the amplitude expansion method. A generalized version of the Stewartson-Stuart equation is derived, which is applicable to any supercritical state as long as the amplitude of the disturbance is small. The modulational instability of plane waves in a two-dimensional jet and wake is investigated on the basis of this equation, and it is found that the plane waves are stable to side-band disturbances in supercritical states except in the close neighbourhood of the critical state.