Destabilizing effects of confinement on homogeneous mixing layers

The absolute and convective instability properties of plane mixing layers are investigated for linearized inviscid disturbances. It is shown that confinement by plates parallel to the flow can enhance the absolute instability so much that even a co-flow plane mixing layer becomes absolutely unstable when the ratio of distances of the plates from the mixing layer lies in a certain range. Even when the plates are placed equidistantly from the mixing layer, a co-flow mixing layer can become absolutely unstable if the velocity profile has an asymmetry about its mid-plane. 'Semiconfinement', where a plate is only added to one side of the mixing layer, is also investigated. A substantial destabilization is possible when the plate is added on the side of the faster stream. Previous investigations seem only to have found absolute instability when the streams flow in opposite directions.