Stabilization and destabilization of channel flow by location of viscosity-stratified fluid layer

The stability of the channel flow of two fluids of different viscosities wi th a mixed layer in between is demonstrated to be qualitatively different f rom both interface dominated flows and stratified flows. More important, th is flow displays unexpected changes in stability when the mixed layer overl aps the critical layer of the disturbance: this feature can be exploited fo r flow control. When these layers are distinct, the flow is mildly destabil ized when the less viscous fluid is in the outer region. When the layers ov erlap, however, there is an order of magnitude stabilization of the flow. T he reverse occurs when the more viscous fluid is in the outer region. This behavior may be explained by the balance of stresses in the critical layer. (C) 2001 American Institute of Physics.