Linear stability and energy growth of viscosity stratified flows

The non-normality of the Orr–Sommerfeld equation leads to the possibility of disturbance growth even though all eigenvalues are stable. In single-fluid flow the disturbance growth converges to a limit once the number of modes exceeds a minimum number. In the case of a two-fluid flow, however, convergence is not found. The problem of nonconvergence is due to the presence of the interface and the corresponding interfacial mode. The interface is replaced with a miscible layer of variable viscosity. When the thickness of the miscible layer is approximately equal to the thickness of the critical layer, the flow resembles two-fluid flow and one of the modes starts behaving like the interfacial mode.