Steady periodic waves in a three-layer fluid with shear in the middle layer

A three-layer intrusion flow is considered, in which all three layers are in motion, with different speeds, relative to the observer. Shear is present in the middle layer, and the lowest fluid may even move oppositely to the upper two (so giving an exchange flow). Two thin interfaces are present, above and below the moving middle layer. A linearized analysis is presented for small wave amplitudes. Nonlinear periodic solutions are then obtained using a Fourier technique, and reveal a range of nonlinear phenomena, including limiting waves, multiple solutions and resonances.