Spectra of the Orr–Sommerfeld equation

It is shown that the continuous spectra of both the temporal and spatial eigenvalues of the Orr–Sommerfeld equation for a Blasius flow can be re-interpreted as discrete spectra of the heat-conduction and diffraction types, respectively. Most of the general properties of the discrete spectrum of the Orr–Sommerfeld equation discovered by Mack can be understood on the basis of a simple model equation.