Interaction of discrete and continuous boundary layer modes to cause transition

The interaction of discrete and continuous Orr–Sommerfeld modes in a boundary layer is studied by computer simulation. The discrete mode is an unstable Tollmien–Schlichting wave. The continuous modes generate jet-like disturbances inside the boundary layer. Either mode alone does not cause transition to turbulence; however, the interaction between them does. The continuous mode jets distort the discrete modes, producing ΛΛ shaped vortices. Breakdown to turbulence is subsequent. The lateral spacing of the ΛΛ’s is sometimes the same as the wavelength of the continuous mode, sometimes it differs, depending on the ratio of wavelength to boundary layer thickness.