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 Λ shaped vortices. Breakdown to turbulence is subsequent. The lateral spacing of the Λ ’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.