Three-dimensional phenomena in the transitional flat-plate boundary layer

Primary instability of the Blasius boundary-layer flow leads to growth of TS waves and to a streamwise almost periodic modulation of the flow. The linear stability of this modulated flow with respect to spanwise periodic three-dimensional disturbances is studied. Of the various possible types of resonance in the Floquet system of stability equations, the case of primary resonance is considered. The associated disturbances lead to peak-valley splitting and formation of a longitudinal vortex system. Growth rates and spatial structure of the disturbances are compared with experimental data of Klebanoff et al. (1959, 1962). The role of the subharmonic instability in these experiments is discussed.