Secondary Instability of Plane Channel Flow to Subharmonic Three-Dimensional Disturbances

A linear secondary instability mechanism is presented that leads to the occurrence of subharmonic three‐dimensional disturbances in wall‐bounded shear flows. The instability originates from the periodic redistribution of vorticity in the shear flow by small but finite‐amplitude Tollmien–Schlichting waves. Low threshold amplitudes and other characteristics of this instability are consistent with experiments and may elucidate various obscure observations.