Free disturbances and spot formation: (i) on a surface roughness, (ii) in a near-wake, (iii) under an adverse pressure gradient

A review is given of recent and continuing theory on initial-value problems arising in a surface-mounted roughness flow, in the near-wake behind a str eamlined body or in a boundary layer under an adverse pressure gradient. Th ese concern instability and transition with entire spectra of modes present . In every case the unsteady disturbances evolve through a transient stage before forming spots. Absolute instability can occur. Nonlinear responses i n the near-wake case drive the transition point extremely close to the body trailing edge; while in the adverse-gradient case they first affect signif icantly the spot leading edge and the calmed region behind the spot.