A narrow low-speed streamwise streak is deliberately introduced into an otherwise extremely spanwise uniform Blasius boundary layer. The streak shares many of the characteristics of Klebanoff modes known to be responsible for bypass transition at moderate Free Stream Turbulence (FST) levels. However, for the low background disturbance level of the free stream (u/U1 < 0.05%), the layer remains laminar to the end of the test section (Rx = 1.4 × 106) and there is no evidence of bursting or other phenomena associated with breakdown to turbulence. A harmonic disturbance is used to excite a sinuous form of instability, which grows over a considerable streamwise distance before breakdown of the streak occurs, which leads to the formation of a turbulent wedge. Detailed measurements show that new streaks are formed on either side during the breakdown process. The characteristics of the wedge are examined over a considerable streamwise distance. A similar mechanism appears to be responsible for the spanwise growth of the wedge since a span-wise succession of new streaks is observed in the early stages of its development.

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