THE GENERATION OF TURBULENT SPOTS IN A LAMINAR BOUNDARY-LAYER

The turbulent spot stage of boundary layer transition was investigated by using computer generated deterministic white noise to excite a lam inar boundary layer. This type of excitation generated amplitude modul ated Tollmien-Schlichting waves at some location downstream of the sou rce that evolved in a manner qualitatively similar to modulated distur bances observed in ''naturally'' excited situations. The initial devel opment of these modulated waves was in good agreement with linear theo ry, however further downstream the velocity signals exhibited large am plitude excursions at isolated time instants that were associated with the formation of localised structures in the flow. These flows appear ed to be highly unstable and admitted bursts of high frequency oscilla tions that lead to the formation of turbulent spots at some downstream location.