LAMINAR-TURBULENT TRANSITION ZONE IN THE BOUNDARY LAYER.

The flow during transition from the laminar to a turbulent state in a boundary layer is best described through the distribution of the intermittency. In constant-pressure, two-dimensional flow, turbulent spots appear to propagate linearly; the hypothesis of concentrated breakdown, together with Emmons's theory, leads to an adequate model for the intermittency distribution over flow regimes ranging all the way from low subsonic to hypersonic speeds. However, when the pressure graident is not zero, or when the flow is not two-dimensional, spot propagation characteristics are more complicated. The resulting intermittency distributions often show peculiarities that may be best viewed as 'subtransitions'. Previous experimental results in such situations are reviewed and recent results and models are discussed.