ON THE NONLINEAR GROWTH OF SINGLE 3-DIMENSIONAL DISTURBANCES IN BOUNDARY-LAYERS

Experiments indicate the importance of three-dimensional action during transition, while high-Reynolds-number-flow theory indicates a multi- structured type of analysis. In line with this, the three-dimensional non-linear unsteady triple-deck problem is addressed here, for slower transition. High-amplitude/high-frequency properties show enhanced dis turbance growth occurring downstream for single nonlinear oblique wave s inclined at angles greater than tan-1 square-root 2 (almost-equal-to 54.7-degrees) to the free stream, in certain interesting special case s. The three-dimensional response there is very 'spiky' and possibly r andom, with sideband instabilities present. A second nonlinear stage, and then an Euler stage, are entered further downstream, although fast er transition can go straight into these more nonlinear stages. More g eneral cases are also considered. Sideband effects, sublayer bursting and secondary instabilities are discussed, along with the relation to experimental observations.