Lighthill and the triple-deck, separation and transition

In 1953 James Lighthill, conducting an investigation into the potential mechanisms for upstream influence within boundary layers in supersonic flow, published a theoretical approach which explicitly took into account the influence of viscosity on a disturbance to an incident boundary-layer profile. In doing so he was able to predict a length-scale for upstream influence which scales with the global Reynolds number R, assumed large, as R-3/8. The physical process he identified is now referred to as a pressure-displacement (or viscous-inviscid) interaction. This article discusses Lighthill's original paper and then proceeds to show how an appreciation of this interaction mechanism can help in the solution of many other problems in fluid mechanics and especially those of flow separation and late-stage laminar-turbulent transition. Finally, the article gives a brief description of the similarities between these two processes as seen from the unifying viewpoint of Lighthill's pressure-displacement interaction.