The transition to turbulence in parallel flows: A personal view

This is a discussion of the present understanding of transition to turbulence in parallel flows, based upon the idea that it arises from a subcritical instability. The result is a coupled set of equations, one amplitude equation in the direction of translational invariance of the geometry coupled with the standard Reynolds equation for the average transfer of momentum. It helps to understand a basic feature of the transition in parallel flows, namely that turbulence manifests itself in localised domains growing at a constant speed depending on the Reynolds number.