From Disturbances to Instabilities, to Breakdown to Turbulence: The Physics of Transition in Boundary Layers

In order to understand the end-stages of boundary layer transition in low as well as high disturbance environments it is desirable to establish a unified view of the sequences of physico-mathematical phenomena that lead from laminar flow to self-sustained "bursting" in wall turbulence. The dominant driving disturbances: oncoming free turbulence, unsteady pressure fields, inhomogeneous density fields, inhomogeneities in wall geometry, all force disturbed motions within the boundary layer via multiple competitive receptivity mechanisms. For small disturbances, a sequence of instabilities then leads to sporadic local bursting very near the wall which can sustain turbulence. The local seeds of turbulence then somehow propagate to engulf quite rapidly the surrounding disturbed but still laminar regions.