Shear flow stability: the physics of spatial development and viscosity stratification

We describe how a nonparallel shear flow presents completely new physics in its stability behaviour, totally absent in a very close parallel shear flow. Roddam Narasimha’s insight in this context is discussed. The critical layer, where most of the disturbance kinetic energy is produced, plays an important role in this process. This critical layer also plays an important role in a viscosity-stratified flow. Boundary layers at low disturbance levels follow a more “traditional” route to turbulence whereas the process is very different in channels and pipes. In recent decades, strides have been made on understanding the latter, and we describe this. We also mention how viscosity stratification enters this process.