Review Note

1. Below a certain Reδ, disturbances of all wavelengths are attenuated. But above this Reynolds number, some disturbances may grow. This identifies the critical Re for onset of instability.

2. Typically, Recrit is lower for velocity profiles with a point of inflection, since such flows may undergo inviscid instability (if damping effect of viscosity is not present).

3. Critical Re for instability is much lower than critical Re for transition to turbulence. This is because the disturbances need to grow in amplitude before they become large enough for turbulence to develop.

4. In the range of Re between Recrit for instability and Recrit for transition, the character of the disturbances is also expected to evolve: e.g from 2D to 3D.