Kelvin Helmholtz instability

A brief introduction

   If sinusoidal waves will increase in amplitude downstream when a sinusoidal disturbance is aroused in a orginally laminar flow, the flow is noticed instable.
   The mechanism of instability is not yet fully understood, but there is certainly a conection with the nonlinear character of the equations of motion.
   Let us consider Kelvin Helmholtz instability.
   The specificity of this instability is that the notion of critical threshold does not exist.
   This instability is interesting, because it appears when there is a inflection point in velocity field.

   The mechanism of instability may be described in the following manner :
 

   Let us consider a longitudinal strip of rotationel fluid, separating the two irrotational regions. Suppose that this rotational zone is perturbed with a longitudinal wave.
   First, pressure differences between the two layers are responsible for the growth of the perturbation amplitude (figure b).
   Second, the crests of disturbance at the top of the interface and the troughs of disturbance at the bottom of the interface travel in opposite directions. This will steepen the vortex the vortex sheet (figure c).
   And velocity-induction will transform the sheet into a spiral (figure d)
 

Summary