The Kelvin-Helmholtz  instability results from velocity shears between two media. These media need not even be of different densities. Any time there is a non-zero curvature, the flow of one fluid around
another will lead to a slight centrifugal force which in turn leads to a change in pressure thereby amplifying the ripple. The most familiar example of this is wind blowing over calm water. Tiny dimples in the smooth surface will quickly be amplified to small waves.

    This repport has been voluntarily made with a simple technical support in order to facilitate the understanding.