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A shear layer (or commonly referred to as a mixing layer) is a fluid dynamic structure that involves two flows of varying velocities interacting with each other. As the fluid from one portion interacts with the fluid from the other portion, there are a multitude of instabilities that can occur. Even without the influence of viscosity, a shear layer can "roll-up" from two parallel flows to coherent vortical structures in a very short time. Analyzed since the the 19th century, shear layers are a fundamental, canonical fluid flow. Lord Kelvin and Helmhotz were two of the first to study this flow. They were later credited with both coming up with the Kelvin-Helmholtz Instability.

The Kelvin-Helmholtz Instability (KHI) is a hydrodynamic, inviscid instability. This means that even in absence of viscosity, the flow geometry will become unstable at some time and location downstream of the origin.