• shear mixing;
  • stability analysis;
  • stratified shear layers;
  • transition to turbulence

When fluids with different or continuously varying properties flow next to one another, turbulent flow can emerge. At its most basic, turbulence takes the form of swirling vortices that occur along the flow direction; these vortices are known as a Kelvin-Helmholtz (K-H) instability. Superimposed on this two-dimensional K-H foundation are a variety of secondary instabilities that extend the mixing into three dimensions and endow it with a more complicated structure. How these secondary instabilities form and the role they play in pulling smoothly flowing fluid into the chaos of turbulence remain an area of active investigation.