One of the challenges of experimental fluid dynamics is capturing information about a flow that varies in three spatial dimensions and time. Experimentalists have developed many techniques over the years–some qualitative and some quantitative–all of.
This photo series shows the development of a Kelvin-Helmholtz instability. It’s formed when two layers of fluid move past one another at different speeds. In this case, the two fluids meet off the back of a flat plate (seen at the left of the top.