Spirals. Vortices. Waves. Cyclones. Turbulence. Ripples. An engineer collected all the classic photographs of hydraulic movement he could find in old scientific volumes and self-published a reference book for engineering students. He’s been surprised that mostly artists, animators and poets have been buying it. I’m not surprised.
Hexagonal smoke ring. The growth of waves around a vortex ring is often called Widnall instability, after the researcher who first analyzed it. Here it has produced a remarkably symmetric pattern of smoke in air at a Reynolds number of about 1000.
Instability of a round jet. Smoke gives a different view of the flow above, at a Reynolds number of about 13,000.
Airplane model in free flight at M=1.1. Shadowgraphs show a winged model launched into atmospheric air from a gun. The bow wave is marginally attached at this slightly supersonic speed. In the plan view above, the wings are lifting, as shown by trailing vorticles from the tips. In the side view below, the herringbone pattern is produced by pressure pulses from grooves in the wing that trip the boundary layer to make it turbulent over the rear half.