Fluids and gases can move in strange and mysterious ways that aren’t always apparent to our unaided eyes. It's only with the use of tracers or dyes, photographic techniques and a bit of luck, that we can capture fluid dynamics in action. Here are some of the weird and beautiful ways in which fluids flow. Above: Supersonic Shock Wave When an object moves faster than the speed of sound, funny things start to happen. The bullet in the image above is moving so fast, the air in front of it can’t get out of the way quickly enough. So it starts to pile up in front of the bullet, forming an area of compressed air — a shock wave. The same thing happens with supersonic jets. As they fly faster than the speed of sound, they build up a shock wave in front of them. The sonic boom that accompanies these aircraft is actually the wave of compressed air hurtling by your ears. Image: Andrew Davidhazy/Rochester Institute of Technology

Von Karman Vortex Streets In the image above, winds blowing cloud cover over the Pacific Ocean encounter obstacles in the form of Alaska's Aleutian Islands. Two of the islands are visible as darker spots on the right side of the image. When flows like air currents encounter an object like an island, they split and go around the obstacle. This sets up a low-pressure area just behind the object. As the air comes around the island, the flows curl in toward the low pressure. This curl, or vortex, acquires a spin and breaks off from the rest of the stream. The flows alternate spinning off vortices, forming a trailing line of swirling clouds. Images: 1) U.S. Geological Survey/NASA. 2) NASA/MODIS.

Inverted Drafting Inverted drafting happens when a leading object encounters less drag than the object behind it. An example is when flows encounter passive objects, like flags, arranged one behind the other. Flags are considered passive because they flop around with every shifting breeze, unlike humans who are considered rigid objects. In this image, two white, S-shaped lines are flags oriented one behind the other. The flag in the follower position cuts into the wake given off by the flag out front. By cutting into the leader’s wake, the follower reduces the overall resistance the leader experiences. Image: Leif Ristroph and Jun Zhang/New York University

Saffman-Taylor Instability These beautiful patterns are the result of interactions between fluids of different viscosity, which is a measure of how thick a liquid is. For example, water is less viscous than honey. In the first image below, a viscous fluid called glycerin is sandwiched between two clear plates. When a less viscous fluid like water is injected into the glycerin, the water intrudes in the form of fingers. The pattern grows when the finger tips split into more fingers. Video: Jessica Rosenkrantz. Images: 1) Dustin Grace, Jessica Todd, Marilyn Poon, and Robert Neilson/University of Colorado. 2) Jessica Rosenkrantz/Flickr

Convection A classic example of convection begins with the goblet in the image above filled with warm water. As the air around the glass heats up, it begins to rise. Eventually the air will lose its heat to the surrounding environment, cool and then sink back down. In the second image, an ice cube floating in a pool of water cools the liquid around it. Since cold water is more dense than warm water, the heavier liquid begins to sink. Images: 1) Andrew Davidhazy/Rochester Institute of Technology. 2) J. Kim Vandiver and Harold E. Edgerton/MIT

Cavitation When an object like a propeller blade moves through a fluid with enough speed, it creates a void in the liquid. That cavity, or bubble, collapses in on itself, generating shock waves and jets of water. Cavitation bubbles can also form when a small area of water is hit by an electric spark or laser, as in the video below. This creates a bubble of gas that quickly implodes, also generating shock waves and water jets. In the image above, a laser created a cavitation bubble in a water droplet. When it collapsed, the bubble formed two jets &emdash; a taller, thinner jet coming up the middle of a shorter, wider jet. The small ring of bubbles inside the droplet is the remains of the initial cavitation bubble. https://www.youtube.com/watch?v=gR0YBAhY2PQ Image: Claus-Dieter Ohl/Nanyang Technological University. Video: ESA, Danail Obreschkow, Philippe Kobel, Nicolas Dorsaz, Aurèle de Bosset, Mohamed Farhat.

Kelvin-Helmholtz Instability The waves in the cloud image above illustrate what happens when two flows right next to each other travel at different speeds, creating what is known as a Kelvin-Helmholtz Instability. When this occurs, the area where the two flows meet builds up friction, which rolls one layer up into vortices. Kelvin-Helmholtz instabilities show up well on planets like Saturn (below), that have alternating atmospheric bands. Images: 1) Kate Calder/Flickr. 2) NASA-JPL.