When a droplet impacts a pool at low speed, a layer of air trapped beneath the droplet can often prevent it from immediately coalescing into the pool. As that air layer drains away, surface tension pulls some of the droplet’s mass into the pool while a smaller droplet is ejected. When it bounces off the surface of the water, the process is repeated and the droplet grows smaller and smaller until surface tension is able to completely absorb it into the pool. This process is called the coalescence cascade.
Videos
Coughing Contagions
Schlieren imaging has applications even in public health. This video demonstrates the spread of contagion via coughing with and without a mask on. Although air from the cougher’s lungs escapes the sides of the mask, it mostly rises on a thermal plume rather than projecting 1 to 2 meters forward in a turbulent jet as in the maskless case. Flu season is just starting. Don’t forget to get your flu shot!
Vortex Street Sim
This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.
Aurora from the ISS
The solar wind, a rarefied stream of hot plasma ejected from the sun, constantly bombards Earth’s magnetic field. This results in the formation of the magnetosphere, which deflects most of these charged particles away from the earth. Some of them, however, are drawn toward the magnetic poles; when these charged particles strike the upper atmosphere, they cause the gases there to release photons, resulting in the lights we know as auroras. This animation shows the International Space Station flying through the aurora australis–the southern lights. The fluid-like motion of the aurora is no accident; though diffuse, the solar wind is still a fluid governed by magnetohydrodynamics.
Sharkskin-Style Swimsuits
Fans of swimming will recall the controversies of the now-banned sharkskin-style swimsuits that helped break so many records in the past few years. The suits decrease drag on a swimmer both by making them more hydrodynamic in form and by drastically reducing skin friction where the water meets the swimmer’s body. In addition to decreasing the two major sources of drag on a swimmer, the compression provided by the material can help increase blood flow to muscles. These improvements came at a high material cost, though, and, since the technology was not viable for all athletes, it has since been banned.
Mixing in Space
Living here on earth, we are so accustomed to gravity’s effects on fluid behaviors that it’s not always obvious how microgravity will affect them. Here astronaut Richard Garriott demonstrates mixing and separating immiscible liquids in space.
Airfoil Boundary Layer
This video shows the turbulent boundary layer on a NACA 0010 airfoil at high angle of attack (15 degrees). Notice how substantial the variations are in the boundary layer over time. At one instant the boundary layer is thick and smoke-filled and in another we see freestream fluid (non-smoke) reaching nearly to the surface. This variability, known as intermittency, is characteristic of turbulent flows, and is part of what makes them difficult to model.
Surface Tension Demo
This simple demonstration shows the power of surface tension, especially at small lengthscales. Another way to break the surface tension holding the water in the sieve would be to spray the top of the jar with soapy water. The soap acts as surfactant, decreasing the surface tension such that the water is unable to counteract the force of gravity.
Blast Waves
[original media no longer available]
Watch closely in this high-speed video of a bomb exploding and you will see the spherical blast wave moving outward as a visual distortion. The increase in temperature caused by the leading shockwave changes the index of refraction of the air, bending the light and distorting our view of the background. The mechanism is similar to schlieren photography, which has been used for more than a century to capture images of compressible flows.
2D Convection
This simulation shows 2D Rayleigh-Benard convection in which a fluid of uniform initial temperature is heated from below and cooled from above. This is roughly analogous to the situation of placing a pot of water on a hot stovetop. (In the case of the water on the stove, the upper boundary is the water-air interface, while, in the simulation, the upper boundary is modeled as a no-slip (i.e. solid) interface.) The simulation shows contours of temperature (black = cool, white = hot). In general, the hot fluid rises and the cold fluid sinks due to differences in density, but, as the simulation shows, the actual mixing that occurs is far more complex than that simple axiom indicates.
