As dangerous as explosions are in air, they are even more destructive in water. Because air is a compressible fluid, some part of an explosion’s energy is directed into air compression. Water, on the other hand, is incompressible, which makes it an excellent conductor of shock waves. In the video above we see some simple underwater explosions using water bottles filled with dry ice or liquid nitrogen. The explosions pulsate after detonation due to the interplay between the expanding gases and the surrounding water. When the gases expand too quickly, the water pressure is able to compress the gases back down. When the water pushes too far, the gases re-expand and the cycle repeats until the explosion’s energy is expended. This pulsating change in pressure is part of what makes underwater explosions so dangerous, especially to humans. Note in the video how the balloons ripple and distort due to the changing pressure. Those same changes in pressure can cause major internal damage to people. (Video credit: The Backyard Scientist; submitted by logicalamaze)
Search results for: “waves”
Jumps in Stratified Flows
One of the factors that complicates geophysical flows is that both the atmosphere and the ocean are stratified fluids with many stacked layers of differing densities. These variations in density can generate instabilities, trap rising or sinking fluids, and transmit waves. The animations above show flow over two ridges with dye visualization (top), velocity (middle), and contours of density (bottom). The upstream influence of the left ridge creates a smooth, focused flow that quickly becomes turbulent after the crest. The jet rebounds as a turbulent hydraulic jump before slowing again upstream of the second ridge. Like the first ridge, the second ridge also generates a hydraulic jump on the lee side. Clearly both stratification and the local topography play a big role in how air moves over and between the ridges. If prevailing winds favor these kinds of flows, it can help generate local microclimates. (Image credit and submission: K. Winters, source videos)
How Loud Can Sound Get?
Sound and acoustics often intersect with fluid dynamics. Most of the sounds we experience are pressure waves traveling through air. In this video, Joe of It’s Okay To Be Smart takes a closer look at sound: what it is; how we measure it; and just how loud a sound can get. For air at sea level, the loudest possible sound is 194 dB. Add any more energy and it distorts the pressure wave from what we recognize as sound into what’s known as a shock wave. (Video credit: It’s Okay To Be Smart/PBS Digital Studios)
Extinguishing Fires With Sound
Engineering students from George Mason University have built a fire extinguisher that uses sound to put out flames. Since sound waves are mechanical pressure waves, they can move the air surrounding a burning material. Through trial and error the students found the high-frequency sound had little effect, but at frequencies between 30-60 Hz the sound waves could jostle enough oxygen away from the flame to extinguish the fire. They’re hoping the solution is scalable and can be applied to larger fires. For other wild ideas for chemical-less fire extinguishers, check out how researchers put out fires with explosions. (Video credit: George Mason University; submitted by @isanaht)
The Dance of the Droplets
Milk and juice vibrating on a speaker can put on a veritable fireworks display of fluid dynamics. Vibrating a fluid can cause small standing waves, called Faraday waves, on the surface of the fluid. Add more energy and the instabilities grow nonlinearly, quickly leading to tiny ligaments and jets of liquid shooting upward. With sufficiently high energy, the jets shoot beyond the point where surface tension can hold the liquid together, resulting in a spray of droplets. (Image credit: vurt runner, source video; h/t to @jchawner)
Dead Water
Sailors have long known about the “dead water” phenomenon, which can bring ships to a near-standstill, but it was only within the last century that an explanation for the behavior was found. The underlying cause is a stratification of fluids of different densities. As seen in the video above, when a boat moves by exerting a constant force, such as with propellers, it generates an internal wave along the interface between two density layers in the water. As the wave grows in amplitude, it speeds up, chasing and eventually breaking against the boat. The energy that drives the internal wave’s growth comes from the energy the boat expends for propulsion; the larger and closer the wave gets, the slower the boat goes because its energy is sapped by the wave. In the ocean, particularly near sources of freshwater run-off, like melting glaciers, the water can be extremely stratified, with many layers of different salinity and density. The end of the video simulates this with a three-fluid demonstration in which the boat’s motion generates internal waves across multiple density interfaces. (Video credit: M. Mercier et al.)
Rowing Water Striders
Water strider insects are light enough that their weight can be supported by surface tension. For some time, they were thought to propel themselves by using their long middle legs to generate capillary waves–ripples– that pushed them forward, but juvenile water striders are too small for this technique to work. Instead researchers found that water striders move by using their middle legs like oars. The leg motion creates vortices about 4 mm below the water surface, and this water moving backward propels the insect forward. In the photos above, the scientists visualized the flow by sprinkling thymol blue on the water and letting the striders move freely. You can learn more about the work here or in this Science Friday episode. (Photo credits: J. Bush et al.)
Sand Dunes
Sand dunes form with a gentle incline facing the wind and a steeper slip face pointing away from the wind. Most slip faces are angled at about 30 to 34 degrees–called the angle of repose. The shape is determined by the dune’s ability to support its own weight; add more sand and it will cascade down the slip face in a miniature avalanche. Similarly, if you disturb sand on the slip face by digging a hole at the base, you get the cascading collapse seen in this video. By removing sand, the dune’s equilibrium is broken and it can no longer support its weight. This makes sand flow down the slip face until enough is shifted that the dune can support itself. Being a granular material, the sand itself appears to flow much like a fluid, with waves, ripples and all. (Video credit: M. Meier; submitted by Boris M.)
Reader Question: Rippling Runoff
Reader junolivi asks:
When shallow water (like runoff from melting snow) flows across pavement, it creates small repeated wave-like ripples. What creates that texture and why isn’t it just a steady flow?
This is a great question that’s probably crossed the mind of anyone who’s seen water running down the gutter of a street after a storm. The short answer is that this gravity-driven flow is becoming unstable.
Fluid dynamicists often like to characterize flows into two main types: laminar and turbulent. Most flows in nature are turbulent, like the wild swirls you see behind cars driving in the rain. But there are laminar flows in nature as well. Often flows that begin as laminar will become turbulent. This happens because those laminar flows are unstable to disturbances.
The classic example of stability is a ball on a hill. If the ball is at the top of the hill and you disturb it, it will roll down the hill because its original position was unstable. If, on the other hand, the ball is in a depression, then you can prod the ball and it will eventually settle back down into its original place because that position was stable. Another way of looking at it is that, in the unstable case, the disturbance–how far the ball is from its original position–grows uncontrollably. In the stable case, on the other hand, the disturbance can be initially large but eventually decays away to nothing.
There are many ways to disturb a laminar flow–surface roughness, vibrations, curvature, noise, etc., etc. These disturbances enter the flow and they can either grow (and become unstable) or decay (because the flow is stable to the disturbance). Just as one can look at the stability of a pendulum, one can mathematically examine the stability of a fluid flow. When one does this for water flowing down an incline, one finds that the flow is quite unstable, even in the ideal case of a pure, inviscid fluid flowing down a smooth wall.
The reason that one sees distinctive waves with a particular wavelength (assuming that they aren’t caused by local obstructions) is directly related to this idea of instability. Essentially, the waves are the disturbance, having grown large enough to see. One could imagine that any wavelength disturbance is possible in a flow, but mathematically, what one finds, is that different wavelengths have different growth rates associated with them. The wavelength we observe is the most unstable wavelength in the flow. This is the wavelength that grows so much quicker than the others that it just overwhelms them and trips the flow to turbulence. This is very common. For example, you can see distinctive waves showing up before the flow goes turbulent in both this mixing layer simulation and this boundary layer flow.
(Image credits: anataman, mo_cosmo; also special thanks to Garth G. who originally asked a similar question via email)
Below a Surfer’s Wave
From below a plunging breaking wave–the classic surfer’s wave–looks like a giant vortex tube. Smaller rib vortices, the rings around the main vortex in the photo above, can form where there are variations along the breaking wave. As the wave rolls on, it stretches the vorticity variations along the wave’s span. When stretched, vortices spin up and intensify; this is a result of conservation of angular momentum. Check out more amazing photos of waves in Ray Collins’ portfolio. (Photo credit: R. Collins; via The Inertia)