Here the U.S. Army Corps of Engineers release 13,000 cubic feet per second (~370 cubic meters per second) of water at a dam in Oklahoma. That’s the equivalent of nine-and-a-half shipping containers a second! Releasing that much water at once has created an enormous hydraulic jump, seen on the right side of the animation. Hydraulic jumps are kind of like the shock wave of open channel flow. On the left side of the image, water is moving smoothly and swiftly down the sluiceway. At the center, the incoming water encounters the large, slow-moving mass of water already in the lake. There’s no way for the incoming water to sustain its kinetic energy while discharging into the lake. Instead a hydraulic jump forms, converting the incoming flow’s kinetic energy into potential energy, as seen in the sudden height increase. Some of the energy is also converted to turbulence and dissipated as heat. (Image credit: U.S. Army Corps of Engineers/AP, source; via Gizmodo)
Search results for: “shock wave”
Re-Entry
Atmospheric re-entry subjects vehicles to extreme conditions. At high Mach numbers, the leading shock wave compresses the air so strongly that it reaches temperatures hotter than the surface of the sun. At these temperatures, oxygen and nitrogen molecules in the air dissociate, bathing a vehicle in a plasma of ionized gas molecules. Often these atoms chemically react with the surface materials of a vehicle causing ablation that removes mass from the vehicle while helping protect the vehicle substructure from re-entry heating. Tests in specialized ground facilities like arc-jet plasma tunnels are necessary to develop thermal protection systems capable of shielding a vehicle during hypersonic flight. (Image credit: D. Ponseggi/NASA)
Early Rocket Launch
Pre-dawn launches provide some of the most dramatic rocket footage. This video is from an October 2nd Atlas V launch, and the really fun stuff starts at about 0:34. As the rocket climbs to higher altitudes, the atmospheric pressure around it decreases. As a result of this low pressure, the rocket’s exhaust gases balloon outward in a giant plume many times larger than the rocket. This happens in every launch, but it’s visible here because the rocket is at such a high altitude that its exhaust is being lit by sunlight while the observers on the ground are still in the dark. The ice crystals in the exhaust–much of the rocket’s exhaust is water vapor–reflect sunlight down to the earth. Around 0:47, a cascade of shock waves ripples through the plume just before the first-stage’s main engine cuts off. Once the engine stops firing, there’s no more exhaust and the plume ends. (Video credit: Tampa Bay Fox 13 News; submitted by Kyle C)
Vapor Cones
Vapor cones typically appear around aircraft flying in the transonic regime–near, but still below, the speed of sound. Air moving over the vehicle accelerates and decelerates as it moves around different parts of the plane; if it didn’t, the plane couldn’t generate lift and wouldn’t fly. When the local flow accelerates past the speed of sound, the accompanying drop in pressure and temperature can be enough to for conditions to fall below the dew point, causing the condensation we see. At the back of the airplane, a shock wave decelerates the airflow back to subsonic speeds and raises local conditions back above the dew point, thereby truncating the cone. (Image credit: C. Caine)
Underwater Explosions
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)
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)
Asteroid Impact
I often receive questions about how fluids react to extremely hard and fast impacts. Some people wonder if there’s a regime where a fluid like water will react like a solid. In reality, nature works the opposite way. Striking a solid hard enough and fast enough makes it behave like a fluid. The video above shows a simulated impact of a 500-km asteroid in the Pacific Ocean. (Be sure to watch with captions on.) The impact rips 10 km off the crust of the Earth and sends a hypersonic shock wave of destruction around the entire Earth. There’s a strong resemblance in the asteroid impact to droplet impacts and splashes. Much of this has to do with the energy of impact. The asteroid’s kinetic (and, indeed, potential) energy prior to impact is enormous, and conservation of energy means that energy has to go somewhere. It’s that energy that vaporizes the oceans and fluidizes part of the Earth’s surface. That kinetic energy rips the orderly structure of solids apart and turns it effectively into a granular fluid. (Video credit: Discovery Channel; via J. Hertzberg)
Simplified Schlieren Set-up
Schlieren photography offers a glimpse into flows that are usually invisible to the human eye. With a relatively simple set-up–a light source, collimating mirror(s), and a razor blade–it becomes possible to see differences in density. The technique lets one visualize temperature-driven flows like the buoyant convection from a flame or other heat source, and it can also be used to visualize shock waves and sound. The video above has several neat schlieren demos, including some non-air examples using hydrogen (lighter than air) and sulfur hexafluoride (denser than air), both of which are transparent to the naked eye. (Video credit: Harvard University, via Jennifer Ouellette)
Cavitation
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Cavitation–the formation and collapse of vapor-filled cavities within a liquid–occurs in a variety of natural and manmade applications. It can shatter bottles, wreak havoc with boat impellers, is used as a hunting mechanism by several shrimp species, and can even generate light and sound. It is the collapse of the cavitation bubble that can be so damaging, and this video shows how. In the experiment, researchers generate a cavitation bubble near the free surface–or, in other words, near the air-water interface. Pressure in the bubble is much lower than the pressure of the surrounding liquid, so the bubble collapses after the momentum from its initial generation is spent. Interaction with the surface generates a jet that projects downward and pierces the cavitation bubble as it collapses. As seen from 0:54 onward, the bubble’s collapse generates a shock wave that propagates outward from the bubble site. It’s this shock wave that so effectively damages materials and stuns underwater prey. (Video credit: O. Supponen et al.)
Sound Interactions
Sound waves often interact with many objects before we hear them. Understanding and controlling those interactions is a major part of acoustic engineering. The animations above show shock waves–sound–from a trumpet interacting with different objects. The sound is made visible using the schlieren optical technique, allowing us to observe the reflection, absorption, and transmission of sound as it hits different surfaces. Fiberboard, for example, is highly reflective, redirecting the sound waves along a new path without a lot of damping. In contrast, the metal grid is only weakly reflective and a small portion of the incoming sound wave is transmitted through the grid. To see more examples, check out the full video, and, if you want to learn more about acoustics, check out Listen To This Noise. (Image credits: C. Echeverria et al., source video)