Tag: shockwave

Shock Waves in Flight
Schlieren optical systems have been used to visualize shock waves in labs for more than a century, but the technique did not translate well to photographing shock structures outside the lab. But now NASA’s Armstrong Research Center and Ames Research Center have developed a method that allows them to capture highly-detailed images of the shock waves around airplanes while they are flying. This is incredible stuff. Be sure to check out the high-resolution versions on this page, along with more description of the coordination necessary to pull off the photos.
The light and dark lines you see emanating from the airplane are places with strong density gradients. The dark lines are mostly shock waves, with the strongest shock waves appearing black due to the large change in air density. Many of the light streaks are expansion fans, areas where the density and pressure drop as air speeds up.
The goal of this research is to better understand shock wave structures around supersonic planes in order to reduce the noise supersonic aircraft cause when flying overhead. As you can see in the photos, the shock waves at the nose and tail of the aircraft persist far away from the aircraft; these are what cause the twin sonic boom heard when the plane flies by. (Photo credit: NASA; via J. Hertzberg)
August 31, 2015
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)
June 22, 2015
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)
May 13, 2015
Blast Waves Visualized
Typically, shock waves are invisible to the human eye. Using sensitive optical techniques like schlieren photography, researchers in a lab can visualize sharp density gradients like shock waves or even the slight density variations caused by natural convection. But it takes some special conditions to make shock waves visible to the naked eye. The blast wave of the explosion in the photo above is a great example. The leading edge of the shock wave and the heat of the explosion create a strong, sharp change in density. That density change is accompanied by a change in the air’s refractive index. As light travels from the distance toward the camera, it’s distorted–more specifically, refracted–when it travels through the blast wave and its wake. And, in this case, that visual distortion is strong enough that we can clearly see the outlines of the shock waves moving out from the explosion. The apparent horizontal line through the blast wave is probably the intersection of a weaker secondary shock wave with the initial expanding shock wave. (Image credit: Defense Research and Development Canada; via io9)
April 8, 2015
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)
March 6, 2015
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)
December 1, 2014
Shooting Droplets with Lasers
Last week we saw what happens when a solid projectile hits a water droplet; today’s video shows the impact of a laser pulse on a droplet. Several things happen here, but at very different speeds. When the laser impacts, it vaporizes part of the droplet within nanoseconds. A shock wave spreads from the point of impact and a cloud of mist sprays out. This also generates pressure on the impact face of the droplet, but it takes milliseconds–millions of nanoseconds–for the droplet to start moving and deforming. The subsequent explosion of the drop depends both on the laser energy and focus, which determine the size of the impulse imparted to the droplet. The motivation for the work is extreme ultraviolet lithography–a technique used for manufacturing next-generation semiconductor integrated circuits–which uses lasers to vaporize microscopic droplets during the manufacturing process. (Video credit: A. Klein et al.)
October 16, 2014
Transonic Flow
In the transonic speed regime the overall speed of an airplane is less than Mach 1 but some parts of the flow around the aircraft break the speed of sound. The photo above shows a schlieren photograph of flow over an airfoil at transonic speeds. The nearly vertical lines are shock waves on the upper and lower surfaces of the airfoil. Although the freestream speed in the tunnel is less than Mach 1 upstream of the airfoil, air accelerates over the curved surface of airfoil and locally exceeds the speed of sound. When that supersonic flow cannot be sustained, a shock wave occurs; flow to the right of the shock wave is once again subsonic. It’s also worth noting the bright white turbulent flow along the upper surface of the airfoil after the shock. This is the boundary layer, which can often separate from the wing in transonic flows, causing a marked increase in drag and decrease in lift. Most commercial airliners operate at transonic Mach numbers and their geometry is specifically designed to mitigate some of the challenges of this speed regime. (Image credit: NASA; via D. Baals and W. Corliss)
September 19, 2014
Mach Diamonds
Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)
June 10, 2014
Putting Out Wildfires Using Explosives
Wildfires damage millions of acres of land per year in the United States alone. Using explosives to put out an uncontrolled wildfire sounds a bit crazy, but it’s actually not that far-fetched. The animations above are taken from high-speed footage of a propane fire interacting with a blast wave. The first animation shows what the human eye would see, and the second is a shadowgraph video, a technique which highlights differences in density and makes the flame’s convection and the blast wave itself visible. At close range, the shock wave from the explosion and the high-speed gas behind it push the flames away from their fuel source, stopping combustion almost immediately. For a flame farther away from the blast, the shock wave introduces turbulent disturbances that can destabilize the flame. Much work remains to be done before the technique could be scaled from the laboratory to the field, but it is an exciting concept. You can read more about the work here. (Research credit: G. Doig/UNSW Australia; original videos: here and here; submitted by @CraigOverend)
May 22, 2014