Facts about Shock Waves: How They Form & What They Do
Shock waves are, at their heart, just a consequence of simple laws of physics. For example, shock waves caused by explosions result from a large, nearly instantaneous “shock” at the epicenter. This shock, a rapid increase in air pressure and heat, causes waves of compressed air to propagate from the center of the explosion.
Hollywood loves to depict shock waves: if you’ve ever watched a film where an explosion picked someone off their feet and threw them across the room, you’ve seen a shock wave. Of course, shock waves come in many different forms and energies. While a firecracker creates a small shock wave, which manifests itself as a loud “crack”, the shock waves from the nuclear bomb dropped on Hiroshima were powerful enough to flatten almost a square mile of buildings. And an earthquake in the Indian Ocean in 2004 created a massive shockwave, a tsunami, that travelled thousands of miles and killed hundreds of thousands of people.
Definition of Shock Wave
A shock wave is a disturbance which propagates itself across a distance. Although shock waves typically move through a medium, they are also capable of propagating through a field.
To the left you see a normal shock from a nuclear explosion.
How Shock Waves Form
Shock waves are created by a near-instantaneous discontinuity, called a shock. The epicenter of a shock wave experiences a rapid change in pressure, temperature, and density. The shock imparts a large amount of energy into a small space; this energy cannot stay in one place for any meaningful amount of time. Instead, the energy quickly propagates outward from the shock, moving in a wave.
Shock Wave Effects
Because shock waves carry large amounts of energy, they are quite capable of interacting with their surroundings. When a shock wave encounters a solid obstruction, it passes through the obstruction and comes out with the same total energy. However, in passing through the obstruction, the shock wave can damage or destroy it. Though the shock wave has the same total energy, it gains entropy (disorder) upon passing through an object, while losing destructive capability. Therefore, the more destruction a shock wave inflicts, the faster it decays.
Shock waves also lose energy fairly rapidly with distance. As they travel from the epicenter, shock waves spread out and are weakened by progressive merging with the expansion wave.
Intersection of Conical Shock Waves
Shock waves don’t necessarily propagate in all directions; they may instead propagate at an angle. Shock waves that propagate at an angle are called Conical (or Oblique) shock waves. These types of shock waves typically occur in supersonic travel, such as by jet planes. In fact, supersonic jets are engineered to create and take advantage of conical shock waves.
The wings of supersonic jets are shaped in a thin wedge (or diamond) shape in order to create two conical shock waves. These shock waves propagate above and below the wing. When they intersect, they generate lift for the airplane; this effect is only present at supersonic speeds.
Earthquakes and Shock Waves
The theory of earthquakes posits that they occur primarily along fault lines. As two or more tectonic plates move past each other, they grind, bump, and push up along irregularities in the faults. This builds up a tremendous amount of strain energy, which is released explosively when the irregularities finally yield. Earthquakes transmit their energy through the Earth’s crust; the energy travels in shock waves. These shock waves can cause tremendous damage above ground in older buildings or ones not built to withstand earthquakes. In general, the closer one is to an earthquake’s epicenter the stronger the earthquake will feel.
Because earthquake shock waves cause a rolling effect in the earth’s crust, engineers must design buildings to withstand significant movement in the event of an earthquake. Buildings that aren’t built to earthquake code can simply topple over, as happened in Haiti during their recent earthquake.
Shock Waves Forming in an Engine Intake
Shock waves can also form in engine intakes in supersonic jets. The shock waves form because air further along the intake pipe is travelling at speeds substantially below the speed of sound, while air in the intake is travelling at supersonic speeds. The rapid deceleration generates shock waves. If the air at the end of the pipe is also supersonic, oblique shock waves are generated; otherwise, only normal shock waves are present. Modern aircraft, including ramjets and scramjets, make use of this phenomenon.
Granular Flow Waves
Granular flow waves can occur when a dense material flows down a channel. Granular flow waves can occur in very dense yet granular materials, such as snow during an avalanche. When the flow encounters an obstruction, rapid changes within the flow generate flow waves in the material. Engineering analysis of granular flow waves is important to predict the intensity of waves that will hit a structure when it is struck by a granular flow. Flow waves are also created when a granular flow falls quickly from supercritical to subcritical.
NASA and Shock Waves
Shock waves don’t just occur on planet Earth. NASA has extensively studied shock waves among the stars, which have vital implications for life on Earth. When a star goes supernova, massive shock waves travel many light years, maintaining enormous quantities of energy as they do so. These shock waves travel as radiation, and can destroy whole solar systems.
NASA has also studied shock waves encountered by the Earth every day. These shocks, called bow shocks, occur when the solar wind of radiation from the sun runs into Earth’s magnetic field. This type of shock creates beautiful colors, which are known as the northern lights.
A more recently discovered type of shock wave is Cherenkov radiation, which occurs in nuclear reactors. This type of shock wave occurs when a particle travels faster than the local speed of light, which causes polarization of the water. The water molecules then quickly return to their original state, emitting a flare of radiation. This effect is the cause of the blue glow seen in nuclear reactors. Although it is impossible to travel faster than the speed of light in a vacuum, c, it is possible to exceed the speed of light in a medium. For instance, the speed of light in water is only 0.75c, so reactions in a nuclear reactor can actually cause movement faster than the local speed of light.
Images are taken from Wikimedia commons