Schlieren photograph of an attached shock on a sharp-nosed supersonic body. Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density

A shock wave (or simply "shock") is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid or gas) or in some cases in the absence of a material medium, through a field such as the electromagnetic field. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter In Physics, a field is a Physical quantity associated to each point of Spacetime. The electromagnetic field is a physical field produced by electrically charged objects. Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different ^ref In supersonic flows, expansion is achieved through an expansion fan. A Prandtl-Meyer expansion fan is a centered expansion process which turns a supersonic flow around a convex corner A Prandtl-Meyer expansion fan is a centered expansion process which turns a supersonic flow around a convex corner A shock wave travels through most media at a higher speed than an ordinary wave.

Unlike solitons (another kind of nonlinear wave), the energy of a shock wave dissipates relatively quickly with distance. In Mathematics and Physics, a soliton is a self-reinforcing solitary wave (a wave packet or pulse that maintains its shape while it travels at constant speed Also, the accompanying expansion wave approaches and eventually merges with the shock wave, partially cancelling it out. Thus the sonic boom associated with the passage of a supersonic aircraft is the sound wave resulting from the degradation and merging of the shock wave and the expansion wave produced by the aircraft. The term sonic boom is commonly used to refer to the shocks caused by the Supersonic flight of an aircraft

When a shock wave passes through matter, the total energy is preserved but the energy which can be extracted as work decreases and entropy increases. This for example creates additional drag force on aircraft with shocks. Wave drag is an Aerodynamics term that refers to a sudden and very powerful form of drag that appears on aircraft and blade tips moving at high-subsonic and supersonic

Terminology

Shock waves can be either

• Normal: at 90° (perpendicular) to the shock medium's flow direction, or
• Oblique: at an angle to the direction of flow. oblique shock wave unlike a Normal shock, is inclined with respect to the incident upstream flow direction

Some other terms

• Shock Front: an alternative name for the shock wave itself
• Contact Front: in a shock wave cause by an impact (including the impact of a high pressure gas flow), the location of the impact. The Contact Front trails the Shock Front.

In supersonic flows

Pressure-time diagram at an external observation point for the case of a supersonic object propagating past the observer. The leading edge of the object causes a shock (left, in red) and the trailing edge of the object causes an expansion (right, in blue).

When an object (or disturbance) moves faster than the information about it can be propagated into the surrounding fluid, fluid near the disturbance cannot react or "get out of the way" before the disturbance arrives. In a shock wave the properties of the fluid (density, pressure, temperature, velocity, Mach number) change almost instantaneously. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Physics, velocity is defined as the rate of change of Position. Mach number (\mathrm{Ma} or M (generally ˈmɑːk sometimes /ˈmɑːx/ or /ˈmæk/ is the speed of an object moving through air or any Fluid Measurements of the thickness of shock waves have resulted in values approximately one order of magnitude greater than the mean free path of the gas investigated. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it In Physics the mean free path of a particle is the average distance covered by a particle ( Photon, Atom or Molecule) between subsequent impacts

Shock waves form when the speed of a gas changes by more than the speed of sound. Sound is a vibration that travels through an elastic medium as a Wave. ^[1] At the region where this occurs sound waves traveling against the flow reach a point where they cannot travel any further upstream and the pressure progressively builds in that region, and a high pressure shock wave rapidly forms.

Shock waves are not conventional sound waves; a shock wave takes the form of a very sharp change in the gas properties on the order of a few mean free paths (roughly micro-meters at atmospheric conditions) in thickness. In Physics the mean free path of a particle is the average distance covered by a particle ( Photon, Atom or Molecule) between subsequent impacts Shock waves in air are heard as a loud "crack" or "snap" noise. Over longer distances a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft. The term sonic boom is commonly used to refer to the shocks caused by the Supersonic flight of an aircraft For other uses see Supersonic. The term supersonic is used to define a speed that is over the Speed of sound ( Mach 1

The shock wave is one of several different ways in which a gas in a supersonic flow can be compressed. For other uses see Supersonic. The term supersonic is used to define a speed that is over the Speed of sound ( Mach 1 Some other methods are isentropic compressions, including Prandtl-Meyer compressions. In Thermodynamics, an isentropic process ( iso = "equal" (Greek Entropy = "disorder" is one during which the entropy of the system Ludwig Prandtl ( 4 February 1875 &ndash 15 August 1953) was a German Physicist. The method of compression of a gas results in different temperatures and densities for a given pressure ratio, which can be analytically calculated for a non-reacting gas. A shock wave compression results in a loss of total pressure, meaning that it is a less efficient method of compressing gases for some purposes, for instance in the intake of a scramjet. A scramjet ( s upersonic c ombustion ramjet) is a variation of a Ramjet with the distinction being that some or all of the combustion process The appearance of pressure-drag on supersonic aircraft is mostly due to the effect of shock compression on the flow.

Due to nonlinear steepening

Shock waves can form due to steepening of ordinary waves. The best-known example of this phenomenon is ocean waves that form breakers on the shore. Ocean surface waves are Surface waves that occur on the Free surface of the Ocean. A shore or shoreline is the fringe of land at the edge of a large body of water such as an Ocean, Sea, or Lake. In shallow water, the speed of surface waves is dependent on the depth of the water. An incoming ocean wave has a slightly higher wave speed near the crest of each wave than near the troughs between waves, because the wave height is not infinitesimal compared to the depth of the water. The crests overtake the troughs until the leading edge of the wave forms a vertical face and spills over to form a turbulent shock (a breaker) that dissipates the wave's energy as sound and heat.

Similar phenomena affect strong sound waves in gas or plasma, due to the dependence of the sound speed on temperature and pressure. An acoustic wave is a weak compression wave (meaning a small pressure change that moves at the Speed of sound. Sound is a vibration that travels through an elastic medium as a Wave. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Strong waves heat the medium near each pressure front, due to adiabatic compression of the air itself, so that high pressure fronts outrun the corresponding pressure troughs. While shock formation by this process does not normally happen to sound waves in Earth's atmosphere, it is thought to be one mechanism by which the solar chromosphere and corona are heated, via waves that propagate up from the solar interior. The Sun (Sol is the Star at the center of the Solar System. The chromosphere (literally "colour sphere" is a thin layer of the Sun 's atmosphere just above the Photosphere, roughly 10000 kilometres deep A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily

Analogies

A shock wave may be described as the furthest point upstream of a moving object which "knows" about the approach of the object. In this description, the shock wave position is defined as the boundary between the zone having no information about the shock-driving event, and the zone aware of the shock-driving event, analogous with the light cone described in the theory of general relativity. In Special relativity, a light cone (or null cone) is the pattern describing the temporal evolution of a flash of Light in Minkowski spacetime General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916

To get a shock wave something has to be travelling faster than the local speed of sound. In that case some parts of the air around the aircraft are travelling at exactly the speed of sound with the aircraft, so that the sound waves leaving the aircraft pile up on each other, similar to a tailback on a road, and a shock wave forms, the pressure increases, and then spreads out sideways. Because of this amplification effect, a shock wave is very intense, more like an explosion when heard (not coincidentally, since explosions create shock waves).

Analogous phenomena are known outside fluid mechanics. For example, particles accelerated beyond the speed of light in a refractive medium (where the speed of light is less than that in a vacuum, such as water) create visible shock effects, a phenomenon known as Cherenkov radiation. Refraction is the change in direction of a Wave due to a change in its Speed. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Čerenkov radiation (also spelled Cerenkov or Cherenkov) is Electromagnetic radiation emitted when a charged particle (such as an

Examples

Below are a number of examples of shock waves, broadly grouped with similar shock phenomena:

Shock wave propogating into a stationary medium, ahead of the fireball of an explosion. The shock is made visible by the shadow effect (Trinity explosion. Shadowgraph is an optical method that reveals non-uniformities in Transparent media like air water or glass )

Moving shock

• usually consists of a shockwave propagating into a stationary medium
• In this case, the gas ahead of the shock is stationary (in the laboratory frame), and the gas behind the shock is supersonic in the laboratory frame. The shock propagates with a wave front which is normal (at right angles) to the direction of flow. The speed of the shock is a function of the original pressure ratio between the two bodies of gas.
• Moving shocks are usually generated by the interaction of two bodies of gas at different pressure, with a shock wave propagating into the lower pressure gas, and an expansion wave propagating into the higher pressure gas. A Moving Shock is a Shock wave that is traveling through a fluid (usually gaseous medium with a velocity relative to the velocity of the fluid already making up the medium
• Examples: Balloon bursting, Shock tube, shock wave from explosion

Detonation wave

Main article: Blast wave

• A detonation wave is essentially a shock supported by a trailing exothermic reaction. A shock tube is a device used primarily to study gas phase Combustion reactions A blast wave in Fluid dynamics is the pressure and flow resulting from the deposition of a large amount of energy in a small very localised volume Detonation is a process of Supersonic Combustion in which a Shock wave is propagated forward due to energy release in a reaction zone behind it It involves a wave traveling through a highly combustible or chemically unstable medium, such as an oxygen-methane mixture or a high explosive. The chemical reaction of the medium occurs following the shock wave, and the chemical energy of the reaction drives the wave forward.
• A detonation wave follows slightly different rules from an ordinary shock since it is driven by the chemical reaction occurring behind the shock wave front. In the simplest theory for detonations, an unsupported, self-propagating detonation wave proceeds at the Chapman-Jouguet velocity. The Chapman-Jouguet condition holds approximately in Detonation waves A detonation will also cause a shock of type 1, above to propagate into the surrounding air due to the overpressure induced by the explosion.

Shadowgraph of the detached shock on a bullet in supersonic flight, published by Ernst Mach in 1887. Shadowgraph is an optical method that reveals non-uniformities in Transparent media like air water or glass

Detached shock

• These shocks are curved, and form a small distance in front of the body. Directly in front of the body, they stand at 90 degrees to the oncoming flow, and then curve around the body. Detached shocks allow the same type of analytic calculations as for the attached shock, for the flow near the shock. They are a topic of continuing interest, because the rules governing the shock's distance ahead of the blunt body are complicated, and are a function of the body's shape. Additionally, the shock standoff distance varies drastically with the temperature for a non-ideal gas, causing large differences in the heat transfer to the thermal protection system of the vehicle. See the extended discussion on this topic at Atmospheric reentry. These follow the "strong-shock" solutions of the analytic equations, meaning that for some oblique shocks very close to the deflection angle limit, the downstream Mach number is subsonic. See also bow shock or oblique shock
• Such a shock occurs when the maximum deflection angle is exceeded. Bow shock is a normal shock that occurs in front of an object within a supersonic flow oblique shock wave unlike a Normal shock, is inclined with respect to the incident upstream flow direction A detached shock is commonly seen on blunt bodies, but may also be seen on sharp bodies at low Mach numbers.
• Examples: Space return vehicles (Apollo, Space shuttle), bullets, the boundary (Bow shock) of a magnetosphere. A bow shock is a boundary between a Magnetosphere and an ambient medium A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. The name "bow shock" comes from the example of a bow wave, the detached shock formed at the bow (front) of a ship or boat moving through water, whose slow surface wave speed is easily exceeded (see ocean surface wave). A bow wave is the Wave that forms at the bow of a ship when it moves through the water Ocean surface waves are Surface waves that occur on the Free surface of the Ocean.

Attached shock

• These shocks appear as "attached" to the tip of a sharp body moving at supersonic speeds.
• Examples: Supersonic wedges and cones with small apex angles
• The attached shock wave is a classic structure in aerodynamics because, for a perfect gas and inviscid flow field, an analytic solution is available, such that the pressure ratio, temperature ratio, angle of the wedge and the downstream Mach number can all be calculated knowing the upstream Mach number and the shock angle. Smaller shock angles are associated with higher upstream Mach numbers, and the special case where the shock wave is at 90 degrees to the oncoming flow (Normal shock), is associated with a Mach number of one. These follow the "weak-shock" solutions of the analytic equations.

Recompression shock on a transonic flow airfoil, at and above critical Mach number. The Critical Mach number (Mcr of an aircraft is the slowest Mach number at which the airflow over a small region of the wing reaches the speed of sound

Recompression shock

• These shocks appear when the flow over a transonic body is decelerated to subsonic speeds.
• Examples: Transonic wings, turbines
• Where the flow over the suction side of a transonic wing is accelerated to a supersonic speed, the resulting re-compression can be by either Prandtl-Meyer compression or by the formation of a normal shock. This shock is of particular interest to makers of transonic devices because it can cause separation of the boundary layer at the point where it touches the transonic profile. This can then lead to full separation and stall on the profile, higher drag, or shock-buffet, a condition where the separation and the shock interact in a resonance condition, causing resonating loads on the underlying structure.

Shock in a pipe flow

• This shock appears when supersonic flow in a pipe is decelerated.
• Examples: Supersonic ramjet, scramjet, needle valve
• In this case the gas ahead of the shock is supersonic (in the laboratory frame), and the gas behind the shock system is either supersonic (oblique shocks) or subsonic (a normal shock) (Although for some oblique shocks very close to the deflection angle limit, the downstream Mach number is subsonic. A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of Jet engine that contains no major Moving parts. A scramjet ( s upersonic c ombustion ramjet) is a variation of a Ramjet with the distinction being that some or all of the combustion process ) The shock is the result of the deceleration of the gas by a converging duct, or by the growth of the boundary layer on the wall of a parallel duct.

See also

• Atmospheric focusing
• Atmospheric reentry
• Explosions
• Hydraulic jump
• Mach wave
• Magnetopause
• Moving shock
• Oblique shock
• Shocks and Discontinuities (MHD)
• Sonic boom
• undercompressive shock wave

References

External links

• Bomb Shock Wave Estimation
• NASA Glenn Research Center information on:
  • Oblique Shocks
  • Multiple Crossed Shocks
  • Expansion Fans
• Selkirk college: Aviation intranet: High speed (supersonic) flight
  • Energy loss in a shock wave, normal and oblique shock waves
  • Formation of a normal shock wave
• Fundamentals of compressible flow, 2007