Shock is an abrupt discontinuity in the flow field and it occurs in flows when the local
flow speed exceeds the local
sound speed.
[Courant, Richard, and K.O. Friedrichs. ]
Supersonic Flow and Shock Waves
'. Springer Science & Business Media, 1999. 2-3. More specifically, it is a flow whose
Mach number exceeds 1.
Explanation of phenomena
Shock is formed due to
coalescence of various small pressure pulses.
Sound wave
In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
s are
pressure waves and it is at the speed of the sound wave the disturbances are ''communicated'' in the medium. When an object is moving in a flow field the object sends out disturbances which propagate at the speed of sound and ''adjusts'' the remaining flow field accordingly. However, if the object itself happens to travel at speed greater than sound, then the disturbances created by the object would not have traveled and ''communicated'' to the rest of the flow field and this results in an abrupt change of property, which is termed as ''shock'' in gas dynamics terminology.
Shocks are characterized by discontinuous changes in flow properties such as
velocity
Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...
,
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
,
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
, etc. Typically, shock thickness is of a few
mean free paths (of the order of 10
−8 m). Shocks are irreversible occurrences in
supersonic flows (i.e. the
entropy
Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...
increases).
Normal shock formulas
:
:
:
:
:
:
:
:
Where, the index 1 refers to upstream properties, and the index 2 refers to down stream properties. The subscript 0 refers to total or stagnation properties. T is temperature, M is the mach number, P is pressure, ρ is density, and γ is the ratio of
specific heats.
See also
*
Mach number
*
Sound barrier
*
supersonic flow
References
{{reflist
Fluid dynamics