Moving shock

 ( Aerodynamics ) 

In fluid dynamics, a moving shock is a shock wave that is travelling through a fluid (often ) medium with a velocity relative to the velocity of the fluid already making up the medium.

As such, the normal shock relations require modification to calculate the properties before and after the moving shock. A knowledge of moving shocks is important for studying the phenomena surrounding detonation, among other applications.

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Theory To derive the theoretical equations for a moving shock, one may start by denoting the region in front of the shock as subscript 1, with the subscript 2 defining the region behind the shock. This is shown in the figure, with the shock wave propagating to the right. The velocity of the gas is denoted by u, pressure by p, and the local speed of sound by a. The speed of the shock wave relative to the gas is W, making the total velocity equal to u₁ + W.

Next, suppose a reference frame is then fixed to the shock so it appears stationary as the gas in regions 1 and 2 move with a velocity relative to it. Redefining region 1 as x and region 2 as y leads to the following shock-relative velocities:

$\backslash \; u\_y\; =\; W\; +\; u\_1\; -\; u\_2,$

$\backslash \; u\_x\; =\; W.$

With these shock-relative velocities, the properties of the regions before and after the shock can be defined below introducing the temperature as T, the density as ρ, and the Mach number as M:

$\backslash \; p\_1\; =\; p\_x\; \backslash quad\; ;\; \backslash quad\; p\_2\; =\; p\_y\; \backslash quad\; ;\; \backslash quad\; T\_1\; =\; T\_x\; \backslash quad\; ;\; \backslash quad\; T\_2\; =\; T\_y,$

$\backslash \; \backslash rho\_1\; =\; \backslash rho\_x\; \backslash quad\; ;\; \backslash quad\; \backslash rho\_2\; =\; \backslash rho\_y\; \backslash quad\; ;\; \backslash quad\; a\_1\; =\; a\_x\; \backslash quad\; ;\; \backslash quad\; a\_2\; =\; a\_y,$

$\backslash \; M\_x\; =\; \backslash frac\{u\_x\}\{a\_x\}\; =\; \backslash frac\{W\}\{a\_1\},$

$\backslash \; M\_y\; =\; \backslash frac\{u\_y\}\{a\_y\}\; =\; \backslash frac\{W\; +\; u\_1\; -\; u\_2\}\{a\_2\}.$

Introducing the heat capacity ratio as γ, the speed of sound, density, and pressure ratios can be derived:

$\backslash \; \backslash frac\{a\_2\}\{a\_1\}\; =\; \backslash sqrt\{1\; +\; \backslash frac\{2(\backslash gamma\; -\; 1)\}\{(\backslash gamma\; +\; 1)^2\}\backslash left\backslash gamma\},$

$\backslash \; \backslash frac\{\backslash rho\_2\}\{\backslash rho\_1\}\; =\; \backslash frac\{1\}\{1-\backslash frac\{2\}\{\backslash gamma\; +\; 1\}\backslash left1\},$

$\backslash \; \backslash frac\{p\_2\}\{p\_1\}\; =\; 1\; +\; \backslash frac\{2\backslash gamma\}\{\backslash gamma\; +\; 1\}\backslash leftM\_x^2.$

One must keep in mind that the above equations are for a shock wave moving towards the right. For a shock moving towards the left, the x and y subscripts must be switched and:

$\backslash \; u\_y\; =\; W\; -\; u\_1\; +\; u\_2,$

$\backslash \; M\_y\; =\; \backslash frac\{W\; -\; u\_1\; +\; u\_2\}\{a\_2\}.$

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See also

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External links

• NASA Beginner's Guide to Compressible Aerodynamics

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