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Moving shock

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, pressurebyp, and the local speed of soundbya. 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:

${\displaystyle \ u_{y}=W+u_{1}-u_{2},}$

${\displaystyle \ u_{x}=W.}$

With these shock-relative velocities, the properties of the regions before and after the shock can be defined below introducing the temperatureasT, the densityasρ, and the Mach numberasM:

${\displaystyle \ p_{1}=p_{x}\quad ;\quad p_{2}=p_{y}\quad ;\quad T_{1}=T_{x}\quad ;\quad T_{2}=T_{y},}$

${\displaystyle \ \rho _{1}=\rho _{x}\quad ;\quad \rho _{2}=\rho _{y}\quad ;\quad a_{1}=a_{x}\quad ;\quad a_{2}=a_{y},}$

${\displaystyle \ M_{x}={\frac {u_{x}}{a_{x}}}={\frac {W}{a_{1}}},}$

${\displaystyle \ M_{y}={\frac {u_{y}}{a_{y}}}={\frac {W+u_{1}-u_{2}}{a_{2}}}.}$

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

${\displaystyle \ {\frac {a_{2}}{a_{1}}}={\sqrt {1+{\frac {2(\gamma -1)}{(\gamma +1)^{2}}}\left[\gamma M_{x}^{2}-{\frac {1}{M_{x}^{2}}}-(\gamma -1)\right]}},}$

${\displaystyle \ {\frac {\rho _{2}}{\rho _{1}}}={\frac {1}{1-{\frac {2}{\gamma +1}}\left[1-{\frac {1}{M_{x}^{2}}}\right]}},}$

${\displaystyle \ {\frac {p_{2}}{p_{1}}}=1+{\frac {2\gamma }{\gamma +1}}\left[M_{x}^{2}-1\right].}$

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:

${\displaystyle \ u_{y}=W-u_{1}+u_{2},}$

${\displaystyle \ M_{y}={\frac {W-u_{1}+u_{2}}{a_{2}}}.}$

See also[edit]

• Shock wave
• Oblique shock
• Normal shock
• Gas dynamics
• Compressible flow
• Bow shock (aerodynamics)
• Prandtl-Meyer expansion fan

References[edit]

External links[edit]

• NASA Beginner's Guide to Compressible Aerodynamics