# Why does stagnation pressure reduce across a normal shock?

I am seeking an explanation for this graph where the subscript "1" refers to the supersonic region and the subscript "2" refers to the subsonic region present beyond a normal shock.  The static pressure curve shows an increasing trend. Shouldn't the same be applicable to the stagnation pressure Po?

Is the entropy generation associated with the stagnating of the kinetic energy term so high?

• Can you describe the figure and terms a little more? What are the vertical axes, for instance? What are $(P_{o})_{1}$ and $P_{1}$? Is this for a hydrodynamic, collision-mediated shock? – honeste_vivere May 20 '16 at 17:41
• P1 stands for the static pressure in region 1 (before the shock) and (P0)1 stands for the stagnation pressure in the same region. The y-axis show the value of the ratios depicted in the graph. This is not a hydrodynamic case, its for the compressible flow of an ideal gas. – DBTKNL May 23 '16 at 7:35

This can be concluded by reviewing Gibbs equation for upstream and downstream stagnation conditions. $$T_0ds_0=dh_0-\frac 1{\rho_0}dP_0$$
Because across the shock wave is an adiabatic process, $dh_0=0$
Then Gibbs equation becomes $$ds_0=-\frac 1{\rho_0T_0}dP_0=-\frac {R}{P_0}dP_0$$