Regarding oblique shocks, which statement is correct?

• A. Total pressure behind is higher than in front
• B. Mach number behind is lower than in front
• C. Both statements are correct
• D. Neither statement is correct

Answer: (D)

Across oblique shocks, the flow undergoes irreversible compression: total pressure decreases, while the flow is deflected and the static temperature rises.

The statement that total pressure behind is higher than in front is not correct because a shock always causes a loss in total (stagnation) pressure.

The statement that the Mach number behind is lower than in front is correct. Upstream the flow has a certain speed, and behind the shock the velocity magnitude decreases while the local speed of sound increases due to heating. Since V2 < V1 and a2 > a1, the downstream Mach number M2 = V2/a2 is less than the upstream M1 = V1/a1.

Thus the correct understanding is that the Mach number behind is lower, while total pressure behind is lower as well, making the option describing the Mach-number change the true one.